US 8,946,683 B2Grant
Medium scale carbon nanotube thin film integrated circuits on flexible plastic substrates
Issue Date:2015-02-03
•55 Claims
•36 Drawing Sheets
Abstract
The present invention provides device components geometries and fabrication strategies for enhancing the electronic performance of electronic devices based on thin films of randomly oriented or partially aligned semiconducting nanotubes. In certain aspects, devices and methods of the present invention incorporate a patterned layer of randomly oriented or partially aligned carbon nanotubes, such as one or more interconnected SWNT networks, providing a semiconductor channel exhibiting improved electronic properties relative to conventional nanotubes-based electronic systems.
Metadata
Assignees
- The Board of Trustees of the University of Illinois
- Purdue Research Foundation
Inventors
- John A. Rogers
- Qing Cao
- Muhammad Alam
- Ninad Pimparkar
Application Information
Application Number:US 12/996,924
Filing Date:2009-06-16
Priority Date:2008-06-16
Art Unit:2815
Classifications
IPC:
H01L51/05H01L51/00B82Y10/00
Field of Search:
B82Y 10/00H01L 51/0048H01L 51/0541
Patent Drawings (36 sheets)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority to U.S. provisional Patent Applications 61/061,710 filed Jun. 16, 2008 and 61/074,254 filed Jun. 20, 2008, which are hereby incorporated by reference in their entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support awarded by the following agencies: National Science Foundation Grant NIRT-0403489 and U.S. Department of Energy Grant DE-FG02-07ER46471. The United States government has certain rights in the invention.
BACKGROUND OF INVENTION
[0003] Since their discovery in the early 1990s, a great deal has been learned about the composition and properties of carbon nanotube materials. This research has demonstrated that carbon nanotubes exhibit extraordinary mechanical, electronic and chemical properties, which has stimulated substantial interest in developing applied technologies exploiting these properties. Accordingly, substantial research is directed at developing techniques for organizing, arranging and incorporating carbon nanotube materials into useful functional devices.
[0004] Carbon nanotubes are allotropes of carbon comprising one or more cylindrically configured graphene sheets and are classified on the basis of structure as either single walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs). Typically having small diameters (≈1-30 nanometers) and large lengths (up to several microns), SWNTs and MWNTs commonly exhibit length to diameter ratios of ≈102 to about 107). Carbon nanotubes exhibit either metallic or semiconductor electrical behavior, and the energy band structure of nanotube materials varies considerably depending on their precise molecular structure and diameter. Doped nanotubes having intercalants, such as potassium, have been prepared and the central cavities of nanotubes have been filled with a variety of materials, including crystalline oxide particles, metals, gases and biological materials.
[0005] Single walled carbon nanotubes (SWNTs), in particular, are identified as candidates for functional materials in a new generation of high performance passive and active nanotube based electronic devices. SWNTs are made up of a single, contiguous graphene sheet joined with itself to form a hollow, seamless tube, in some cases with capped ends similar in structure to smaller fullerenes. SWNTs typically have very small diameters (≈1 nanometer) and are often present in curled, looped and bundled configurations. SWNTs are chemically versatile materials capable of functionalization of their exterior surfaces and encapsulation of materials within their hollow cores, such as gases or molten materials.
[0006] A number of unique properties of SWNTs make these materials particularly attractive for a variety of emerging applied technologies, including sensors, light emissive systems, flexible electronics and novel composite materials. First, SWNTs are believed to have remarkable mechanical properties, such as tensile strengths at least 50 times that of steel. Second, the electron transport behavior in SWNTs is predicted to be essentially that of a quantum wire, and the electrical properties of SWNTs have been observed to vary upon charge transfer doping and intercalation, opening up an avenue for potentially tuning the electrical properties of nanotube materials. Finally, SWNTs have also been demonstrated to have very high intrinsic field affect mobilities (e.g., about 10,000 cm2V−1s−1) making them interesting for possible applications in nanoelectronics.
[0007] The astonishing electronic and mechanical properties of SWNTs, together with the ability to deposit nanotubes onto plastics and other unusual device substrates, make them well-suited for use in large-scale distributed electronics for steerable antenna arrays, flexible displays, and other systems. Recent work indicates that random networks of SWNTs can form effective semiconductor layers for thin-film transistor- (TFT-) type devices. Device geometries accessed using random networks of SWNTs are particularly promising for enabling low cost, high-performance devices and device arrays for the field of large area electronics. First, random SWNT networks can be effectively assembled at relatively low costs using a range of solution deposition-based fabrication techniques, including solution casting, ink jet printing and screen printing. Second, device geometries employing random SWNT networks for semiconductor channels in electronic devices are also compatible with low temperature assembly on a range of substrates, including flexible polymer substrates desirable for applications of flexible electronics.
[0008] Despite substantial progress in developing a SWNT-based electronic device platform using random SWNT networks, several factors impede commercialization of these systems. First, the device mobilities that have been achieved with these networks are far below the intrinsic tube mobilities inferred from measurements of transistors that incorporate an individual tube (or small number of tubes) spanning the gap between the source and drain electrodes. The resistance at the many tube-tube contacts that are inherent in the networks may limit charge transport. Second, films comprising random SWNT networks are typically a mixture of metallic tubes and semiconducting tubes. The presence of metallic tubes in the network often results in a significant extent of purely metallic conductive pathways between the source/drain (S/D) electrodes of SWNT-based thin-film transistor (TFT) devices. Such metallic conductive pathways decrease the device on/off ratio attainable and generally increase the static power consumption, thereby preventing their applications for important classes of electronics systems.
[0009] U.S. Pat. No. 7,226,818, issued on Jun. 5, 2007, discloses field-effect transistors based on random SWNT networks enriched in semiconducting nanotubes relative to metallic nanotubes. Enrichment of the semiconducting nanotube component is reported as providing enhanced on/off ratios for thin film transistors based on random SWNT networks. Techniques for enriching the semiconducting nanotube component described in this reference include solution fractionation techniques and selective chemical removal of metallic nanotubes. The authors also report that the channel length can be adjusted to assure that no individual tube spans its length, thus precluding a metallic tube from directly short-circuiting the thin film transistor.
[0010] U.S. Pat. No. 6,918,284, issued on Jul. 19, 2005, discloses electronic devices having a semiconductor component comprising an interconnected network or array of carbon nanotubes. The authors exemplify very dilute networks such that at least 75% or substantially all the carbon nanotubes are at least partially in contact with the substrate. The authors report that applying a large source-drain bias while gating off any semiconductive nanotubes can be used to selectively burn metallic nanotubes.
[0011] The reference “p-Chanel, n-Channel Thin Film Transistors and p-n Diodes Based on Single Wall Carbon Nanotube Networks”, Nanoletters, Vol. 4, No. 10 (2004) 2031-2035, by Zhou et al., discloses single wall carbon nanotube networks providing semiconductor channels for thin film transistors. The disclosed device geometry is designed to electrically isolate adjacent devices, and the strip widths are wider than the channel lengths.
[0012] It will be appreciated from the foregoing that there is currently a need in the art for improved device geometries, components, and fabrication methods to enable passive and active carbon nanotube electronic devices based on random SWNT networks.
SUMMARY OF THE INVENTION
[0013] The present invention provides device component geometries and fabrication strategies for enhancing the electronic performance of electronic devices based on thin films of randomly oriented or partially aligned semiconducting nanotubes. In certain aspects, devices and methods of the present invention incorporate a patterned layer of randomly oriented or partially aligned carbon nanotubes, such as one or more interconnected SWNT networks, providing a semiconductor channel exhibiting improved electronic properties relative to conventional nanotube-based electronic systems.
[0014] Embodiments of this aspect include, for example, electronic devices and device components comprising a patterned layer of randomly oriented or partially aligned carbon nanotubes having void regions, such as etched regions, channels and/or cavities in the nanotube layer, that reduce the number and extent of purely metallic conductive pathways between electrodes of an electronic device. Methods and devices of certain aspects reduce the number and extent of purely metallic conductive pathways in one or more interconnected carbon nanotube networks so as to access nanotube-based transistors exhibiting improved on/off ratios and decreased static power consumption. In specific embodiments, void regions are patterned into a monolayer or sub-monolayer film of randomly oriented or partially aligned nanotubes, for example by photolithography and etching methods, that define a plurality of physically isolated or interconnected strips comprising interconnected carbon nanotube networks (e.g., bands of interconnected nanotube networks). In some embodiments, strips are provided, for example, comprising discrete bands of interconnected carbon nanotube networks that extend lengths from the first electrode to the second electrode. Optionally, each of the bands extend an average width in a dimension orthogonal to the average length, wherein the average width of the band is less than the length of the strip, for example, 10 times less or preferably for some embodiments 50 times less or preferably for some embodiments 100 times less. As semiconducting SWNTs are more abundant than metallic SWNTs in the nanotube layer, the formation of strips by patterning voids in the interconnection carbon nanotube network effectively limits the extent and number of purely metallic conductive pathways between the source and drain electrodes of the transistor. Accordingly, the patterned nanotube layer device geometries of the present invention provide an effective means of increasing device on/off ratios by at least an order of magnitude (and in some embodiments up to 4 orders of magnitude) while only slightly impacting the overall effective device mobility of these systems.
[0015] The present fabrication methods and device geometries are versatile, thereby providing a platform enabling a new class of nanotube-based electronic devices and systems well suited for a range of device applications, including thin film electronics, large area electronics (e.g., macroelectronics), flexible electronics, and sensing. Methods and devices of the present invention are compatible with low temperature, solution-based processing and assembly on a wide range of device substrates, including mechanically flexible substrates such as polymer substrates. Processing methods and design strategies of the present invention are complementary to conventional microfabrication and nanofabrication platforms, and can be effectively integrated into existing photolithographic, etching and thin film deposition patterning strategies, systems and infrastructure. In specific embodiments, methods and device geometries of the present invention enable low cost fabrication of high performance nanotube based semiconductor devices, such as thin film transistors, transistor arrays, and integrated electronic circuits.
[0016] In an aspect, the present invention provides electronic devices and components thereof having a semiconductor channel comprising a layer of randomly oriented or partially aligned carbon nanotubes that is patterned so as to provide a reduction in the number of purely metallic conductive pathways through the nanotube layer. In a specific embodiment, for example, an electronic device of the present invention comprises: a first electrode; a second electrode; and a patterned layer of randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with the first electrode and the second electrode. In an embodiment, the patterned layer has a thickness less than or equal to 10 nanometers and has one or more void regions without carbon nanotubes. In some embodiments, for example, void region(s) are provided in the patterned layer so as to reduce by at least 20% the number of purely metallic conductive pathways between the first electrode and the second electrode in the one or more interconnected carbon nanotube networks. As used herein, interconnected carbon nanotube network refers to a network of carbon nanotubes having at least one nanotube crossing. As used herein, interconnected nanotube networks of the present invention include networks wherein only, a portion and not all, of the nanotubes are interconnected.
[0017] In the context of this description, the reduction of purely metallic conductive pathways between first and second electrodes is defined relative to a corresponding interconnected nanotube network layer not having void regions (e.g., a corresponding layer having the same or similar, physical dimensions (e.g., length, width and thickness), nanotube composition, nanotube surface concentration, nanotube size distribution, electrical connection to the electrodes, etc.). In an embodiment, the patterned layer has a plurality of void regions, optionally provided in a preselected pattern. In an embodiment, the electronic device further comprises a substrate, such as a mechanically flexible substrate, provided to support the first electrode, second electrode and patterned layer. In the context of this description, the substrate can support these device elements directly or via one or more intermediate structures, such as thin film layers, device components or other structures. In an embodiment, the patterned nanotube layer provides a semiconductor channel between the first and second electrodes, wherein the semiconductor channel has a length selected over the range of 50 nanometers and 1000 microns, preferably for some applications 100 nanometers to 100 microns, preferably for some applications 1 micron to 10 microns.
[0018] The presence of void regions in patterned nanotube layers of the present device provides a means of effectively controlling, limiting, and/or reducing the number and extent of purely metallic conductive pathways through the patterned nanotube layer. In the context of this description, the term “void region” refers to a region of the patterned layer that does not have carbon nanotubes present. Void regions useful in the present systems and components thereof include cavities, grooves, openings, channels and/or holes provided in the layer of randomly oriented or partially aligned carbon nanotubes. Void regions in some embodiments electrically isolate discrete regions of the interconnected nanotube network of the patterned layer. For example, void regions in some embodiments, electrically isolated strips of interconnected nanotube network of the patterned layer. In an embodiment, void regions of the patterned layer are regions wherein carbon nanotubes have been selectively removed via etching, such as oxygen plasma etching, or other material removal techniques such as laser ablation patterning. For example, the present invention includes embodiments wherein the void regions comprise a pattern etched into the patterned layer, such as an etch pattern comprising one or more lines, channels, parallel lines, serpentine lines, dashed lines, diagonal lines, or dots. Optionally, void regions of the present patterned layer may be filled with one or more insulating and/or semiconducting material. In an embodiment, for example, void regions in the patterned layer are filed with a polymer and/or dielectric layer or thin film. In an embodiment, the invention provides a semiconductor device having a patterned layer comprising the semiconductor channel in the device, wherein the patterned layer has 2 to 1000 void regions, preferably for some applications 5 to 500 void regions and preferably for some applications 10 to 100 void regions.
[0019] In electronic devices of the present invention, the presence of void regions in the patterned nanotube layer provides semiconductor channels between first and second electrodes having less purely metallic conductive pathways relative to a corresponding nanotube layer not patterned with the void regions (e.g., a corresponding layer having the same or similar, physical dimensions such as length, width and thickness). In some embodiments, the reduction in purely metallic conductive pathways provides transistors having high on/off ratios and low static power consumption. The physical dimensions, positions and/or pattern of the void regions in the patterned layer determine, at least in part, the extent to which the number of purely metallic conductive pathways is reduced in semiconductor channels of the present invention. In some embodiments, for example, the position, physical dimensions and/or pattern of void regions in the patterned layer is selected so that the number of purely metallic conductive pathways between electrodes through the carbon nanotube layer is reduced by more than 20%, 25%, 30%, 35%, 40,%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 95%, 97%, 99% or 99% relative to a corresponding layer not having the void regions of the patterned layer.
[0020] Void regions in the patterned layer of the present invention can have a range of physical dimensions. Useful physical dimensions in some embodiments are such that the void regions disrupt one or more metallic conductive pathway in the nanotube layer. In some embodiments, void regions of the patterned layer extend the entire thickness of the patterned layer, or substantially the entire thickness (e.g., within 10%) of the patterned layer. Use of void regions extending the entire thickness of the patterned layer is useful for some applications to electrically isolate discrete regions of interconnected nanotube networks, such as electrically isolated strips of interconnected carbon nanotubes. In some embodiments, the thickness (e.g., depth into the patterned layer) of the void region is selected from the range of 2 to 10 nanometers. In some embodiments, the lateral dimensions, such as length, width, radius, diameter etc., of the void regions is selected over the range of 50 nanometers to 1000 microns, preferably for some applications 100 nanometers to 10 microns, and preferably for some applications 1 micron to 10 microns. As used herein lateral dimensions are dimensions along axes orthogonal to the thickness dimension, such as length and width dimensions in a plane parallel to the substrate surface supporting the interconnected nanotube network) Void regions of the present invention may have a range of shapes, including void regions having a lateral cross sectional shape selected from the group consisting of rectangle, square, circle, and oval. Void regions are optionally provided in a preselected pattern in the patterned layer. A range of patterns for void regions are useful in the present invention. In an embodiment, void regions are provided in a pattern having a periodic spatial distribution. Alternatively, the present invention includes devices and device components having a patterned layer wherein void regions are provided in a pattern having an aperiodic spatial distribution. Patterns of void regions useful in the present invention include, but are not limited to, parallel lines, grids, and arrays of lines, dots and/or dashes.
