Thin-film field-effect transistor with organic semiconductor requiring low operating voltages

[0002] This is a continuation of application Ser. No. 08/827,018, filed Mar. 25, 1997, U.S. Pat. No. 5,981,970.

[0003] U.S. Application Serial No. 08/827,015 entitled, “FABRICATION OF THIN FILM FIELD EFFECT TRANSISTOR COMPRISING AN ORGANIC SEMICONDUCTOR AND CHEMICAL SOLUTION DEPOSITED METAL OXIDE GATE DIELECTRIC” to C.D. Dimitrakopoulos et al. filed on the same day herewith U.S. Pat. No. 5,946,551, is incorporated herein by reference.

[0004] This invention pertains to the field of organic thin film field effect transistors (TFT), in particular to flat panel liquid crystal displays using such transistors.

[0005] Thin film field effect transistors (TFT) used in liquid crystal display (LCD) applications typically use amorphous silicon (a-Si:H) as the semiconductor and silicon oxide and/or silicon nitride as the gate insulator. Recent developments in materials have led to the exploration of organic oligomers such as hexathiophene and derivatives, and organic molecules such as pentacene (G. Horowitz, D. Fichou, X. Peng, Z. Xu, F. Garnier, Solid State Commun. Volume 72, pg. 381, 1989; F. Garnier, G. Horowitz, D. Fichou, U.S. Pat. No. 5,347,144) as potential replacements for amorphous silicon as the semiconductor in thin-film field-effect transistors.

[0006] The highest field effect mobility in thiophene-oligomer-based TFT's is usually about 0.06 cm²V⁻¹sec⁻¹(F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science, Volume 265, pg. 1684, 1994), which is substantially lower than the mobility of standard a-Si:H TFT's. Only in the case that the organic insulator cyanoethylpullulane was used, was a higher field effect mobility measured (0.4 cm²V⁻¹sec⁻¹, F. Garnier, G. Horowitz, D. Fichou, U.S. Pat. No. 5,347,144). However, that insulator exhibits some undesirable characteristics such as inferior dielectric strength, mobile charges, (G. Horowitz, F. Deloffre, F. Garnier, R. Hajlaoui, M. Hmyene, A. Yassar, Synthetic Metals, Volume 54, pg 435, 1993) and sensitivity to humidity. Hence it is not suitable for use as a gate insulator in the fabrication of practical TFT devices. Field effect mobility up to 0.6 cm²V⁻¹sec⁻¹has recently been achieved in pentacene based TFT's with SiO₂as the gate insulator (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54^thAnnual Device Research Conference Digest, 1996 pg. 80), making them potential candidates for such applications. Major drawbacks of these pentacene-based organic TFT's are high threshold voltage, high operating voltages required to achieve high mobility and simultaneously produce high current modulation (typically about 100 V when 0.4 μm thick SiO₂insulator is used), and high sub-threshold slope, S, which is approximately 14 V per decade of current modulation (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54^thAnnual Device Research Conference Digest, 1996, pg. 80) as compared to about 0.3 V per decade of current modulation achieved in a-Si:H based TFT's (C.-Y. Chen, J. Kanicki, 54^thAnnual Device Research Conference Digest, 1996, pg. 68). Reducing the thickness of the gate insulator would improve the above mentioned characteristics but there is a limit to the decrease of the insulator thickness, which is imposed by ease of manufacturing and reliability issues. For example in the current generation of TFT LCD devices the thickness of the TFT gate insulator is typically 0.4 μm.

[0007] The electrical characteristics of TFT's having pentacene as the semiconductor, a heavily doped Si-wafer as the gate electrode, thermally grown SiO₂on the surface of the Si-wafer as the gate insulator, and Au source and drain electrodes, are adequately modeled by standard field effect transistor equations (S. M. Sze “Physics of Semiconductor Devices”, Wiley, N.Y., 1981, pg. 442), as shown previously (G. Horowitz, D. Fichou, X. Peng, Z. Xu, F. Garnier, Solid State Commun. Volume 72, pg. 381, 1989; C. D. Dimitrakopoulos, A. R. Brown, A. Pomp, J. Appl. Phys. Volume 80, pg. 2501, 1996). The pentacene used in these devices behaves as a p-type semiconductor. FIG. 1, cited from Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54^thAnnual Device Research Conference Digest, 1996, pg. 80, shows the dependence of the current flowing between the source and drain electrodes (I_D) on the voltage applied to the drain electrode (V_D), at discrete voltages applied to the gate electrode (V_G). When the gate electrode is biased negatively with respect to the grounded source electrode, pentacene-based TFT's operate in the accumulation mode and the accumulated carriers are holes. At low V_D, I_Dincreases linearly with V_D(linear region) and is approximately given by the equation: $\begin{array}{cc}{I}_D=\frac{W{C}_i}{L}\mu \left(V_G-V_T-\frac{V_D}{2}\right)V_D& \left(1\right)\end{array}$

