JP4164562B2 - Transparent thin film field effect transistor using homologous thin film as active layer - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a transparent field effect using a single crystal film or an amorphous film of a homologous compound containing ZnO as a main constituent, which is transparent to visible light and can be used as an element constituting a transparent electronic circuit, as an active layer. Type transistor.
[0002]
[Prior art and problems]
Field effect transistors are used as unit electronic elements, high frequency signal amplifying elements, liquid crystal driving elements and the like of semiconductor memory integrated circuits, and are the most widely used electronic devices at present. As a material, silicon semiconductor compounds are most widely used. Single-crystal silicon is used for high-frequency amplifying elements and integrated circuit elements that require high-speed operation, and amorphous silicon is used for liquid crystal driving that is sufficient for low-speed operation due to the demand for large area. Yes.
[0003]
Field effect transistors using silicon have sufficient performance for many applications. However, the transistor is opaque to visible light based on the optical characteristics of silicon and cannot constitute a transparent circuit. In addition, since conductive carriers are generated by irradiation with visible light, transistor characteristics deteriorate under high light irradiation. For example, when the transistor is applied as a switching element for driving a liquid crystal of a liquid crystal display, the device is opaque to visible light, so that the aperture ratio of the display pixel is small. Moreover, a photo-induced current is generated by backlight irradiation, and the switching characteristics are deteriorated. In order to prevent such deterioration, it is necessary to provide a light shielding film for cutting back light.
[0004]
Such problems of the silicon field effect transistor can be solved in principle by using a semiconductor material having a large energy bandwidth instead of silicon. Actually, an attempt has been made to manufacture a field effect transistor using ZnO which is a transparent oxide semiconductor (for example, Non-Patent Document 1). However, ZnO has drawbacks that it is difficult to reduce electrical conductivity, and a normally-off field effect transistor cannot be formed. Further, since it is difficult to form an amorphous state, an amorphous transistor suitable for a large area cannot be manufactured.
[0005]
[Non-Patent Document 1]
Nanase et al., Proceedings of the 2000 Spring Conference of the Japan Society of Applied Physics, 2000.3, 29p-YL-16
[0006]
[Means for Solving the Problems]
The present inventors previously used InGaO ₃ (ZnO) _m containing ZnO as a main constituent, which shows n-type electrical conduction in an amorphous state by film formation at room temperature using a pulse laser thin film deposition method. Homologous compound transparent thin films such as (m is a natural number) were grown (JP 2000-44236 A, Hosono et al. Philosophical Magazine B. 81.501-515 (2001)).
[0007]
Furthermore, the present inventors have deposited an amorphous homologous thin film on a ZnO single crystal ultrathin film grown on a YSZ (yttria stabilized zirconia) substrate, and heat-diffused the obtained multilayer film at a high temperature. Developed a method for growing a homologous single crystal thin film by the “chemical solid phase epitaxial method”, named “natural superlattice homologous single crystal thin film and method for producing the same”, and applied for a patent (Japanese Patent Application No. 2001-340066).
[0008]
In the same way as the above-described method for producing a homologous single crystal thin film, the present inventor has replaced the conventional field effect transistor using silicon by using a means for heating and diffusing a complex oxide thin film epitaxially grown on a ZnO thin film. It has been found that an excellent field effect transistor can be provided.
[0009]
The present invention provides a field effect transistor using a homologous compound single crystal thin film containing ZnO as a main constituent or a homologous amorphous thin film containing ZnO as a main constituent grown by a reactive solid phase epitaxial method as an active layer To do.
[0010]
That is, the present invention provides a transparent film characterized in that (1) a homologous compound InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) is used as an active layer. It is a thin film field effect transistor. (2) The transparent thin film field effect transistor according to (1) above, wherein a single crystal or amorphous homologous compound thin film having a flat surface at an atomic level is used. (3) The transparent thin film field effect transistor according to (1) above, wherein the homologous compound is a heat resistant, single crystal thin film formed on a transparent oxide single crystal substrate. (4) The transparent thin film field effect transistor according to (1) above, wherein the homologous compound is an amorphous thin film formed on a glass substrate.