[0021] In a specific embodiment, patterned layers of randomly oriented or partially aligned carbon nanotubes comprise a plurality of void regions that define one or more interconnected or physically isolated strips comprising interconnected carbon nanotube networks extending between the first and second electrode. Embodiments of the present invention having a patterned layer comprising a plurality of physically isolated strips, for example, enable nanotube-based transistors exhibiting enhanced on/off ratios relative to conventional transistors based on random networks of SWNTs. In an embodiment, for example, void regions comprise a plurality of cavities or channels in the patterned layer extending the entire distance from the first electrode to the second electrode, thereby defining a plurality of strips of interconnected carbon nanotube networks in the patterned layer. Accordingly, this configuration provides a plurality of strips positioned to directly connect the first and second electrodes. In an embodiment, for example, cavities in the patterned layer separate the strips such that the strips do not physically contact each other. Optionally, cavities may be provided having lateral dimensions, such as widths, large enough to prevent electron transport directly between adjacent strips of the patterned layer. In an embodiment, for example, adjacent strips of the patterned layer are separated from each other by an average distance selected over the range of 50 nanometers to 1000 microns, preferably for some applications 100 nanometers to 10 microns, preferably for some applications 100 nanometers to 1 micron.
[0022] Selection of the number, physical dimensions and pattern of strips in patterned nanotube layers of this aspect determines, at least in part, the overall conductance of the patterned layer, and in some embodiments the electronic properties (e.g., on/off ratios, field effect mobility, etc.) of transistors of the present invention. Patterned nanotube layers of this aspect of the present invention may comprise a plurality of strips providing useful properties for a desired electronic device application, such a semiconductor channel for a transistor exhibiting useful properties. In an embodiment, for example, the patterned layer has a plurality of void regions comprising cavities that define from 2 to 1000 strips of interconnected carbon nanotubes extending from first to second electrodes, in some embodiments 10-500 strips of interconnected carbon nanotubes extending from first to second electrodes, and in some embodiments 20 to 100 strips of interconnected carbon nanotubes extending from first to second electrodes. In some embodiments, each of the strips of the patterned layer has an average width and extends an average length from first to second electrodes at least 10 times greater than the average width, preferably for some applications a length at least 50 times greater than the average width, preferably for some applications a length at least 100 times greater than the average width and preferably for some applications a length at least 1000 times greater than the average width. Strips useful in this aspect of the present invention may have aspect ratios (aspect ratio=(average length)/(average width), wherein the length corresponds to the direction from first to second electrodes (e.g., from source electrode to drain electrode) selected over the range of 10 to 1000, and preferably for some applications aspect ratios selected over the range of 50 to 500, preferably for some applications aspect ratios selected over the range of 100 to 250 and preferably for some applications 20 to 100. Strips useful in this aspect of the present invention may have an average width selected over the range of 50 nanometers to 1000 microns (preferably for some embodiments 100 nanometers to 10 microns) and an average length from first to second electrode selected over the range of 500 nanometers to 10000 microns (preferably for some embodiments 100 nanometers to 10 microns) wherein the length corresponds to the direction from first to second electrodes. Strips useful in this aspect of the present invention may have a lateral cross sectional shape selected from the group consisting of rectangular, circular, oval, and trapezoidal. In some embodiments, strips of the patterned layer are provided in a parallel orientation, wherein each strip extends a length from first to second electrodes that is parallel to a central alignment axis. Alternatively, strips of the patterned layer are provided in a serpentine configuration extending from first to second electrodes. Alternatively, strips of the patterned layer are interconnected with each other. In an embodiment, the strips of interconnected carbon nanotubes of the patterned layer extend from first electrode to second electrode and have an area of 1 micron2 to 100,000 micron2, preferably for some embodiments 10 micron2 to 10,000 micron2, and preferably for some embodiments 100 micron2 to 1,000 micron2.
[0023] Carbon nanotubes of the present invention may be single walled carbon nanotubes, multiwalled carbon nanotubes or a mixture of both. Use of single walled nanotubes (SWNTs) is preferred for some applications given their particularly useful semiconducting properties. In an embodiment, the patterned layer is a monolayer or sub-monolayer of carbon nanotubes. In an embodiment, the carbon nanotubes of the patterned layer have an average length selected over the range of 20 nanometers to 100 microns, preferably for some applications selected over the range of 100 nanometers to 10 microns. In an embodiment, the carbon nanotubes of the patterned layer have a surface concentration selected over the range of 0.2 carbon nanotubes micron−2 to 100 carbon nanotubes micron−2, preferably for some embodiments 0.5 carbon nanotubes micron−2 to 20 carbon nanotubes micron−2, preferably for some embodiments 1 carbon nanotubes micron−2 to 10 carbon nanotubes micron−2. As used herein, the terms “nanotube surface concentration” and “nanotube density” are used interchangeably and refer to the number of nanotubes per area of substrate having the nanotubes. In an embodiment, carbon nanotubes of the patterned layer are a mixture of semiconducting nanotubes and metallic nanotubes, wherein there are more semiconducting nanotubes than metallic nanotubes. Conventional sources of carbon nanotubes, such as SWNTs, typically generate mixtures having more semiconducting nanotubes than metallic nanotubes, for example mixtures having between 60-80% semiconducting nanotubes and 40-20% metallic nanotubes or mixtures having between 65-75% semiconducting nanotubes and 35-25% metallic nanotubes. In an embodiment, carbon nanotubes of the patterned layer are a mixture of semiconducting nanotubes and metallic nanotubes, wherein there are more semiconducting nanotubes than metallic nanotubes, for example a mixture wherein there are at least 1.5 times more semiconducting nanotubes than metallic nanotubes, and in some embodiments wherein there are 1.5-4 times more semiconducting nanotubes than metallic nanotubes. Carbon nanotubes of the patterned layer can be generated by a range of synthetic methods including, chemical vapor deposition, pyrolysis, arc discharge, catalytic methods and laser ablation methods. Patterned layers of carbon nanotubes of the present invention may further comprise additional components, such as dopants or components enhancing the mechanical properties of the nanotube layer. In an embodiment, for example, carbon nanotubes of the patterned layer are optionally provided in a polymeric matrix, such as a polymer encapsulation layer encapsulating the nanotubes of the patterned layer. Use of an encapsulation layer encapsulating the nanotubes of the patterned layer is beneficial in some embodiments for providing mechanical robust, flexible and/or deformable electronic devices.
[0024] Devices and device components of the present invention may be supported by a substrate. Useful substrates for supporting devices and device components of the present invention include but are not limited to mechanically flexible substrates such as polymer substrates, dielectric substrates, metal substrates, ceramic substrates, glass substrates, semiconductor substrates and functional substrates prepatterned with one or more device components. The present invention also includes devices and device components provided on (i.e. supported by) contoured substrates, including curved substrates, curved rigid substrates, concave substrates, and convex substrates.
[0025] In an embodiment, the present invention provides a transistor wherein the patterned nanotube layer provides a semiconductor channel between first and second electrodes comprising source and drain electrodes. Transistors of the present invention may further comprise a gate electrode and dielectric layer; wherein the dielectric layer is provided between the gate electrode and the patterned layer. In some embodiments, the gate electrode is electrically isolated from, and positioned close enough to, the semiconductor channel such that electron transport through the channel is modulated by application of an electric potential to the gate electrode. In some embodiments, the patterned layer has a strip geometry and comprises a plurality of strips of interconnected carbon nanotube networks, wherein strips of interconnected carbon nanotubes extend lengths from source to drain electrodes and are aligned in the electron transport direction of the transistor, optionally in a parallel strip orientation. In an embodiment, a transistor of this aspect is a thin film transistor. In an embodiment, a transistor of this aspect has an on/off ratio greater than or equal to 100, and preferably for some applications greater than or equal to 1000. In an embodiment, a transistor of this aspect has a field effect mobility greater than or equal to 0.1 cm2 V−1 s−1, and preferably for some applications a field effect mobility greater than or equal to 10 cm2 V−1 s−1. The invention provides nanotube-based transistor arrays and integrated circuits comprising a plurality of nanotube-based transistors.
[0026] In another aspect, the invention provides a method of making an electronic device comprising the steps of: (i) providing a first electrode; (ii) providing a second electrode; and (iii) providing a patterned layer of randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with the first electrode and the second electrode, the patterned layer having a thickness less than 10 nanometers and having one or more void regions in the patterned layer without carbon nanotubes; wherein the one or more void regions provided between the first and second electrodes reduces by at least 20% the number of purely metallic conductive pathways between the first electrode and the second electrode in the one or more interconnected carbon nanotube networks. In some embodiments, the void regions comprise a plurality of cavities or channels in the patterned layer that extend entirely from the first electrode to the second electrode, thereby defining a plurality of strips of interconnected carbon nanotube networks in the patterned layer. In some embodiments, the cavities in the patterned layer separate the strips such that they do not physically contact each other, and/or optionally each of the strips has an average width and extends an average length from first to second electrode at least 10 times greater than the average width and optionally 100 times greater than the average width.
[0027] In a specific method of this aspect, the step of providing a patterned layer of randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with the first electrode and the second electrode comprises the steps of: (i) providing a precursor layer of the randomly oriented carbon nanotubes in electrical contact with the first and second electrodes; and (ii) patterning the precursor layer so as to generate the patterned layer. In an embodiment, the step of providing the precursor layer in electrical contact with the first and second electrodes is selected from the group consisting of: (i) growing the carbon nanotubes on a device substrate, thereby generating the precursor layer; (ii) dispersing the carbon nanotubes in a solvent, thereby generating a carbon nanotube solution comprising a suspension of the carbon nanotubes, and depositing the carbon nanotube solution on to a device substrate, thereby generating the precursor layer; and (iii) contact printing the carbon nanotubes on to a device substrate, thereby generating the precursor layer. In an embodiment, contact printing of carbon nanotubes is achieved using soft lithography methods, such as dry transfer printing techniques using a conformable transfer device such as an elastomeric stamp.
[0028] In a specific method of this aspect, the step of patterning the precursor layer is carried out using one or more photolithography techniques. In an embodiment, for example, the step of patterning the precursor layer comprises the steps: (i) providing a layer of resist on the precursor layer; (ii) patterning the resist layer by selectively removing regions of the resist layer, thereby generating exposed regions of the precursor layer; and (iii) removing carbon nanotubes from the exposed regions of the precursor layer, thereby generating the patterned layer comprising the one or more strips extending between the first and second electrodes. Methods of this aspect may further comprise the step of removing the resist layer, for example by dissolution in an appropriate solvent. In an embodiment, the step of patterning the resist layer is carried out via photolithography, soft lithography, phase shift lithography, electron beam writing lithography or deep ultraviolet lithography. In an embodiment, the step of removing carbon nanotubes from the exposed regions of the precursor layer comprises etching the exposed regions of the precursor layer.
[0029] In a specific method of this aspect, the step of providing a patterned layer of randomly oriented carbon nanotubes in electrical contact with the first electrode and the second electrode comprises ink jet printing the carbon nanotubes, thermal transfer printing the carbon nanotubes, contact printing the carbon nanotubes, or screen printing the carbon nanotubes.
[0030] Electronic devices of the present invention include a range of nanotube-based devices. Electronic devices of the present invention include for example, transistors, diodes, light emitting diodes and photodetectors comprising one or more patterned layer of randomly oriented or partially aligned carbon nanotubes provided in the present interconnected carbon nanotube networks.
[0031] In another aspect, the present invention provides a transistor comprising: (i) a source electrode; (ii) a drain electrode; (iii) a gate electrode; (iv) a patterned layer of randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with the source electrode and the drain electrode, the patterned layer having a thickness less than 10 nanometers and having one or more void regions in the patterned layer without carbon nanotubes; wherein the one or more void regions provided between the first and second electrodes reduces by at least 20% the number of purely metallic conductive pathways between the source and drain electrodes in the one or more interconnected carbon nanotube networks; and (v) a dielectric layer positioned between the patterned layer and the gate electrode. In this embodiment, the patterned layer provides the semiconductor channel of the transistor. A transistor of this aspect comprises a thin film transistor wherein source electrode, gate electrode, drain electrode and dielectric layer transistor components are thin film structures. The present invention also provides arrays of transistors, including thin film transistor arrays, and integrated circuits.
[0032] In another aspect, the present invention provides a method for reducing the number of purely metallic conductive pathways in one or more interconnected carbon nanotube networks provided between a first electrode and second electrode comprising the step(s) of: providing a patterned layer of randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with the first electrode and the second electrode, the patterned layer having a thickness less than 10 nanometers and having one or more void regions in the patterned layer without carbon nanotubes; wherein the one or more void regions provided between the first and second electrodes reduces by at least 20% the number of purely metallic conductive pathways between the first and second electrodes in the one or more interconnected carbon nanotube networks.
[0033] Without wishing to be bound by any particular theory, there can be discussion herein of beliefs or understandings of underlying principles or mechanisms relating to the invention. It is recognized that regardless of the ultimate correctness of any explanation or hypothesis, an embodiment of the invention can nonetheless be operative and useful.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 . FIGS. 1A-1J provide schematic diagrams illustrating plan views of electronic devices of the present invention, and demonstrate useful configurations of patterned carbon nanotube layers comprising a semiconductor channel of an electronic device.
[0035] FIG. 2 provides a vertical cross sectional view of a field-effect transistor or thin film transistor incorporating the present invention.
[0036] FIG. 3 provides a schematic diagram of an exemplary method for making an electronic device incorporating the present invention.
[0037] FIG. 4 provides an array of SWNT strips formed by phase shift lithography on SiO2/Si substrate.
[0038] FIG. 5 shows the influence of the geometry of the etch lines on electrical characteristics of devices with 100 μm channel length, which is selected to be compatible with low-cost patterning techniques for flexible electronics, and moderate high density of SWNTs. For strips having a width of ˜5 μm, incorporation of the etched lines increases the on/off ratios (lon/loff) by three orders of magnitude, while reducing the transconductance (gm) by only ˜40%. Moreover, this influence can be predicted, with reasonable accuracy, according to a heterogeneous percolative theory modeling as shown in FIG. 5B .
[0039] FIG. 6 shows the relationship among gm, on/off ratio, channel length (LC), and strip width (WS) for two different tube densities and average tube lengths (LS). FIG. 6 shows that an optimal WS can be identified for any given combination of LS, LC, and tube density to achieve an on/off ratio sufficiently high for real applications (e.g. on/off ratio>1,000) and maintain the highest possible gm.
[0040] FIG. 7 provides simulated (FIG. 7A ) and experimental (FIG. 7B ) ON current (ION) vs. effective stripe width stripe (Wstripe/Lstick, for tube density D=40 μm−2 and effective channel length LC/Wstripe=15. The ON current decreases as the stripe width is reduced as more and more intersecting parallel paths are broken. The simulations match the experiments quite well. Experimental (FIG. 7C ) and simulated (FIG. 7D ) ON/OFF ratio vs. effective channel length LC/Wstripe for various values of effective stripe width Wstripe/Lstick for tube density D=40 μm−2 are also shown.
[0041] FIG. 8 provides a schematic illustration, scanning electron microscope (SEM) images, theoretical modeling results and photographs of flexible SWNT integrated circuits on plastic. FIG. 8A provides a schematic vertical cross-sectional view of a SWNT PMOS inverter on a PI substrate (PI: Polyimide, PU: Polyurethane, PAA: Polyamic acid). FIG. 8B provides a SEM image of a part of the SWNT circuit, collected before deposition of the gate dielectric, gate or gate level interconnects. A magnified SEM image of the network strips (FIG. 8C ) corresponding to a region of the device channel highlighted with the white box in FIG. 8B , and theoretical modeling results for the normalized current distribution (FIG. 8D ) in the ‘on’ state of the device, where color indicates current density (yellow: high; red: medium; blue: low) are also shown. FIG. 8E provides a photograph of a collection of SWNT transistors and circuits on a thin sheet of plastic (PI).
