OXIDE SINTERED BODY, SPUTTERING TARGET, AND OXIDE SEMICONDUCTOR THIN FILM OBTAINED USING SPUTTERING TARGET

Abstract

Provided are an oxide sintered compact whereby low carrier density and high carrier mobility are obtained when the oxide sintered compact is used to obtain an oxide semiconductor thin film by a sputtering method, and a sputtering target which uses the oxide sintered compact. This oxide sintered compact contains oxides of indium, gallium, and aluminum. The gallium content is from 0.15 to 0.49 by Ga/(In+Ga) atomic ratio, and the aluminum content is from 0.0001 to less than 0.25 by Al/(In+Ga+Al) atomic ratio. A crystalline oxide semiconductor thin film formed using this oxide sintered compact as a sputtering target is obtained at a carrier density of 4.0×10.sup.18 cm.sup.−3 or less and a carrier mobility of 10 cm.sup.−2V.sup.−1sec.sup.−1 or greater.

Claims

1: An oxide sintered body comprising indium, gallium, and aluminum as oxides, wherein the content of the gallium is 0.15 or more and 0.49 or less in terms of Ga/(In+Ga) atomic ratio, and the content of the aluminum is 0.0001 or more and less than 0.25 in terms of Al/(In+Ga+Al) atomic ratio, the oxide sintered body includes an In.sub.2O.sub.Q phase having a bixbvite-type structure, and a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure as a formed phase other than the In.sub.2O.sub.3 phase, or a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as a formed phase other than the In.sub.2O.sub.3 phase.

2. (canceled)

3: The oxide sintered body according to claim 1, wherein the content of the aluminum is 0.01 or more and 0.20 or less in terms of Al/(In+Ga+Al) atomic ratio.

4: The oxide sintered body according to claim 1, wherein the content of the gallium is 0.20 or more and 0.45 or less in terms of Ga/(In+Ga) atomic ratio.

5: A sputtering target obtained by machining the oxide sintered body according to claim 1.

6: An amorphous oxide semiconductor thin film formed on a substrate by a sputtering method using the sputtering target according to claim 5 and then subjected to a heat treatment in an oxidizing atmosphere.

7: The oxide semiconductor thin film according to claim 6, wherein a carrier density is less than 4.0×10.sup.18 cm.sup.−3 and a carrier mobility is 10 cm.sup.2 V.sup.−1 sec.sup.−1 or more.

8: The oxide semiconductor thin film according to claim 7, wherein the carrier density is 6.0×10.sup.17 cm.sup.−3 or less.

9: The oxide semiconductor thin film according to claim 7, wherein the carrier mobility is 15 cm.sup.2 V.sup.−1 sec.sup.−1 or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a graph showing an X-ray diffraction measurement spectrum of an oxide sintered body of Example 6.

[0029] FIG. 2 is a graph showing an X-ray diffraction measurement spectrum of an oxide sintered body of Example 9.

[0030] FIG. 3 is a crystal grain photograph of the oxide sintered body of Example 9 captured by using a scanning transmission electron microscope.

[0031] FIG. 4 is an electron beam diffraction photograph of white crystal grains of the oxide sintered body of Example 9 captured by using a scanning transmission electron microscope.

[0032] FIG. 5 is an electron beam diffraction photograph of black crystal grains of the oxide sintered body of Example 9 captured by using a scanning transmission electron microscope.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

[0033] An oxide sintered body, a sputtering target, and an oxide semiconductor thin film obtained by using the target in the present invention will be described below in detail.

(1) Oxide Sintered Body

(a) Composition

[0034] The oxide sintered body of the present invention is an oxide sintered body containing indium, gallium, and aluminum as oxides. A gallium content is 0.15 or more and 0.49 or less in terms of Ga/(In+Ga) atomic ratio, and an aluminum content is 0.0001 or more and less than 0.25 in terms of Al/(In+Ga+Al) atomic ratio.

[0035] The gallium content, in terms of Ga/(In+Ga) atomic ratio, is 0.15 or more and 0.49 or less and preferably 0.20 or more and 0.45 or less. Gallium has the effect of increasing the crystallization temperature of the amorphous oxide semiconductor thin film of the present invention. Further, gallium has high bonding strength to oxygen and has the effect of reducing the oxygen loss in the amorphous oxide semiconductor thin film of the present invention. When the gallium content is less than 0.15 in terms of Ga/(In+Ga) atomic ratio, these effects are not sufficiently obtained. On the other hand, when the gallium content is more than 0.49, the carrier mobility is not high enough for an oxide semiconductor thin film.

