WO2007005618A2 - High performance organic thin film transistor - Google Patents

High performance organic thin film transistor Download PDF

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WO2007005618A2
WO2007005618A2 PCT/US2006/025598 US2006025598W WO2007005618A2 WO 2007005618 A2 WO2007005618 A2 WO 2007005618A2 US 2006025598 W US2006025598 W US 2006025598W WO 2007005618 A2 WO2007005618 A2 WO 2007005618A2
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layer
thin film
film transistor
metal
organic thin
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PCT/US2006/025598
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French (fr)
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WO2007005618A3 (en
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Yang Yang
Chih-Wei Chu
Sheng-Han Li
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The Regents Of The University Of California
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having potential barriers
    • H10K10/80—Constructional details
    • H10K10/82—Electrodes
    • H10K10/84—Ohmic electrodes, e.g. source or drain electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K10/00—Organic devices specially adapted for rectifying, amplifying, oscillating or switching; Organic capacitors or resistors having potential barriers
    • H10K10/40—Organic transistors
    • H10K10/46—Field-effect transistors, e.g. organic thin-film transistors [OTFT]
    • H10K10/462—Insulated gate field-effect transistors [IGFETs]
    • H10K10/468—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics
    • H10K10/472—Insulated gate field-effect transistors [IGFETs] characterised by the gate dielectrics the gate dielectric comprising only inorganic materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02225—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer
    • H01L21/0226—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a deposition process
    • H01L21/02263—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a deposition process deposition from the gas or vapour phase
    • H01L21/02271—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a deposition process deposition from the gas or vapour phase deposition by decomposition or reaction of gaseous or vapour phase compounds, i.e. chemical vapour deposition
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/60—Organic compounds having low molecular weight
    • H10K85/615—Polycyclic condensed aromatic hydrocarbons, e.g. anthracene

Definitions

  • This application relates to organic thin film transistors, and more particularly to organic thin film transistors that have a transition-metal oxide layer in a source and/or drain electrode thereof.
  • An organic thin film transistor has a gate electrode, a gate insulator formed on the gate electrode, a layer of organic semiconducting material formed on the gate insulator, a source electrode formed on the layer of organic semiconducting material, and a drain electrode formed on the layer of organic semiconducting material.
  • the source electrode and/or the drain electrode have a layer of transition-metal oxide formed on the organic semiconducting material and a layer of metal formed on the layer of transition-metal oxide.
  • a method of producing an organic thin film transistor includes providing a gate electrode, forming a gate insulator on the gate electrode, forming a layer of semiconducting material on the gate insulator, forming a source electrode on the layer of organic semiconducting material, and forming a drain electrode on the layer of organic semiconducting material.
  • Forming the source electrode and/or forming the drain electrode includes forming a layer of transition-metal oxide on the organic semiconducting material and forming a layer of metal on the layer of transition-metal oxide.
  • Figure l(a) is a schematic illustration of an OTFT according to an embodiment of this invention.
  • Figure l(b) is an energy level diagram for the OTFT of Figure l(a) when particular materials are used;
  • Figure 2(a) shows the source-drain current-voltage characteristics of the OTFT 0 with MOO 3 /AI electrodes according to an embodiment of this invention
  • Figure 2(b) shows the transfer characteristics of the OTFT with MOO3/AI electrodes at a constant drain voltage of -40 V according to an embodiment of this invention
  • Figure 3 (a) shows source-drain current-voltage characteristics of the OTFT where the source-drain electrodes are Au according to an embodiment of this 5 invention
  • Figure 3(b) shows source-drain current-voltage characteristics of the OTFT where the source-drain electrodes are Al according to an embodiment of this invention
  • Figure 5 is a table of electrical parameters of the OTFTs produced as examples according to an embodiment of this invention.
  • Figure 6 shows transfer characteristics (-I SD vs V G ) of OTFTS using MoO 3 5 covered with different metals as the electrodes: MoO3/Au (•); MoO3/Ag ( ⁇ );
  • transition-metal oxides offer a unique opportunity to control the work function, and hence, the charge-injection properties. Therefore, modification of the organic/electrode interface by inserting a transition-metal oxide has received considerable attention in the case of organic electroluminescent (EL) devices (J. Kido, T. Matsumoto, T, Nakada, J. Endo, K. Mori, N. Kawamura, A. Yoko, SID 2003, pp. 964 (2003); G. L. Frey, K. J. Reynolds, and R. H. Friend, Adv. Mater. 14, 265 (2002)).