[0042] FIG. 9 provides plots showing electrical properties of TFTs that use SWNT network strips for the semiconductor, on thin plastic substrates. FIG. 9A shows the measured (solid dots) and simulated (open dots) influence of the width of the strips on the on/off ratio (lon/loff) and normalized transconductance (gm/gm0; where gm0 represents the response without strips), of transistors with channel lengths of 100 μm. Error bars represent s.d. of n=6 TFTs. FIG. 9B shows a plot of measured (solid lines) and simulated (dashed lines) VGS−IDS characteristics of depletion-mode and enhancement-mode SWNT TFTs, whose channel widths are 200 μm and channel lengths are 100 μm. (VDS: Drain-source voltage=−1 V, IDS: Drainsource current, VGS: Gate-source voltage). Inset: VGS−IDS curve of the enhancement mode device plotted on a logarithmic scale with VDS=−0.5 V (navy), −2 V (green), and −5 V (magenta) from bottom to up. FIG. 9C provides measured (solid lines) and simulated (dashed lines) VDS−IDS characteristics of an enhancement-mode TFT (VGS changed from −2 V to 2 V in steps of 0.5 V.). FIG. 9D provides a plot of gm/gm0 of a TFT and normalized voltage gain (G/G0, in both cases “0” represents the response in the unbent state) in an inverter as a function of bend radius. FIG. 9E provides measured (VDS=−0.2 V, solid lines) and simulated (dashed lines) distribution of lon ( l on: averaged on current). FIG. 9F provides a two dimensional histogram showing the correlation between lon/loff (measured at VDS=−0.2 V) and threshold voltage (VT) distributions. Inset: Correlation between lon/loff and normalized n branch transconductance (gmn/ g mn). The dashed line depicts a linear fitting result. The dashed area shows the distribution of lon/loff predicted by percolation models that do not explicitly account for the influence of source/drain contacts.
[0043] FIG. 10 provides circuit diagram, optical micrographs, output-input characteristics and circuit simulation results for different logic gates, such as inverter (FIGS. 10A-C ), NOR (FIGS. 10D-F ), and NAND (FIGS. 10G-I ) gates. We adopt a negative logic system. The Vdd applied to these logic gates is −5 V relative to GND. The logic-‘0’ and -‘1’ input signals of two terminals of the NOR and NAND gates are driven by 0 V and 25 V, respectively. The logic-‘0’ and -‘1’ outputs of the NOR gate are −(0.88-1.39) V and −3.85 V, respectively. The logic-‘0’ and -‘1’ outputs of the NAND gate are −1.47 V and −(4.31-4.68) V, respectively. FIG. 10B provides a plot of Vout and gain measurements. In FIGS. 10F and 10I , any specific combination of input-output signals is indicated as (logic address level inputs)logic address level outputs, and the timescales on the x axes are omitted because data collection involved the switching of voltage settings by hand. In FIGS. 10B , 10F and 10I, dashed lines represent circuit simulation results. Scale bars in FIGS. 10E and 10H are 100 mm.
[0044] FIG. 11 provides medium scale integrated circuits based on SWNT network strips on thin plastic substrates. FIG. 11A provides an optical image of a flexible SWNT IC chip bonded onto a curved surface. An optical micrograph (11B) and circuit diagram (11C) of a four-bit row decoder with sixteen outputs (0-15) are also provided. The bits are designated as most significant bit (MSB), second bit (SB), third bit (TB), and least significant bit (LSB). Vdd applied was −5 V versus ground (GND). FIG. 11D provides characteristics of the four-bit decoder. In descending order, the first four traces are inputs, labeled LSB, TB, SB, and MSB on the right axis; the remaining traces, labeled 0-15, show the output voltages of the sixteen outputs. Inset: Measured (blue) and SPICE simulated (red dashed) dynamic response of one output line under square wave input pulse (black) at clock frequency of 1 kHZ.
[0045] FIG. 12 provides channel length scaling properties of SWNT thin-film transistors. FIG. 12A provides IDS−VGS characteristics (VDS: Drain-source voltage=−0.2 V) of devices with channel widths (W) of 200 μm and channel lengths (LC), from top to bottom, of 10 μm, 25 μm, 50 μm, 75 μm, and 100 μm. (IDS: Drain-source current, VGS: Gate-source voltage). Inset: Width-normalized ON resistance (RONW) under different gate voltages as a function of channel length (LC). The solid lines represent the linear least square fit of the data. FIG. 12B provides effective mobility (μeff), extracted from both linear region and saturation region, and device on/off ratio (lon/loff, measured with VDS=−0.2 V) as a function of channel length (LC). FIG. 12C provides IDS1/2−VGS characteristics (VDS=−2 V) of a device with W of 200 μm and LC of 100 μm. Dashed lines serves as a visual guide to extract threshold voltage (VT).
[0046] FIG. 13 shows capacitance-voltage characteristics of a dish-shaped metal-insulator-semiconductor capacitor formed by gold, HfO2, and SWNT network. The inset shows a schematic of the capacitor array (CNT: Carbon nanotube, RP: Parasitic resistance).
[0047] FIG. 14 provides IDS−VGS characteristics (VDS: Drain-source voltage=−0.2 V) in both linear scale (left axis) and logarithmic scale (right axis) of three representative devices with typical (dark yellow), high (blue), and low (dark red) on/off ratios (14A) and IDS−VDS characteristics (VGS: Gate-source voltage changed from −2 V to 0 V) (14B) of a top gate device with a high on/off ratio. For all three devices, channel widths are 200 μm and channel lengths are 100 μm, with photolithographically defined 5 μm wide strips and source/drain electrodes patterned after transfer (IDS: Drain-source current).
[0048] FIG. 15 provides off state current (loff) as a function of drain-source voltage (VDS) for devices with channel length (LC) of 100 μm (15A) and 10 μm (15B) respectively, shown in both linear scale (right axis) and logarithmic scale (left axis).
[0049] FIG. 16 provides histograms of normalized mobilities (μ/ μ ) (16A) and subthreshold swing variations (S- S ) (16B) determined from measurements of over 100 SWNT transistors. (The bars over the quantities μ and S refer to average values and the dotted lines represent Gaussian fits to the data.)
[0050] FIG. 17 provides plots of dynamic switching characteristics of the four-bit decoder. The data show the output voltage of the least significant bit of the No. 7 output line, and the square wave input pulse, at clock frequency of 150 Hz (17A) and 1 kHz (17B), and constant bias from other input lines.
[0051] FIG. 18 provides a plot of drain-source current (IDS) of a typical SWNT thin-film transistor modulated by a triangle waveform (−2 V to 0 V) applied to the gate with a constant drain-source bias (−0.2 V), showing the operational stability of SWNT devices in air.
[0052] FIG. 19 provides a schematic illustration of the process for fabricating circuits on plastics with transfer-printed single-walled carbon nanotube (SWNT) networks (S/D: Source/Drain electrodes, PI: Polyimide, PU: Polyurethane, PAA: Polyamic acid).
[0053] FIG. 20 provides a plot of measured (solid line) and simulated (dashed line) dependence of inverter current load (I) on input voltage (Vin).
[0054] FIG. 21 (a) Schematic illustration showing the CNT nanonet thin film transistor. (b) SEM images of the network before and after striping. The stripes are etched using the oxygen plasma process. The scale bar in both SEM images is 5 μm. (c) Nanonet with density (DCNT) higher than the percolation threshold (DP), but m-CNT density (DM) less than DP. The right panel is a plot of conductance (σ) vs density (DCNT) illustrating various densities. The conductance is negligible if the density is below percolation density. (d) Nanonet with DCNT>DM>DP. (e) Striped nanonet obtained from that in (d). As the striping breaks conducting paths, the entire solid curve in the right panel shifts to the dotted curve.
[0055] FIG. 22 Variation in (a) simulated and (b) experimental ON-current (ION) with effective stripe width (Wstripe/Lstick) for tube density DCNT=40/μm2 and effective channel length LC/Wstripe=15. The ON-current decreases as the stripe width is reduced as more and more intersecting parallel paths are broken. The simulations match the experiments quite well. Variation in (c) experimental and (d) simulated ON/OFF ratio with effective channel length LC/Wstripe for various values of effective stripe width Wstripe/Lstick for tube density DCNT=40/μm2.
[0056] FIG. 23 Plots of (a), (c) simulated and (b), (d) experimental ON-current (ION) and ON/OFF ratio vs tube density DCNT as a function of normalized stripe widths Wstripe/Lstick. The normalized channel length LC/Lstick is held fixed at 25 for the simulations. The tube density plays an important role in determining the device performance. Note that the simulated curves for ON-current and ON/OFF ratio are statistical averages of 200 transistors. While the sample size is adequate for almost all simulations, the slight non-monotonicity of the ON/OFF ratio (b) at Wstripe/Lstick=0.5 and at lower densities (25-30) is most likely an artifact of finite sample size.
[0057] FIG. 24 (a) Plot of normalized standard deviation of ON-current (ION) vs tube density for various Wstripe/Lstick. Here, LC=25 μm. The normalized standard deviation is high for lower density and smaller Wstripe/Lstick. (b) Fraction of samples with high ON/OFF ratio as a function of tube density for various Wstripe/Lstick.
[0058] FIG. 25 (a) Schematic showing transition of channels from 1-D to 2-D with width i. Random sections of the grid are occupied (black) with a probability of 0.5 (bond percolation threshold for an infinite 2-D network). The percolation threshold in finite width samples is defined by the condition that one certain occupation probability of the grid ensures current flow from S/D.
DETAILED DESCRIPTION OF THE INVENTION
[0059] Referring to the drawings, like numerals indicate like elements and the same number appearing in more than one drawing refers to the same element. In addition, hereinafter, the following definitions apply:
[0060] “Carbon nanotube” and “nanotube” are used synonymously and refer to allotropes of carbon comprising one or more cylindrically configured graphene sheets. Carbon nanotubes include single walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). Carbon nanotubes typically have small diameters (≈1-10 nanometers) and large lengths (up to several microns), and therefore may exhibit length to diameter ratios ≈102 to about 107. The longitudinal dimension of a nanotube is its length and the cross sectional dimension of a nanotube is its diameter (or radius). Carbon nanotubes include semiconducting carbon nanotubes, metallic carbon nanotubes and mixtures of both semiconducting carbon nanotubes and metallic carbon nanotubes. In some embodiments, the compositions and methods of the present invention include a mixture of semiconducting and metallic carbon nanotubes, for example, wherein the ratio of semiconducting nanotubes to metallic nanotubes varies over the range of 9-0.5. In some embodiments, the compositions and methods of the present invention include a mixture of semiconducting and metallic carbon nanotubes, for example, wherein the ratio of semiconducting nanotubes to metallic nanotubes is greater than or equal to 1, preferably for some application greater than or equal to 2. In some embodiments, the compositions and methods of the present invention include a mixture of semiconducting and metallic carbon nanotubes, for example, wherein the extent of semiconducting nanotubes is enriched, for example using fractionation or other purification techniques.
[0061] The compositions and methods of the invention include carbon nanotube networks comprising a plurality of randomly oriented and/or partially aligned carbon nanotubes, including interconnected nanotube networks having at least one carbon nanotube crossing. The present invention includes interconnected nanotube networks having a surface concentration of carbon nanotubes selected over the range of 0.2 carbon nanotubes micron−2 to 500 carbon nanotubes micron−2, preferably for some embodiments selected over the range of 0.5 carbon nanotubes micron−2 to 20 carbon nanotubes micron−2. Nanotube crossings in this context refers to a configuration wherein two or more nanotubes are in electrical contact, physical contact and/or in an overlapping configuration. For example, nanotube crossings in some embodiments refers to a configuration with two, three or four different nanotubes are provided on top of or underneath each other. In some embodiments, interconnected nanotube networks of the invention include nantoube networks having 0.1 to 100 nanotube crossings μm−2, preferably for some applications having 0.2 to 10 nanotube crossings μm−2.
[0062] “Solution processing” is intended to refer to processes whereby one or more structures, such as carbon nanotubes, are dispersed into a carrier medium and delivered in a concerted manner to a substrate surface. In an exemplary solution printing method, delivery of structures to selected regions of a substrate surface is achieved by methods that are independent of the morphology and/or physical characteristics of the substrate surface undergoing patterning. Solution printing methods useable in the present invention include, but are not limited to, ink jet printing, thermal transfer printing, and capillary action printing.
[0063] “Supported by a substrate” refers to a structure that is present at least partially on a substrate surface or present at least partially on one or more intermediate structures positioned between the structure and the substrate surface. The term “supported by a substrate” may also refer to structures partially or fully embedded in a substrate, structures partially or fully immobilized on a substrate surface via an encapsulating layer (e.g., polymer layer) and structures partially or fully laminated on a substrate surface.
[0064] “Partially aligned nanotubes” have lengths extending in longitudinal directions that are aligned with respect to each other but not provided in an absolutely parallel configuration. In some embodiments, for example, partially aligned nanotubes have a partially linear geometry wherein their lengths assume a configuration with deviations from absolute linearity greater than about 10%, and in some embodiments with deviations from absolute linearity greater than about 20%. As used in this context, the term “parallel” refers to a geometry in which the lengths of carbon nanotubes are equidistant from each other for at least a portion of the points along their respective lengths and have the same direction or curvature. In one embodiment, for example partially aligned nanotubes are aligned relative to each other with deviations from absolute parallelism that are greater than or equal to 20 degrees, and in some embodiments deviations from absolute parallelism that are greater than or equal to 10 degrees. In some embodiments, compositions and methods of the invention includes carbon nanotube networks comprising partially aligned nanotubes having at least one nanotube crossing. Alternatively, compositions and methods of the invention includes carbon nanotube networks comprising randomly oriented nanotubes having at least one nanotube crossing.
[0065] “Monolayer of nanotubes” refers to a layer nanotubes on a substrate surface wherein the coverage of the area of the surface of the substrate having nanotubes is less than 100%, preferably for some embodiment substantially less than 100%. In some embodiments, for example, a monolayer refers to a layer of nanotubes wherein the coverage of the area of the surface of the substrate having nanotubes is less than 10%, preferably for some applications less than 2%, and preferably for some applications less than 1%. In some embodiments, for example, a monolayer refers to a layer of nanotubes wherein the coverage of the area of the surface of the substrate having nanotubes is selected over the range of 0.1-10%, or preferably for some embodiments selected over the range of 0.5-2%. As used herein, monolayers of carbon nanotubes includes carbon nanotube networks having overlapping nanotubes, for example, layers having a plurality of nanotube crossings and/or discrete regions characterized by two, three or four overlapping nanotubes. In this sense, monolayers include layers of nanotubes having discrete bilayer and trilayer regions or components. In some embodiments a monolayer of carbon nanotubes has a thickness less than or equal to 20 nanometers, preferably for some applications less than of equal to 10 nanometers and preferably for some applications less than or equal to 5 nanometers. Use of monolayers of carbon nanotubes in some embodiments of the invention are useful for achieving effective gate modulation in a nanotube-based electronic devices.
[0066] “Flexible” refers to a property of an object, such as a substrate, which is deformable in a reversible manner such that the object or material does not undergo damage when deformed, such as damage characteristic of fracturing, breaking, or inelastically deforming. Flexible polymers are useful with the methods described herein. Specific flexible polymers include, but are not limited to: rubber (including natural rubber, styrene-butadiene, polybutadiene, neoprene, ethylene-propylene, butyl, nitrile, silicones), acrylic, nylon, polycarbonate, polyester, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyolefin, elastomers and other flexible polymers known to those of skill in the art. In certain embodiments, flexible objects or materials can undergo strain levels selected over the range of 1% to 1300%, 10% to 1300%, or 100% to 1300% without resulting in mechanical failure (e.g., breaking, fracturing or inelastically deforming). In some embodiments, flexible objects or materials can be deformed to a radius of curvature selected over the range of 100 μm to 3 m without resulting in without resulting in mechanical failure (e.g., breaking, fracturing or inelastically deforming).
[0067] The present invention provides methods and device geometries to enhance the device on/off ratio for thin-film transistors based on patterned layers comprising randomly oriented or partially aligned, and optionally unsorted single-walled carbon nanotubes. In some embodiments, isolated strips comprising interconnected carbon nanotube networks are provided between source and drain electrodes of a transistor. Strips may be aligned in electron transport direction with optimal strip width, determined by nanotube surface concentration, average tube length, and device channel length, to minimize the effect on device transconductance. Also provided are electronic devices and circuits compromising nanotube strips.