[0036] The oxide sintered body of the present invention contains aluminum in addition to indium and gallium in the composition ranges defined above. The concentration of aluminum, in terms of Al/(In+Ga+Al) atomic ratio, is 0.0001 or more and less than 0.25 and preferably 0.01 or more and 0.20 or less. Aluminum has the effect of increasing the crystallization temperature of the amorphous oxide semiconductor thin film of the present invention. Further, doping the oxide sintered body with aluminum suppresses the carrier density of the amorphous oxide semiconductor thin film of the present invention. However, when the concentration of aluminum is more than 0.25, the bulk resistance value of the sputtering target increases. Thus, a homogeneous film cannot be obtained due to abnormal discharge such as arc discharge (arcing) at the time of deposition when sputtering is performed.

[0037] According to the effect, when the amorphous oxide semiconductor thin film of the present invention is applied to TFTs, the on/off ratio of TFTs can be increased.

(b) Structure of Sintered Body

[0038] The oxide sintered body of the present invention includes an In.sub.2O.sub.3 phase having a bixbyite-type structure and a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure, and may contain a (Ga, In).sub.2O.sub.3 phase to a certain extent in addition thereto.

[0039] Herein, gallium is preferably dissolved in the In.sub.2O.sub.3 phase or preferably makes up the GaInO.sub.3 phase. In the case of being dissolved in the In.sub.2O.sub.3 phase, gallium, which is basically a trivalent cation, substitutes for indium, which similarly is a trivalent cation, at the lattice position. In the case of making up the GaInO.sub.3 phase and the (Ga, In).sub.2O.sub.3 phase, basically, Ga occupies the original lattice position, but may be slightly dissolved to substitute at the lattice position of In as a defect. Further, it is not preferred that gallium is difficult to dissolve in the In.sub.2O.sub.3 phase, or the GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure and the (Ga, In).sub.2O.sub.3 phase are difficult to generate, and as a result, the Ga.sub.2O.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure is formed because of unsuccessful sintering or the like. Since the Ga.sub.2O.sub.3 phase has low conductivity, abnormal discharge arises.

[0040] Aluminum is preferably dissolved in the In.sub.2O.sub.3 phase or the GaInO.sub.3 phase. In the case of being dissolved in the In.sub.2O.sub.3 phase, aluminum, which is basically a trivalent cation, substitutes for indium, which is similarly a trivalent cation, at the lattice position. In the case of being dissolved in the GaInO.sub.3 phase and the (Ga, In).sub.2O.sub.3 phase, basically, In or Ga substitutes at the lattice position.

[0041] The oxide sintered body may be composed of at least an In.sub.2O.sub.3 phase, and contain a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase. Crystal grains of these phases preferably have a mean particle size of 5 μm or less. Crystal grains of these phases are difficult to subject to sputtering as compared to the crystal grains of the In.sub.2O.sub.3 phase having a bixbyite-type structure so that they remain and generate nodules, and thus the nodules may cause arcing.

(2) Method for Producing Oxide Sintered Body

[0042] The oxide sintered body of the present invention is produced by using an oxide powder consisting of an indium oxide powder and a gallium oxide powder, and an aluminum oxide powder as raw material powders.

[0043] In the process for producing the oxide sintered body of the present invention, these raw material powders are mixed and then compacted, and the compact is sintered by ordinary-pressure sintering. The formed phases in the structure of the oxide sintered body of the present invention strongly depend on the conditions in each step for producing the oxide sintered body, for example, the particle size of the raw material powders, the mixing conditions, and the sintering conditions.

[0044] The structure of the oxide sintered body of the present invention is preferably controlled so that each crystal grain of the InAlO.sub.3 phase, the GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure, and the (Ga, In).sub.2O.sub.3 phase is 5.0 μm or less. For this reason, the mean particle size of the raw material powder is adjusted to more preferably 3.0 μm or less and even more preferably 1.0 μm or less.

[0045] Indium oxide powder is a raw material for ITO (tin-doped indium oxide), and fine indium oxide powder having good sintering properties has been developed along with improvements in ITO. Since indium oxide powder has been continuously used in large quantities as a raw material for ITO, a raw material powder having a mean particle size of 1.0 μm or less is available these days.

[0046] Since aluminum oxide (alumina) powder is widely used as a raw material for ceramics or sapphire, raw material powder having a mean particle size of 1.0 μm or less is available.

[0047] However, since the amount of gallium oxide powder used is still smaller than that of indium oxide powder used, it is difficult to obtain raw material powder having a mean particle size of 1.0 μm or less. When only coarse gallium oxide powder is available, the powder needs to be pulverized into particles having a mean particle size of 1.0 μm or less.

