JP2004103957A - Transparent thin film field effect type transistor using homologous thin film as active layer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that in ZnO as a transparent oxide semiconductor, it is difficult to reduce an electric conductivity and it is impossible to constitute a normally off field effect type transistor, or as it is difficult to form an amorphous state, an amorphous transistor adaptive for a large area cannot be manufactured. <P>SOLUTION: In a homologous compound InMO<SB>3</SB>(ZnO)<SB>m</SB>(M=In, Fe, Ga or Al; m=an integer of 1 to 49) single crystal thin film manufactured by a reactive solid-phase epitaxial method, a deviation from a stoichiometry is very small and a good insulator is obtained near room temperatures. By using the homologous compound single crystal InMO<SB>3</SB>(ZnO)<SB>m</SB>(M=In, Fe, Ga or Al; m=an integer of 1 to 49) thin film as an active layer, a transparent thin film field effect type transistor having a good switching characteristic can be manufactured by a normally off operation. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a transparent electric field effect that uses a single crystal film or an amorphous film of a homologous compound containing ZnO as a main component as an active layer and is transparent to visible light and can be used as an element constituting a transparent electronic circuit. Type transistor.
[0002]
[Prior art and its problems]
2. Description of the Related Art A field effect transistor is used as a unit electronic element of a semiconductor memory integrated circuit, a high-frequency signal amplifying element, an element for driving a liquid crystal, and the like, and is the most widely used electronic device. As a material, a silicon semiconductor compound is most widely used. Single crystal silicon is used for high-frequency amplifiers and integrated circuit elements that require high-speed operation, and amorphous silicon is used for low-speed operation and sufficient liquid crystal driving due to the need for a large area. I have.
[0003]
A field effect transistor using silicon has sufficient performance for many applications. However, the transistor is opaque to visible light due to the optical characteristics of silicon and cannot form a transparent circuit. In addition, since conductive carriers are generated by irradiation with visible light, transistor characteristics are deteriorated under irradiation with high light. 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 display pixels is reduced. Further, light-induced current is generated by backlight irradiation, and the switching characteristics are degraded. In order to prevent such deterioration, it is necessary to provide a light-shielding film for cutting backlight light.
[0004]
These 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 in that it is difficult to reduce the electrical conductivity, and a normally-off field-effect transistor cannot be formed. In addition, 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]
Nanakin, et al., Proceedings of the 2000 Spring Meeting of the Japan Society of Applied Physics, 2000.3, 29p-YL-16
[0006]
[Means for Solving the Problems]
The present inventors have previously made InGaO ₃ (ZnO) _m containing ZnO as a main component showing n − -type electric conduction in an amorphous state by film formation at room temperature using a pulsed laser thin film deposition method. (M is a natural number) and the like, and a transparent thin film of a homologous compound was grown (Japanese Patent Application Laid-Open No. 2000-44236, Hosono et al., Philosophical Magazine B. 81.501-515 (2001)).
[0007]
Furthermore, the present inventors deposit an amorphous homologous thin film on a ZnO single crystal ultrathin film grown on a YSZ (yttria-stabilized zirconia) substrate, and heat-diffuse the resulting multilayer film at a high temperature. A method for growing a homologous single crystal thin film was developed by the "active solid phase epitaxial method," and a patent application was filed under the name of "Natural superlattice homologous single crystal thin film and its manufacturing method" (Japanese Patent Application No. 2001-340066).
[0008]
The present inventor has proposed a new alternative to a 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 in the same manner as the above-described method for producing a homologous single crystal thin film. It has been found that an excellent field effect transistor can be provided.
[0009]
The present invention provides a field-effect transistor in which a homologous compound single crystal thin film containing ZnO as a main component or a homologous amorphous thin film containing ZnO as a main component grown as an active layer grown by a reactive solid phase epitaxial method. I do.
[0010]
That is, the present invention provides (1) a transparent material characterized by using a thin film of a homologous compound InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to less than 50) as an active layer. It is a thin film field effect transistor. (2) The transparent thin film field effect transistor according to (1), 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), 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), wherein the homologous compound is an amorphous thin film formed on a glass substrate.