  • EL organic electroluminescent
  • the source-drain (S-D) contacts in the organic-TFTs have significant influence on device operation, through their contribution to the contact resistance arising from mismatch of the work functions, and/or interaction between the metal electrodes and the organic semiconductor (R. Hajlaoui, G. Horowitz, F. Gamier, A. Arce-Brouchet, L. Laigre, A. El Kassmi, F. Demanze, and F. Kouki, Adv. Mater. 9, 389 (1997); C. Waldauf, P. Schilinsky, M. Perisutti, J. Hauch, and C. J. Brabec, Adv. Mater. 15, 2084 (2003)). Inserting a transition-metal oxide, such as MoO 3 , between the S-D contact and organic active layer can greatly reduce the contact resistance for the organic-TFTs.
  • a transition-metal oxide such as MoO 3
  • Aluminum (Al) is a well-known contact material in integrated circuits and exhibits good corrosion resistance. However, its low-work function precludes the application of Al to high performance OTFT for ⁇ -ty ⁇ e semiconductors.
  • high performance OTFTs can be achieved by inserting a transition metal oxide layer between Al S-D electrodes and organic semiconductors.
  • the performance of OTFTs with the MoO 3 /Al S-D electrodes are greatly improved over the ones with Al, or even gold (Au), as the S-D electrodes.
  • Au gold
  • An organic thin film transistor 100 according to an embodiment of this invention is illustrated schematically in Figure l(a).
  • the organic thin film transistor 100 has a gate electrode 102, a gate insulator 104 formed on the gate electrode 102 and a layer of organic semiconducting material 106 formed on the gate insulator 104.
  • a source electrode 108 and a drain electrode 110 are formed on the layer of organic semiconducting material 106.
  • At least one of the source electrode 108 and drain electrode 110 has a layer of transition metal oxide 112 formed between the layer of organic semiconducting material 106 and a metal layer 114 of the source electrode 108, for example.
  • the drain electrode 110 may also have a transition metal oxide layer 116 between the layer of organic semiconducting material 106 and a metal layer 118.
  • the transition metal oxide layer 112 may also be formed of an alkaline metal and/or alkaline earth metal complex.
  • the transition-metal oxide may be an alkaline metal or alkaline earth metal complex from the group consisting of LiF, Ca(acac) 2 , Cs 2 COs, BaO.
  • the transition metal oxide layer 112 and/or 116 may also be made from Mo ⁇ 3 , WO3 or V 2 ⁇ 5 and/or combinations thereof.
  • the metal layer 114 and/or 118 of the source and drain electrodes 108 and 110 may be selected from metals according to the desired application.
  • metal layers 114 and/or 118 may be selected from Cu, Ag, Al or Au, and/or mixtures thereof.
  • one may select a transition metal oxide layer 112 and 116 that consists essentially of MOO 3 and metal layers 114 and 118 that consist essentially of Al.
  • pentacene was found to be suitable for the layer of organic semiconductor material 106.
  • Silicon oxide was found to be suitable for the gate insulator 104.
  • hydrogenated amorphous silicon was found to be suitable for the gate electrode 102 in a specific example.
  • FIG. l(a) and l(b) The schematic structure of a top-contact pentacene TFT and the energy level diagram are illustrated schematically in Figures l(a) and l(b), respectively.
  • the devices were made using heavily doped p-type silicon wafers with a 300 nm thick SiO 2 , which functioned as the gate electrode and the gate insulator, respectively.
  • the SiO 2 surface was cleaned by UV/O 3 cleaner and chemically-modified using silane coupling agent octadecyltrichlorosilane (OTS) (Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, IEEE Trans. Electron
  • OTS silane coupling agent octadecyltrichlorosilane
  • Figure 2 (a) shows the source-drain current (/ DS ) VS. source-drain voltage (F DS ) of the OTFT with Al/MoO 3 as the S-D electrodes at different gate voltages (F 0 ).