[0008] where L is the channel length, W is the channel width, C_iis the capacitance per unit area of the insulating layer, V_Tis a threshold voltage, and μ is the field effect mobility. μ can be calculated in the linear region from the transconductance: $\begin{array}{cc}{g}_m={\left(\frac{\partial I_D}{\partial V_G}\right)}_{V_D=\mathrm{const}}=\frac{W{C}_i}{L}\mu V_D,& \left(2\right)\end{array}$

[0009] by plotting I_Dvs. V_Gat a constant low V_Dand equating the value of the slope of this plot to g_m.

[0010] When the drain electrode is more negatively biased than the gate electrode (i.e. −V_D≧−V_G), with the source electrode being grounded (i.e. V_s=0), the current flowing between source and drain electrodes (I_D) tends to saturate (does not increase any further) due to the pinch-off in the accumulation layer (saturation region), and is modeled by the equation: $\begin{array}{cc}{I}_D=\frac{W{C}_i}{2L}\mu {\left(V_G-V_T\right)}^2.& \left(3\right)\end{array}$

[0011] FIG. 2 a shows the dependence of I_Don V_Gin saturation (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54^thAnnual Device Research Conference Digest, 1996, pg. 80). The field effect mobility can be calculated from the slope of the {square root over (|I_D+L |)} vs. V_Gplot. FIG. 2bshows a plot of the square root of I_Dvs V_G. A mobility of 0.62 cm²V⁻¹sec⁻¹can be calculated from this plot. The sub-threshold slope, S, is approximately 14 volts per decade of current modulation (Y. Y. Lin, D. J. Gundlach, T. N. Jackson, 54^thAnnual Device Research Conference Digest, 1996, pg. 80).

[0012] It is an object of this invention is to demonstrate TFT structures that overcome the need to use high operating voltages in order to achieve the desirable combination of high field effect mobility, low threshold voltage, low sub-threshold slope, and high current modulation, without having to reduce the thickness of the insulator. Such structures contain an inorganic high dielectric constant gate insulator layer (for example, barium strontium titanate) in combination with an organic semiconductor (for example, pentacene).

[0013] It is another object of this invention to produce organic TFT structures wherein the high dielectric constant gate insulator is deposited and processed at temperatures compatible with glass and plastic substrates (150 to 400° C.), which are substantially lower than the processing temperatures of these materials when they are used for memory applications (up to 650° C.).

[0014] The proposed TFT structures make use of a high dielectric constant thin film gate insulator, an organic semiconductor such as pentacene, and a metal, conducting polymer, highly doped high conductivity material or a combination thereof as the gate, source, and drain electrodes.

[0015] There are many candidate materials with high dielectric constant that can be used as gate insulator layers in the above structures, including but not restricted to Ta₂O₅, Y₂O₃, TiO₂, and the family of ferroelectric insulators, including but not restricted to PbZr_xTi_1−xO₃(PZT), Bi₄Ti₃O₁₂, BaMgF₄, barium zirconate titanate (BZT) and Ba_xSr_1−xTiO₃(BST). These materials have been studied and used in the past in combination with inorganic semiconductors mainly for memory device applications (P. Balk, Advanced Materials, Volume 7, pg. 703, 1995 and references therein) but never in combination with organic semiconductors. Typically these insulators are annealed at 600° C. or higher to achieve dielectric constant (ε) values exceeding 150.

[0016] In general the proposed structure uses an inorganic, high dielectric constant gate insulator in combination with an organic semiconductor (e.g. pentacene) in a TFT structure. The high ε insulator is annealed at 400° C. to achieve an ε≧15, which makes possible the use of glass or plastic substrates. If a high dielectric constant organic insulator was available (ε≧15) which also fulfilled other requirements, such as environmental stability, high breakdown voltage, good film-forming capability, absence of mobile charges, it could also be used instead of the previously mentioned inorganic insulators for the proposed structures.