[0048] In the process for sintering the oxide sintered body of the present invention, ordinary-pressure sintering is preferably employed. Ordinary-pressure sintering is a simple and industrially advantageous method, and is also an economically preferable means.

[0049] When ordinary-pressure sintering is used, a compact is first produced as described above. Raw material powders are placed in a resin pot and mixed with a binder (for example, PVA) and the like by wet ball milling or the like. The oxide sintered body of the present invention may be composed of the InAlO.sub.3 phase, the In.sub.2O.sub.3 phase having a bixbyite-type structure, and the GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure, and may further include the (Ga, In).sub.2O.sub.3 phase. The crystal grains of these phases are preferably controlled to have a mean particle size of 5 μm or less and to be finely dispersed. It is preferable to suppress formation of the (Ga, In).sub.2O.sub.3 phase as much as possible. In addition, it is necessary that the Ga.sub.2O.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure, which causes arcing, other than these phases is not generated. The ball mill mixing is preferably performed for 18 hours or longer in order to satisfy these requirements. At this time, hard ZrO.sub.2 balls may be used as mixing balls. After mixing, the slurry is taken out, filtered, dried, and granulated. Subsequently, the resultant granulated material is compacted under a pressure of about 9.8 MPa (0.1 ton/cm.sup.2) or more and 294 MPa (3 ton/cm.sup.2) or less by cold isostatic pressing to form a compact.

[0050] The sintering process by ordinary-pressure sintering is preferably preformed in an atmosphere containing oxygen. The volume fraction of oxygen in the atmosphere is preferably over 20%. In particular, when the volume fraction of oxygen is over 20%, the oxide sintered body is further densified. n excessive amount of oxygen in the atmosphere causes the surface of the compact to undergo sintering in advance during the early stage of sintering. Subsequently, sintering proceeds while the inside of the compact is reduced, and a highly dense oxide sintered body is finally obtained.

[0051] In an atmosphere free of oxygen, the surface of the compact does not undergo sintering and as a result, densification of the sintered body does not proceed. If oxygen is absent, indium oxide decomposes particularly at about 900° C. to 1000° C. to form metal indium, which makes it difficult to obtain a desired oxide sintered body.

[0052] The temperature range of ordinary-pressure sintering is preferably 1200° C. or higher and 1550° C. or lower and more preferably 1350° C. or higher and 1450° C. or lower in an atmosphere obtained by introducing oxygen gas into air in a sintering furnace. The sintering time is preferably 10 hours or longer and 30 hours or shorter, and more preferably 15 hours or longer and 25 hours or shorter.

[0053] When the sintering temperature is adjusted in the above range and the oxide powder consisting of an indium oxide powder and a gallium oxide powder and the aluminum oxide powder which are controlled to have a mean particle size of 1.0 μm or less are used as raw material powders, the oxide sintered body is mainly composed of the In.sub.2O.sub.3 phase having a bixbyite-type structure and the GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure or the (Ga, In).sub.2O.sub.3 phase.

[0054] At a sintering temperature lower than 1200° C., the sintering reaction does not proceed well. On the other hand, densification increase with more difficulty at a sintering temperature higher than 1550° C., and the components of the sintering furnace and the oxide sintered body react with each other, which makes it difficult to obtain a desired oxide sintered body. In particular, since the gallium content in the oxide sintered body of the present invention is 0.15 or more in terms of Ga/(In+Ga) atomic ratio, the sintering temperature is preferably set to 1450° C. or lower. This is because formation of the (Ga, In).sub.2O.sub.3 phase is significant in the temperature range around 1500° C. in some cases. There is no problem as long as the amount of the (Ga, In).sub.2O.sub.3 phase is small, but when the amount thereof is large, a decrease in deposition rate and arcing may occur, which is not preferable.

[0055] The temperature elevation rate until the sintering temperature is reached is preferably in the range of 0.2° C. to 5° C./min in order to cause debinding without forming cracks in the sintered body. As long as the temperature elevation rate is this range, the temperature may be increased to the sintering temperature in a combination of different temperature elevation rates as desired. During the temperature elevation process, a particular temperature may be maintained for a certain time in order for debinding and sintering to proceed. After sintering, oxygen introduction is stopped before cooling. The temperature is preferably decreased to 1000° C. at a temperature drop rate in the range of preferably 0.2 to 5° C./min, and particularly 0.2° C./min or more and less than 1° C./min.

