Patent Application
20100059755
This invention relates to oxide-based field-effect transistors. It is particularly applicable, but by no means limited, to thin-film transistors.
Semiconducting thin-film transistors (TFTs) comprise a substrate, a semiconducting layer, a dielectric layer, and conducting materials for the source, drain and gate electrodes. Depending on the gate potential (VG) and the drain potential (VD), the channel current (i.e. the current flowing from the source electrode to the drain electrode, often referred to as ID) can be modulated. Such TFTs are used in applications such as pixel engine and integrated drivers in active matrix flexible displays, integrated microelectronic circuits, and many other applications that will be familiar to those skilled in the art.
Depending on the application, an inorganic or organic material can be employed as the semiconductor layer. For example, organic materials have the potential for flexible large-area devices and low manufacturing costs, since they can be deposited using solution processing techniques. Despite this advantage, however, the performance of most organic semiconductors is only moderate and is not yet suitable for practical applications. On the other hand, traditional high-performance inorganic semiconductors such as silicon and germanium require highly controlled growth techniques that give rise to relatively high overall manufacturing costs, while their potential applications are limited due to their poor mechanical flexibility and the demanding processing required during fabrication.
There is a desire for a combination of the attractive processing properties of organic semiconductors with the high performance characteristics of inorganic semiconductors.
According to a first aspect of the present invention there is provided a field-effect transistor as defined in claim 1 of the appended claims. Thus there is provided a field-effect transistor comprising: a source region; a drain region; a semiconductor layer disposed between the source and drain regions; a gate region; and a dielectric region disposed between the semiconductor layer and the gate region; wherein the semiconductor layer comprises a titanium dioxide (TiO2) film.
The term “between”, in the context of “the semiconductor layer disposed between the source and drain regions”, should be interpreted broadly, to encompass both spatially between (i.e. physically located between the source and the drain regions) and also functionally between (i.e. arranged to form a semiconducting channel from the source to the drain via the semiconductor layer).
By forming the semiconductor layer as a TiO2 film, this provides the advantage that the semiconductor layer may be fabricated using a solution processing technique, such as spray pyrolysis, or by spin coating, doctor-blading or screen-printing. This circumvents the problem of expensive manufacturing typically required for the deposition of inorganic semiconductors such as silicon and germanium, etc. The TiO2 semiconductor layer may also be deposited on a flexible substrate, to form a flexible device. We have also unexpectedly found from our experiments that transistors having a TiO2 semiconductor layer exhibit light sensitivity, without the need for an organic layer to act as an optical sensitizer. Furthermore, TiO2 is biocompatible, hence making it a good candidate for use in electronic or optoelectronic circuits for biomedical applications.
With all the aspects of the invention, preferable, optional, features are defined in the dependent claims.
Thus, the semiconductor layer may be substantially dense, and/or may be mesoporous. As those skilled in the art will appreciate, a mesoporous material is a material containing pores with diameters between about 2 nm and 50 nm.
An organic dye/semiconductor layer may be disposed adjacent to the titanium dioxide semiconductor layer. This may be beneficial (although not necessary) if the transistor is to be used in light-sensing applications. The organic semiconductor could be polymers, oligomers, small molecules, co-polymers, dendrimers etc. Alternatively, a mesoporous titanium dioxide semiconductor layer may be coated with a layer of dye molecules to act as an optical sensitizer, and infiltrated with a molecular hole transporting organic semiconductor.
Alternatively, the semiconductor layer may further comprise molecular adsorbates, such as dye pigments or protein molecules, to enable the transistor to be used for sensing gas or biological molecules. Receptors may be provided on the surface of the semiconductor layer to enhance reaction between the semiconductor layer and gas or biological molecules.
According to a further aspect of the invention there is provided a method of forming a field-effect transistor comprising: forming a dielectric layer adjacent a gate; forming a source region and a drain region; and forming a semiconductor layer on the dielectric layer, the semiconductor layer comprising titanium dioxide.