[0011]
1. In a homologous compound single crystal InMO ₃ (ZnO) _m (M = In, Fe, Ga, Al, m = 1 or more and less than 50) thin film produced by a reactive solid phase epitaxy method, _1.5 layers of InO have an atomic level. Since a flat thin film surface is formed by the above method, defects are less likely to be present at the interface between the gate and the active layer, and a thin film field effect transistor having a small gate leak current can be manufactured. The value of m of InMO ₃ (ZnO) _m is preferably an integer of 1 or more and less than 50. In principle, the value of m can be up to infinity, but in practice, if the value of m is too large, the dispersion of m in the film will increase and oxygen vacancies will easily occur, As a result, the electrical conductivity of the film increases, making it difficult to produce a normally-off type FET.
[0012]
2. The bandgap energy of the homologous compound InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to less than 50) containing ZnO as a main component is greater than 3.3 eV. It is transparent to visible light having a wavelength of 400 nm or more. Therefore, by using a thin film of a homologous compound single crystal InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to less than 50) as the active layer, the visible light transmittance is high and the visible light transmittance is high. A thin-film field-effect transistor without generation of light-induced current by light can be manufactured.
[0013]
3. Further, the homologous compound InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to less than 50 and an integer less than 50) single crystal thin film produced by the reactive solid phase epitaxy is Since the deviation is extremely small and is a good insulator near room temperature, a thin film of a homologous compound single crystal InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to less than 50) is formed. By using the active layer as an active layer, a normally-off transparent thin-film field-effect transistor having 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 this state is 10 times higher than 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, operates more stably against light irradiation, and operates at a higher speed than a silicon amorphous field-effect transistor. Can be expected.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
The substrate of the homologous compound single crystal thin film containing ZnO as a main component 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. And so on. Among them, YSZ, which 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, is most preferable. It is preferable to use a substrate having a surface mean square roughness Rms of 1.0 nm or less. Rms can be calculated by scanning an 1 μm square with an atomic force microscope, for example.
[0016]
When a homologous compound amorphous thin film containing ZnO is used, 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 the glass substrate used for the amorphous silicon field effect transistor.
[0017]
By heating an oxide single crystal substrate such as YSZ to 1000 ° C. or higher in air or vacuum, an ultra-flat surface can be obtained. A structure reflecting the crystal structure appears on the surface of the ultra-flattened oxide single crystal substrate. That is, the structure is composed of a terrace having a width of about several hundred nm and a step having a height of about sub-nm, and is generally called a structure flattened in an atomic state.
[0018]
The terrace portion is composed of atoms arranged on a plane, and is a completely planarized surface if the existence of some defects is ignored. The presence of the steps does not result in a completely planarized surface over the entire substrate. If this structure is expressed in terms of roughness Rms by the mean square roughness measurement method, Rms is 1.0 nm or less. Rms is a value calculated, for example, by scanning a range of 1 μm square with an atomic force microscope.
[0019]
A ZnO single crystal thin film having an atomically flat surface is epitaxially grown on the obtained heat-resistant transparent oxide substrate having an atomically flat surface by MBE, pulse 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 to less than 50) is used as a target on the ZnO thin film to form the oxide. Is grown by MBE method, pulsed laser deposition method (PLD method) or the like using the polycrystalline sintered body of (1).
[0020]
The obtained thin film need not be a single crystal film, but 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 more at atmospheric pressure containing ZnO vapor. The reason why the high melting point compound is covered is to avoid contact between the ZnO vapor described later and the surface of the thin film.
[0021]
Since the vapor pressure of ZnO is high, unless ZnO vapor is added to the atmosphere, ZnO evaporates from the film during the heat diffusion process, and the film composition after the heat diffusion process changes significantly, resulting in a high quality crystalline film. Can not be obtained. For this purpose, the reaction vessel is filled with at least 5% by volume of ZnO based on the volume of the vessel. The ZnO to be filled is preferably a powder or a sintered body. The ZnO powder evaporates from the ZnO powder by the heat diffusion treatment at a high temperature, the vapor pressure of ZnO 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 to less than 50) and the ZnO film diffuse and react with each other, and if the temperature is appropriately set, the uniform composition InMO ₃ (ZnO) _{m ′} (M = In, Fe, Ga, or Al, m ′ = 1 to an integer less than 50). m ′ is determined from InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to an integer less than 50) and the ZnO film thickness ratio, and when the ZnO film thickness is less than 5 nm and the InMO ₃ If _{(ZnO) m (M = in} , Fe, Ga, or the Al, m = 1 or more 50 than an integer) thickness exceeds 100nm is m = m '.