[0011]
1. Homologous compound single crystal InMO ₃ (ZnO) _m (M = In, Fe, Ga, Al, m = 1 or more and less than 50) manufactured by reactive solid phase epitaxy, InO _1.5 layer is atomic level Since a flat thin film surface is formed, a thin film field effect transistor with little gate leakage current and less gate defects can be produced. The value of m in InMO ₃ (ZnO) _m is preferably an integer of 1 or more and less than 50. In principle, the value of m can be infinite, but in practice, if the value of m becomes too large, the variation of m in the film will increase, and oxygen defects will easily occur, As a result, the electrical conductivity of the film increases, making it difficult to make a normally-off FET.
[0012]
2. The band gap energy of the homologous compound InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, where m is an integer from 1 to less than 50) containing ZnO as the main component is larger than 3.3 eV. Transparent to visible light having a wavelength of 400 nm or more. Therefore, by using a homologous compound single crystal InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) thin film as an active layer, visible light transmittance is high and visible. A thin-film field-effect transistor that does not generate light-induced current due to light can be manufactured.
[0013]
3. In addition, the homologous compound InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) produced by reactive solid phase epitaxial method has a stoichiometric composition. The homologous compound single crystal InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) is a very good insulator near room temperature. By using a thin film as an active layer, a transparent thin film field effect transistor having a normally-off operation and good switching characteristics can be manufactured.
[0014]
An amorphous state in which a homologous compound containing ZnO is formed at room temperature by a reactive solid phase epitaxial method is stable up to a high temperature of about 1000 ° C., and the electron carrier mobility in that state is 10 times that of amorphous silicon. More than double. Therefore, a field effect transistor using a homologous amorphous thin film as an active layer has a higher visible light transmittance than a silicon amorphous field effect transistor, operates stably against light irradiation, and operates at a higher speed. I can expect that.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
The substrate of a homologous compound single crystal thin film containing ZnO as a main constituent used in the present invention has heat resistance and is a transparent oxide single crystal substrate, for example, YSZ (yttria stabilized zirconia), sapphire, MgO, ZnO. Etc. are used. Among these, YSZ is most preferable because it has a lattice constant close to that of a homologous compound containing ZnO and does not chemically react with the compound at a temperature of 1400 ° C. or lower. The surface mean square roughness Rms of these substrates is preferably 1.0 nm or less. Rms can be calculated by scanning a 1 μm square with an atomic force microscope, for example.
[0016]
When using a homologous compound amorphous thin film containing ZnO, the substrate does not need to have heat resistance, and an inexpensive glass substrate can be used. The flatness may be about the same as that of a glass substrate used for an amorphous silicon field effect transistor.
[0017]
A super-flattened surface can be obtained by heating an oxide single crystal substrate such as YSZ to 1000 ° C. or higher in the air or in vacuum. A structure reflecting the crystal structure appears on the surface of the ultra flattened oxide single crystal substrate. That is, it is a structure composed of a terrace having a width of about several hundred nm and steps having a height of about sub-nm, and is generally called an atomically flattened structure.
[0018]
The terrace portion is composed of atoms arranged on a plane, and is a completely flattened surface if the presence of some defects is ignored. Due to the presence of steps, the entire substrate is not a completely planarized surface. If this structure is expressed as roughness Rms by the mean square roughness measurement method, Rms is 1.0 nm or less. Rms is a value calculated by, for example, scanning a 1 μm square range with an atomic force microscope.
[0019]
A ZnO single crystal thin film having an atomic flat surface is epitaxially grown on the obtained heat-resistant transparent oxide substrate having an atomic flat surface by MBE, pulsed laser deposition (PLD), or the like. Next, a homologous compound thin film described as InMO ₃ (ZnO) _m (M = In, Fe, Ga, Al, m = 1 or more and less than 50) is used as a target on the ZnO thin film, and the oxide The polycrystalline sintered body is used to grow by MBE method, pulse laser deposition method (PLD method) or the like.