[0068] FIGS. 1A-1J provide lateral cross section views of electronic devices of the present invention, wherein the channel of the device comprises a patterned layer of randomly oriented or partially aligned carbon nanotubes. In these embodiments, the patterned layer has a plurality of void regions provided to reduce the number of purely conductive pathways between first and second electrodes. Referring to FIGS. 1A-1H , an electronic device 100 of the present invention comprises a first electrode 101, a second electrode 102, and a channel region comprising the patterned layer 110. Optionally, first electrode 101, second electrode 102, and the channel region are supported by a flexible substrate 111. The patterned layer 110, positioned between the first and second electrodes 101 and 102, includes regions having one or more interconnected carbon nanotube networks 103 provided in electrical contact with the electrodes, and void regions 104 without carbon nanotubes, such as cavities, grooves, openings and/or holes in the patterned layer 110. In some embodiments, the void regions 104 comprise an etch pattern in patterned layer 110. In some embodiments, the void regions 104 extend through the entire thickness of patterned layer 110, or substantially all (e.g., within 10%) of the thickness of patterned layer 110. Electrical contact between the patterned layer 110 and electrodes 101 and 102 is achieved in these embodiments by overlapping region 109 wherein the patterned layer 110 overlaps the two electrodes 101 and 102 (e.g., either above or below the electrodes). In some embodiments, the regions of interconnected nanotube networks 103 may have an average length 105 extending from first electrode 101 to second electrode 102, and an average width 106 extending between void regions 104. In some embodiments, the void regions 104 comprise one or more cavities extending through the entire thickness of patterned layer 110, and have appropriate dimensions (e.g., width 107) to disrupt metallic conductive pathways by preventing physical contact between and electron transport through nanotube networks 103 on opposing sides of the void regions. However, the void regions 104 are patterned such that most semiconducting pathways of the interconnected carbon nanotube network extend from the first electrode 101 to the second electrode 102 without being disrupted by the void regions 104, wherein the pathways contains at least one semiconducting nanotube. As used herein, the width and length of strips of void regions refer to lateral dimensions (e.g., dimensions in a plane parallel to the substrate surface supporting the nanotubes), and thickness refers to a vertical dimension orthogonal to the lateral dimensions. Accordingly, patterned layer 110 of the present invention is capable of reducing the number of purely metallic conductive pathways between the first and second electrodes relative to a system having no void regions, while at the same time not dramatically impacting the field effect mobilities achievable. This aspect is useful for providing nanotube-based transistors having large on/off ratios and useful field effect mobilities.
[0069] Useful configurations of void regions 104 in the patterned layer 110, occupying the channel of an electronic device 100, are provided in FIGS. 1A-1H , although additional configurations are possible. As shown in FIG. 1A , an embodiment has void regions 104 patterned in a periodic manner extending the entire distance from first electrode 101 to second electrode 102 and arranged parallel to an alignment axis 108, that is optionally provided orthogonal to both first and second electrodes. Void regions 104 in this embodiment define strips 103 of interconnected carbon nanotube networks in the patterned layer 110 that extend between, and are in electrical contact with the two electrodes 101 and 102. In the specific embodiment shown in FIG. 1A , the void regions 104 and strips 103 have a rectangular shape in a lateral cross section. In a similar embodiment, shown in panel 1B of FIG. 1 , the void regions 104 extend from the first 101 to the second electrode 102 parallel to an axis that is inclined at a non-orthogonal angle to the two electrodes. Further embodiments comprise a patterned layer 110 having voids 104 that define strips of interconnected nanotube networks 103 with non-rectangular shapes. For example, the strips 103 may have a lateral cross sectional shape that is trapezoidal (FIG. 1C ), or oval (FIG. 1D ), or provided in a serpentine configuration (FIG. 1E ). In these embodiments, the strips 103 have an average length 105 extending from the first electrode 101 to the second electrode 102, an average width 106, and are separated from adjacent strips by an average distance 107.
[0070] In other embodiments of the present invention, the void regions 104 may extend only a portion of the distance from the first electrode 101 to the second electrode 102. For example, as shown in FIGS. 1F-1H , the void regions 104 may have lateral dimensions 107a and 107b that are smaller than the channel length 105. These void regions 104 may be patterned in a periodic or aperiodic manner. The void regions 104 may have lateral cross-sectional shapes that are rectangular (FIG. 1F ), circular (FIG. 1G ), or any other shape as shown in FIG. 1H . In such embodiments, the void regions 104, similarly act to disrupt metallic conductive pathways in the interconnected carbon nanotube networks 103.
[0071] In some embodiments, the spatial distribution of void regions 104 is uniform throughout the patterned layer. Alternatively, the invention includes embodiments wherein the spatial distribution of void regions 104 varies with respect to position in the channel region comprising the patterned layer. For example, the invention include embodiments wherein the patterned layer has a plurality of void regions, wherein the density of void regions (e.g. voids regions per micron2) varies in the patterned layer, for example, wherein the density of void regions is smaller adjacent to or proximate to the electrodes than away from the electrodes (e.g., in the center of the channel region comprising the patterned layer). In an embodiment, the density of void regions decreases by greater than 10% from the center of the channel to a region of the patterned layer adjacent to the electrodes. In the embodiments shown in FIGS. 1I and 1J , for example, the void regions 104 are positioned closer to each other in a region of the channel region positioned away from the electrodes. As shown in these figures, the density of void regions 104 is greatest in the center of the channel relative to regions proximate to the electrodes.
[0072] FIG. 2 provides a vertical cross sectional view of a field-effect transistor comprising a thin film transistor of the present invention, having a source electrode 101, a drain electrode 102, a patterned layer of randomly oriented or partially aligned carbon nanotubes 110 in electrical contact with the source and drain electrodes 101 and 102, a gate electrode 116, and a dielectric layer 115 insulating the patterned layer 110 from the gate electrode 116. The patterned layer 110, having void regions 104 that disrupt metallic conductive pathways in the semiconductor channel, forms one or more interconnected carbon nanotube networks to provide a semiconducting channel between the source and drain electrodes. As shown in FIG. 2 , the electronic device is supported by a flexible substrate 111, optionally having additional layers 111a and 111b. In an embodiment, the flexible substrate is a polyimide 111 coated with polyurethane 111a, beneath a layer of polyamic acid 111b. The source and drain electrodes 101 and 102, and patterned layer of nanotubes 110 are coated with and embedded in the polyamic acid layer 111b. The gate electrode 116 may be positioned above or below the patterned carbon nanotube layer.
[0073] FIG. 3 provides a schematic diagram of an exemplary method for making an electronic device having a channel region comprising a patterned layer of randomly oriented or partially aligned carbon nanotubes. The method comprises the following steps: (i) providing a first and second electrode which optionally may comprise a source and drain electrode; (ii) providing a patterned layer of randomly oriented carbon nanotubes, having regions of interconnected nanotube networks and void regions without carbon nanotubes, in electrical contact with the first and second electrodes; and (iii) optionally depositing a dielectric layer and gate electrode above or below the patterned nanotube layer. The first and second electrode may optionally be deposited and patterned over an already existing layer of patterned nanotubes (i.e. after step ii), and the gate electrode and dielectric material may optionally be provided in step (i).
[0074] In an embodiment, step (ii) may be carried out by a number of different methods; the first method utilizing printing techniques such as: ink jet printing, thermal transfer printing, contact printing, or screen printing. Alternatively, a second method for providing a patterned layer of carbon nanotubes comprises the following steps: providing a precursor sub-monolayer of nanotubes by growing the nanotubes on a substrate, dispersing the nanotubes in a solvent and depositing the nanotube solution on a device substrate, or contact printing the nanotubes on a device substrate; spin coating a layer of resist on the precursor nanotube layer; patterning the resist layer by photolithography, soft lithography, phase shift lithography, electron beam writing or deep ultraviolet lithography to selectively remove regions of the resist layer and expose regions of the precursor layer; removing exposed regions of the precursor nanotube layer by O2 plasma etching; and removing the remaining resist layer such that a patterned layer of carbon nanotubes remains.
[0075] Nanotube strips useful in patterned layers of the present invention can be formed by firstly patterning a photoresist (PR) layer with either conventional photolithography or novel nanofabrication methods such as phase shift lithography. The shape and position of strips can be controlled in this step. Oxygen plasma etching removes nanotubes in exposed area and PR layer can be subsequently removed with acetone. FIG. 4 shows an array of SWNT strips formed by phase shift lithography on SiO2/Si substrate.
[0076] FIG. 5 shows the influence of the geometry of the etch lines on electrical characteristics of devices with 100 μm channel length, which is selected to be compatible with low-cost patterning techniques for flexible electronics, and moderate high density of SWNTs. For a strip width of ˜5 μm, on/off ratios are increased by three orders of magnitude, while reducing the transconductance (gm) by only ˜40%. Moreover, this influence can be predicted, with reasonable accuracy, according to a heterogeneous percolative theory modeling as shown in FIG. 5B .
[0077] The effectiveness of this approach depends on choosing optimal strip width (WS) according to nanotube density, average tube length (LS), and device channel length (LC). FIG. 6 shows the relationship among gm, on/off ratio, LC, and WS for two different tube densities and LS. An optimal WS can be identified for any given combination of LS, LC, and tube density to achieve on/off ratio sufficiently high for real applications, e.g. on/off ratio>1,000, and maintain the highest possible gm.
[0078] To avoid the time consuming “trail-and-try” strategy, percolative modeling can be used to find the optimal WS with its ability to accurately represent experimental data as shown in FIG. 7 . FIG. 7 provides: (a) Simulated and (b) experimental ON current (ION) vs. effective stripe width (Wstripe/Istick) for tube density D=40 μm−2 and effective channel length LC/Wstripe=15. The ON current decreases as the stripe width is reduced as more and more intersecting parallel paths are broken. The simulations match the experiments quite well. (c) Experimental and (d) simulated ON/OFF ratio vs. effective channel length LC/Wstripe for various values of effective stripe width Wstripe/Lstick for tube density D=40 μm−2.
[0079] This approach can be applied over large size SWNT films, making them suitable for constructing integrated circuits/systems.
[0080] The invention may be further understood by the following non-limiting example.
Example 1
Medium Scale Carbon Nanotube Thin Film Integrated Circuits on Flexible Plastic Substrates
[0081] The ability to form integrated circuits (ICs) on flexible sheets of plastic enables attributes (e.g. conformal and flexible formats, lightweight and shock resistant construction) in electronic devices that are difficult or impossible to achieve with technologies that use semiconductor wafers or glass plates as substrates.1 Organic small molecule and polymer based materials represent the most widely explored types of semiconductors for such flexible ICs.2 Although these materials and those that use films or nanostructures of inorganics have promise for certain applications, existing demonstrations of them in ICs on plastic indicate modest performance characteristics that might restrict the application possibilities. In this Example we disclose advanced implementations of a comparatively high-performance carbon-based semiconductor comprising sub-monolayer, random networks of single-walled carbon nanotubes (SWNTs) to yield small-scale to medium-scale, integrated digital circuits, composed of up to nearly one hundred transistors on plastic substrates. Transistors in these ICs demonstrate excellent properties: mobilities as high as 80 cm2/Vs, subthreshold slopes as low as 140 mV/dec, operating voltages <5 V together with deterministic control over the threshold voltages, on/off ratios as high as 105, switching speeds well in the kHz range even for coarse (˜100 μm) device geometries and good mechanical bendability, all with levels of uniformity and reproducibility that enable high yield fabrication of ICs. Theoretical calculations, ranging from percolation transport through the networks to compact models for the transistors to circuit level simulations, provide quantitative and predictive understanding of these systems. Taken together, these results demonstrate that sub-monolayer films of SWNTs provide an attractive type of carbon-based semiconductor for flexible ICs, with many potential areas of application in consumer and other areas of electronics.
[0082] Efforts to develop polymer and small molecule semiconductors for electronics have yielded several impressive demonstrations, including ICs with more than one thousand transistors3, flexible displays3,4, sensor sheets5 and other systems6,7. In all cases, however, the field effect mobilities of the transistors are modest: typically ˜1 cm2/Vs for isolated devices8,9, and <0.05 cm2/Vs in ICs3-7. Although these properties are sufficient for electrophoretic displays and certain other applications, improvements in the materials would expand the possibilities.1 Separately, for any given application, increases in mobility relax the requirements on critical feature sizes in the circuits (e.g. transistor channel lengths) and tolerances on their multilevel registration, which can be exploited to reduce the cost of the plastic substrates and patterning systems to achieve roll-to-roll fabrication by dry printing10 or ink-jet printing11. Recently developed carbon-based semiconducting nanomaterials, especially SWNTs, provide an opportunity to achieve extremely high intrinsic mobilities, high current carrying capacities and exceptional mechanical/optical characteristics,12 in bendable formats on plastic substrates. Sub-monolayer random networks13-16 or aligned arrays17,18 of SWNTs can serve as thin film semiconductors which, in the best cases, inherit the exceptional properties (e.g. device mobilities up to ˜2,500 cm2/Vs, on current above several milliamperes, and cut-off frequency above 1 GHz for devices on plastic) of the tubes. The network geometry is of particular interest for flexible electronics because it is achievable by printing SWNTs from solution suspensions.19 The present Example demonstrates advanced implementations of SWNT networks in flexible ICs on plastic that have attractive characteristics, together with corresponding theoretical models and simulation tools that capture all of the key aspects.
[0083] The system layouts, as schematically illustrated in FIG. 8 a, exploit architectures similar to those in established silicon ICs. A thin (50 μm) sheet of polyimide serves as the substrate. Random networks of SWNTs grown by chemical vapour deposition and subsequently transfer printed onto the polyimide forms the semiconductor layer.17 Source and drain (S/D) electrodes of Au serve as low resistance contacts to these networks, as determined by scaling studies (FIG. 12 ). Although roughly one third of the SWNTs are metallic, purely metallic transport pathways between source and drain can be eliminated by suitably engineering the average tube lengths and the network layouts. For present purposes, we used soft lithography and reactive ion etching to cut fine lines into the networks. The resulting network strips are oriented along the overall direction of transport, with widths designed to reduce the probability of metallic pathways below a practical level without significantly reducing the effective thin film mobility of the network. FIG. 8 b shows a scanning electron micrograph (SEM) of a region of an IC just before deposition of the gate dielectric. A magnified view of a part of the SWNT network in the channel of one of the devices (FIG. 8 c, the S/D electrodes are to the right and left, outside of the field of view) reveals narrow, dark horizontal lines, corresponding to the etched regions. The critically important role of these features on the electrical characteristics can be quantified through first principles modeling studies that consider percolation transport through sticks with average lengths and layouts (e.g. etched lines, densities of SWNTs, etc) corresponding to experiment.20 FIG. 8 d shows the distribution of current flow for a typical case, in which the color indicates the current density in the ‘on’ state of the device. In addition to providing guidance on optimal design (FIG. 9 a), these simulations reveal that networks in this geometry and coverage (˜0.6%) distribute current evenly, thereby serving as an effective film for transport. A typical device incorporates ˜16,000 individual SWNTs. The circuits are completed in top gate configurations by depositing and patterning high capacitance, hysteresis-free dielectrics (˜40 nm of HfO2) directly on the tubes, followed by gate metallization, vias and interconnects. FIG. 8 e shows a representative system, complete with arrays of isolated enhancement-mode (lower right region) and depletion mode (lower middle region) transistors, various logic gates (lower left part), and two twenty-logic-gate size four-bit row decoders (middle and upper parts). Fabrication details are further described in the Methods Section.
