Method for forming an electrostatically-doped carbon nanotube device

[0001] The present invention relates generally to the field of nanotechnology. More specifically, the present invention relates to a method and associated structure for forming an electrostatically-doped carbon nanotube device. The electrostatically-doped carbon nanotube device of the present invention is suitable for use as a light-emitting diode (“LED”), as well as in other applications.

[0002] Carbon nanotubes have attracted a great deal of attention in recent years due to their possibilities for use as nanoscale electronic devices, such as diodes, transistors and semiconductor circuits. Structurally, a carbon nanotube resembles a hexagonal lattice of carbon rolled into a cylinder and may belong to one of two varieties, a single-walled variety and a multi-walled variety. Either of these varieties may, in whole or in part, exhibit the behavior of a metal or a semiconductor material, depending upon their chirality (i.e., conformational geometry).

[0003] Carbon nanotubes that exhibit the behavior of a semiconductor material are typically doped using various chemical methods. In other words, different chemicals are used to create p-type (hole majority carrier) regions and n-type (electron majority carrier) regions in the carbon nanotube. This results in a P-N junction that, when an appropriate voltage is applied, emits light (in the case of a light-emitting diode (“LED”)). The chemical methods for doping a carbon nanotube, however, suffer from the problem that the p-type regions and the n-type regions are typically not well characterized, resulting in nanoscale electronic devices with reduced performance characteristics.

[0004] Thus, what is needed are a method and associated structure for forming an electrostatically-doped carbon nanotube device having well characterized p-type regions and n-type regions, allowing for the creation of nanoscale electronic devices, such as LEDs and the like, with enhanced performance characteristics.

[0005] The present invention provides a method and associated structure for forming an electrostatically-doped carbon nanotube device having well characterized p-type regions and n-type regions, allowing for the creation of nanoscale electronic devices, such as light-emitting diodes (“LEDs”) and the like, with enhanced performance characteristics. More specifically, the present invention provides for the use of a plurality of doping electrodes that are decoupled from a plurality of bias electrodes. Thus, the doping of a carbon nanotube may be finely tuned by varying the bias of each of the plurality of bias electrodes. Advantageously, the method and associated structure of the present invention are capable of providing a carbon nanotube having a P-N junction, a P-I-P junction, a P-I-N junction, an N-I-P junction, an N-I-N junction, a P-N-P junction or an N-P-N junction.

[0006] In one embodiment of the present invention, a method for forming an electrostatically-doped carbon nanotube device includes providing a carbon nanotube having a first end and a second end. The method also includes disposing a first metal contact directly adjacent to the first end of the carbon nanotube, wherein the first metal contact is electrically coupled to the first end of the carbon nanotube, and disposing a second metal contact directly adjacent to the second end of the carbon nanotube, wherein the second metal contact is electrically coupled to the second end of the carbon nanotube. The method further includes disposing a first metal electrode adjacent to and at a distance from the first end of the carbon nanotube, wherein the first metal electrode is capacitively coupled to the first end of the carbon nanotube, and disposing a second metal electrode adjacent to and at a distance from the second end of the carbon nanotube, wherein the second metal electrode is capacitively coupled to the second end of the carbon nanotube. The method still further includes selectively applying a first bias to the first metal electrode to electrostatically dope the first end of the carbon nanotube and selectively applying a second bias to the second metal electrode to electrostatically dope the second end of the carbon nanotube.

[0007] In another embodiment of the present invention, a structure for forming an electrostatically-doped carbon nanotube device includes a carbon nanotube having a first end and a second end. The structure also includes a first metal contact disposed directly adjacent to the first end of the carbon nanotube, wherein the first metal contact is electrically coupled to the first end of the carbon nanotube, and a second metal contact disposed directly adjacent to the second end of the carbon nanotube, wherein the second metal contact is electrically coupled to the second end of the carbon nanotube. The structure further includes a first metal electrode disposed adjacent to and at a distance from the first end of the carbon nanotube, wherein the first metal electrode is capacitively coupled to the first end of the carbon nanotube, and a second metal electrode disposed adjacent to and at a distance from the second end of the carbon nanotube, wherein the second metal electrode is capacitively coupled to the second end of the carbon nanotube. The first metal electrode is operable for receiving a first bias to electrostatically dope the first end of the carbon nanotube and the second metal electrode is operable for receiving a second bias to electrostatically dope the second end of the carbon nanotube.