[0020]
The obtained thin film does not need to be a single crystal film, and may be a polycrystalline film or an amorphous film. Finally, a high melting point compound such as YSZ or Al ₂ O ₃ is covered so as to cover the entire thin film, and a heat diffusion treatment is performed at a high temperature of 1300 ° C. or higher and in an atmospheric pressure containing ZnO vapor. The reason for covering with the high melting point compound is to avoid contact between ZnO vapor, which will be described later, and the surface of the thin film.
[0021]
Since ZnO has a high vapor pressure, if no ZnO vapor is added to the atmosphere, ZnO will evaporate from the film during the heat diffusion process, and the film composition after the heat diffusion process will change drastically, resulting in a good quality crystalline film. Cannot be obtained. Therefore, the reaction vessel is filled with 5% by volume or more of ZnO with respect to the vessel volume. The ZnO to be filled is preferably a powder or a sintered body. ZnO is evaporated from the ZnO powder by heat diffusion treatment at a high temperature, the ZnO vapor pressure inside the reaction vessel is increased, and the evaporation of ZnO in the thin film can be suppressed.
[0022]
InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) and ZnO film diffuse and react with each other, and if the temperature is set appropriately, uniform composition InMO ₃ (ZnO) _{m ′} (M = In, Fe, Ga, or Al, m ′ = 1 or more and less than 50). m ′ is determined from InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) and a ZnO film thickness ratio. The ZnO film thickness is less than 5 nm, and InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) When the film thickness exceeds 100 nm, m = m ′.
[0023]
A suitable temperature is 800 ° C or higher and 1600 ° C or lower, more preferably 1200 ° C or higher and 1500 ° C or lower. Diffusion is slow below 800 ° C, and uniform composition of InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = integer between 1 and 50) cannot be obtained. Further, if the temperature exceeds 1600 ° C., the evaporation of ZnO cannot be suppressed, and uniform composition InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and less than 50) cannot be obtained.
[0024]
Single crystal thin film obtained, since the MO ₃ (ZnO) _m layer can be regarded as a natural superlattice structure sandwiched by InO _1.5 layers, present at the interface between the MO ₃ (ZnO) _m layer and InO _1.5 layer A quantum effect occurs in electrons. For this reason, the obtained single crystal thin film can be used as a high-frequency electronic device material similarly to the artificial superlattice structure.
[0025]
In addition, the homologous single crystal film containing ZnO obtained by reactive solid phase epitaxial growth is close to the stoichiometric composition, exhibits high insulation of 10 ⁸ W · cm or more at room temperature, and is a normally-off field effect type. Suitable for transistors.
[0026]
A top-gate MIS field effect transistor using the obtained homologous single crystal thin film containing ZnO as a main constituent component as an active layer can be produced. As shown in FIG. 3, first, a metal film for a gate insulating film 3 and a gate electrode 4 is formed on a homologous single crystal thin film 2 containing ZnO epitaxially grown on a substrate 1 as a main component. Al ₂ O ₃ is most suitable for the gate insulating film 3. Au, Ag, Al, Cu or the like can be used for the metal film for the gate electrode 4. The gate electrode 4 is produced by the optical lithography method and dry etching or lift-off method, and finally the source electrode 5 and the drain electrode 6 are produced. The shape of the field effect transistor of the present invention is not limited to the top gate type MIS field effect transistor (MIS-FET), and includes a J-FET and the like.
[0027]
Similarly, a top-gate MIS field effect transistor can be produced using a homologous amorphous thin film containing ZnO as a main constituent. In the case of an amorphous thin film, since it is not necessary to perform epitaxial growth, ZnO epitaxial growth and a high temperature annealing process can be eliminated. Therefore, a gate electrode can be formed between the substrate and the film, and a bottom gate type MIS field effect transistor can also be manufactured.
