Patent Application
20090134386
The present invention relates to an organic field-effect transistor and to a method of fabricating an organic field-effect transistor.
Flat-panel displays, electronic paper, radio frequency identification tags, environmental sensors and some other forms of electronic device employ thin film transistors as switching elements.
Currently, thin film transistors are fabricated from amorphous or polycrystalline silicon using a variety of well-established semiconductor processing techniques, such as photolithography, chemical vapour deposition and laser annealing. However, these processes have a number of disadvantages. For example, the processes tend to be expensive, consume large amounts of power and/or suffer from low production yields. Furthermore, at least some of these processes require the use of substrates which are capable of being subjected to elevated temperatures (e.g. >300° C.).
Thin film transistors fabricated from organic semiconductors and gate insulating materials offer advantages in that they use cheaper processing techniques, such as coating and printing, which are carried out at lower temperatures (e.g. <200° C.). Thus, transistors can be formed on flexible substrates and yield devices which are lighter, flexible and more resilient to mechanical shock.
Attention is being directed to finding organic semiconductors with higher values of mobility (μ), which helps to increase switching speed. However, the choice of material used for the gate insulating layer also influences performance. Therefore, efforts are being made to find materials having desirable gate dielectric properties, such as high resistivity (e.g. >1×1015 Ωcm at 50 V), high breakdown voltage (e.g. >5 MVcm−1) and low hysteresis (e.g. exhibiting a shift in the turn-on voltage, Vth, greater than 1 V when cycled from |Vth| to |Vth|+20 V for 1000 s), as well as having favourable processing characteristics, such as being solution processable and curable at a low temperature (e.g. <200° C.).
Existing gate dielectric materials tend to fall short in respect of at least one of these requirements.
Polyimides are commonly used as dielectrics in organic devices. However, most polyimides require high curing temperatures, above 200° C., and so are unsuitable for use with many plastic substrates. Although pre-converted soluble polyimides may be used to achieve a lower curing temperature, these usually have a low solubility and so cannot be used to fabricate films thicker than about 1 μm which are usually necessary for organic transistors to operate at high voltages (e.g. >50 V).
Epoxy resins have been used as insulators in a wide variety of applications such as in printed circuit boards and as sealing agent in integrated circuit packages. A photo-cured epoxy resin has been used as a gate insulating layer in a Si-FET as described in “A printable form of silicon for high performance thin film transistors on plastic substrates”, E. Menard, K. J. Lee, D.-Y. Khang, R. G. Nuzzo, and J. A. Rogers, Applied Physics Letters, vol. 84, Num. 26, pages 5398 to 5400.
Divinyl-siloxane-bis-benzocyclobutene (BCB) is also used as a dielectric in organic devices and can be applied by spin-coating. However, BCB needs to be cured at high temperatures in an inert atmosphere.
Polymethylmethacrylates (PMMAs) have also been used as dielectrics in organic devices. However, PMMAs tend to have low glass transition temperatures and also tend to dissolve easily in other organic solvents used during device processing.
A phenol-based or phenol-derivative-based epoxy resin can also be used as a dielectric. An epoxy resin can be applied as a solution and cured at low temperatures. However, FETs formed using this type of epoxy resin suffer from poor electrical properties, such as significant hysteresis.
Recently, a poly(vinylphenol) has been used as a gate dielectric because it has a high dielectric constant. However, the material suffers from low solubility and exhibits significant hysteresis. Moreover, when used with pentacene, the device exhibits low mobility. For example, all-polymer logic circuits have been made using poly(vinylphenol), as described in “Printable Organic and Polymeric Semiconducting Materials and Devices”, Zhenan Bao, John A. Rogers and Howard E. Katz, Journal of Materials Chemistry, vol. 9, pages 1895 to 1904 (1999). However, these devices exhibit current-voltage characteristics which change significantly with time.
The present invention seeks to provide an organic field-effect transistor.
According to a first aspect of certain embodiments of the present invention there is provided an organic field-effect transistor having a gate insulating layer comprising a cured epoxy resin and which exhibits, in a current-gate voltage transfer characteristic, a hysteretic shift in turn-on voltage of less than 1 V in magnitude.
The organic field-effect transistor may exhibit a hysteretic shift in turn-on voltage of less than 0.5 V in magnitude or less than 0.2 V in magnitude.
The cured epoxy resin may comprise silyl groups. The cured epoxy resin may be substantially free of hydroxyl groups. The cured epoxy resin may comprise halo-methyl groups or fluoro-alkyl groups, such as trifluoromethyl groups. For the purposes of the present invention, the term “halo-methyl” encompasses methyl groups substituted with one, two or three halogens independently selected from fluorine, chlorine, bromine and iodine. The term “fluoro-alkyl” encompasses alkyl groups (preferably C1-6 alkyl groups) substituted with one, two, three or more fluorines.