[0008] In a further embodiment of the present invention, a method for forming an electrostatically-doped carbon nanotube device includes providing a semiconductor layer having a surface and disposing a first insulating layer having a surface on the surface of the semiconductor layer. The method also includes patterning and selectively disposing a metal electrode material having a surface on the surface of the first insulating layer and disposing a second insulating layer having a surface on the surface of the first insulating layer and the surface of the metal electrode material. The method further includes patterning and selectively disposing a metal contact material having a surface on the surface of the second insulating layer and patterning and selectively disposing a catalyst material on the surface of the metal contact material. The method still further includes growing a carbon nanotube from the catalyst material, wherein the carbon nanotube is aligned substantially parallel to the surface of the second insulating layer, and wherein a portion of the carbon nanotube is in contact with a portion of the metal contact material.

[0017] The present invention provides a method and associated structure for forming an electrostatically-doped carbon nanotube device having well characterized p-type regions and n-type regions, allowing for the creation of nanoscale electronic devices, such as light-emitting diodes (“LEDs”) and the like, with enhanced performance characteristics. More specifically, the present invention provides for the use of a plurality of doping electrodes that are decoupled from a plurality of bias electrodes. Thus, the doping of a carbon nanotube may be finely tuned by varying the bias of each of the plurality of bias electrodes. Advantageously, the method and associated structure of the present invention are capable of providing a carbon nanotube having a P-N junction, a P-I-P junction, a P-I-N junction, an N-I-P junction, an N-I-N junction, a P-N-P junction or an N-P-N junction.

[0018] Referring to FIG. 1, in one embodiment of the present invention, a structure for forming an electrostatically-doped carbon nanotube device 10 includes a carbon nanotube 12 having a first end 14 and a second end 16. The carbon nanotube 12 may be either a single-walled carbon nanotube (“SWCNT”) or a multi-walled carbon nanotube (“MWCNT”). The carbon nanotube 12 has a length of between about 0.1 microns and about 10 microns and a diameter of between about 0.4 nm and about 20 nm, however other suitable dimensions may be used. In general, a carbon nanotube may act as a metal or a semiconductor material, depending upon its chirality (i.e., conformational geometry). Preferably, the carbon nanotube 12 of the present invention acts as a semiconductor material. The first end 14 of the carbon nanotube 12 is disposed adjacent to and in direct electrical contact with a first metal contact 18. Likewise, the second end 16 of the carbon nanotube 12 is disposed adjacent to and in direct electrical contact with a second metal contact 20. The first metal contact 18 and the second metal contact 20 are each made of Ti, Mo, Au, Cr or the like, and each has an area or size of between about 0.1 microns by about 10 microns and about 1 micron by about 10 microns. In general, any dimensions that provide adequate electrical contact with the first end 14 of the carbon nanotube 12 and the second end 16 of the carbon nanotube 12 may be used. The first metal contact 18 and the second metal contact 20 may be disposed either above or below the first end 14 of the carbon nanotube 12 and the second end 16 of the carbon nanotube 12, respectively.

[0019] The first metal contact 18 and the second metal contact 20 are disposed on the surface of a dielectric material 22. The dielectric material 22 includes SiO₂, Si₃N₄, Al₂O₃, ZrO₂ or the like. A first metal electrode 24 and a second metal electrode 26 are disposed within the dielectric material 22, adjacent to and at a distance from the first metal contact 18 and the second metal contact 20, respectively. Because of this separation, the first metal electrode 24 is capacitively coupled to the first end 14 of the carbon nanotube 12 and the second metal electrode 26 is capacitively coupled to the second end 16 of the carbon nanotube 12. Preferably, the distance between the first metal electrode 24 and the first end 14 of the carbon nanotube 12 and the second metal electrode 26 and the second end 16 of the carbon nanotube 12 is between about 2 nm and about 100 nm, respectively. The first metal electrode 24 and the second metal electrode 26 are each made of Mo, Ti, Pt, Au, Cr or the like, and each has an area or size of between about 0.1 microns by about 10 microns and about 1 micron by about 10 microns. Advantageously, the area or size of the first metal electrode 24 and the second metal electrode 26 may be selected to achieve a desired spacing between the first metal electrode 24 and the second metal electrode 26. The significance of this spacing is described in detail below. Preferably, the first metal electrode 24 is separated from the second metal electrode by a distance of between about 100 nm and about 1 micron.