(3) Target

[0056] The target of the present invention can be obtained by machining the oxide sintered body of the present invention to a predetermined size. When the oxide sintered body is used as the target, the target can be obtained by further grinding the surface thereof and bonding the oxide sintered body to a backing plate. The target preferably has a flat shape, but may have a cylindrical shape. When a cylindrical target is used, it is preferred to suppress particle generation due to target rotation. In addition, the oxide sintered body is machined, for example, into a circular cylindrical shape to form a tablet, and the tablet can be used for film deposition by a vapor-deposition method or an ion plating method.

[0057] For use as a sputtering target, the density of the oxide sintered body of the present invention is preferably 6.3 g/cm.sup.3 or more and more preferably 6.7 g/cm.sup.3 or more. When the density is less than 6.3 g/cm.sup.3, nodules are formed during use in mass production. For use as a tablet for ion plating, the density of the oxide sintered body is preferably less than 6.3 g/cm.sup.3 and more preferably 3.4 g/cm.sup.3 or more and 5.5 g/cm.sup.3 or less. In this case, the sintering temperature is preferably lower than 1200° C.

(4) Oxide Semiconductor Thin Film and Method for Depositing Oxide Semiconductor Thin Film

[0058] The amorphous oxide semiconductor thin film of the present invention is mainly obtained as follows: first forming an amorphous oxide thin film on a substrate by a sputtering method using the sputtering target; and then subjecting the amorphous oxide thin film to an annealing treatment.

[0059] The sputtering target is formed from the oxide sintered body of the present invention. The structure of the oxide sintered body, that is, the structure composed basically of an In.sub.2O.sub.3 phase having a bixbyite-type structure and a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure or a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase, is important. In order to obtain the amorphous oxide semiconductor thin film of the present invention, it is important that the crystallization temperature of the amorphous oxide semiconductor thin film is high. This is related to the structure of the oxide sintered body. That is, when the oxide sintered body includes not only an In.sub.2O.sub.3 phase having a bixbyite-type structure but also a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure or a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase as in the oxide sintered body of the present invention, the oxide thin film obtained from this oxide sintered body has a high crystallization temperature, namely, a crystallization temperature of 300° C. or higher, more preferably 350° C. or higher, and is a stable amorphous film. In contrast, when the oxide sintered body includes only an In.sub.2O.sub.3 phase having a bixbyite-type structure, the oxide thin film obtained from this oxide sintered body has a low crystallization temperature of about 200° C. and is not amorphous. Incidentally, in this case, microcrystals are already generated after the film deposition so that the amorphous properties are not maintained. Thus, it is difficult to perform patterning by wet etching.

[0060] In the film deposition process of the amorphous oxide semiconductor thin film of the present invention, a general sputtering method is used. In particular, according to a direct current (DC) sputtering method, thermal influence in film deposition is small, and high rate film deposition is possible, which is thus industrially advantageous. To form the oxide semiconductor thin film of the present invention by the direct current sputtering method, a gas mixture of inert gas and oxygen, particularly argon and oxygen, is preferably used as the sputtering gas. Sputtering is preferably performed in a chamber of a sputtering apparatus at an internal pressure of 0.1 Pa or more and 1 Pa or less, particularly, 0.2 Pa or more and 0.8 Pa or less.

[0061] The substrate is typically a glass substrate and is preferably an alkali-free glass. In addition, any resin sheet and resin film that withstands the above process condition can be used.

[0062] In the process for forming the amorphous oxide thin film, presputtering can be performed as follows: for example, after evacuation to 2×10.sup.−4 Pa or less, introducing a gas mixture of argon and oxygen until the gas pressure reaches 0.2 Pa or more and 0.8 Pa or less; and generating a direct current plasma by applying direct current power so that the direct current power with respect to the area of the target, namely, the direct current power density, is in the range of about 1 W/cm.sup.2 or more and 7 W/cm.sup.2 or less. It is preferred that, after this presputtering for 5 minutes or longer and 30 minutes or shorter, the substrate position be corrected as desired and then sputtering be performed.

[0063] In sputter deposition in the film deposition process, the direct current power applied is increased in order to increase the deposition rate.

[0064] The amorphous oxide semiconductor thin film of the present invention is obtained by forming the amorphous oxide thin film and then performing an annealing treatment on the amorphous oxide thin film. As a method until the annealing treatment, for example, an amorphous oxide thin film is first formed at a low temperature, such as at around room temperature, and then an annealing treatment is performed at a temperature lower than the crystallization temperature to obtain an oxide semiconductor thin film while maintaining the amorphous state. As another method, a substrate is heated to a temperature lower than the crystallization temperature, preferably, to 100° C. or higher and 300° C. or lower to form an amorphous oxide semiconductor thin film. Subsequently thereto, an annealing treatment may be further performed. The heating temperature in these two methods is sufficiently about 600° C. or lower and can be set to be equal to or lower than the strain point of an alkali-free glass substrate.