  • the device showed typical p-channel characteristics. As can be noted, the output characteristics displayed very good saturation, clear saturation currents which behaved quadratically as a function of the gate bias.
  • the field-effect mobility ( ⁇ ) was calculated at the saturation regions from the following equation:
  • J DS ( WC ⁇ /2L) ⁇ ( V G - V T ) 2
  • Q is the capacitance per unit area of the insulator
  • V- ⁇ is the threshold voltage
  • the field-effect mobility and the threshold voltage of the OTFT were 0.40 cm 2 /V-s and -10.43V, respectively.
  • Figures 3(a) and 3(b) show / DS vs. F D s curves for the OTFTs with pure Au and Al as the S-D electrodes, respectively. Both of these devices showed typical p-channel characteristics as well.
  • MOO3 is a wide-gap semiconductor with band gap of 3-3.1 eV, and an electron affinity of around 2.2 eV (T. Yasuda, T. Goto. K. Fujita. and T. Tsutsui. Appl. Phys. Lett. 85, 2098, (2004); P. A. Cox, Transition Metal Oxides, An Introduction to Their Electronic Structure and Properties (Claredon, Oxford, 1992)), which implies the valance band position is around 5.3 eV.
  • the highest occupied molecular orbital (HOMO) of pentacene lies at 5.0 eV (A. Galtayries, S. Wisniewski, and J. Grimblot, J. Electron Spectrosc. Relat. Phenom. 87, 31 (1997)) and is aligned with the valence band of MOO 3 , resulting in no barrier for injection of holes into the pentacene layer. It has been suggested that the excess constituent can act as doping centers and that this dopant controls the electrical properties of the film (J. G. Simmons, Phys. Rev., 166, 912 (1968) ; G. S. Nadkarni, and J. G. Simmons, J. Appl.
  • MoO 3 layer Since the MoO 3 layer was deposited by evaporation, Mo ⁇ 3 yields a film containing species from MoO to MOO3 as well as free Mo. Additionally, impurities might be introduced into the film, arising from boats used during thermal evaporation, as well as from the impurities present in MOO 3 powder. The width of the depletion region between metal and MOO 3 junction decreases as the doping concentration in the MOO3 film increases; as a result, the probability of tunneling through the barrier increases dramatically. Therefore, an ohmic contact is likely to result between at the metal and MoO 3 interface. To demonstrate it further, devices were fabricated using MOO 3 covered with different metals (Au, Ag, and Al) as the electrodes.
  • Figure 6 shows the transfer characteristics of OTFTs using MoO 3 covered with different metals as the electrodes. They show similar electrical characteristics and improved performance compared to metal only electrodes. Because of a small mismatch between the work function of Au and the HOMO level of pentacene, Au is one of the most promising metal electrodes for pentacene- TFTs. However, metals deposited onto the pentacene surface either penetrate the surface, thereby doping the upper layer of pentacene, or, diffuse into pentacene to form a metallic overlayer, which is a mixture of metal and pentacene instead of pure metal. The interface dipole immediately forms increasing the barrier height between metal and pentacene (N. J. Watkins, L. Yan, and Y.
  • the modified interface by using MOO3 layer provides protection against metal diffusion into the organic layer and unfavorable chemical reaction between organic and metal electrodes.
  • the modification decreases the intensity of interface dipole and enhances charge injection.
  • the OTFTs with Mo ⁇ 3 as a hole injection layer between the metal electrodes and the organic semiconductor layer were fabricated through thermal evaporation according to an embodiment of this invention. Compared with OTFTs without the metal oxide, the current and the field-effect mobility were significantly improved. Therefore, using a transition metal oxide as the hole injection layer is an effective way to improve the characteristics of OTFTs, making the device suitable for commercial applications.

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  • Chemical & Material Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Thin Film Transistor (AREA)

Abstract

An organic thin film transistor has a gate electrode on which a gate insulator is formed, a layer of organic semiconducting material formed on the gate insulator and a source electrode and a drain electrode formed on the layer of organic semiconducting material. The source and/or drain electrode have a layer of transition-metal oxide formed on the organic semiconducting material and a layer of metal formed on the layer of transition-metal oxide. A method of producing an organic thin film transistor includes providing a gate electrode on which a gate insulator is formed, forming a layer of semiconducting material on the gate insulator, and forming a source electrode and a drain electrode on the layer of organic semiconducting material. Forming the source electrode and/or drain electrode include forming a layer of transition metal oxide on the organic semiconducting material and forming a layer of metal on the layer of transition- metal oxide.