[0020] The dielectric material 22 is disposed on the surface of a semiconductor material 28, such as Si, SiC or the like. Alternatively, the dielectric material 22 is disposed on the surface of a metal layer 28, such as Al, Cr, Mo, Ti, Pt or the like. As described above, the carbon nanotube 12 has a first end 14 and a second end 16. Accordingly, a center section 30 is disposed between the first end 14 of the carbon nanotube 12 and the second end 16 of the carbon nanotube 12. In one embodiment of the present invention, a portion of the semiconductor material 28 is disposed adjacent to and at a distance from the center section 30 of the carbon nanotube 12, with the dielectric material 22, a portion of the first metal electrode 24 and a portion of the second metal electrode 26 disposed between the semiconductor material 28 and the center section 30 of the carbon nanotube 12. In an alternative embodiment of the present invention, a portion of the semiconductor material 28 is disposed adjacent to and at a distance from the center section 30 of the carbon nanotube 12, with only the dielectric material 22 disposed between the semiconductor material 28 and the center section 30 of the carbon nanotube 12. Again, this difference relates to the spacing between the first metal electrode 24 and the second metal electrode 26 and its significance is described in detail below.

[0021] Referring to FIG. 2, the structure for forming an electrostatically-doped carbon nanotube device 10 (FIG. 1) is represented by a circuit diagram. The first metal contact (“M1”) 18 is electrically coupled to the first end 14 of the carbon nanotube 12 and the second metal contact (“M2”) 20 is electrically coupled to the second end 16 of the carbon nanotube 12. Similarly, the first metal electrode (“VC1”) 24 is capacitively coupled to the first end 14 of the carbon nanotube 12 and the second metal electrode (“VC2”) 26 is capacitively coupled to the second end 16 of the carbon nanotube 12. In this respect, VC124 and VC226 form a first gate and a second gate, respectively. In the alternative embodiment of the present invention described above, with only the dielectric material 22 (FIG. 1) disposed between the semiconductor material 28 and the center section 30 of the carbon nanotube 12, the semiconductor material (“SI”) 28 is capacitively coupled to the center section 30 of the carbon nanotube 12 and forms a third gate, which otherwise does not exist.

[0022] In operation, a first bias is applied to VC124, resulting in the electrostatic doping of the first end 14 of the carbon nanotube 12. Likewise, a second bias is applied to VC226, resulting in the electrostatic doping of the second end 16 of the carbon nanotube 12. Depending upon the bias applied, the first end 14 of the carbon nanotube 12 and the second end 16 of the carbon nanotube 12 may each be made a p-type semiconductor (hole majority carrier) or an n-type semiconductor (electron majority carrier). If the first end 14 of the carbon nanotube 12 is made a p-type semiconductor and the second end 16 of the carbon nanotube 12 is made an n-type semiconductor, or vice versa, the result is a P-N junction. A P-N junction may be used to form a light-emitting diode (“LED”), as is well known to those of ordinary skill in the art. The preferred voltage range of the structure for forming an electrostatically-doped carbon nanotube device 10 is between about 1 V and about 30 V.

[0023] In the alternative embodiment of the present invention described above, with only the dielectric material 22 disposed between SI 28 and the center section 30 of the carbon nanotube 12, SI 28 is used to modulate the doping of the center section 30 of the carbon nanotube 12. Thus, the center section 30 of the carbon nanotube 12 may be made a p-type semiconductor, an I-type (intrinsic) semiconductor or an n-type semiconductor. This results in a number of possible configurations, summarized in Table I below, and a number of possible devices, well known to those of ordinary skill in the art.