The organic field-effect transistor may comprise an organic semiconductor layer which comprises pentacene.
According to a second aspect of certain embodiments of the present invention there is provided an organic field-effect transistor having a gate insulating layer comprising a cured epoxy resin which is substantially free of hydroxyl groups at the interface between the gate insulating layer and an organic semiconductor layer.
According to a third aspect of certain embodiments of the present invention there is provided an organic field-effect transistor having a gate insulating layer comprising a cured epoxy resin comprising silyl groups and/or trifluoromethyl groups.
The transistor may have a bottom gate, top electrode configuration or a top gate, bottom electrode configuration.
According to a fourth aspect of certain embodiments of the present invention there is provided a method of fabricating an organic field-effect transistor, the method comprising depositing an epoxy resin on a substrate, wherein the organic field-effect transistor exhibits, in a current-gate voltage transfer characteristic, a hysteretic shift in turn-on voltage of less than 1 V in magnitude.
Depositing the epoxy resin on a substrate may comprise spin-coating the epoxy resin on the substrate.
The method may comprise curing an epoxy component using a silyl-substituted hardener. The silyl-substituted hardener may be an alcohol or a phenol derivative substituted with a silyl group. The silyl group may comprise R3Si, R2Si or RSi, where R is a phenyl, alkyl or alkyl siloxane group.
The method may comprise curing an epoxy component using a fluoro-alkyl-substituted hardener. The fluoro-alkyl-substituted hardener may comprise trifluoromethyl groups as the pendant groups.
The method may comprise curing a fluoro-alkyl-substituted epoxy component with a hardener. The fluoro-alkyl-substituted epoxy component may comprise trifluoromethyl groups. The trifluoromethyl-substituted epoxy component may comprise 4,4′-(hexafluoroisopropylidene)diphenol epoxy.
According to a fifth aspect of certain embodiments of the present invention there is provided a method of fabricating an organic field-effect transistor, the method comprising depositing an epoxy resin on a substrate, wherein the epoxy resin comprises an epoxy component and a silyl-substituted hardener.
According to a sixth aspect of certain embodiments of the present invention there is provided a method of fabricating an organic field-effect transistor, the method comprising depositing an epoxy resin on a substrate, wherein the epoxy resin comprises an epoxy component and a trifluoromethyl-substituted hardener.
According to a seventh aspect of certain embodiments of the present invention there is provided method of fabricating an organic field-effect transistor, the method comprising depositing an epoxy resin on a substrate, wherein the epoxy resin comprises a trifluoromethyl-substituted epoxy component and a hardener.
Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:
a is a schematic diagram of an interface between a gate insulating layer and an organic semiconductor layer of the organic field-effect transistor shown in
a to 4d illustrate fabrication of the field-effect transistor shown in
a to 7f are plots of source-drain current against gate bias for some embodiments of the present invention and comparative examples.
Structure
Referring to
Referring to
In some embodiments of the present invention, the epoxy resin is formed by curing an epoxy component using a silyl-substituted hardener or a trifluoromethyl-substituted hardener. In the former case, an epoxy group and a silyl group form a silyl ether group instead of a hydroxyl group. In the later case, trifluoromethyl groups aggregate in the surface region 9 due to differences in affinity between these groups and the rest of the polymer and, thus, force hydroxyl groups away from the interface 10, out of the surface region 9.
The epoxy component can be any reagent having epoxy groups, for example having a structure:
Preferred epoxy components are bisphenol A-type epoxy, bisphenol AD-type epoxy, bisphenol F-type epoxy and noborac-type epoxy.
The silyl-substituted hardener may be an alcohol or a phenol derivative substituted with a silyl group, such as R3Si, R2Si, RSi, where R is a phenyl, alkyl or alkyl siloxane group, for example having a structure:
wherein 1≦m≦500 and each repeating unit X is independently:
A preferred silyl-substituted hardener is the silicone based phenolic resin having the structure (wherein 1≦m≦500):
By curing an epoxy component with this silicone based phenolic hardener, the following hardened epoxy resin may be formed:
The trifluoromethyl-substituted hardener may be any component having trifluoromethyl group(s) as the pendant group(s), for example having a structure:
A preferred trifluoromethyl-substituted hardener is 4,4′-(hexafluoroisopropylidene) diphenol having the structure:
By curing an epoxy component with 4,4′-(hexafluoroisopropylidene)diphenol hardener, the following hardened epoxy resin may be formed:
A catalyst can be used for the curing, such as an imidazole, amine or Lewis acid.