Description

HIGH PERFORMANCE ORGANIC THIN FILM TRANSISTOR
CROSS-REFERENCE OF RELATED APPLICATION This application claims priority to U.S. Provisional Application No. 60/695,513 filed June 30, 2005, the entire contents of which are hereby incorporated by reference. The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of ONR Grant No. NOOO 14-04- 1-0434.
BACKGROUND
1. Field of Invention
This application relates to organic thin film transistors, and more particularly to organic thin film transistors that have a transition-metal oxide layer in a source and/or drain electrode thereof.
2. Discussion of Related Art
The contents of all references cited anywhere in this specification are hereby incorporated by reference.
Substantial progress has been made in developing high-performance organic semiconductors in the past decade. (See, for example, CW. Tang and S. Van Slyke, Appl. Phys. Letts., 51, 913 (1987); P. Peumans, A. Yakimov, and S. R. Forrest, J. Appl. Phys. 93, 3693 (2003); and H. E. Katz and Z. Bao J. Phys. Chem. B 104, 671 (2000).) Organic semiconducting materials have been used to fabricate transistors with electronic properties similar to hydrogenated amorphous silicon (a-Si:H), a material often used for flat panel displays. For example, field-effect mobilities greater than 1 cm2/V-s with large on/off current ratio have been reported for pentacene-TFTs (Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, DEEE Trans. Electron Devices 18, 606 (1997)). Such comparable electronic characteristics, together with an advantage of low temperature and low cost fabrication on various conformable substrates, make organic thin film transistors (OTFTs) attractive candidates for use in commercial products. Although, a large number of studies have focused on improving the intrinsic electrical properties of the organic semiconductors and towards the development of device fabrication techniques (Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, IEEE Trans. Electron Devices 18, 606 (1997); M. Shtein, J. Mapei, J. B. Benziger, and S. R. Forrest, Appl. Phys. Lett. 81, 268 (2002)), the contact between the electrodes and the active layer, which is critical to OTFT device performance, has not received much attention. There is thus a need for improved OTFTs having improved contacts between electrodes and active layers.
SUMMARY
Further objectives and advantages will become apparent from a consideration of the description, drawings, and examples.
An organic thin film transistor according to an embodiment of this invention has a gate electrode, a gate insulator formed on the gate electrode, a layer of organic semiconducting material formed on the gate insulator, a source electrode formed on the layer of organic semiconducting material, and a drain electrode formed on the layer of organic semiconducting material. The source electrode and/or the drain electrode have a layer of transition-metal oxide formed on the organic semiconducting material and a layer of metal formed on the layer of transition-metal oxide.
A method of producing an organic thin film transistor according to an embodiment of this invention includes providing a gate electrode, forming a gate insulator on the gate electrode, forming a layer of semiconducting material on the gate insulator, forming a source electrode on the layer of organic semiconducting material, and forming a drain electrode on the layer of organic semiconducting material. Forming the source electrode and/or forming the drain electrode includes forming a layer of transition-metal oxide on the organic semiconducting material and forming a layer of metal on the layer of transition-metal oxide. BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be better understood by reading the following detailed description with reference to the accompanying figures in which:
5 Figure l(a) is a schematic illustration of an OTFT according to an embodiment of this invention;
Figure l(b) is an energy level diagram for the OTFT of Figure l(a) when particular materials are used;
Figure 2(a) shows the source-drain current-voltage characteristics of the OTFT 0 with MOO3/AI electrodes according to an embodiment of this invention;
Figure 2(b) shows the transfer characteristics of the OTFT with MOO3/AI electrodes at a constant drain voltage of -40 V according to an embodiment of this invention;
Figure 3 (a) shows source-drain current-voltage characteristics of the OTFT where the source-drain electrodes are Au according to an embodiment of this 5 invention;
Figure 3(b) shows source-drain current-voltage characteristics of the OTFT where the source-drain electrodes are Al according to an embodiment of this invention;
Figures 4(a) and 4 (b) show transfer characteristics for different materials as O the source-drain contacts MoO3/Al (■); Au (•);• Al ( A):(a) -ISD-VG and (b) (-ISD)1/2- VGplots at VD=-40 V;
Figure 5 is a table of electrical parameters of the OTFTs produced as examples according to an embodiment of this invention; and
Figure 6 shows transfer characteristics (-ISD vs VG) of OTFTS using MoO3 5 covered with different metals as the electrodes: MoO3/Au (•); MoO3/Ag (■);
MoO3/Al (A) according to embodiments of this invention.