[0017] A typical sequence used in the fabrication of the proposed TFT structures includes the following steps:

[0018] preparation of the gate electrode, which can be either the substrate itself, in such case being heavily doped Si, or a patterned metal (or conducting polymer or other conductive material) gate electrode deposited and patterned on a substrate;

[0019] deposition of the high dielectric constant gate insulator on top of the gate electrode by one of various processes including but not restricted to sol gel spin coating and baking, sputtering, chemical vapor deposition (CVD), laser ablative deposition, and physical vapor deposition;

[0020] optionally annealing the films at a suitable temperature in the range of 150 to 400° C. to improve the film quality and enhance the dielectric constant;

[0021] deposition of the organic semiconductor on top of the gate insulator by one of various processes including but not restricted to vapor deposition, spin-coating from solution or self assembly of layers from solution;

[0022] fabrication of the electrically conducting source and drain electrodes on top of the organic semiconductor;

[0023] and optionally applying a passivation coating of an insulator by chemical vapor deposition (CVD), physical vapor deposition or spin coating and curing.

[0024] The sequence of the steps comprising the deposition of the organic semiconductor and the fabrication of the source and drain electrodes could be reversed to allow process compatibility and ease of fabrication.

[0025] FIG. 1 depicts the operating characteristics of a pentacene-based TFT device with SiO₂gate insulator. Dependence of drain current as a function of source-drain voltage is shown for different discrete values of the gate voltage. Cited from Lin et al., 1996.

[0026] FIG. 2ashows the same data as in FIG. 1 replotted to show the dependence of the drain current as a function of the gate voltage in the saturation regime.

[0027] FIG. 2bis a plot of the square root of the drain current from FIG. 2aas a function of the gate voltage in the saturation regime to calculate the field effect mobility.

[0028] FIG. 3 is the measured operating characteristics of a TFT device using as-received (97+% pure) pentacene as semiconductor and an 120 nm thick SiO₂as gate insulator.

[0029] FIGS. 4ais a replot of the data from FIG. 3 showing the dependence of the drain current on gate voltage in the saturation regime.

[0030] FIGS. 4bdepicts the data from FIG. 4ain a semilogarithmic scale used to calculate the current modulation and sub-threshold slope.

[0031] FIG. 4C is a plot of the square root of the drain current from FIG. 4aas a function of the gate voltage in the saturation regime to calculate the field effect mobility.

[0032] FIG. 5 is a plot of the field effect mobility calculated for the devices characterized by FIG. 3 at different gate voltages but at the same source-drain voltage (−100V), showing that there is a strong dependence of the mobility on gate voltage.

[0033] FIG. 6 is a schematic sketch of the pentacene-based TFT devices with a high dielectric constant gate insulator proposed in this application.

[0034] FIG. 7ais the measured operating characteristics of a TFT device using as-received (97+% pure) pentacene as semiconductor and a 90 nm thick layer of barium strontium titanate film (deposited by sol gel processing from an isopropoxide based precursor solution) as the gate insulator. Dependence of the drain current on the gate voltage at a fixed source-drain voltage is shown.

[0035] FIG. 7bis the plot of the data from FIG. 7ain a semilogarithmic scale used to calculate the current modulation and sub-threshold slope.

[0036] FIG. 7cis a plot of the square root of the drain current from FIG. 7aas a function of the gate voltage in the saturation regime to calculate the field effect mobility.

[0037] FIG. 8 is the measured operating characteristics of a TFT device using as-received (97+% pure) pentacene as semiconductor and a barium strontium titanate film (deposited by sol gel processing from an alkoxide based precursor solution) as the gate insulator. Dependence of the drain current on the source-drain voltage is shown at different gate voltage levels.

[0038] FIG. 9ashows the drain current as a function of gate voltage at fixed source-drain voltage, from a TFT comprising a pentacene semiconductor layer and a BST gate insulator layer, disposed by spin coating a solution different than the one used in the sample of FIG. 7 and 8.

[0039] FIG. 9bis the plot of the data from FIG. 9ain a sernilogarithmic scale used to calculate the current modulation and sub-threshold slope.

[0040] FIG. 9cis a plot of the square root of the drain current from FIG. 9bas a function of the gate voltage in the saturation regime to calculate the field effect mobility.

[0041] FIG. 3, which corresponds to a device having as-received pentacene (97+% purity, purchased from FLUKA Chemical Co.) as semiconducting layer, a heavily doped Si wafer as gate electrode, 120 nm thick thermally grown SiO₂as gate insulator, and Au source and drain electrodes, shows the dependence of the current flowing between source and drain electrodes (I_D) on the voltage applied to the drain electrode (V_D), at discrete voltages applied to the gate electrode (V_G). The linear region of the plots (i.e. at low V_D) can be modeled by Equation 1 as discussed previously.