[0028]
【Example】
Example 1 in which the present invention is described in detail with reference to the following examples
1. Preparation of single crystal InGaO ₃ (ZnO) ₅ thin film
A 2 nm thick ZnO thin film was epitaxially grown on a YSZ (111) single crystal substrate at a substrate temperature of 700 ° C. by the PLD method. Next, the substrate temperature was cooled to room temperature, and a 150 nm thick polycrystalline InGaO ₃ (ZnO) ₅ thin film was deposited on the ZnO epitaxial thin film by PLD. The two-layer film thus produced was taken out into the atmosphere, subjected to heat diffusion treatment at 1400 ° C. for 30 minutes in the atmosphere using an electric furnace, and then cooled to room temperature.
[0029]
As a result of XRD measurement, as shown in FIG. 1, the thin film obtained by heating is single-crystal InGaO ₃ (ZnO) ₅ , and as shown in FIG. It was a flat surface at an atomic level consisting of a flat terrace and a step of 2 nm in height. An attempt was made to measure the conductivity of a single crystal InGaO ₃ (ZnO) ₅ thin film by the DC four-terminal method, but the measurement was not possible due to the high insulating properties of the film. The produced single crystal InGaO ₃ (ZnO) ₅ thin film can be said to be an insulator. The band gap of InGaO ₃ (ZnO) ₅ was estimated to be about 3.3 eV from the light absorption spectrum measured at room temperature.
[0030]
2. Fabrication of MISFET element A top-gate MISFET element was fabricated by photolithography.
Au and amorphous Al ₂ O ₃ were used for the source and drain electrodes and the gate insulating film, respectively. The channel length and channel width are 0.05 mm and 0.2 mm, respectively.
[0031]
3. Characteristic evaluation of MISFET element Fig. 4 shows the current-voltage characteristics of the MISFET element measured at room temperature. When the gate voltage V _G = 0V, I _DS = 10 ⁻⁸ A (V _DS = 2.0 V), and so-called normally OFF characteristics were obtained. When V _G = 10 V, a current of I _DS = 1.6 × 10 ⁻⁶ A flowed. This corresponds to the fact that carriers could be induced in the insulator InGaO ₃ (ZnO) ₅ single crystal thin film by the gate bias. A similar measurement was performed by irradiating the fabricated device with visible light, but no change in the numerical value was observed. Generation of photoinduced current in visible light was not observed.
[0032]
【The invention's effect】
The transparent thin film field effect transistor of the present invention is transparent to visible light / infrared light having a wavelength of 400 nm or more and can be normally switched off. By applying the transparent thin film field effect transistor of the present invention as a switching element of an LCD, it is possible to use backlight light effectively without any loss, and development to a see-through type display can be expected.
[Brief description of the drawings]
FIG. 1 is a graph showing the results of XRD measurement of a single crystal InGaO ₃ (ZnO) ₅ thin film produced in Example 1. FIG.
FIG. 2 is a drawing-substituting AFM observation photograph showing the surface structure of a single crystal InGaO ₃ (ZnO) ₅ thin film produced in Example 1. FIG.
FIG. 3 is a schematic diagram showing the structure of a MISFET element according to an embodiment of the present invention.
4 is a graph showing current-voltage characteristics of the MISFET element fabricated in Example 1 measured at room temperature. FIG.

Claims (4)

A transparent thin-film field effect transistor using a thin film of a homologous compound InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 or more and an integer less than 50) as an active layer. 2. The transparent thin film field effect transistor according to claim 1, wherein a single crystal or amorphous homologous compound thin film having a flat surface at an atomic level is used. 2. The transparent thin film field effect transistor according to claim 1, wherein the homologous compound is a heat resistant, single crystal thin film formed on a transparent oxide single crystal substrate. 2. The transparent thin film field effect transistor according to claim 1, wherein the homologous compound is an amorphous thin film formed on a glass substrate.

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