The semiconductor layer may be fabricated using a solution processing technique, such as by spray pyrolysis using a precursor solution, or by spin coating, doctor-blading or screen-printing using a sol-gel colloidal suspension. These techniques enable the transistor to be fabricated relatively inexpensively, and also enable the semiconductor layer to be deposited on a flexible substrate if it is desired to form a flexible device. Spray pyrolysis in particular may be used to form a substantially dense titanium dioxide semiconductor layer with enhanced transport characteristics, i.e. high charge mobility.
Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:
The present embodiments represent the best ways known to the applicants of putting the invention into practice. However, they are not the only ways in which this can be achieved.
The present embodiments relate to the development of field-effect transistors based on films of titanium dioxide (TiO2) fabricated using a solution processing technique, such as by spray pyrolysis. Our approach circumvents the problem of expensive manufacturing typically required for the deposition of inorganic semiconductors such as silicon, germanium, etc. Additionally, due to the attractive properties of TiO2, a number of potential applications can be envisioned. Example applications include switching pixel engine and integrated drivers in active matrix flexible displays, bio-sensing FETs, and light-sensing FETs. To the best of our knowledge no field-effect transistor based on a TiO2 film as the semiconductor has yet been reported in the open literature.
The present embodiments provide a TiO2-based TFT device and some potential technological applications. As illustrated in
Preliminary experiments have been performed on devices employing gold source and drain electrodes 14, 16, highly doped silicon (Si++) as the gate electrode 20, and silicon dioxide (SiO2) as the gate dielectric 18.
The TiO2 layer was deposited by spray pyrolysis onto the pre-fabricated transistor structure at 450° C. in oxygen, followed by a further annealing step at 500° C. for 30 minutes in ambient atmosphere. Alternative deposition and annealing temperatures in the range of 140-700° C. are also possible, preferably in the range of 250-600° C., and more preferably in the range of 400-500° C. We employ the spray pyrolysis technique as it provides dense films with enhanced transport characteristics, i.e. high charge mobility. Freshly prepared FETs were then transported into a glove-box for electrical characterization.
Fabrication of TiO2-based TFT Devices
The device of
Step (1): The substrate 22 may be rigid or flexible depending on the application. In the present embodiment highly conductive Si++ is employed as the substrate 22, which also acts as the gate electrode 20, but this may be replaced by other materials.
Step (2): If the substrate 22 is not conducting then a conductive gate 20 has to be deposited. This can be a conductive polymer, metal, or any type of solid conductive substance (e.g. silicon, metal oxides, transparent metal oxides, etc). In the present embodiment the gate 20 is made using conductive doped silicon. However, flexible gates and substrates could also be used, made of metal foil or plastic, which would enable fabrication of flexible devices or arrays.
Step (3): The dielectric 18 is then deposited on the top of the gate 20. This is the standard process for a bottom-contact bottom-gate FET. In the present embodiment the dielectric is standard thermally-grown SiO2. However this layer can be any inorganic material having good insulating properties, or similarly-performing organic materials (small molecules, oligomers and polymers).
Step (4): The source electrode 14 and the drain electrode 16 (“S” and “D”) are then deposited on top of the dielectric 18. In the present embodiment the source and drain electrodes 14, 16 are each made of chromium and gold layers, the chromium and gold layers having thicknesses of 10 nm and 100 nm respectively, which are vacuum deposited and patterned using standard photolithographic techniques. Here the chromium acts as an adhesion layer for the gold, since the latter will not stick to the SiO2 by itself. The role of the thin chromium layer is therefore is not functional in terms of the electronic functionality of the device. However, other contact metals may alternatively be employed, as those skilled in the art will appreciate.