[0065] The amorphous oxide semiconductor thin film of the present invention is obtained by first forming an amorphous oxide thin film and then subjecting the amorphous oxide thin film to an annealing treatment. The condition for the annealing treatment is a temperature lower than the crystallization temperature in an oxidizing atmosphere. The oxidizing atmosphere is preferably an atmosphere containing oxygen, ozone, water vapor, or nitrogen oxides. The temperature for annealing is 200° C. or higher and 600° C. or lower and preferably 300° C. or higher and 500° C. or lower. The time for annealing, i.e. the time during which the amorphous thin film is held at the temperature for annealing, is 1 minute or longer and 120 minutes or shorter and preferably 5 minutes or longer and 60 minutes or shorter.

[0066] The composition of indium, gallium, and aluminum of the amorphous oxide thin film and the amorphous oxide semiconductor thin film substantially corresponds to the composition of the oxide sintered body of the present invention. That is, the amorphous oxide semiconductor thin film contains indium and gallium as oxides and further contains aluminum. The gallium content is 0.15 or more and 0.49 or less in terms of Ga/(In+Ga) atomic ratio and the aluminum content is 0.0001 or more and less than 0.25 in terms of Al/(In+Ga+Al) atomic ratio.

[0067] The amorphous oxide semiconductor thin film of the present invention has a reduced carrier density of 4.0×10.sup.18 cm.sup.−3 or less and a carrier mobility of 10 cm.sup.2 V.sup.−1 sec.sup.−1 or more when the oxide sintered body with the controlled composition and structure as described above is deposited by using a sputtering target or the like and the annealing treatment is performed thereon under the appropriate conditions. The amorphous oxide semiconductor thin film more preferably with a carrier mobility of 15 cm.sup.2 V.sup.−1 sec.sup.−1 or more, particularly preferably with a carrier mobility of 20 cm.sup.2 V.sup.−1 sec.sup.−1 or more is obtained.

[0068] The amorphous oxide semiconductor thin film of the present invention is subjected to micromachining, which is required in applications such as TFTs by wet etching or dry etching. Generally, it is possible to perform micromachining by wet etching after the formation of an amorphous oxide thin film first when a temperature lower than the crystallization temperature, for example, an appropriate substrate temperature in the range of from room temperature to 300° C. is selected. Most weak acids can be used as the etchant, but a weak acid composed mainly of oxalic acid or hydrochloric acid is preferably used. For example, commercial products, such as ITO-06N available from Kanto Chemical Co., Inc., can be used. Dry etching may be selected depending on the configuration of TFT.

[0069] Although the thickness of the amorphous oxide semiconductor thin film of the present invention is not limited, the thickness is 10 nm or more and 500 nm or less, preferably 20 nm or more and 300 nm or less, and more preferably 30 nm or more and 100 nm or less. When the thickness is less than 10 nm, favorable semiconductor characteristics are not obtained, and as a result, high carrier mobility is not achieved. On the other hand, when the thickness is more than 500 nm, it is disadvantageous in that a problem associated with productivity arises.

EXAMPLES

[0070] A more detailed description is provided below by way of Examples of the present invention, but the present invention is not limited by these Examples.

<Evaluation of Oxide Sintered Body>

[0071] The composition of the metal elements in the obtained oxide sintered body was determined by ICP emission spectroscopy. The formed phases were identified by a method using a powder X-ray diffractometer (available from Philips) using rejects of the obtained oxide sintered body. The formed phases were identified by making the obtained oxide sintered body into a thin piece by using a focused ion beam apparatus, and performing observation of crystal grains and electron beam diffraction measurement with a scanning transmission electron microscope (available from Hitachi High-Technologies Corporation). Further, the composition of each crystal grain was determined by energy dispersive X-ray analysis (available from Hitachi High-Technologies Corporation).

<Evaluation of Basic Properties of Oxide Thin Film>

[0072] The composition of the obtained oxide thin film was determined by ICP emission spectrometry. The thickness of the oxide thin film was determined with a surface profilometer (available from KLA-Tencor Corporation). The deposition rate was calculated from the film thickness and the film deposition time. The carrier density and mobility of the oxide thin film were determined with a Hall-effect measurement apparatus (available from TOYO Corporation). The formed phases in the film were identified by X-ray diffraction measurement.