DETAILED DESCRIPTION
In describing embodiments of the present invention illustrated in the drawings, O specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. It is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
The range of electronic properties of the transition-metal oxides offers a unique opportunity to control the work function, and hence, the charge-injection properties. Therefore, modification of the organic/electrode interface by inserting a transition-metal oxide has received considerable attention in the case of organic electroluminescent (EL) devices (J. Kido, T. Matsumoto, T, Nakada, J. Endo, K. Mori, N. Kawamura, A. Yoko, SID 2003, pp. 964 (2003); G. L. Frey, K. J. Reynolds, and R. H. Friend, Adv. Mater. 14, 265 (2002)). Similarly, the source-drain (S-D) contacts in the organic-TFTs have significant influence on device operation, through their contribution to the contact resistance arising from mismatch of the work functions, and/or interaction between the metal electrodes and the organic semiconductor (R. Hajlaoui, G. Horowitz, F. Gamier, A. Arce-Brouchet, L. Laigre, A. El Kassmi, F. Demanze, and F. Kouki, Adv. Mater. 9, 389 (1997); C. Waldauf, P. Schilinsky, M. Perisutti, J. Hauch, and C. J. Brabec, Adv. Mater. 15, 2084 (2003)). Inserting a transition-metal oxide, such as MoO3, between the S-D contact and organic active layer can greatly reduce the contact resistance for the organic-TFTs.
Aluminum (Al) is a well-known contact material in integrated circuits and exhibits good corrosion resistance. However, its low-work function precludes the application of Al to high performance OTFT for ρ-tyρe semiconductors. On the other hand, high performance OTFTs can be achieved by inserting a transition metal oxide layer between Al S-D electrodes and organic semiconductors. In the Examples section below, we show that the performance of OTFTs with the MoO3/Al S-D electrodes are greatly improved over the ones with Al, or even gold (Au), as the S-D electrodes. The presence of MOO3 layer at the organic/Al interface significantly reduces the contact barrier and provides protection from diffusion and other chemical reactions between the organic layer and the metal in this example. However, the broad concepts of this invention are not limited to the embodiments described in the specific examples. An organic thin film transistor 100 according to an embodiment of this invention is illustrated schematically in Figure l(a). The organic thin film transistor 100 has a gate electrode 102, a gate insulator 104 formed on the gate electrode 102 and a layer of organic semiconducting material 106 formed on the gate insulator 104. A source electrode 108 and a drain electrode 110 are formed on the layer of organic semiconducting material 106. At least one of the source electrode 108 and drain electrode 110 has a layer of transition metal oxide 112 formed between the layer of organic semiconducting material 106 and a metal layer 114 of the source electrode 108, for example. Alternatively, or in addition to the source electrode having a transition metal oxide layer 112 and metal layer 114, the drain electrode 110 may also have a transition metal oxide layer 116 between the layer of organic semiconducting material 106 and a metal layer 118. The transition metal oxide layer 112 may also be formed of an alkaline metal and/or alkaline earth metal complex. The transition-metal oxide may be an alkaline metal or alkaline earth metal complex from the group consisting of LiF, Ca(acac)2, Cs2COs, BaO.
The transition metal oxide layer 112 and/or 116 may also be made from Moθ3, WO3 or V2θ5 and/or combinations thereof. Furthermore, the metal layer 114 and/or 118 of the source and drain electrodes 108 and 110 may be selected from metals according to the desired application. For example, metal layers 114 and/or 118 may be selected from Cu, Ag, Al or Au, and/or mixtures thereof. For example, in one particular embodiment, one may select a transition metal oxide layer 112 and 116 that consists essentially of MOO3 and metal layers 114 and 118 that consist essentially of Al. In a particular example, pentacene was found to be suitable for the layer of organic semiconductor material 106. Silicon oxide was found to be suitable for the gate insulator 104. In addition, hydrogenated amorphous silicon was found to be suitable for the gate electrode 102 in a specific example.