[0042] FIGS. 4aand 4bcorrespond to the same device as above and show the dependence of I_Don V_Gin the saturation region. The field effect mobility, μ, was calculated from the slope of the {square root over (|I_D+L |)} vs. V_Gplot (FIG. 4c) to be 0.52 cm²V⁻¹sec⁻¹. The sub-threshold slope, S, is 13.7 volts per decade of current modulation. These data are comparable to the ones for similar TFT structures described earlier as part of the prior art review. As explained earlier, although the mobility is acceptable for practical TFT applications, the operating voltage and the sub-threshold slope are far too high compared to a-Si:H TFT's. The field effect mobility measured from TFT devices such as the one corresponding to FIG. 3 and FIG. 4, shows a gate voltage dependence. In particular, high mobilities are obtained at higher gate voltages. This in turn results in the need for using impractically high operating voltages in these devices to obtain acceptable mobilities. To solve this problem we have examined the dependence of the field effect mobility, μ, measured from TFT devices such as the one corresponding to FIG. 3 and FIG. 4, on the applied gate voltage, V_Gin greater detail. FIG. 5 shows the dependence of μ on the maximum V_Gused at different gate-voltage-sweep experiments while V_Dwas kept constant at −100 V and V_swas always set to 0 volts.

[0043] The observed behavior is attributable to a dependence of mobility on the gate field, E, where E=dV/dy and y is the thickness of the insulator, in which case the high gate fields can be achieved by using a thinner gate dielectric layer. This is typically the methodology used in the prior art for inorganic semiconductor TFT devices but suffers from the limitation that thin dielectrics are prone to pinhole defects and exhibit lower dielectric breakdown voltages and higher leakage currents. Hence this approach is not a fruitful one to harness the gate voltage dependent mobility of pentacene based TFT's.

[0044] Alternatively it is possible, although not expected, that the gate voltage dependence is a manifestation of the changes in the accumulated carrier concentration at the semiconductor/insulator interface. It is probable, that in organic semiconductors the extra accumulation of charge carriers facilitates the filling of trap states thereby allowing the additional carriers to move more easily without being hampered by the trapping process. We propose to facilitate charge accumulation by replacing SiO₂with an insulator that has a similar thickness but a much higher dielectric constant. In this case, a similar accumulated carrier concentration is attained as in the SiO₂case, but at much lower gate fields and thus gate voltages, with all the other parameters kept the same.

[0045] If this hypothesis is correct, a high mobility should be achieved in these devices at lower voltages compared to TFT's using comparable thickness of SiO₂. In the opposite case, in other words if the mobility was field dependent but independent of the carrier concentration, a relatively low mobility should be observed at the low gate voltages used in the case of the latter sample. As will be shown in the following, comparing the measured field effect mobilities from devices based on the two different insulators with comparable thickness but different dielectric constants, supports our hypothesis. The following example details the fabrication of a pentacene based TFT using a high dielectric constant inorganic film as the gate insulator and the resulting high field effect mobilities at low operating voltages.

[0046] We have fabricated TFT's having as gate insulator a thin film of barium strontium titanate (BST), which was deposited using sol gel techniques. The organic semiconductor used in these devices was pentacene deposited by vacuum sublimation. The gate electrode was aluminum or a Pt/Ti bilayer while the source and drain electrodes consisted of Au. The substrates used were either quartz disks or Si wafers covered with a thermally grown SiO₂layer.

[0047] Oxidized silicon or quartz substrates were cleaned in an isopropanol bath using ultrasonic agitation and dried with nitrogen. They were then assembled with a metal mask with openings corresponding to the gate lines and placed and pumped down to high vacuum in an electron beam evaporator. Gate metallization of either 40 nm of aluminum or a bilayer of 15 nm titanium followed by 30 nm of Pt was deposited on the substrates by electron beam evaporation. The samples were unloaded from the assembly and coated with a layer of high dielectric constant insulator by the sol gel process.

[0048] In this process metal oxide films are fabricated using a precursor comprising either a short chain metal alkoxide exemplified by but not restricted to metal isopropoxide, or a second type of organometallic precursor solution. The solution is applied by liquid deposition (for example spin coating) onto a substrate. The coated substrate is baked to dry the precursor, and then annealed. Specifically, the spin solution was loaded into a syringe with 0.45 μm and 0.2 μm Whatman syringe filters attached on it. The solution was syringed onto the substrate until the substrate was completely wetted. The substrate was then spun for 45 sec at 2500 rpm. The coated substrate was dried on a hot plate at 200-400° C. and then annealed at temperatures up to 400° C. for 10-20 min. The process can be repeated to fabricate thicker films by successive coating and annealing steps.