[0023]
A suitable temperature is from 800 ° C to 1600 ° C, more preferably from 1200 ° C to 1500 ° C. If the temperature is lower than 800 ° C., the diffusion is slow, and a uniform composition of InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to less than 50) cannot be obtained. On the other hand, if the temperature exceeds 1600 ° C., evaporation of ZnO cannot be suppressed and InMO ₃ (ZnO) _m (M = In, Fe, Ga, or Al, m = 1 to less than 50) having a uniform composition cannot be obtained.
[0024]
The obtained single crystal thin _film, since the MO 3 _{(ZnO) m} layer can be regarded as a natural superlattice structure sandwiched by _{InO 1.5 layers,} the MO 3 _{(ZnO) m} layer and _{InO 1.5} layer A quantum effect occurs in electrons present at the interface. 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 the reactive solid phase epitaxial growth method has a close stoichiometric composition, exhibits a high insulating property of 10 ⁸ W · cm or more at room temperature, and has 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 component as an active layer can be manufactured.
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 as a main component grown epitaxially on a substrate 1. 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. A gate electrode 4 is formed by photolithography and dry etching or a lift-off method, and finally, a source electrode 5 and a drain electrode 6 are formed. The shape of the field-effect transistor of the present invention is not limited to a top-gate MIS field-effect transistor (MIS-FET), but includes a J-FET and the like.
[0027]
Even if a homologous amorphous thin film containing ZnO as a main component is used, a top-gate MIS field-effect transistor can be similarly produced. In the case of an amorphous thin film, since it is not necessary to perform epitaxial growth, it is possible to eliminate the ZnO epitaxial growth and the high-temperature annealing process. For this reason, a gate electrode can be formed between the substrate and the film, and a bottom-gate MIS field-effect transistor can also be manufactured.
[0028]
【Example】
Example 1 Hereinafter, the present invention will be described in detail with reference to 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 by a PLD method at a substrate temperature of 700 ° C. Next, the substrate temperature was cooled to room temperature, and a polycrystalline InGaO ₃ (ZnO) ₅ thin film having a thickness of 150 nm was deposited on the ZnO epitaxial thin film by the PLD method. The two-layer film thus produced was taken out into the air, and subjected to heat diffusion treatment at 1400 ° C. for 30 minutes in the air 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 was a single crystal InGaO ₃ (ZnO) ₅ , and as shown in FIG. And a flat terrace at the atomic level consisting of a flat terrace and a step of 2 nm in height.
An attempt was made to measure the conductivity of the single-crystal InGaO ₃ (ZnO) ₅ thin film by a DC four-terminal method, but the measurement was not possible due to the high insulating property of the film. It can be said that the manufactured single crystal InGaO ₃ (ZnO) ₅ thin film is an insulator. From the light absorption spectrum measured at room temperature, the band gap of InGaO ₃ (ZnO) ₅ was estimated to be about 3.3 eV.
[0030]
2. Production of MISFET device A top gate MISFET device was produced 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. Evaluation of Characteristics of MISFET Device FIG. 4 shows current-voltage characteristics of the MISFET device measured at room temperature. When the gate voltage V _G = 0 V, I _DS = 10 ⁻⁸ A (V _DS = 2.0 V), and a so-called normally-off characteristic was obtained. _Also, V G = 10 V at times _flowed a current of I DS = 1.6X ^{10 -6} A. This corresponds to the fact that carriers can 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 was observed in the numerical values. No photo-induced current was generated in visible light.
[0032]
【The invention's effect】
The transparent thin-film field-effect transistor of the present invention is transparent to visible light and infrared light having a wavelength of 400 nm or more, and can be switched normally off. By applying the transparent thin-film field-effect transistor of the present invention as a switching element of an LCD, backlight light can be used effectively without loss, and furthermore, development to a see-through 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. 2 is an AFM observation photograph as a drawing showing the surface structure of the single crystal InGaO ₃ (ZnO) ₅ thin film produced in Example 1.
FIG. 3 is a schematic view showing the structure of a MISFET device according to one embodiment of the present invention.
FIG. 4 is a graph showing current-voltage characteristics of the MISFET device manufactured in Example 1 measured at room temperature.

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 to 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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