[0084] FIG. 9 summarizes measurements on individual transistors. FIG. 9 a illustrates the predicted and measured influence of the geometry of the etched lines described above on devices with coarse dimensions (i.e. 100 μm channel lengths), selected to be compatible with established low cost patterning techniques such as screen printing,21 and with sufficiently high densities of SWNTs to achieve good performance and uniformity as a thin film type semiconductor. For carbon nanotube strips having widths of ˜5 μm, the etched lines increase the on/off ratios by up to four orders of magnitude, while reducing the transconductances (gm) by only ˜40%. FIG. 9 b and c show characteristics of transistors with this geometry, illustrating well behaved responses with minimal hysteresis and with excellent transconductances (gm as high as 0.15 μS μm−1 and typically 0.12 μS μm−1 for LC≧50 μm, which corresponds to an estimated cut-off frequency of >100 kHz.), device mobilities (μeff as high as ˜80 cm2V−1s−1 and typically ˜70 cm2V−1s−1 as calculated using standard MOSFET models with measured gate capacitances (FIG. 13 ), for both the linear and the saturation regimes) and subthreshold swings (S as low as 160 mV/dec and typically ˜200 mV/dec). The transconductances and the subthreshold behaviors, in particular, exceed those that have been demonstrated in flexible ICs on plastic with organic thin film semiconductors (gm<0.02 μS μm−1 for LC˜50 μm, S>140 mV/dec)22,23 or with Si nanowires (gm<0.01 μS μm−1 for LC˜50 μm, S>280 mV/dec)24 and are competitive with the best reports of p-channel single crystalline Si ribbons (gm˜0.25 μS μm−1 for LC=50 μm, S ˜230 mV/dec)25. Under low to moderate bias conditions, the on/off ratios are as high as 105 (see FIG. 9 f and FIG. 14 a), and typically ˜103, for transistors with this geometry. The inset to FIG. 9 b and FIG. 15 a show a decrease in the on/off ratio with increasing drain-source voltage (VDS), due primarily to the slightly ambipolar nature of the device operation. These ratios also decrease with channel length (FIG. 12 b). The favorable direct-current (DC) properties of long channel devices are achieved at short channel lengths, for improved operating speeds, by use of correspondingly shorter SWNTs and narrower etched stripes, as suggested by modeling results, or utilizing pre-enriched semiconducting SWNTs.26 The threshold voltage (VT) are controlled by using gate metals with different work functions, because the high capacitance gate dielectrics reduce the relative contribution of voltage across the dielectric to VT.27 For example, replacing Au with Al as the gate metal shifts VT by −0.6˜0.8V, thereby hanging the device operation from depletion mode to enhancement mode (FIG. 9 b). Systematic bending tests of individual devices and inverters showed no significant change in device performance during inward or outward bending to radii as small as ˜5 mm (FIG. 9 d). Collectively, these properties are as good as or better than those of previously reported SWNT network devices, in spite of the moderate decreases in gm associated with the etching procedures. Transistors that use aligned arrays of SWNTs have improved performance, but these layouts cannot be formed readily with solution printing techniques. As such, they are not relevant for the type of printed, flexible electronics applications contemplated here.
[0085] For use in ICs, the yields and performance uniformity of the transistors are critically important. We examined these aspects through measurements on more than 100 devices, as summarized in FIG. 9-9 f and FIG. 16 . The results show standard deviations of ˜20% for the normalized on-state current (Ion) and ˜0.05V for VT. The former result is quantitatively in agreement with percolation theory, illustrated also in FIG. 9 e. Although on/off ratios vary by roughly two orders of magnitude, most of values are >103. The distribution, as shown in FIG. 9 f, indicates no correlation with VT (suggesting the importance of extrinsic doping effects on SWNTs28), and is much larger than what is predicted by percolation models (FIG. 9 f, right inset) that do not explicitly include effects of S/D contacts. These results suggest strongly that the variation in on/off ratio results from electron conduction caused by tunneling through the Schottky barriers at S/D contacts (FIG. 9 f, right inset).29 Doping techniques similar to those demonstrated in single SWNT devices can be used to suppress the ambipolar behaviour and improve on/off ratio uniformity.30 Such doping methods also help to eliminate decreases in on/off ratio with increasing VDS as mentioned previously and illustrated in FIG. 9 b and FIG. 15 a.
[0086] We find that standard models for silicon device technologies can capture the device behaviors. FIG. 9 b & c illustrate the level of agreement that can be achieved with a level 3 PMOS SPICE model that uses a parallelly connected exponential current source controlled by both gate voltage and VDS to mimic the electron tunneling current. This level of compatibility with established simulation tools allows the use of existing, sophisticated computer aided design platforms developed for Si ICs. As the first step toward large scale integration, we modeled and then built “universal” logic gates. FIG. 10 a provides a circuit diagram of a PMOS inverter with enhancement load. The inverter exhibits well defined static voltage transfer characteristics, consistent with simulation, at a supply voltage of −5V as shown in FIG. 10 b. The rise in output voltage with increasing positive input voltage arises from the ambipolar behavior of the driving transistor. Maximum voltage gains of ˜4, together with good noise immunity with a width of the transition region <0.8 V and a logic swing >3 V are achieved, indicating that the inverter can be employed to switch subsequent logic gates without losing logic integrity. Measuring their alternating-current (AC) responses generated a magnitude Bode plot closely resembling the characteristics of low-pass amplifiers (FIG. 10 c), with operation in kHz range even for devices with long (˜100 μm) channel length and significant (˜40 fF/μm) overlap capacitance. The ability to achieve switching speeds in the kHz range with device geometries that are compatible with techniques such as screen printing is important for use of such SWNT networks in low cost, printed electronics. By adding another driving transistor to the inverter, either in parallel with the pulling down transistor to incorporate OR logic (FIG. 10 d, e) or in series to incorporate AND logic (FIG. 10 g, h), NOR and NAND logic gates are achievable, respectively. The output characteristics and simulation results are presented in FIG. 10 f & i. Voltage amplification is observed in all cases.
[0087] All of these experimental and computational components can be used together to yield SWNT based digital circuits, as shown in FIG. 11 a. The largest circuit in this chip is a four-bit row decoder, designed using modeling tools and measured characteristics of standalone logic gates. This circuit incorporates eighty eight transistors, in four inverters and a NOR array with the output of the inverter serving as one of the inputs for the NOR gate. The circuit diagram, depicted in FIG. 11 c, is configured such that any given set of inputs only give one logic “1” output. The input-output characteristics of the decoder are shown in FIG. 11 d and FIG. 17 , which demonstrates its ability to decode a binary encoded input of four data bits into sixteen individual data output lines, at frequency in kHz regime. These results demonstrate that SWNT networks can form the basis for a potentially interesting and scalable alternative to conventional organic or other classes of semiconductors for flexible IC applications.
Methods Summary
[0088] The process flow for fabricating SWNT IC on plastics is depicted in FIG. 19 . SWNTs are synthesized by chemical vapor deposition on SiO2/Si wafers and then etched into strips using an experimentally simple, optical soft lithography technique. Standard photolithography, electron-beam evaporation, Au wet chemical etching, and oxygen plasma etching are used to pattern S/D electrodes and isolate each device. A film of polyamic acid (PAA) is then used to encapsulate predefined S/D electrodes and SWNT networks on the growth wafers for transfer to a polyimide (PI) substrate coated with liquid polyurethane (PU). Subsequent curing of the PU and PAA complete the transfer process. Metal gates are defined on top of a high capacitance dielectric layer of HfO2 (˜40 nm). Vias and windows for probing are opened by wet etching (dip into concentrated HF aqueous solution) through patterned photoresist. Finally, another level of interconnect metallization formed local interconnections defined previously with the gate and source/drain metal layers. Electrical measurements are carried out in air using a semiconductor parameter analyzer (Agilent 4155C). AC input was provided by a function generator (GWinstek, GFG-8219A) and output was read by a standard oscilloscope (Tektronix, TDS 3012B). The stick percolation simulations involved finite size, first principles two-dimensional numerical models based on generalized random network theory. Device and circuit simulation used the commercial software package HSPICE.
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Methods
[0119] Synthesis of SWNT Network:
[0120] SWNT random networks were grown by chemical vapor deposition (CVD) on Si wafers with 100 nm thick layers of thermal oxide. The process began with cleaning the SiO2/Si wafer with piranha solution (A 3:1 volumetric mixture of concentrated sulphuric acid to 30% hydrogen peroxide solution). This process not only removed organic contaminants but also hydroxylated the reoxidized SiO2 surface, making it extremely hydrophilic to enable uniform deposition of catalyst.31 This catalyst consisted of ferritin (Aldrich, diluted by deionized water at a volumetric ratio of 1:20 to control the density of catalyst) deposited onto the SiO2/Si surface by adding methanol.32 The wafer was then heated to 800° C. in a quartz tube to oxidize ferritin into iron oxide nanoparticles. After cooling down to room temperature, the quartz tube was flushed with a high flow of argon gas (1500 sccm) for cleaning and then heated up to 925° C. in hydrogen atmosphere (120 sccm), which reduced iron oxide to iron. After reaching 925° C., methane (1500 sccm) was released into the quartz tube as carbon source while maintaining the hydrogen flow. Growth was terminated after 20 min, and the chamber was then cooled in hydrogen and argon flow. The density of the SWNT networks formed in this fashion was controlled by the dilution ratio of the ferritin solution, while leaving the other aspects of the growth and processing unchanged.
[0121] Cutting Strips into the SWNT Networks with Phase Shift Lithography and Reactive Ion Etching:
[0122] Elastomeric phase masks with depths of 1.8 μm, widths of 5 μm and periodicity of 10 μm were fabricated from relief structures defined by lithography and anisotropic etching through a casting and curing procedure.33 AZ5214 photoresist, diluted with AZ1500 thinner in a 1:1 volumetric ratio, was spin casted onto SiO2/Si wafer with SWNT networks at 5,000 rpm and then baked at 95° C. for 1 min to afford a flat and solid 300 nm thick photoresist layer. After cleaning the surface of phase mask with scotch tape, we placed it into conformal contact with the photoresist layer, flood exposed the resist by shining the i-line (365 nm) output of a Hg ultraviolet (UV) lamp through the mask, and then removed the mask. The SiO2/Si substrate was then baked at 112° C. for another minute, followed by a flood exposure of UV light. Developing in AZ MIF327 developer for 40 s created a regular array of submicron wide spacings in the photoresist layer, with 5 μm periodicity. 5 μm wide photoresist strips could also be generated by conventional photolithography with much wider spacings (˜5 μm). Although large spacings lead to reduction in effective channel width and increase in parasitic capacitance, we utilized it instead of phase shift lithography in fabricating transistors used in decoder circuits as conventional photolithography is easier to perform. After that oxygen reactive ion etching (200 mTorr, 20 sccm, O2 flow, 100 W Radio Frequency power) removed the exposed SWNTs. Finally, the photoresist layer was removed by soaking in acetone for one hour. Successfully utilizing optical soft lithography to pattern the only sub-10 μm features in our circuits suggests the potential to use low cost, low resolution printing-like processes to define all features in the circuits.34
[0123] Source/Drain (S/D) Patterning and Device Isolation:
[0124] A gold film (30 nm) was deposited by electron-beam evaporation (Temescal BJD 1800; base pressure of 3×10−6 Torr) onto a SiO2/Si substrate with predefined nanotube strips. We then used standard UV photolithography to pattern the S/D electrodes and interconnects using an etch-back scheme with a commercial wet etchant (TFA, Transcene Co.) to remove gold in exposed areas. We then used oxygen reactive ion etching (200 mTorr, 20 sccm, O2 flow, 100 W Radio Frequency power) to remove SWNTs outside of channel regions that were protected by a patterned layer of photoresist (Shipley 1805).
[0125] This step can also be carried out on the plastic substrate after transfer, which avoids the dimensional instability associated with polymer shrinkage in curing process and the device failure due to incomplete transfer of S/D electrodes. However, it will lead to inferior device performances, due to synergetic effect of smaller contact area between S/D electrodes and partially embedded SWNTs as well as smaller effective channel width when we use photolithography to defines SWNT strips as described above on relatively rough (compared with surface roughness of Si wafers) polymer surface (FIG. 14 ). Therefore, this approach is only utilized in fabricating row-decoder circuits, which has the highest requirements on device yield.
[0126] Transfer Printing Process:
[0127] The transfer printing process involved spin casting (1,500 rpm, 60 s) polyamic acid (PAA, Aldrich) onto the SiO2/Si wafer with SWNTs and S/D patterns, and then heating at 110° C. for 3 min to remove the solvent. On the target polyimide (PI) substrate (Kapton E, thickness ˜50 μm, DuPont Co.), we spin cast (5,000 rpm, 60 s) a film of polyurethane (PU, NEA 121). Before this step, we thermally cycled the PI between 30° C. and 270° C. to improve its dimensional stability.35 We laminated this PU coated substrate on top the PAN SiO2/Si wafer with PU facing toward the PAA film and applied pressure on back of the wafer to remove air bubbles. Heating them together to 135° C. for 30 min thermally cured the PU film, thereby binding the PI substrate to the PAA film. Peeling off the PI substrate lifted the film of PU/PAA with embedded SWNT networks and S/D electrodes off the SiO2/Si wafer, with one side of S/D electrodes exposed. In the final step a vacuum oven (base pressure of 300 mTorr) with nitrogen flow (500 sccm) was used to thermally cure the PAA to PI through imidization reaction.36
[0128] Gate Dielectric Deposition:
[0129] The gate dielectric was deposited on top of the PAA after curing to PI. In the first step, 30 nm of HfO2 was deposited by electron-beam evaporation (Temescal BJD 1800; base pressure of 2×10−6 Torr) at a relatively low deposition rate (<0.5 Å/s) as measured by a quartz crystal thickness monitor. This layer served as a protective layer for SWNTs against highly reactive precursors used in a subsequent atomic layer deposition (ALD) process.37 After evaporation, the sample was transferred immediately to the ALD chamber to preserve the hydrophilicity of freshly deposited HfO2, which facilitates the growth of high quality pin-hole free ALD film. The ALD HfO2 film (12 nm) was deposited with a commercial ALD reactor (Savannah 100, Cambridge Nanotech. Inc.). One ALD reaction cycle consists of one dose of water followed by 5 s exposure and 300 s purge and then one dose of Hf(NMe2)4 followed by another 5 s exposure and 270 s purge. During deposition, the nitrogen flow was fixed at 20 sccm and the chamber temperature was set at 120° C. The low deposition temperature prevents cracking of HfO2 due to thermal expansion coefficient mismatch but requires very long purging time to remove excess precursors absorbed on the surface to prevent CVD type reactions in the chamber.38
[0130] Via Opening and Gate/Interconnect Pattering:
[0131] After depositing the dielectric, the gate pattern was defined by another photolithography step. A lift-off scheme was used to allow alignment of gate electrodes to S/D using previously patterned alignment markers. Metal for the gate electrodes (120 nm Al or 2 nm Cr/120 nm Au) was deposited by electron-beam evaporation (Temescal BJD 1800; base pressure of 3×10−6 Torr). In this metallization step, as well as the next step for defining interlayer interconnects, two angled evaporations (incidence angle=60°) with substrates placed at opposite orientations and a blanket evaporation (incidence angle=90°) were performed to insure that the metal layers covered the underlying surface topography, thereby avoiding open points that would otherwise form in the interconnect lines. In all cases, the deposition rate must be within 4˜7 Å/s. If the evaporation rate is lower than 4 Å/s, heat accumulated can lead to cracking of the PU layer; if the evaporation rate is higher than 7 Å/s, the strain accumulated in the metal film can lead to defect formation in the lift-off process.
[0132] Following deposition, the lift-off was accomplished by soaking in acetone for 10 min, followed by a short ultrasonic treatment (30 s) to assure that the lift-off process was complete. Since the SWNTs were covered by HFO2, the ultrasonic treatment did not damage the nanotube network. (Prolonged acetone soaking can dissolve, at a low rate, the PI cured from PAA due, presumably, to incomplete imidization.) Contact pads for probing and vias for interlayer interconnects were exposed by photolithography using AZ 5214 photoresist. A hard bake (120° C., 2 min) of the photoresist was performed before hydrofluoric acid etching (4 s in concentrated HF solution) of HfO239 to improve the adhesion between photoresist and HfO2. Note here that the Au pads patterned in S/D layer under vias must be larger in size than via holes to protect PU from being etched by HF in this step through acidolysis reaction. The interlayer interconnect (5 nm Cr/100 nm Au) was patterned using a lift-off process and photolithography. The patterning of gate electrodes and interconnects were carried out separately because 1) the predefined gate layer can also serve as a protective layer for gate dielectric against possible defects existing in the photoresist mask layer to prevent creating pin-holes in channel region in wet etching step, and 2) aluminum tends to form a poor contact to the gold S/D electrodes, possibly due to intermetallic formation,40 such that a different interconnect metal, such as Cr/Au, was necessary when using Al gates. Finally, the completed device/circuit was aged in air for 24 hours, followed by a thermal anneal at 120° C. for 30 min, to achieve stable operation.
[0133] Device and Circuit Characterizations:
[0134] DC measurements of SWNT transistors and circuits were carried out in air using a semiconductor parameter analyzer (Agilent 4155C), operated by Agilent Metrics I/CV Lite program and GBIP communication. Triaxial and coaxial shielding was incorporated into a Signatone probe station to achieve better signal/noise ratio. An Agilent 4282A precision LCR meter was used for capacitance and impedance measurements. AC input signals were generated by a function generator (GWinstek, GFG-8219A). The output signals were measured using a standard oscilloscope (Tektronix, TDS 3012B).