EXAMPLES
The schematic structure of a top-contact pentacene TFT and the energy level diagram are illustrated schematically in Figures l(a) and l(b), respectively. The devices were made using heavily doped p-type silicon wafers with a 300 nm thick SiO2, which functioned as the gate electrode and the gate insulator, respectively. Prior to pentacene active layer deposition, the SiO2 surface was cleaned by UV/O3 cleaner and chemically-modified using silane coupling agent octadecyltrichlorosilane (OTS) (Y. Lin, D. J. Gundlach, S. F. Nelson, and T. N. Jackson, IEEE Trans. Electron
Devices 18, 606 (1997)). Following the gate insulator treatment, a 40-nm-thick layer of pentacene (Sigma- Aldrich, -98% purity) was thermally evaporated at the rate of 0.5A/sec from a molybdenum boat. Finally, thermal evaporation of MOO3 (Sigma- Aldrich, 99.99% purity) and Al were sequentially deposited onto the pentacene film through a shadow mask to form the S-D electrodes with a channel length of 100 μm and width of 5 mm. The thicknesses of MOO3 and Al films were 20nm and 50nm, respectively. In addition, Au and Al were investigated as alternatives to the MoO3/Al as S-D contacts. All the materials were used without any further purification. All thermal evaporations were done under a pressure of less than 6xlO"6 Torr while monitoring the film thickness with a quartz oscillator. Electrical measurements were performed in a vacuum of 10"5 Torr at the room temperature using HP 4155B semiconductor parameter analyzer.
Figure 2 (a) shows the source-drain current (/DS) VS. source-drain voltage (FDS) of the OTFT with Al/MoO3 as the S-D electrodes at different gate voltages (F0). The device showed typical p-channel characteristics. As can be noted, the output characteristics displayed very good saturation, clear saturation currents which behaved quadratically as a function of the gate bias. The corresponding plots of -/DS and (- -føs)1/2 vs. VQ for the device are shown in Fig. 2(b). Strong field-effect modulation of the channel conductance was observed, with on/off current ratios (/On//Off) as high as 104 (measured between gate voltage, VQ = -40 to 10). The field-effect mobility (μ) was calculated at the saturation regions from the following equation:
JDS = ( WC±/2L) μ ( VG- VT) 2 where Q is the capacitance per unit area of the insulator and V-γ is the threshold voltage (S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981)). The field-effect mobility and the threshold voltage of the OTFT were 0.40 cm2/V-s and -10.43V, respectively. On the other hand, Figures 3(a) and 3(b) show /DS vs. FDs curves for the OTFTs with pure Au and Al as the S-D electrodes, respectively. Both of these devices showed typical p-channel characteristics as well. The transfer characteristics for different materials for the S-D electrodes are shown in Figures 4(a) and 4(b). The device with Au S-D electrode had similar I0JIoH compared to the device with MOO3/AI bilayer electrode, while IoJIoπ for the pure Al electrode was only 102. In addition, the decreased slope of the transfer characteristics corresponded to a twofold decrease in the field-effect mobility to μ^ = 0.18 cm2/V-s for the Au contacts. For the case of Al, the saturation current of the channel further reduced, and field- effect mobility of 2.8x 10"3 cm2/V-s was calculated. These results demonstrate that the introduction of the metal oxide in our OTFTs played an important role in decreasing the contact resistance. Other metal oxides were combined with Al for the S-D electrodes and the electrical characteristics of the OTFTs with different S-D electrodes in these examples according to an embodiment of this invention are summarized in the table of Figure 5.