(Production of Oxide Sintered Body and Oxide Thin Film)

[0073] An indium oxide powder, a gallium oxide powder, and an aluminum oxide powder were prepared as raw material powders so that each powder has a mean particle size of 1.0 μm or less. These raw material powders were prepared so as to obtain the Ga/(In+Ga) atomic ratio and the Al/(In+Ga+Al) atomic ratio of Examples and Comparative Examples shown in Table 1 and Table 3. The raw material powders were placed in a resin pot together with water and mixed by wet ball milling. In this case, hard ZrO.sub.2 balls were used, and the mixing time was 18 hours. After mixing, the slurry was taken out, filtered, dried, and granulated. The granulated material was compacted by cold isostatic pressing under a pressure of 3 ton/cm.sup.2.

[0074] Next, the compact was sintered as described below.

[0075] The compact was sintered at a sintering temperature of between 1350 and 1450° C. for 20 hours in an atmosphere obtained by introducing oxygen into air in a sintering furnace at a rate of 5 L/min per 0.1 m.sup.3 furnace volume. At this time, the temperature was increased by 1° C./min, oxygen introduction was stopped during cooling after sintering, and the temperature was decreased to 1000° C. by 1° C./min.

[0076] The composition of the obtained oxide sintered body was analyzed by ICP emission spectrometry. As a result, it was confirmed that the proportion of the metal elements was substantially the same as the composition prepared at the time of mixing raw material powders in all Examples.

[0077] Next, results obtained by performing the phase identification of the oxide sintered body by X-ray diffraction measurement (X-ray diffraction, XRD) and the observation of crystal grains by a scanning transmission electron microscope (STEM) and analyzing the composition of each crystal grain by energy dispersive X-ray spectrometry (EDX) analysis are shown in Table 1.

[0078] In addition, the results obtained by performing X-ray diffraction measurement and phase identification on Example 6 are shown in FIG. 1, the results obtained by performing X-ray diffraction measurement and phase identification on Example 9 are shown in FIG. 2, and the observation result of Example 9 with a scanning transmission electron microscope is shown in FIG. 3. Through observation with a scanning transmission electron microscope, it was confirmed that there are two types of crystal grains, that is, white crystal grains and black crystal grains. Further, an electron beam diffraction photograph of white crystal grains in FIG. 3 is shown in FIG. 4 and an electron beam diffraction photograph of black crystal grains in FIG. 3 is shown in FIG. 5. Incidentally, in FIGS. 4 and 5, the positions of transparent spots and analyzed diffraction spots are represented by “0” and the transparent spots and the diffraction spots are connected by white lines. The plane distance and the plane angle of each crystal grain are obtained from FIGS. 4 and 5 and results obtained by comparing the plane distance and the plane angle with literature values of a JCPDS card are shown in Table 2. From the comparison with a JCPDS card, it is found that the white crystal grains are In.sub.2O.sub.3 phases (JCPDS card number: 00-006-0416) and the black crystal grains are GaInO.sub.3 phases (JCPDS card number: 04-017-1567). Further, from the results of X-ray diffraction measurement and energy dispersive X-ray analysis, it is also found that the white crystal grains are In.sub.2O.sub.3 phases and the black crystal grains are GaInO.sub.3 phases.

[0079] The oxide sintered body was machined to a size of 152 mm in diameter and 5 mm in thickness. The sputtering surface was grinded with a cup grinding wheel so that the maximum height Rz was 3.0 μm or less. The machined oxide sintered body was bonded to an oxygen-free copper backing plate by using metal indium to provide a sputtering target.

[0080] Film deposition by direct current sputtering was performed at a substrate temperature described in Table 3 by using the sputtering targets of Examples and Comparative Examples and an alkali-free glass substrate (Eagle XG available from Corning). The sputtering target was attached to the cathode of a direct current magnetron sputtering apparatus (available from Tokki Corporation) having a direct current power supply with no arcing control function. At this time, the target-substrate (holder) distance was fixed at 60 mm. After evacuation to 2×10.sup.−4 Pa or less, a gas mixture of argon and oxygen was introduced at an appropriate oxygen ratio, which depends on the gallium content and the aluminum content in each target. The gas pressure was set to 0.6 Pa. A direct current plasma was generated by applying a direct current power of 300 W (1.64 W/cm.sup.2). After presputtering for 10 minutes, the substrate was placed directly above the sputtering target, namely, in the stationary opposing position, and an oxide semiconductor thin film having a thickness of 50 nm was formed. At this time, the presence or absence of the occurrence of arcing was confirmed. The composition of the obtained oxide semiconductor thin film was confirmed to be substantially the same as that of the target.