In other embodiments of the present invention, the epoxy resin is formed by curing a trifluoromethyl-substituted epoxy component with a hardener. In this case, the hardener can be any agent capable of reacting with and forming linkages with epoxy groups of the epoxy component, typically having two phenolic hydroxyl groups in a molecule, such as a bisphenol A-type reagent, a bisphenol AD-type reagent, a bisphenol F-type reagent or a phenol derivative. However, the hardener may be a silyl-substituted hardener or a trifluoromethyl-substituted hardener. The trifluoromethyl-substituted epoxy may be any epoxy having trifluoromethyl group(s) as the pendant group(s), such as 4,4′-(hexafluoroisopropylidene)diphenol epoxy. A catalyst can be used for the curing, such as an imidazole, amine or Lewis acid.
Fabrication
Embodiments of FETs in accordance with certain embodiments of the present invention and FETs used as comparative examples were fabricated using commercially-available materials, as will now be described:
Bisphenol-A noborac type epoxy (N865) and bisphenol-A noborac (VH-4170) were obtained from Dainippon Ink and Chemicals, Inc.
4,4′-(Hexafluoroisopropylidene)diphenol, 2-ethyl-4-methyl-imidazole (2E4MZ), anhydrous anisole and anhydrous 1-methyl-2-pyrrolidinone were obtained from Sigma-Aldrich Chemistry Co.
Silyl-substituted hardener, oligomer(diphenylsiloxane-co-bis-(4-hydroxyphenyl)methane), was synthesized in Hitachi Chemical Ltd. from bisphenol F and diphenyl dimethoxy silane.
Pentacene was obtained from Sigma-Aldrich Chemistry Co. and was used without further purification.
Referring to
The thermally-oxidized silicon substrates 11 comprise a surface layer 2 of SiO2 having a thickness of 100 nm on highly-doped p-type silicon 3 having a conductivity of 0.002-0.004 Ωcm. Chromium-coated silicon wafers (not shown) were also obtained from KN plats Ltd. The chromium wafers comprised a silicon wafer and evaporated surface layer of chromium having a thickness of 20 nm.
Referring to
Bisphenol-A noborac type epoxy N865, silyl-substituted hardener, anisole, and 1-methyl-2-pyrrolidinone were weighed, put in a sample tube and dissolved by shaking in a clean room. 2-Ethyl-4-methyl-imidazole (2E4MZ) solution in anisole, 0.1 phr, is added to the solution. Weights are in Table I, below:
The solution was filtered through a membrane filter with 0.02 m pores, Anotop 10 (Whatman Ltd.), just before spin coating. The solution was spin-coated on a thermally-oxidized silicon wafer in a N2 glove box by two-step spinning, in particular 500 rpm for 5 s and 2000 rpm for 60 s to form a coated substrate. The coated substrate was dried at 100° C. for 10 min and cured at 200° C. for 60 min on a hot plate in a N2 glove box. The resulting structure is shown in
Pentacene and the source and drain electrodes were evaporated using a thermal deposition system (Kurt J. Lesker Company Ltd.) in a N2 glove box (not shown). Thickness and deposition rate were monitored by a quartz crystal oscillator.
Pentacene was deposited through a metal mask (not shown) at a rate of 0.4 Å/s to a thickness of 60 nm. The resulting structure is shown in
The source and drain electrodes were formed by evaporating gold through a different metal mask (not shown) at a rate of 0.4 Å/s to a thickness of 20 nm. The resulting structure is shown in
Referring again to
A second embodiment of an FET in accordance with the present invention was made using the same method used to fabricate the first embodiment, except that a different hardener was used. In particular, 4,4′-(hexafluoroisopropylidene)diphenol was used as hardener instead of the silyl-substituted hardener.
A first comparative example of an FET was made using the same method used to fabricate the first embodiment of an FET according to the invention, except that a different, conventional hardener was used. In particular, VH-4170 was used as hardener in place of the silyl-substituted hardener.
A second comparative example of an FET was made using the same method used to fabricate the first embodiment of an FET according to the invention, except that a first type of conventional organic dielectric was used.
Divinyl-siloxane-bis-benzocyclobutene (Dow Chemical Company) (30 mass %) was diluted to 1.9 mass % with mesitylene and filtered through a membrane filter to provide a diluted solution. The diluted solution was spin-coated on a thermally-oxidized silicon wafer with a rotation speed 2000 rpm for 40 s in a N2 glove box to provide a coated wafer. The coated wafer was cured at 290° C. for 1 min on a hot plate in the N2 glove box.
After curing, a layer of pentacene was deposited and the source and drain electrodes were defined as described earlier.
A third comparative example of an FET was made using the same method used to fabricate the first embodiment of an FET according to the invention, except that a second type of conventional dielectric was used.
Poly(4-vinylphenol) 0.02 g and 1,6-bis (trichlorosilyl)hexane 0.02 g were mixed with tetrahydrofuran 9.6 g. The solution was filtered through a membrane filter with 0.02 m pores, Anotop 10 (Whatman Ltd.), just before spin coating. The solution was spin-coated on a thermally-oxidized silicon wafer in a N2 glove box by two-step spinning, in particular at 500 rpm for 5 s and 2000 rpm for 60 s to provide a coated substrate. The coated substrate was cured at 200° C. for 60 min on a hot plate in the N2 glove box.