[0049] The films were baked intentionally at only moderate temperatures (up to 400° C.) in order to be compatible with glass and plastic substrates. This results in an amorphous insulator film which has a dielectric constant of about 16 as compared to values close to 300 or more achieved when the films are heat treated at 650° C. leading to crystallization. As will be seen shortly, the modest increase in dielectric constant obtained by using these amorphous films is adequate for the purpose of the organic TFT application. However, the scope of the present invention is not limited to such deposition process only. BST films and films of most of the previously mentioned high dielectric constant gate insulators can be alternately deposited using sputter deposition techniques, laser ablation, or CVD deposition and these methods could be used without deviating from the spirit of our invention.

[0050] The organic semiconductor layer (pentacene) was deposited using vapor deposition in an ultra high vacuum (UHV) chamber. We have also used deposition in lower cost high vacuum chambers and comparable results are also obtained in this case. Alternatively, this film could be deposited using a soluble precursor of pentacene which is converted to pentacene by heating up to 140° C. in vacuo (A. R. Brown, A. Pomp, D. M. de Leeuw, D. B. M. Klaassen, E. E. Havinga, P. Herwig, K. Mullen Journal of Applied Physics, Volume 79, pg. 2136, 1996). The samples are then assembled with a mask provided with openings for source and drain contact electrodes, placed and pumped down in an electron beam evaporator and coated with 60 nm of gold to produce the source/drain contacts. The resulting TFT structure is shown schematically in FIG. 6. Other source drain contact materials such as chromium, titanium, copper, aluminum, molybdenum, tungsten, nickel, gold, platinum, palladium, conducting polymers, oligomers and organic molecules, can be used without deviating from the spirit of the invention.

[0051] Completed TFT samples were then tested electrically using a Hewlett Packard Model 4145B semiconductor parameter analyzer to determine their operating characteristics.

[0052] FIGS. 7aand 7bare typical examples of a pentacene-based TFT, represented by the schematic drawing in FIG. 6, in which the thickness of the BST gate-insulator was approximately 90 nm and its dielectric constant, ε≈16. The insulator was deposited from an isopropoxide based solution in isopropanol as described earlier. The source drain separation (channel length, L) was 83 μm and the channel width, W, was 1500 μm. Both figures show the dependence of I_Don V_Gin saturation. FIG. 7cshows a plot of the square root of I_Dvs V_G. The field effect mobility, μ, is calculated from the slope of the {square root over (|I_D+L |)} vs. V_Gplot to be 0.38 cm²V⁻¹sec⁻¹. The current modulation is more than 3×10⁵for a gate voltage variation of 4 volts. The sub-threshold slope, S, is approximately 0.4 volts per decade of current modulation.

[0053] FIG. 8 shows the dependence of the current flowing between source and drain electrodes (I_D) on the voltage applied to the drain electrode (V_D), at discrete voltages applied to the gate electrode (V_G) for the device described in the previous paragraph.

[0054] FIGS. 9aand 9brefer to a pentacene-based TFT, represented by the drawing in FIG. 6, in which capacitance per unit area of the BST gate-insulator layer was similar to the BST film in the previous device. The BST layer was deposited as described before from the second type of sol gel solution of Ba, Sr, and Ti organometallic precursors. Both figures show the dependence of I_Don V_Gin saturation. FIG. 9cshows a plot of the square root of I_Dvs V_G. The field effect mobility, μ, is calculated from the slope of the {square root over (|I_D+L |)} vs. V_Gplot to be 0.62 cm²V⁻¹sec⁻¹. The subthreshold slope, S, is approximately 0.4 volts per decade of current modulation. The channel length was 109 μm and the channel width, W, was 250 μm.

[0055] Thus, it is clear that high mobilities and low sub-threshold slopes can be achieved in pentacene based TFT devices when one uses high dielectric constant films as gate insulators. This substantiates our hypothesis that the gate voltage dependence in these devices is a result of higher concentration of the charge carriers achieved with these insulators, since the applied gate fields were kept very low.

[0056] Notwithstanding the specific mechanism of how the characteristics are achieved, we claim that we have demonstrated a structure and a process to fabricate the same to achieve high field effect mobilities, high current modulation and low sub-threshold slope in pentacene based organic TFT's. While the present invention has been described with respect to preferred embodiments, numerous modifications, changes, and improvements will occur to those skilled in the art without departing from the spirit and scope of the invention.