[0135] Stick-percolation Simulation: We constructed a sophisticated first-principle numerical stick-percolation model for the above random CNT network by generalizing the random-network theory.20,41,42 The model randomly populates a 2D grid by sticks of fixed length (LS) and random orientation (θ) and determines ION through the network by solving the percolating electron transport through individual sticks. In contrast to classical percolation, the CNT network is a heterogeneous network: ⅓rd of the CNTs are metallic and remaining ⅔rd are semiconducting. Since, LC and LS are much larger than the phonon mean free path; linear-response transport obviates the need to solve the Poisson equation explicitly. The system is well described by drift-diffusion theory within individual stick segments of this random stick-network. The low bias drift-diffusion equation, J=qμn dφ/ds, when combined with current continuity equation, dJ/ds=0, gives the non-dimensional potential φi along tube i as
[0136]
[0137] Here, s is the length along the tube and Cij=G0/G1 is the dimensionless charge-transfer coefficient between tubes i and j at their intersection point, and G0(˜0.1 e2/h) and G1(=qnμ/Δx) are mutual- and self-conductance of the tubes, respectively. Here, n is carrier density, μ is mobility and Δx is grid spacing. The density (D) of the random stick network is measured in /μm2 and the density of the SWNT network ˜20/μm2 according to SEM measurements.
[0138] SPICE Simulation:
[0139] The behavior of SWNT TFTs is described as a PMOS field-effect transistor (FET) parallelly connected with a voltage (VGS and VDS) dependent exponential current source. The p-FET is modeled using standard square law model with channel length modulation and source-drain resistance effects in consideration. The exponential current source is utilized to mimic the ambipolar current which leads to exponential increase in Ioff with increasing VDS. The exponential term is expressed in form of Taylor's series,
[0140]
where Vx is defined as Vx=Vthreshold+αVGS−βVDS, and first three terms are incorporated in the SPICE model. All fitting parameters are extracted from measured I-V characteristics (summarized in Table 1). The channel length scaling behavior of these SWNT random network transistors can only be captured by percolation modeling. The results of such models (e.g. off state resistances) can be used as inputs to the SPICE models to capture the full range of behaviors.
where Vx is defined as Vx=Vthreshold+αVGS−βVDS, and first three terms are incorporated in the SPICE model. All fitting parameters are extracted from measured I-V characteristics (summarized in Table 1). The channel length scaling behavior of these SWNT random network transistors can only be captured by percolation modeling. The results of such models (e.g. off state resistances) can be used as inputs to the SPICE models to capture the full range of behaviors.
[0141]
| TABLE 1 | ||||
|---|---|---|---|---|
| Fitted SPICE Model Parameters | ||||
| Component | Parameter | Value | ||
| Voltage Controlled | Vthreshold | −4 | V | |
| Current Source | α | 0.45 | ||
| β | 3 | |||
| KN | 10−9 | |||
| VG0 | 1 | V | ||
| p-FET | λ | 0.1 | ||
| VT | −0.4 | V | ||
| KP | 20 | μA/V2 | ||
| RS | 11 | KΩ | ||
| RD | 11 | KΩ | ||
[0142] The above model was then utilized in designing and simulating digital logic circuits.43 In time response simulation, load capacitance is calculated automatically from measured overlap capacitance (330 nF/cm2) and gate capacitance (80 nF/cm2) as well as estimated contact resistance (11 KΩ) by HSPICE program. Although the measured voltage responses of fabricated circuits agree well with designing specifications, the current load responses show behavior only qualitatively similar to simulation results as shown in FIG. 20 . This deviation may come from relatively large batch-to-batch variations in device performances, which influences the current load more significantly.
Supplementary Discussion
Scaling Analysis of SWNT TFTs
[0143] Channel length (LC) scaling properties of SWNT Thin-film Transistors (TFTs) are explored prior to successful circuit design and integration. The device on current scales linearly with LC, which can be easily understood since the channel width only changes the number of the stripes in the channel. FIG. 12 a shows the transfer characteristics of an array of SWNT TFTs with different LCs. The LC normalized on resistance under different gate voltages increase linearly with the decrease of LCs (FIG. 12 a, inset). The contact resistance, as determined from the y axis intercept, is much smaller compared to the channel resistance, which indicates that these transistors are not contact limited. FIG. 12 b presents the effective mobility (μeff) and device on/off ratio (lon/loff) as a function of LC. μeff decreases only slightly at small LCs, confirming that the Schottky barrier between SWNT and Au has limited effects on device performance. More importantly, μeff and threshold voltage (VT) extracted from linear region and saturation region doesn't show significant difference (FIG. 12 b & c). It is an advantage compared with devices built on organic transistors, whose mobility is dependent on gate voltage as a result of their variable range hopping transport mechanism.44 Device on/off ratios show, on the other hand, very strong variation with LC, increasing sharply and then gradually saturating for LC larger than 50 μm. This correlation derives from purely metallic pathways between Source/drain (S/D) electrodes that increase in number with decreasing LC, for a given tube density and average tube length. These variations are quantitatively consistent with percolation modeling. The decrease of on/off ratio at high VDS bias is caused by band-to-band tunneling current and the off current can be reduced through doping scheme reported for single SWNT devices.45 In conclusion, the scaling properties of SWNT TFTs allow us to predict properties of a given SWNT TFT based on measurements performed on another device with different geometries, which forms the basis for our rational circuit design efforts.
Gate Capacitance of SWNT TFTs
[0144] Gate capacitance is a crucial parameter for transistors, serving as the basis to extract effective field effect mobility and estimate the transient behavior of transistors/circuits. Here we demonstrate the first direct capacitance measurements of SWNT random networks. We used a structure similar to the layout of the present SWNT TFTs, where the capacitor forms between the top round-shaped electrodes and the nanotube network beneath it (FIG. 13 , Inset). The measurements were performed at 100 kHz under serial connection model because the leakage current is small but the contact resistance, especially the contact resistance between SWNTs and Au electrodes is significant. Measured capacitance-voltage (C-V) characteristics closely resemble previous results based on individual tubes, with symmetric C-V curves in the depletion region and accumulation region reflecting the underlying band structure symmetry of SWNTs (FIG. 13).46 The dip of capacitance at the device off state is wide and shallow, possibly due to the diameter distribution of nanotubes in the network. The measured gate capacitance, 85 nF/cm2, is about one third of measured thin film capacitance of 42 nm thick HfO2, which is in good agreement with previous modeling results.47
Distribution of Device On/off Ratios
[0145] Compared with other device parameters, the on/off ratios show much larger variation, as shown in FIG. 9 f and FIG. 14 . The origin of this distribution is likely due to electron injection current, from the slightly ambipolar nature of the device operation. Devices with high electron conduction current generally have low on/off ratios and, as a consequence, larger subthreshold slopes (FIG. 14 a).
The Dependence of Off-state Current (loff) on Drain-source Voltage (VDS)
[0146] The p-channel SWNT top-gate transistors show slight ambipolar behavior. As a result, loff contains, in general, contributions from electron injection and transport through semiconducting pathways through the networks as well as ohmic transport of electrons through metallic pathways.48 For devices with long channel lengths, whose on/off ratio is high (˜>1000) for low VDS, metallic tube paths do not exist or are very few. In this case, loff mainly comes from undesired electron injection. This behavior leads to superlinear increases in loff with linearly increasing VDS as shown in FIG. 15 a. By contrast, for devices with small channel lengths, which usually have low on/off ratio even for low VDS, metallic pathways become the dominant contributor to loff. Here, loff increases linearly with VDS, as shown in FIG. 15 b.
Distribution of Effective Device Mobility and Subthreshold Swing
[0147] The distribution of effective device mobility (μ) and subthreshold swing (S) were examined through measurements on more than 100 devices (channel length 100 μm, channel width 200 μm), as summarized in FIG. 16 . Standard deviations of ˜10% for μ and ˜15% for S are observed, which are sufficient for most envisioned applications of flexible electronics.
Switching Speed Characteristics of the Four-bit Decoder Circuit
[0148] The operation speed of the decoder circuit was characterized by measuring one output signal with one alternating current input. Results show that the decoder can be successful switched in kHz regime, with a T≈50 μs rise time and a T≈100 μs fall time (FIG. 17 ). The experimental results agree reasonably well with time domain simulation results based on previously described SPICE model for SWNT TFTs.
Operational Stability Test of SWNT TFTs
[0149] Good stability is observed in the top-gate SWNT transistors as illustrated by data in FIG. 18 . These measurements involved electrically cycling the devices in ambient conditions (relative humidity≈90% and T=20° C.˜25° C.) between VGS=−2 and 0 V at VDS=−0.2 V. The data indicate negligible changes in Ion or on/off ratio for more than two hundred cycles.
Estimation of Some Properties of SWNT TFTs
[0150] Estimated total number of SWNTs involved in transport for a typical device:
[0151]
where W is channel width, LC is channel length, LS is average tube length and D is area nanotube density normalized by LS respectively.
where W is channel width, LC is channel length, LS is average tube length and D is area nanotube density normalized by LS respectively.
[0152] Estimated total surface coverage by SWNTs in channel region:
[0153]
where N is number of nanotubes in channel region, and dNT is nanotube diameter.
where N is number of nanotubes in channel region, and dNT is nanotube diameter.
[0154] Estimated cut-off frequency of a device:
[0155]
where gm is transconductance, Ci is gate capacitance, and Cparasitic is source/drain-gate overlap capacitance respectively.
where gm is transconductance, Ci is gate capacitance, and Cparasitic is source/drain-gate overlap capacitance respectively.
REFERENCES
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- [0170] 45. Chen, J., Klinke, C., Afzali, A. & Avouris, P. Self-aligned carbon nanotube transistors with charge transfer doping. Appl. Phys. Lett. 86, 123108 (2005).
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Example 2
Theory and practice of “Striping” for Improved ON/OFF Ratio in Carbon Nanonet Thin Film Transistors
Abstract
[0174] A new technique to reduce the influence of metallic carbon nanotubes (CNTs)—relevant for large-scale integrated circuits based on CNT-nanonet transistors—is described and verified. Historically, electrical and chemical filtering of the metallic CNTs have been used to improve the ON/OFF ratio of CNT-nanonet transistors; however, the corresponding degradation in ON-current has made these techniques somewhat unsatisfactory. Here, we diverge significantly from the classical approaches in favor of a new approach based on relocation of asymmetric percolation threshold of CNT-nanonet transistors by a technique called “striping”; this allows fabrication of transistors with ON/OFF ratio >1000 and ON-current degradation no more than a factor of 2. We offer first principle numerical models, experimental confirmation, and renormalization arguments to provide a broad theoretical and experimental foundation of the disclosed methods and device configurations.
Introduction
[0175] Since the year 2000, there have been many reports of a new class of devices called nanonet network thin-film transistors (NN-TFTs), whose channel material is composed of nanocomposites of carbon nanotubes (CNTs) or Si/ZnO nanowires (NWs) (FIGS. 21( a) and 21(b)) [1-11]. It has often been suggested that this technology might be a higher performance alternative to now-dominant amorphous silicon (a-Si) and polysilicon (poly-Si) technologies for applications in flexible electronics, transparent displays, etc. While the progress has been rapid—especially for CNT NNTFTs, evolving from single resistors to RF transistors within a span of a few years—there has been a persistent perception that large scale integration of CNT NN-TFT technology will be challenging (if not impossible) due to “contamination from metallic tubes” [12, 13].
[0176] It is well known that in a typical ensemble of CNTs, approximately two-thirds of the tubes are semiconducting and one-third metallic [14]. Since the conductivity of the metallic tubes cannot be controlled by gate voltage, the ON/OFF ratio of short channel transistors—where a proportionate fraction of both metallic and semiconducting tubes bridge the source (S) and drain (D) directly—cannot exceed ˜3. This ratio is unacceptably small for large scale integration of transistors that demand an ON/OFF ratio of at least ˜103-104. Such a high ON/OFF ratio is possible for longer channel CNT NN-TFTs where individual tubes do not thread the S/D directly, so that carriers must percolate from source to drain by hopping through a percolating path of a network of sticks. As long as the percolation threshold (DP, see FIG. 21( c)) is smaller than the density of CNTs (DCNT) but larger than that of the metallic tubes (DM), all percolation paths from S/D must involve the at least one semiconducting segment. Gate modulation of this “weakest-link” ensures high a ON/OFF ratio.
[0177] There are however three specific challenges to this approach of ensuring a high ON/OFF ratio for all transistors in an integrated circuit (IC): First, the requirement that DM<DP<DCNT dictates use of relatively low-density nets with correspondingly reduced ON-currents. Second, since an IC typically includes transistors of various channel lengths (LC), widths (WC), and stick length (Lstick) and since the finite size percolation threshold depends on LC, WC, and Lstick (i.e., DP=f(WC,LC,Lstick)) [15], ensuring DM<DP<DS requires LC- and WC-specific tailoring of Lstick for every group of transistors. Otherwise, the shift in DP could violate DM<DP<DS and degrade the ON/OFF ratio to unacceptably low values. Even if a combination of such stick lengths could be produced, solution processing precludes delivery of tubes of specific lengths to specific transistors. Third and finally, Lstick of CNTs produced by classical techniques is not monodisperse and the inherent statistical distribution of Lstick in as-processed CNTs translates to a statistical distribution of DP in various transistors across the IC. Once again, the distribution of DP enhances the probability of accidental shorts in one of the many transistors in the IC [13], which would render the entire IC non-functional.
[0178] To solve this “metallic-contamination” issue and to address the three limitations of NN-TFTs discussed above, researchers have developed several “purification” techniques to ensure DS/DM>>2 [16]. Briefly, these techniques include removal of metallic CNTs (m-CNTs) by chemical [17] or mechanical means prior to device fabrication [18] or in situ resistive [12, 19] and inductive filtering of the m-CNTs after fabrication of the transistors [20]. There is a debate whether chemical filtering modifies the properties of remaining semiconducting CNTs (s-CNTs), because device properties of such purified films have not been reported in Ref. [21]. On the other hand, both resistive and inductive filtering has been shown to degrade ON-current significantly (because the protocol burns at least some s-CNTs along with m-CNTs) [20]. Moreover, it is not clear if the technique is scalable because while it is possible to burn m-CNTs in individual transistors by turning off the s-CNTs by gate bias and by adjusting the drain voltage, it is not clear how to translate this protocol to an IC configuration where the electrodes are interconnected and there is only a limited access to individual transistors.
[0179] In this Example, we describe the theory and demonstration of a new in situ technique called “striping” that simultaneously addresses the three concerns of “metallic contamination” discussed above. This method has also been recently used to demonstrate the first practical medium scale circuit using NN-TFTs [1]. Striping is based on the idea that instead of reducing DM to address the “metallic contamination” problem, one might alternatively tailor DP for individual transistors to solve the problem of ON/OFF ratio. In simple terms, since DP˜k/L2stick [22] (where k is a constant), DP can be modified by tailoring Lstick. This modification of DP cannot be done prior to fabrication of the devices, because each LC requires a different Lstick, as discussed above. This modification is also difficult to achieve after device fabrication, because one cannot possibly find and cut to size all the randomly oriented tubes after fabrication. Striping resolves this dilemma by recognizing that percolation theory requires that Lstick in the expression for DP refers to effective tube length and so long as the “average” tube-length <Lstick,eff> is reduced by any means, the in situ transistor-specific modification of percolation threshold can be easily achieved.
[0180] Briefly, the technique of striping involves the procedure shown in FIG. 21( a). As used in FIG. 21 source electrode is designated as “S”, drain electrode is designated as “D” and gate electrode is designated as “G”. Given a high-density CNT network of a given LC, Lstick, and DS/DM, striping involves defining specific channel width (Wstripe) in strips. Striping reduces Lstick of a fraction of tubes near the stripe, which in turn translates into an overall reduction in effective <Lstick,eff> in the channel. This procedure—as we have mentioned before—allows transistor-specific modulation in DP of the transistors. Intuitively speaking, in a transistor with a density of tubes above the percolation threshold (see FIG. 21( d)), there are many parallel paths carrying current from source to drain. In the striping method, the goal is to break these paths (FIG. 21( e)) by using finite stripes for individual transistors. Note that striping not only solves the LC-specific percolation problem, but also reduces the intrinsic process-induced fluctuation of DP due to the distribution of Lstick. The reduced spread of DP reduces the chances of accidental shorting of S/D in large scale ICs.