After curing, a layer of pentacene was deposited and the source and drain electrodes were defined as described earlier.
A fourth comparative example of an FET was made using the same method used to fabricate the first embodiment of an FET according to the invention, except that a third type of conventional organic dielectric was used.
A thermally-oxidized silicon wafer was treated with 1,1,1,3,3,3-hexamethyldisilazane vapour on a glass holder in a glass bottle attached with a water condenser and a hot plate set at 170-180° C.
After curing, a layer of pentacene was deposited and the source and drain electrodes were defined as described earlier.
Measurements
The embodiments of FETs according to certain embodiments of the present invention and the FETs used as comparative examples were characterised using the following measurements.
Polymer Thickness
Values for the thickness of the hardened epoxy resins were obtained using a surface profile measuring device (Dektak) (not shown) by measuring a scratch made by a sharp knife.
Surface Morphology
Surface morphologies of the hardened epoxy resin and the deposited pentacene were observed using atomic force microscope (AFM (not shown).
Surface Energy
Values of surface energy for the polymer surface were calculated by Van Oss model from the contact angles measured by optical contact angle and surface tension meter (CAM200, KSV Instruments Ltd.) (not shown) with three probe liquid, water, formamide and glycerol.
Breakdown Voltage and Leak Current
Values of break down voltage and resistivity were obtained from I-V curves measured using a Model 237 High-Voltage Source-Measure Unit (Keithley Instruments Ltd.) (not shown) and test samples comprising a layer of dielectric sandwiched between electrodes.
The test samples were fabricated on chromium-coated substrate (not shown) which provided a back electrode. Different dielectrics were deposited on different substrates (not shown) using the same processes described earlier and gold discs (not shown) having a diameter of 1 mm were evaporated through a shadow mask so as to provide top electrodes.
A needle probe station (not shown) was used to make contact with the electrodes and measurements of break down voltage, resistivity and dielectric constant were made, in air, at room temperature.
Threshold Voltage and Mobility
Referring to
Mobility and threshold voltage were calculated using equation 1 below using values of slope and intercept taken from plots (not shown) of the square root of current, ID, against gate voltages, VG:
where μ is mobility, Vth is threshold voltage, ID is source-drain current, VG is gate voltage and C is capacitance calculated using C=εε0(A/d), where A is the area of the pentacene layer 5 (
Hysteresis Property (Vth Shift)
Hysteretic properties were measured using values of Vth before and after a gate bias, VG=−1(|Vth|+20) V is applied. The gate bias is a function of the threshold voltage, Vth, measured for each transistor.
Referring to
Results of the measurements are shown in Table II below:
It is noted that in the first and second embodiments of the invention and the first, second and third comparative examples, the dielectric is spin-coated on a thermally-oxidized silicon wafer. In the fourth comparative example, the dielectric is deposited by vapour deposition on a thermally-oxidized silicon wafer.
Values of breakdown voltage and resistivity are shown in Table III below:
Referring to
It will be appreciated that many modifications may be made to the embodiments hereinbefore described.
Other hydroxyl protecting groups, for example an ester group such as —COCH3, can be used to mask hydroxyl groups. Other suitable hydroxyl protecting groups are described, for example, in “Protective Groups in Organic Synthesis” by T. W. Greene and P. G. M. Wuts (Wiley-Interscience, 3rd edition, 1999).
Other halo-methyl groups, such as trichloromethyl groups, or other fluoro-alkyl groups, such as trifluoroethyl or pentafluoroethyl groups, may be used instead of the trifluoromethyl groups in the trifluoromethyl-substituted hardener or the trifluoromethyl-substituted epoxy component.
Other device configurations may be employed. For example, instead of a top contact configuration, a bottom-contact configuration may be used in which the organic semiconductor layer is deposited over source and drain contacts. Examples of device configurations can be found in U.S. Pat. No. 6,433,359, which is incorporated herein by reference.
Other substrates may be used, such as glass or polycarbonate.
Other inorganic dielectrics may be used, such as Si3N4. The inorganic dielectric layer may be omitted.
Other organic semiconductors may be used. For example, other semiconducting oligomers may be used, such as anthracene or rubrene. A semiconducting polymer may be used such as poly(3-hexylthiophene), poly p-phenylene vinylene) or poly(9,9′-dioctylfluorene-co-benzothiadiazole) (“F8BT”). The organic semiconductor may be p-type or n-type.
Other source and drain contact materials may be used, such as conducting polymers.
Other layer thicknesses and device geometries may be used.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07121569.3 | Nov 2007 | EP | regional |