[0081] The deposited oxide semiconductor thin film was subjected to a heat treatment at 300° C. or higher and 500° C. or lower for 30 minutes or longer and 60 minutes or shorter in oxygen as described in Table 3. The crystallinity of the oxide semiconductor thin film after the heat treatment was examined by X-ray diffraction measurement. As a result, the oxide semiconductor thin films of Comparative Examples 1 and 2 were crystallized and the In.sub.2O.sub.3 phase having a bixbyite-type structure was generated. However, Examples and Comparative Examples except Comparative Examples 1 and 2 maintained amorphous properties. For crystallized oxide semiconductor thin films, the crystalline phases in the oxide semiconductor thin films were identified. The Hall-effect measurement was performed on the oxide semiconductor thin films of Examples and Comparative Examples except Comparative Examples 1 and 2 to obtain the carrier density and the carrier mobility. The obtained evaluation results are summarized in Table 3.

[Evaluation]

[0082] From the results of Table 1, when the gallium content was 0.15 or more and 0.49 or less in terms of Ga/(In+Ga) atomic ratio and the aluminum content was 0.0001 or more and less than 0.25 in terms of Al/(In+Ga+Al) atomic ratio, the oxide sintered bodies of Examples 1 to 17 included an In.sub.2O.sub.3 phase having a bixbyite-type structure, and a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure or a GaInO.sub.3 phase having a β-Ga.sub.2O.sub.3-type structure and a (Ga, In).sub.2O.sub.3 phase. On the other hand, in Comparative Examples 1 to 3, the gallium content or aluminum content of the oxide sintered body was smaller than the range of the present invention. For this reason, in Comparative Example 1, the oxide sintered body included only an In.sub.2O.sub.3 phase having a bixbyite-type structure. Further, in Comparative Examples 4 to 8, since the aluminum content was excessive, arcing occurs at the time of sputter deposition and a homogeneous film cannot be obtained. Thus, a desired oxide sintered body of the present invention cannot be obtained.

[0083] From the results of Table 3, the oxide semiconductor thin film was an amorphous oxide semiconductor thin film containing indium, gallium, and aluminum, in which the gallium content is set to 0.15 or more and 0.49 or less in terms of Ga/(In+Ga) atomic ratio, the aluminum content is controlled to 0.0001 or more and less than 0.25 in terms of Al/(In+Ga+Al) atomic ratio.

[0084] It is found that all of the oxide semiconductor thin films of Examples are amorphous. In addition, it is found that the oxide semiconductor thin films of Examples have a carrier density of 4.0×10.sup.18 cm.sup.−3 or less and a carrier mobility of 10 cm.sup.2 V.sup.−1 sec.sup.−1 or more, and particularly, the oxide semiconductor thin films of Examples 6, 7, 9, 11, and 12 in which the gallium content is 0.20 or more and 0.45 or less in terms of Ga/(In+Ga) atomic ratio and the aluminum content is 0.01 or more and 0.20 or less in terms of Al/(In+Ga+Al) atomic ratio have excellent properties in that the carrier density is 6.0×10.sup.17 cm.sup.−3 or less and the carrier mobility is 15 cm.sup.2 V.sup.−1 sec.sup.−1 or more.

[0085] On the other hand, in Comparative Examples 1 and 2, the oxide semiconductor thin films after annealing were crystallized and were not amorphous due to the generation of an In.sub.2O.sub.3 phase having a bixbyite-type structure. It is found that, in Comparative Example 3, the aluminum content in terms of Al/(In+Ga+Al) atomic ratio does not satisfy the range of the present invention, and as a result, the carrier density exceeds 4.0×10.sup.18 cm.sup.−3. Regarding the oxide semiconductor thin films of Comparative Examples 4 to 8, as a result of the aluminum content, arcing occurred so that a homogeneous film was not obtained. Thus, the evaluation of carrier density and carrier mobility was not performed on the oxide semiconductor thin films of Comparative Examples 4 to 8. It is found that, since the Ga/(In+Ga) of the oxide semiconductor of Comparative Example 9 exceeds the upper limit, the carrier mobility is less than 10 cm.sup.2 V.sup.−1 sec.sup.−1.