[0181] It is obvious from the above discussion that the critical element of the striping is the definition of stripe width Wstripe as a specific function of LC, Lstick, and DCNT. Below we use the stick percolation model to develop a prescription for Wstripe and then validate our predictions by systematic experiments.
- [0182] 1. Theoretical Models and Summary of the Fabrication Process
[0183] We constructed a sophisticated first principles numerical stick percolation model for NN-TFTs by generalizing the random-network theory which has been described in several earlier publications [8, 22, 23]. Briefly, the model randomly populates a two-dimensional (2-D) grid by sticks of fixed length (Lstick) and random orientation (a), and determines the ON-current ION through the network by solving the percolating electron transport through individual sticks. In contrast to classical percolation of homogenous sticks, the NN-TFT is a heterogeneous network: as noted above, one-third of the CNTs are metallic and remaining two-thirds are semiconducting. Since LC and Lstick are much larger than the phonon mean free path, linear-response transport within individual stick segments of this random stick-network system is well described by drift-diffusion theory [8]. Also, small source drain voltage (VSD) and gate valtage (VG) obviate the need to solve the Poisson equation explicitly.
[0184] The key difference between the previously published models and the model used in this Example is this: Typically the width of the simulation domain is much smaller than the actual width of the transistors. Previously, this necessitated the use of periodic boundary conditions (PBC) for all sticks that crossed the edge of the transistors. Finite width Wstripe related effects are fundamental to the operation of the transistors produced through striping and as such these transistors can no longer be treated by PBC. Instead, we use reflecting boundary conditions (RBC) for the potential and charge for all sticks that are “cut” by stripe lines to simulate the performance of striped NN-TFTs.
[0185] In order to validate the theory of striping, we fabricated arrays of SWNT TFTs with various combinations of LC, Wstripe, and tube density (DCNT). Uniform SWNT thin films were first synthesized by the chemical vapor deposition method on SiO2 (100 nm) Si wafers. The tube density was controlled by adjusting the dilution ratio of the catalyst solution. Source/drain electrodes were patterned by the liftoff method with standard photolithography and electron-beam evaporation. Each device was isolated through oxygen reactive ion etching (RIE) while CNTs in the channel region were protected by a patterned layer of photoresist. Either phase-shift lithography or photolithography generated photoresist stripes, with variable Wstripe, aligned to the electron transport direction of each TFT. Subsequent oxygen RIE removed CNTs in the exposed area and transformed the stripe pattern to an underlying nanonet network as shown in FIG. 21( b). Removing the photoresist layer by acetone soaking completed the device fabrication process. The measurements of device ON-current and ON/OFF ratio were carried out in air, using a semiconductor parameter analyzer (Agilent, 4155C). Details of the methods of fabrication and characterization are provided throughout the present description [24].
- [0186] 2. Results and Discussion
[0187] FIG. 22 shows the simulation results for ON-current (ION) and ON/OFF ratio of striped transistors for various widths, channel lengths, and stick lengths, plotted against the scaling variables Wstripe/Lstick and LC/Wstripe, respectively. The nanonets have several intermingled parallel paths and the stripes break some of them depending on the stripe width decreasing both ON- and OFF-currents. FIG. 22( a) shows that for a given channel length, the decrease in ON-current is relatively minor—approximately a factor of two as the Wstripe/Lstick is reduced from 15 to 0.3. The relatively benign effect on the ON-current of striping should be compared with about an order of magnitude reduction in the ON-current due to electrical filtering methods [12, 19]. Experimental results for transistors with the same dimensions are plotted in FIG. 22( b) and these measurements validate the scaling predictions from the percolation theory remarkably well.
[0188] Obviously, high ON-current is of little value if the ON/OFF ratio is not improved simultaneously. FIG. 22( c) plots the ON/OFF ratio as a function of the scaling variable LC/Wstripe, the channel length normalized by the width (Wstripe), which is the effective stick length after the striping. The different curves are for different Wstripe/Lstick. At large widths with Wstripe/Lstick>˜1, the ON/OFF ratio is small, because in a wide device (compared to the stick length), there is always a finite probability that a metallic subnetwork would be able to bridge the S/D directly. Only a negligible fraction of the tubes is affected by striping and the length of the sticks after striping <Lstick,eff> is hardly suppressed by this process. As such DP does not change appreciably and the ON/OFF ratio remains pegged at small values (˜10), as seen in the black and green curves in FIG. 22( c). On the other hand, for Wstripe/Lstick<1, the many cross-bridging percolation paths are interrupted, and the probability of an all-metallic subnetwork threading the S/D is reduced. Thus, the striping process pushes the percolation threshold into the DM-DCNT interval, i.e., DM<DP<DCNT, with a corresponding dramatic increase in the ON/OFF ratio (FIG. 22( c), red and blue curves). However, if the channel length remains small (i.e., LC/Wstripe<5) only a few sticks are needed to bridge S/D. A metallic subnetwork can still bridge the S/D with finite probability when the ON/OFF ratio is small (<102) as shown in FIG. 22( c). FIG. 22( d) shows the corresponding experimental results and they match the simulation results closely.
[0189] In this Example, we set out to explore the fundamentals of the technique so that the trade-offs and scaling issues of the methodology become well-documented. The average tube length can be adjusted via controlling the CVD (chemical vapor deposition) synthesis parameters, e.g., catalyst species and growth time duration. In addition, since Wstripe can be reduced to tens of nanometers, which is solely defined through lithography, the requirement of LC/Wstripe>10 can generally be satisfied without sacrificing LC through adjusting Wstripe (even if Lstick cannot be adjusted effectively).
[0190] To explore the predictions discussed above, a large number of devices with various LC, Wstripe, and DCNT were fabricated and characterized. Despite all the idealization inherent in the model and its inability to mimic the details of the physical process, once plotted against the scaled variables identified by theory (Wstripe/Lstick), the experimental results for the ON/OFF ratio of the corresponding transistors (FIG. 22( b)) support the theoretical trends almost exactly, showing very similar trends in the ON/OFF ratio to those anticipated by the theory.
- [0191] 3. Optimization and Generalization
[0192] The wide ranging simulation and measurement data discussed above suggest that the condition for excellent ON/OFF ratio and good ON-current is possible with striping provided that Wstripe/Lstick˜1 and LC/Wstripe>10. Specifically, for example, for a technology with 5 μm design rules, i.e., LC≧5 μm, stripe separation should be ˜0.5 μm, and the CNT length should be chosen as ˜0.5 μm to ensure high yield ICs.
[0193] To complete the discussion on optimization of the striping method, let us briefly discuss the fluctuation in the tube density DCNT, which plays an important role in determining the device performance (FIG. 23 ) and device-to-device fluctuations (FIG. 24 ). A higher density network with lower LC is desired for higher ON-current, although an increased probability of all metallic paths decreases the ON/OFF ratio of the device. FIG. 23 shows the simulation and experimental results for ON-current and ON/OFF ratio as a function of tube density for different stripe widths. Note that the ON-current has linear dependence on density for the higher density networks shown in FIG. 23 . Note that the average tube length is difficult to control for a random CNT network and we have normalized the results with respect to the average tube length [22]. FIGS. 23( a) and 23(b) show that the ON-current has a linear dependence on density. This is expected for a network with densities much higher than the percolation threshold. The increased density causes a larger number of percolating paths. These simulations can be used to optimize the device parameters such as density (DCNT), channel length (LC), and stripe width (Wstripe), given the required constraints on the device performance. The ON/OFF ratio decreases with higher tube density and is found to be appreciably high (>103) for all densities if Wstripe/Lstick<1 and if the channel length is at least 10 times the width of the transistor. The ON/OFF ratios of these devices depend mainly on the fraction of useful (>103) devices, e.g., a single metallic path in a total of 100 paths can significantly decrease the ON/OFF ratio of the device to ˜100. In FIGS. 23( c) and 23(d) we see that the ON/OFF ratio decreases monotonically with the tube density. Higher densities increase the probability of an all metallic path, decreasing the ON/OFF ratio. Also, lower striping widths (Wstripe/Lstick) have a larger chance of cutting an all metallic path, hence the ON/OFF ratio is higher for lower Wstripe/Lstick.
[0194] Due to statistical nature of nanonet-TFTs (and by analogy to the random dopant fluctuation issue in classical complementary metal oxide semiconductor (CMOS) devices), we expect some variation in ON-current from one transistor to next. Apart from issues related to average ON-current and ON/OFF ratio discussed in relation to FIG. 23 , the fluctuation in the ON-current and ON/OFF ratio must also be within acceptable limits for practical large scale IC design. FIG. 24( a) shows the variation in normalized standard deviation (NSD) for the ON-current (normalized with respect to the average ON-current) with tube density for various values of Wstripe/Lstick. The variation in ON-current is due to the variation in the number of connecting paths from S/D. Devices with lower numbers of connecting paths are affected far more by these variations than devices with higher numbers of connecting paths. In the devices with higher numbers of connecting paths, the variation reduces due to an averaging effect. The standard deviation in FIG. 24( a), shows two trends consistent with this argument. Firstly, the standard deviation increases for lower tube densities. Secondly, striping with smaller Wstripe/Lstick leads to fewer connected paths (FIG. 23( a)) which causes higher standard deviations. For unstriped devices of large width, NSD is small (˜0.05) which means only about 5%-10% variation in ON-current between different transistors. FIG. 24( a) shows that NSD values do change with striping; however, as long the design rule (i.e., Wstripe/Lstick˜1) is followed; specifically, provided Wstripe/Lstick=1-1.25, NSD variation remains essentially unchanged with respect to the unstriped transistors (5%-10%) even in the unlikely scenario of 50% fluctuation in the original target density of 25 sticks/μm2.
[0195] The importance of following the design rule of Wstripe/Lstick ˜1 is further illustrated in FIG. 24( b) which shows the fraction (fON/OFF) of devices with high (>103) ON/OFF ratios for various densities. It is clear that striping reduces the sensitivity of the ON/OFF ratio to fluctuations in tube density. Indeed, while for Wstripe/Lstick>1, ON/OFF decreases monotonically with density, when Wstripe/Lstick<1, fON/OFF is close to 1, i.e., approximately 100% of the transistors have high ON/OFF ratios, even with 20%-50% variation in the target tube density of 25 sticks/μm2.
[0196] The numerical simulation and experimental confirmation of the scaling relationship above provide simple, intuitive guidelines for transistor scaling. A simple renormalization argument of the above scaling relationship may aid in understanding the physical basis of asymmetric percolation that underlies the robustness and generality of the striping technique. Understanding the renormalization argument may also allow development of other techniques that achieve the same result, without following the specific prescription proposed in this Example.
[0197] The reason striping allows an easy manipulation of the percolation threshold is because a finite stripe allows tailoring of the percolation threshold in between that of a 1-D chain of resistors (DP=1) and a 2-D resistor network (DP=0.5). The precise value of DP for a finite width resistor on a square lattice is obtained from the recurring renormalization condition that (see FIG. 25 ) [15] DP,i=p,i+15−5Dp,i+14+2Dp,i+13+2Dp,i+12 where DP,i is the percolation threshold of the resistor of width 2i and length N. Assuming DP,1→1 for a quasi-1-D conductor, the solution of the recurrence relationship (DP,1→DP,2→DP,3→DP,4 . . . →DP,a, etc.) as plotted in FIG. 25( b) shows a dramatic transition in percolation threshold from 1 to 0.5 as width of the resistor (i.e., stripe width) passes through a critical value. Moreover, for a lattice whose elements are filled with probability, DP,i the condition of conductivity, scaling requires that [15]
[0198]
Here σi is the conductance of the network with width 2i and percolation threshold DP,i. The presence of the “2” in the denominator of the left-hand side represents the factor of two renormalizations of the network at successive levels. For very wide transistors, a factor of two scaling of transistor width scales the current by 2μ/v˜2×1.93=3.86, as expected from the 2-D conductivity exponent [25]. However, as the width of the transistor is reduced, many of the percolation paths are broken and the 2-D conductivity exponent reduces from 1.93 to 1; in other words, current scales linearly with conductor width. This procedure explains how the factor of two reductions in the ON-current due to striping (FIGS. 22( a), 23(a), and 25(c)) is a consequence of the finite-size percolation threshold and is not specific to the particular network or simulation model being discussed.
Here σi is the conductance of the network with width 2i and percolation threshold DP,i. The presence of the “2” in the denominator of the left-hand side represents the factor of two renormalizations of the network at successive levels. For very wide transistors, a factor of two scaling of transistor width scales the current by 2μ/v˜2×1.93=3.86, as expected from the 2-D conductivity exponent [25]. However, as the width of the transistor is reduced, many of the percolation paths are broken and the 2-D conductivity exponent reduces from 1.93 to 1; in other words, current scales linearly with conductor width. This procedure explains how the factor of two reductions in the ON-current due to striping (
- [0199] 4. Conclusions
[0200] In summary, we have provided a theory of a highly effective alternative method called striping to increase the ON/OFF ratio of a CNT NN-TFT without significantly reducing the per-width ON-current. Compared to traditional techniques of increasing ON/OFF ratio like electrical or chemical filtering, the in situ method appears scalable and versatile, and appropriate for large scale integrated circuits. Both theory and experiments suggest that if the stripe width is so defined that Wstripe/Lstick <1 and LC/Wstripe>10, high ON/OFF ratio and high ON-current are easily ensured. Note that reduction in Wstripe/Lstick significantly below unity may not be acceptable from the point of view of parameter fluctuation (FIG. 24 ) and may otherwise have to be managed by various fluctuation-resilient circuit techniques. Apart from the numerical simulation and experimental results, our renormalization argument suggests that this technique is really a general approach, fundamentally rooted in the asymmetric percolation threshold of heterogeneous networks.
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STATEMENTS REGARDING INCORPORATION BY REFERENCE AND VARIATIONS
[0227] All references throughout this application, for example patent documents including issued or granted patents or equivalents; patent application publications; and non-patent literature documents or other source material; are hereby incorporated by reference herein in their entireties, as though individually incorporated by reference, to the extent each reference is at least partially not inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference except for the partially inconsistent portion of the reference).
[0228] The following references relate generally to methods of synthesizing and purifying carbon nanotubes: (1) A. M. Cassell, J. A. Raymakers, J. Kong, H. J. Dai, J. Phys. Chem. B 103(31), (1999), pp. 6484-6492; (2) M. Su, B. Zheng, J. Liu, Chem. Phys. Lett. 322(5), (2000), pp. 321-326; (3) B. Kitiyanan, W. E. Alvarez, J. H. Harwell, D. E. Resasco, Chem. Phys. Lett. 317(3-5), (2000), pp. 497-503; (4) J. H. Hafner, M. J. Bronikowski, B. R. Azamian, P. Nicolaev, A. G. Rinzler, D. T. Colbert, K. A. Smith, R. E. Smalley, Chem. Phys. Lett. 296(1-2), (1998) pp. 195-202 (5) H. M. Cheng, F. Li, G. Su, H. P. Pan, L. L. He, X. Sun, M. S. Dresselhaus, Appl. Phys. Lett. 72(25), (1998), pp. 3282-3284; (6) B. Zheng, Y. Li, J. Liu, Applied Physics A 74, 345-348 (2002); (7) C. Journet, W. K. Maser, P. Bernier, A. Loiseau, M. Lamy de la Chapelle, S. Lefrant, R. Lee, J. E. Fischer, Nature 388, 756-758 (1997); (8) A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tomanek, J. E. Fischer, R. E. Smalley, Science 273, (1996) 483-487; (9) M. Holzinger, A. Hirsch, P. Bernier, G. S. Duesberg, M. Burghard, Applied Physics A 70, (2000) 599-602; (10) G. S. Duesberg, J. Muster, V. Krstic, M. Burghard, S. Roth, Appl. Phys. A 67, (1998), 117-119; (11) J. Liu, A. G. Rinzler, H. Dai, J. H. Hafner, R. K. Bradley, P. J. Boul, A. Lu, T. Iverson, K. Shelimov, C. B. Huffman, F. Rodriuez-Macias, Y. S. Shon, T. R. Lee, D. T. Colbert, R. E. Smalley, Science 280, (1998) 1253-1256; (12) A. G. Rinzler, J. Liu, H. Dai, P. Nikolaev, C. B. Huffman, F. J. Rodriguez-Macias, P. J. Boul, A. H. Lu, D. Heymann, D. T. Colbert, R. S. Lee, J. E. Fischer, A. M. Rao, P. C. Eklund, R. E. Smalley, Appl. Phys. A 67, (1998) 29-37; (13) Y. Feng, G. Zhou, G. Wang, M. Qu, Z. Yu, Chem, Phys, Lett, 375, (2003) 645-648; (14) T. Takenobu, M. Shiraishi, A. Yamada, M. Ata, H. Kataura, Y. Iwasa, Synthetic Metals 135-136, (2003) 787-788; (15)1. W. Chiang, B. E. Brinson, A. Y. Huang, P. A. Willis, M. J. Bronikowski, J. L. Margrave, R. E. Smalley, R. H. Hauge, J. Phys. Chem. B 105, (2001) 8297-8301.