Step (5): Finally, the TiO2 semiconductor layer 12 is formed on the top of the prefabricated structure. The TiO2 semiconductor layer may be deposited using spray pyrolysis, which is a technique commonly used for the deposition of TiO2, or may be formed as a mesoporous film of nanocrystalline morphology by spin coating, doctor-blading or screen-printing techniques. These alternative techniques for forming the TiO2 semiconductor layer will now be described:
Spray pyrolysis is a widely-used technique for TiO2 thin film deposition. The advantage of this method is that it is relatively easy to scale up for large area production. There are several spraying methods to generate aerosols with droplet size in the micrometre to sub-micrometre range. In our work, experiments were performed with pneumatic pressure nozzles.
Spray pyrolysis is based on evaporation of a precursor in the vicinity of a substrate heated by a hotplate. With TiO2 deposition, a temperature of 450° C. was used, although alternative deposition temperatures in the range of 140-700° C. are also possible, preferably in the range of 250-600° C., and more preferably in the range of 400-500° C. Aerosols have been widely used as the material source for the deposition of thin films. The advantages of using aerosols are relatively low equipment costs and wider options for precursor materials. Deposition can be achieved in an open atmosphere without the need for sophisticated equipment. Unlike CVD (Chemical Vapour Deposition) processes, precursor materials with high vapour pressure are not required in aerosol deposition. However, spray pyrolysis requires a large volume of carrier gas to deliver the aerosol onto the substrate during film deposition. The large flow of carrier gas may result in turbulence near the substrate, which may affect the efficiency of deposition and the uniformity of the resultant film, and so care should be taken to produce a well-defined uniform film.
A TiO2 film may be fabricated from a precursor solution containing titanium(IV) isopropoxide (Ti-iPr), 2,4 pentanedione (PD) in absolute ethanol with concentration of 5 vol. % at Ti-iPr:PD molar ratio of 1:2. The solution may be deposited employing a Badger airbrush system (as illustrated in
The TiO2 film is preferably deposited at 450° C. by a pulsed solution feed. For example, the pulses may consist of 20 seconds of spray time followed by 20 seconds of pause, and five pulses may be performed. Other pulsed deposition procedures will be known to those skilled in the art. The film is subsequently heat treated for 30 minutes at 500° C. in air in order to remove any residual un-reacted precursor. Alternative deposition and heat treatment temperatures in the range of 140-700° C. are also possible, preferably in the range of 250-600° C., and more preferably in the range of 400-500° C.
With our TiO2-based FETs, the deposited films on pre-patterned FET substrates were post-annealed under ambient atmosphere at 500° C. for 30 minutes. The TFT devices were then placed inside a glove-box, followed by annealing at 150° C. for several hours prior to electrical characterization.
Mesoporous TiO2 films of nanocrystalline morphology may be prepared by spin coating, doctor-blading or screen-printing a TiO2 paste consisting of 10 nm sized TiO2 particles, this being prepared from a sol-gel colloidal suspension containing 12.5 wt % TiO2 particles and 6.2 wt % Carbowax 20,000.
The TiO2 nanoparticles may be synthesized by employing the following procedure: 20 ml of titanium iso-propoxide is injected into 5.5 g of glacial acetic acid under argon atmosphere and stirred for 10 minutes. The mixture is then injected into 120 ml of 0.1 M nitric acid under anhydrous atmosphere at room temperature in a conical flask and stirred vigorously. The flask is left uncovered and heated at 80° C. for 8 hours. After cooling, the solution is filtered using a 0.45 μm syringe filter, diluted to 5 wt % TiO2 by the addition of H2O and then autoclaved at 220° C. for 12 hours. The colloids are re-dispersed with a 60 s cycle burst from a LDU Soniprobe horn. The solution is then concentrated to 5% on a rotary evaporator using a membrane vacuum pump at a temperature of 40° C. Next, 6.2 wt % Carbowax 20,000 is added and the resulting paste is stirred slowly overnight to ensure homogeneity. Finally, an appropriate volume of this suspension is deposited onto the substrates. The resulting films are dried in air, and then sintered at 450° C. for 20 minutes in air. Alternative processing temperatures in the range of 140-700° C. are also possible, preferably in the range of 250-600° C., and more preferably in the range of 400-500° C.