[0024]

TABLE 1
Electrostatically-Doped Carbon Nanotube Junctions and Devices
	Bias Modes
	VC1	SI	VC2	Junction	Device(s)
	Low	—	Low	P-I-P	Back-to-Back Junctions
	Low	—	High	P-I-N	Light-Emitting Diode (“LED”)
	High	—	Low	N-I-P	Light-Emitting Diode (“LED”)
	High	—	High	N-I-N	Back-to-Back Junctions
	Low	High	Low	P-N-P	Bipolar Junctions
	High	Low	High	N-P-N	Bipolar Junctions

[0025] Referring to FIGS. 3 and 4, in another embodiment of the present invention, a method for forming an electrostatically-doped carbon nanotube device includes first providing the semiconductor layer 28 described above. Again, the semiconductor layer 28 includes Si, SiC or the like. Alternatively, a metal layer 28 may be provided, such as Al, Cr, Mo, Ti, Pt or the like. Preferably, the semiconductor layer 28 has a thickness of between about 1 micron and about 550 microns. A first insulating layer 40 is deposited or grown on the surface of the semiconductor layer 28 using a thermal oxide, a chemical vapor deposition dielectric, a plasma-enhanced chemical vapor deposition dielectric, a low-pressure chemical vapor deposition dielectric or the like. The first insulating layer 40 includes SiO₂, Si₃N₄, Al₂O₃, ZrO₂ or the like. Preferably, the first insulating layer 40 has a thickness of between about 2 nm and about 100 mm. Following the deposition or growth of the first insulating layer 40, a metal electrode material is patterned and deposited on the surface of the first insulating layer 40 to form the first metal electrode 24 and the second metal electrode 26 described above. The metal electrode material includes Mo, Ti, Pt, Au, Cr or the like. Preferably, the first metal electrode 24 and the second metal electrode 26 each have a thickness of between about 10 nm and about 100 nm.

[0026] Referring to FIG. 5, a second insulating layer 42 is then deposited or grown on the surface of the first insulating layer 40, substantially surrounding the first metal electrode 24 and the second metal electrode 26, using a chemical vapor deposition dielectric, a plasma-enhanced chemical vapor deposition dielectric, a low-pressure chemical vapor deposition dielectric or the like. The second insulating layer 42 includes SiO₂, Si₃N₄, Al₂O₃, ZrO₂ or the like. Preferably, the second insulating layer 42 has a thickness of between about 2 nm and about 100 nm. Collectively, the first insulating layer 40 and the second insulating layer 42 form the dielectric layer 22 described above. Following the deposition or growth of the second insulating layer 42, a metal contact material is patterned and deposited on the surface of the second insulating layer 42 to form the first metal contact 18 and the second metal contact 20 described above. The metal contact material includes Ti, Mo, Au, Cr or the like. Preferably, the first metal contact 18 and the second metal contact 20 each have a thickness of between about 10 nm and about 100 nm.

[0027] Referring to FIG. 6, a catalyst material 44 suitable for growing a carbon nanotube is then patterned and deposited on the surfaces of the first metal contact 18 and the second metal contact 20 using, for example, a lift-off technique, well known to those of ordinary skill in the art. The catalyst material 44 may take the form of a thin film or a nanoparticle and includes Ni, Fe, Co, Mo, Al₂O₃ in Fe nitrate or the like. Preferably, the catalyst material 44 has a thickness of between about 0.1 nm and about 1 nm. Prior to depositing the catalyst material 44 on the surfaces of the first metal contact 18 and the second metal contact 20, the surfaces of the first metal contact 18 and the second metal contact 20, as well as the dielectric layer 22, may be selectively coated with photo-resist. This photo-resist forms the appropriate pattern for the deposition of the catalyst material 44 and is subsequently removed. It should be noted that the catalyst material may be selectively deposited on the surface of only one of the first metal contact 18 and the second metal contact 20. Following the deposition of the catalyst material 44, the carbon nanotube 12 described above is grown, as illustrated in FIG. 7. Preferably, the carbon nanotube 12 is aligned substantially parallel to the surface of the dielectric layer 22. In general, the carbon nanotube 12 is grown in a chemical vapor deposition (CVD) tube coupled to a flowing carbon (hydrocarbon) source, such as a methane source or an acetylene source, at between about 700 degrees C. and about 1000 degrees C. The catalyst material 44 forms a plurality of “islands” at these temperatures and becomes supersaturated with carbon. Eventually, the carbon nanotube 12 grows from these catalyst islands. This process is well known to those of ordinary skill in the art.

[0028] Although the present invention has been illustrated and described with reference to preferred embodiments and examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve similar results. All such equivalent embodiments and examples are within the spirit and scope of the present invention and are intended to be covered by the following claims.