[0229] The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments, exemplary embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims. The specific embodiments provided herein are examples of useful embodiments of the present invention and it will be apparent to one skilled in the art that the present invention may be carried out using a large number of variations of the devices, device components, methods steps set forth in the present description. As will be obvious to one of skill in the art, methods and devices useful for the present methods can include a large number of optional composition and processing elements and steps.
[0230] When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all combinations and subcombinations possible of the group are intended to be individually included in the disclosure. When a compound is described herein such that a particular isomer, enantiomer or diastereomer of the compound is not specified, for example, in a formula or in a chemical name, that description is intended to include each isomers and enantiomer of the compound described individual or in any combination. Additionally, unless otherwise specified, all isotopic variants of compounds disclosed herein are intended to be encompassed by the disclosure. For example, it will be understood that any one or more hydrogens in a molecule disclosed can be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological research related to the molecule or its use. Methods for making such isotopic variants are known in the art. Specific names of compounds are intended to be exemplary, as it is known that one of ordinary skill in the art can name the same compounds differently.
[0231] Many of the molecules disclosed herein contain one or more ionizable groups [groups from which a proton can be removed (e.g., —COON) or added (e.g., amines) or which can be quaternized (e.g., amines)]. All possible ionic forms of such molecules and salts thereof are intended to be included individually in the disclosure herein. With regard to salts of the compounds herein, one of ordinary skill in the art can select from among a wide variety of available counterions those that are appropriate for preparation of salts of this invention for a given application. In specific applications, the selection of a given anion or cation for preparation of a salt may result in increased or decreased solubility of that salt.
[0232] Every formulation or combination of components described or exemplified herein can be used to practice the invention, unless otherwise stated.
[0233] Whenever a range is given in the specification, for example, a range of integers, a temperature range, a time range, a composition range, or concentration range, all intermediate ranges and subranges, as well as all individual values included in the ranges given are intended to be included in the disclosure. As used herein, ranges specifically include the values provided as endpoint values of the range. As used herein, ranges specifically include all the integer values of the range. For example, a range of 1 to 100 specifically includes the end point values of 1 and 100. It will be understood that any subranges or individual values in a range or subrange that are included in the description herein can be excluded from the claims herein.
[0234] All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. References cited herein are incorporated by reference herein in their entirety to indicate the state of the art as of their publication or filing date and it is intended that this information can be employed herein, if needed, to exclude specific embodiments that are in the prior art. For example, when composition of matter are claimed, it should be understood that compounds known and available in the art prior to Applicant's invention, including compounds for which an enabling disclosure is provided in the references cited herein, are not intended to be included in the composition of matter claims herein.
[0235] As used herein, “comprising” is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. As used herein, “consisting of” excludes any element, step, or ingredient not specified in the claim element. As used herein, “consisting essentially of” does not exclude materials or steps that do not materially affect the basic and novel characteristics of the claim. In each instance herein any of the terms “comprising”, “consisting essentially of” and “consisting of” may be replaced with either of the other two terms. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein.
[0236] One of ordinary skill in the art will appreciate that starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, assay methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resort to undue experimentation. All art-known functional equivalents, of any such materials and methods are intended to be included in this invention. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention as defined by the appended claims.
Claims
We claim:
1. An electronic device comprising:
a first electrode;
a second electrode; and
a patterned layer comprising randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with said first electrode and said second electrode, wherein said partially aligned carbon nanotubes are aligned relative to each other with deviations from absolute parallelism that are greater than 20 degrees; said one or more interconnected carbon nanotube networks characterized by a plurality of nanotube intersections; said patterned layer having a thickness less than or equal to 10 nanometers and having one or more void regions without carbon nanotubes; wherein said void regions comprise one or more cavities in said patterned layer extending entirely from said first electrode to said second electrode, thereby defining a plurality of strips of said interconnected carbon nanotube networks, wherein said one or more void regions provided between said first and second electrodes reduces by at least 50% the number of purely metallic conductive pathways from said first electrode and second electrode in said one or more interconnected carbon nanotube networks relative to a corresponding layer of randomly oriented or partially aligned carbon nanotubes provided in another one or more interconnected carbon nanotube networks and not having said void regions.
2. The electronic device of claim 1 wherein said one or more cavities extend through the entire thickness of said patterned layer.
3. The electronic device of claim 1 wherein said void regions are provided in said patterned layer in a periodic spatial distribution.
4. The electronic device of claim 1 wherein said void regions are provided in said patterned layer in an aperiodic spatial distribution.
5. The electronic device of claim 1 wherein said void regions comprise a pattern etched into said carbon nanotube networks of said patterned layer.
6. The electronic device of claim 1 wherein said void regions are filled with one or more insulating or semiconducting materials capable of disrupting metallic conductive pathways in said one or more interconnected carbon nanotube networks from said first and second electrodes.
7. The electronic device of claim 1 wherein said void regions comprise a plurality of said cavities in said patterned layer.
8. The electronic device of claim 7 wherein said cavities in said patterned layer have lateral dimensions selected from the range of 50 nanometers to 1000 microns.
9. The electronic device of claim 7 wherein said cavities in said patterned layer have a lateral cross sectional shape selected from the group consisting of rectangle, square, circle, and oval.
10. The electronic device of claim 7 wherein cavities in said patterned layer separate said strips such they do not physically contact each other.
11. The electronic device of claim 7 wherein each of said strips has an average width and extends an average length at least 10 times greater than said average width.
12. The electronic device of claim 11 wherein each of said strips extends an average length at least 100 times greater than said average width.
13. The electronic device of claim 11 wherein each of said strips extends an average length at least 1000 times greater than said average width.
14. The electronic device of claim 7 wherein said cavities in said patterned layer define 2 to 1000 of said strips in said patterned layer.
15. The electronic device of claim 14 wherein at least a portion of said strips extend between first and second electrodes and are provided in a parallel orientation.
16. The electronic device of claim 14 wherein at least a portion of said strips of interconnected carbon nanotube networks extend between first and second electrodes and are provided in a serpentine orientation.
17. The electronic device of claim 14 wherein adjacent strips are separated from each other by one or more of said cavities having lateral dimensions large enough to prevent electron transport directly between adjacent strips.
18. The electronic device of claim 7 wherein adjacent strips are separated from each other by an average distance selected over the range of 50 nanometers to 1000 microns.
19. The electronic device of claim 7 wherein at least a portion of said strips has an aspect ratio selected over the range of 10 to 1000.
20. The electronic device of claim 19 wherein at least a portion of said strips has an aspect ratio selected over the range of 50 to 500.
21. The electronic device of claim 7 wherein each of said strips has an average width selected over the range of 50 nanometers to 1000 microns and a length selected over the range of 500 nanometers to 10000 microns.
22. The electronic device of claim 21 wherein each of said strips has an average width of 5 microns and a length of 100 microns.
23. The electronic device of claim 7 wherein each of said strips have a lateral cross sectional shape selected from the group consisting of rectangular, circular, oval, and trapezoidal.
24. The electronic device of claim 7 wherein said strips of said patterned layer provide a semiconductor channel between said first and second electrodes, wherein said semiconductor channel has a length selected over the range of 50 nanometers and 1000 microns.
25. The electronic device of claim 1 wherein said patterned layer is a monolayer or sub-monolayer film of said carbon nanotubes.
26. The electronic device of claim 1 wherein said carbon nanotubes of said patterned layer have an average length selected over the range of 20 nanometers to 100 microns.
27. The electronic device of claim 1 wherein said carbon nanotubes of said patterned layer have a surface concentration selected over the range of 0.2 carbon nanotubes micron−2 to 100 carbon nanotubes micron−2.
28. The electronic device of claim 1 wherein said carbon nanotubes of said patterned layer comprise single walled carbon nanotubes.
29. The electronic device of claim 1 wherein said carbon nanotubes of said patterned layer comprise a mixture of semiconducting nanotubes and metallic nanotubes, wherein there are at least 1.5 times more semiconducting nanotubes than metallic nanotubes.
30. The electronic device of claim 29 wherein there are 1.5 to 4 times more semiconducting nanotubes than metallic nanotubes.
31. The electronic device of claim 1 further comprising a substrate provided to support said first electrode, second electrode and said patterned layer, said substrate selected from the groups consisting of a flexible substrate, a rigid substrate, a semiconductor substrate, polymer substrate, a ceramic substrate and a contoured substrate.
32. The electronic device of claim 1 comprising a transistor, wherein said first electrode is a source electrode, said second electrode is a drain electrode and said patterned layer is a semiconductor channel of said transistor.
33. The electronic device of claim 1 wherein said one or more void regions reduces by at least 70% the number of purely metallic conductive pathways in said one or more interconnected carbon nanotube networks relative to said corresponding layer of randomly oriented or partially aligned carbon nanotubes provided in said one or more interconnected carbon nanotube networks and not having said void regions.
34. The electronic device of claim 1 wherein said void regions have a depth in said patterned layer selected over the range of 2 nanometers to 10 nanometers.
35. The electronic device of claim 1 wherein said patterned layer comprises randomly oriented carbon nanotubes.
36. The electronic device of claim 35 wherein said carbon nanotubes of said patterned layer comprise single walled carbon nanotubes.
37. The electronic device of claim 35 wherein said electronic device has an on/off ratio greater than or equal to 1000.
38. The electronic device of claim 35 wherein said electronic device has a field effect mobility greater than or equal to 0.1 cm2 V−1 s−1.
39. A method for making an electronic device comprising the steps of:
providing a first electrode;
providing a second electrode; and
providing a patterned layer of comprising randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with said first electrode and said second electrode, wherein said partially aligned carbon nanotubes are aligned relative to each other with deviations from absolute parallelism that are greater than 20 degrees; said one or more interconnected carbon nanotube networks characterized by a plurality of nanotube intersections; said patterned layer having a thickness less than or equal to 10 nanometers and having one or more void regions in said patterned layer without carbon nanotubes; wherein said void regions comprise one or more cavities in said patterned layer extending entirely from said first electrode to said second electrode, thereby defining a plurality of strips of said interconnected carbon nanotube networks, wherein said one or more void regions provided between said first and second electrodes reduces by at least 50% the number of purely metallic conductive pathways from said first and second electrodes in said one or more interconnected carbon nanotube networks relative to a corresponding layer of randomly oriented or partially aligned carbon nanotubes provided in another one or more interconnected carbon nanotube networks and not having said void regions.
40. The method of claim 39 wherein cavities in said patterned layer separate said strips such they do not physically contact each other.
41. The method of claim 39 wherein each of said strips has an average width and extends an average length at least 10 times greater than said average width.
42. The method of claim 39 wherein said step of providing a patterned layer of randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with said first electrode and said second electrode comprises the steps of:
providing a precursor layer of said randomly oriented or partially aligned carbon nanotubes in electrical contact with said first and second electrodes;
patterning said precursor layer so as to generate said patterned layer.
43. The method of claim 42 wherein said step of providing said precursor layer in electrical contact with said first and second electrodes is selected from the group consisting of:
growing said carbon nanotubes on a device substrate, thereby generating said precursor layer;
dispersing said carbon nanotubes in a solvent, thereby generating a carbon nanotube solution comprising a suspension of said carbon nanotubes, and depositing said carbon nanotube solution on to a device substrate, thereby generating said precursor layer; and
contact printing said carbon nanotubes on to a device substrate, thereby generating said precursor layer.
44. The method of claim 42 wherein said step of patterning said precursor layer is carried out using one or more photolithography techniques.
45. The method of claim 42 wherein said step of patterning said precursor layer comprises the steps:
providing a layer of resist on said precursor layer;
patterning said resist layer by selectively removing regions of said resist layer, thereby generating exposed regions of said precursor layer; and
removing carbon nanotubes from said exposed regions of said precursor layer, thereby generating said patterned layer comprising said one or more strips extending between said first and second electrodes.
46. The method claim 45 wherein said step of patterning said resist layer is carried out via photolithography, soft lithography, phase shift lithography, electron beam writing lithography or deep ultraviolet lithography.
47. The method claim 45 wherein said step of removing carbon nanotubes from said exposed regions of said precursor layer comprises etching said exposed regions of said precursor layer.
48. The method of claim 39 wherein said step of providing a patterned layer of randomly oriented or partially aligned carbon nanotubes in electrical contact with said first electrode and said second electrode comprises ink jet printing said carbon nanotubes, thermal transfer printing said carbon nanotubes, contact printing said carbon nanotubes, dry transfer printing or screen printing said carbon nanotubes.
49. A transistor comprising:
a source electrode;
a drain electrode;
a gate electrode;
a patterned layer comprising randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with said source electrode and said drain electrode, wherein said partially aligned carbon nanotubes are aligned relative to each other with deviations from absolute parallelism that are greater than 20 degrees; said one or more interconnected carbon nanotube networks characterized by a plurality of nanotube intersections; said patterned layer having a thickness less than or equal to 10 nanometers and having one or more void regions in said patterned layer without carbon nanotubes; wherein said void regions comprise one or more cavities in said patterned layer extending entirely from said source electrode to said drain electrode, thereby defining a plurality of strips of said interconnected carbon nanotube networks, wherein said one or more void regions provided between said source and drain electrodes reduces by at least 50% the number of purely metallic conductive pathways between said source electrode and said drain electrode in said one or more interconnected carbon nanotube networks relative to a corresponding layer of randomly oriented or partially aligned carbon nanotubes provided in another one or more interconnected carbon nanotube networks and not having said void regions; and
a dielectric layer positioned between said patterned layer and said gate electrode.
50. The electronic device of claim 49 comprising a thin film transistor.
51. The transistor of claim 49 wherein said transistor has an on/off ratio greater than or equal to 100.
52. The thin film transistor of claim 51 wherein said transistor has an on/off ratio greater than or equal to 1000.
53. The transistor of claim 49 wherein said transistor has a field effect mobility greater than or equal to 0.1 cm2 V−1 s−1.
54. The thin film transistor of claim 53 wherein said transistor has a field effect mobility equal to or greater than 10 cm2 V-1 s-1.
55. A method for reducing the number of purely metallic conductive pathways in one or more interconnected carbon nanotube networks provided between a first electrode and second electrode, said method comprising the steps of:
providing a patterned layer comprising randomly oriented or partially aligned carbon nanotubes provided in one or more interconnected carbon nanotube networks positioned between and in electrical contact with said first electrode and said second electrode, wherein said partially aligned carbon nanotubes are aligned relative to each other with deviations from absolute parallelism that are greater than 20 degrees; said one or more interconnected carbon nanotube networks characterized by a plurality of nanotube intersections; said patterned layer having a thickness less than or equal to 10 nanometers and having one or more void regions in said patterned layer without carbon nanotubes; wherein said void regions comprise one or more cavities in said patterned layer extending entirely from said first electrode to said second electrode, thereby defining a plurality of strips of said interconnected carbon nanotube networks, wherein said one or more void regions provided between said first and second electrodes reduces by at least 50% the number of purely metallic conductive pathways between said first electrode and said second electrode in said one or more interconnected carbon nanotube networks relative to a corresponding layer of randomly oriented or partially aligned carbon nanotubes provided in another one or more interconnected carbon nanotube networks and not having said void regions.
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