Details of the synthesis of the TiO2 particles and paste, for producing mesoporous TiO2 films, are described in the following three papers:
As well as making the bottom-gate bottom-contact transistor based on gold contact electrodes as described above, we have also fabricated TiO2-based TFTs in a bottom-gate top-contact configuration as shown schematically in
Our TiO2-based transistors are also found to be relatively air stable. This is clearly evident from the experimental data displayed in
Although the experimental results shown in
Applications for TiO2-based TFTs are envisioned in the area of light-sensing. For light-sensing applications, a dense or mesoporous (or a combination of both) film of TiO2 may be employed, together with an organic dye/semiconductor as in the case of dye-sensitized solar cells. The conductivity of the TiO2/dye can be altered by the intensity of the light incident on the surface due to free carrier photogeneration within the active layers. As a result the overall transistor current can be modulated (increased or decreased). Since dye-sensitized solar cells are highly efficient, a large response may be achieved.
Photo-detecting TiO2-based TFTs can be fabricated employing different device architectures broadly similar to those shown in
Structure (c) of
The role of the organic material is to absorb the light so formation of bound hole-electron pairs (excitons) is possible, and also to transport the photogenerated holes. These excitons will eventually dissociate under the influence of the built-in potential present at the interface between the TiO2 and the organic material. The resulting free carriers (holes and electrons) will be transported to the collecting electrodes through the TiO2 (electrons) and the organic material (holes), hence contributing to the overall current flowing through the transistor channel. This change in the operating characteristics of the device can then be detected electronically, i.e. the electrical signal.
Although the use of organic material may be beneficial when fabricating photo-detecting TiO2-based TFTs, our experiments have unexpectedly revealed that organic material is not necessary in order for TiO2-based FETs to exhibit light sensitivity. Specifically, we have found that transistors having a TiO2 semiconductor layer, with no organic layer or organic content, exhibit light sensitivity—without the need for an organic layer to act as an optical sensitizer. Our experiments investigated the sensitivity of TiO2-based transistors to ultraviolet light.
We also envision the use of TiO2-based FETs in gas/bio-sensing applications. A key feature of the TiO2 layer is that it can be functionalized with molecular adsorbates such as dye pigments and protein molecules. Such molecules can promote changes in the channel current (ID) flowing through the FET device in response to a gaseous or biological analyte. Reaction between the TiO2 and the analyte may be promoted through the incorporation of special receptors on the surface of the semiconductor layer. Since the channel profile, that is the distance from the dielectric to the surface of the semiconductor through which all channel current flows, may be only a few nanometers, a significant change in the operating characteristics of the device is expected and hence great signal enhancement capabilities may be realised. However, a number of different detection schemes can be envisioned including electro-optical methods.
Summary of Some Potential Applications for TiO2-based FETs
An important advantage of TiO2-based FETs lies in the solution processing properties of the TiO2 layer and the low manufacturing cost associated with the solution processing technique. Another interesting feature of TiO2 is its high optical transparency in the visible spectrum, thus making it suitable for application in transparent electronics.
An important advantage of TiO2-based FETs lies in the solution processing properties of the TiO2 layer and the low manufacturing cost associated with the solution processing technique. The ability to process TiO2 in large areas provides a further advantage.
An important advantage of TiO2-based FETs for light sensing-applications is the potentially high sensitivity of the devices. Combination with electro-optical detection of bio-molecules is also envisioned.
The TiO2-based FET approach offers the potential for ease of fabrication, signal enhancement, stability, and the possibility of surface chemistry. The unique operating principle of the TiO2-based FET may be suitable for detection at low concentrations, even at ultra low levels. In addition TiO2 offers chemical reactions suitable for surface modification covalently. This is highly desirable for linking specific atoms/molecules with the surface of the TiO2. The latter may lead to enhance sensor specificity/selectivity.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0707714.2 | Apr 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB08/01298 | 4/11/2008 | WO | 00 | 10/9/2009 |