In order to understand the performance improvement in the devices with MOO3/AI as electrodes, we must consider the electronic properties of the transition metal oxide. The energy level diagrams for pentacene, MOO3, Au and Al are illustrated in Figure l(b). MOO3 is a wide-gap semiconductor with band gap of 3-3.1 eV, and an electron affinity of around 2.2 eV (T. Yasuda, T. Goto. K. Fujita. and T. Tsutsui. Appl. Phys. Lett. 85, 2098, (2004); P. A. Cox, Transition Metal Oxides, An Introduction to Their Electronic Structure and Properties (Claredon, Oxford, 1992)), which implies the valance band position is around 5.3 eV. The highest occupied molecular orbital (HOMO) of pentacene lies at 5.0 eV (A. Galtayries, S. Wisniewski, and J. Grimblot, J. Electron Spectrosc. Relat. Phenom. 87, 31 (1997)) and is aligned with the valence band of MOO3, resulting in no barrier for injection of holes into the pentacene layer. It has been suggested that the excess constituent can act as doping centers and that this dopant controls the electrical properties of the film (J. G. Simmons, Phys. Rev., 166, 912 (1968) ; G. S. Nadkarni, and J. G. Simmons, J. Appl. Phys., 41, 545 (1970)). Since the MoO3 layer was deposited by evaporation, Moθ3 yields a film containing species from MoO to MOO3 as well as free Mo. Additionally, impurities might be introduced into the film, arising from boats used during thermal evaporation, as well as from the impurities present in MOO3 powder. The width of the depletion region between metal and MOO3 junction decreases as the doping concentration in the MOO3 film increases; as a result, the probability of tunneling through the barrier increases dramatically. Therefore, an ohmic contact is likely to result between at the metal and MoO3 interface. To demonstrate it further, devices were fabricated using MOO3 covered with different metals (Au, Ag, and Al) as the electrodes. Figure 6 shows the transfer characteristics of OTFTs using MoO3 covered with different metals as the electrodes. They show similar electrical characteristics and improved performance compared to metal only electrodes. Because of a small mismatch between the work function of Au and the HOMO level of pentacene, Au is one of the most promising metal electrodes for pentacene- TFTs. However, metals deposited onto the pentacene surface either penetrate the surface, thereby doping the upper layer of pentacene, or, diffuse into pentacene to form a metallic overlayer, which is a mixture of metal and pentacene instead of pure metal. The interface dipole immediately forms increasing the barrier height between metal and pentacene (N. J. Watkins, L. Yan, and Y. L Gao, Appl. Phys. Letts., 80, 4384 (2002)). The modified interface by using MOO3 layer provides protection against metal diffusion into the organic layer and unfavorable chemical reaction between organic and metal electrodes. The modification decreases the intensity of interface dipole and enhances charge injection. '
In conclusion, the OTFTs with Moθ3 as a hole injection layer between the metal electrodes and the organic semiconductor layer were fabricated through thermal evaporation according to an embodiment of this invention. Compared with OTFTs without the metal oxide, the current and the field-effect mobility were significantly improved. Therefore, using a transition metal oxide as the hole injection layer is an effective way to improve the characteristics of OTFTs, making the device suitable for commercial applications.
The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art the best way known to the inventors to make and use the invention. Nothing in this specification should be considered as limiting the scope of the present invention. The above-described embodiments of the invention may be modified or varied, and elements added or omitted, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.

Claims

WE CLAIM:
1. An organic thin film transistor, comprising: a gate electrode; a gate insulator formed on said gate electrode; a layer of organic semiconducting material formed on said gate insulator; a source electrode formed on said layer of organic semiconducting material; and a drain electrode formed on said layer of organic semiconducting material, wherein at least one of said source electrode and said drain electrode comprises a layer of transition-metal oxide formed on said organic semiconducting material and a layer of metal formed on said layer of transition- metal oxide.
2. An organic thin film transistor according to claim 1, wherein said drain electrode and said source electrode each comprises a layer of transition-metal oxide formed on said organic semiconducting material and a layer of metal formed on said layer of transition-metal oxide.
3. An organic thin film transistor according to claim 2, wherein said layer of transition-metal oxide comprises a transition-metal oxide selected from the group consisting of MOO3, WO3 and V2Os.
4. An organic thin film transistor according to claim 2, wherein said layer of metal comprises a metal selected from the group consisting of Cu, Ag, Al and Au.
5. An organic thin film transistor according to claim 3, wherein said layer of metal comprises a metal selected from the group consisting of Cu, Ag, Al and Au.
6. An organic thin film transistor according to claim 2, wherein said layer of transition-metal oxide consists essentially of MOO3 and said layer of metal consists essentially of Al.
7. An organic thin film transistor according to claim 1 , wherein said layer of organic semiconductor consists essentially of pentacene.
8. An organic thin film transistor according to claim 6, wherein said layer of organic semiconductor consists essentially of pentacene.
9. An organic thin film transistor according to claim 8, wherein said gate electrode consists essentially of hydrogenated amorphous silicon.
10. An organic thin film transistor according to claim 9, wherein said gate insulator consists essentially Of SiO2.
11. A method of producing an organic thin film transistor, comprising: providing a gate electrode; forming a gate insulator on said gate electrode; forming a layer of semiconducting material on said gate insulator; forming a source electrode on said layer of organic semiconducting material; and forming a drain electrode on said layer of organic semiconducting material; wherein at least one of said forming said source electrode and forming said drain electrode comprises forming a layer of transition-metal oxide on said organic semiconducting material and forming a layer of metal on said layer of transition-metal oxide.
12. A method of producing an organic thin film transistor according to claim
11, wherein said forming said drain electrode and said forming said source electrode each comprises forming a layer of transition-metal oxide on said organic semiconducting material and forming a layer of metal on said layer of transition-metal oxide.
13. A method of producing an organic thin film transistor according to claim
12, wherein said layer of transition-metal oxide comprises a transition-metal oxide selected from the group consisting of Moθ3, WO3 and V2O5.
14. A method of producing an organic thin film transistor according to claim 12, wherein said layer of metal comprises a metal selected from the group consisting of Cu, Ag, Al and Au.
15. A method of producing an organic thin film transistor according to claim
B3, wherein said layer of metal comprises a metal selected from the group consisting of Cu, Ag, Al and Au.
16. A method of producing an organic thin film transistor according to claim 12, wherein said layer of transition-metal oxide consists essentially of MOO3 and said layer of metal consists essentially of Al.
17. A method of producing an organic thin film transistor according to claim 11, wherein said layer of organic semiconductor consists essentially of pentacene.
18. A method of producing an organic thin film transistor according to claim 16, wherein said layer of organic semiconductor consists essentially of pentacene.
19. A method of producing an organic thin film transistor according to claim 18, wherein said gate electrode consists essentially of hydrogenated amorphous silicon.
20. A method of producing an organic thin film transistor according to claim 19, wherein said gate insulator consists essentially of SiO2.
21. A method of producing an organic thin film transistor, comprising: providing a gate electrode; forming a gate insulator on said gate electrode; forming a layer of semiconducting material on said gate insulator; forming a source electrode on said layer of organic semiconducting material; and forming a drain electrode on said layer of organic semiconducting material; wherein at least one of said forming said source electrode and forming said drain electrode comprises forming a layer of alkaline metal or alkaline earth metal complex on said organic semiconducting material and forming a layer of metal on said layer of transition-metal oxide.
22. An organic thin film transistor according to claim 21 , wherein said drain electrode and said source electrode each comprises a layer of alkaline metal or alkaline earth metal complex formed on said organic semiconducting material and a layer of metal formed on said layer of transition-metal oxide.
23. An organic thin film transistor according to claim 22, wherein said layer of transition-metal oxide comprises an alkaline metal or alkaline earth metal complex from the group consisting of LiF, Ca(acac)2, Cs2CO3, BaO.
24. An organic thin film transistor according to claim 22, wherein said layer of metal comprises a metal selected from the group consisting of Cu, Ag, Al and Au.
25. An organic thin film transistor according to claim 23, wherein said layer of metal comprises a metal selected from the group consisting of Cu, Ag, Al and Au.
26. An organic thin film transistor according to claim 22, wherein said layer of alkaline metal consists essentially Of Cs2CO3 and said layer of metal consists essentially of Al.
27. An organic thin film transistor according to claim 21 , wherein said layer of organic semiconductor consists essentially of pentacene.
28. An organic thin film transistor according to claim 26, wherein said layer of organic semiconductor consists essentially of pentacene.
29. An organic thin film transistor according to claim 28, wherein said gate electrode consists essentially of hydrogenated amorphous silicon.
30. An organic thin film transistor according to claim 29, wherein said gate insulator consists essentially of SiO2.
PCT/US2006/025598 2005-06-30 2006-06-30 High performance organic thin film transistor WO2007005618A2 (en)

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