TABLE-US-00001 TABLE 1 Sintered body EDX In.sub.2O.sub.3 PHASE GalnO.sub.3 PHASE Ga/ Al/(In + Sintering STEM In/(In + Ga/(In + Al/(In + In/(In + Ga/(In + Al/(In + (In + Ga) Ga + Al) temper- Type of Ga + Al) Ga + Al) Ga + Al) Ga + Al) Ga + Al) Ga + Al) Atomic Atomic ature XRD crystal Atomic Atomic Atomic Atomic Atomic Atomic ratio ratio (° C.) Formed phase grain ratio ratio ratio ratio ratio ratio Comparative Example 1 0.15 0 1450 In.sub.2O.sub.3 1 Not measured — — — Comparative Example 2 0.1 0.01 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.89 0.10 0.01 0.48 0.51 0.01 Comparative Example 3 0.15 0.00005 1450 In.sub.2O.sub.3 GalnO.sub.3 2 0.88 0.11 0.01 0.49 0.50 0.01 Example 1 0.15 0.0001 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.88 0.11 0.01 0.47 0.52 0.01 Example 2 0.15 0.01 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.88 0.11 0.01 0.46 0.52 0.02 Example 3 0.15 0.2 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.85 0.12 0.03 0.37 0.51 0.12 Example 4 0.15 0.24 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.86 0.11 0.03 0.37 0.50 0.13 Comparative Example 4 0.15 0.25 1350 In.sub.2O.sub.3 GalnO.sub.3 2 0.86 0.11 0.03 0.36 0.30 0.14 Example 5 0.2 0.0001 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.87 0.12 0.01 0.48 0.51 0.01 Example 6 0.2 0.01 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.87 0.12 0.01 0.47 0.52 0.01 Example 7 0.2 0.2 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.85 0.11 0.04 0.35 0.53 0.12 Example 8 0.2 0.24 1350 In.sub.2O.sub.3 GalnO.sub.3 2 0.84 0.11 0.05 0.35 0.52 0.13 Comparative Example 5 0.2 0.25 1350 In.sub.2O.sub.3 GalnO.sub.3 2 0.83 0.12 0.03 0.35 0.51 0.14 Example 9 0.25 0.05 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.84 0.13 0.03 0.39 0.52 0.09 Example 10 0.45 0.0001 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.85 0.14 0.01 0.48 0.51 0.01 Example 11 0.45 0.01 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.85 0.14 0,01 0.47 0.52 0.01 Example 12 0.45 0.2 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.85 0.14 0.01 0.35 0.53 0.12 Example 13 0.45 0.24 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.82 0.14 0.04 0.33 0.54 0.13 Comparative Example 6 0.45 0.25 1350 In.sub.2O.sub.3 GalnO.sub.3 2 0.82 0.14 0.04 0.35 0.51 0.14 Comparative Example 7 0.45 0.25 1350 In.sub.2O.sub.3 GalnO.sub.3 2 0.82 0.14 0.04 0.34 0.53 0.13 Example 14 0.49 0.0001 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.85 0.14 0.01 0.46 0.53 0.01 Example 15 0.49 0.01 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.85 0.14 0.01 0.43 0.54 0.01 Example 16 0.49 0.2 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.83 0.14 0.03 0.45 0.52 0.03 Example 17 0.49 0.24 1400 In.sub.2O.sub.3 GalnO.sub.3 2 0.83 0.14 0.03 0.36 0.51 0.13 Comparative Example 8 0.5 0.25 1350 In.sub.2O.sub.3 GalnO.sub.3 2 0.83 0.14 0.03 0.35 0.53 0.12

TABLE-US-00002 TABLE 2 Plane distance (nm) Plane angle (°) JCPDS JCPDS In.sub.2O.sub.3 GalnO.sub.3 In.sub.2O.sub.3 GalnO.sub.3 Measure- Plane Measure- Card Card Measure- Card Card ment orientation ment number number ment number number place h k l value 00-006-0416 04-017-1567 Measurement place value 00-006-0416 04-017-1567 White 1 4 4 0 0.18 0.179 — Angle formed by 1 and 2 63.9 64.8 — crystal 2 2 6 2 0.15 0.1453 — Angle formed by 2 and 3 51.1 50.5 — grains 3 6 2 2 0.15 0.153 — Black 1 3 1 3 0.14 — 0.145 Angle formed by 1 and 2 29.2 — 29.6 crystal 2 2 2 5 0.09 — 0.096 Angle formed by 2 and 3 36.6 — 37.1 grains 3 5 1 2 0.17 — 0.178

TABLE-US-00003 TABLE 3 Heat treat- Crystal Ga/ Al/(In + ment Film structure (In + Ga) Ga + Al) Substrate temper- thick- of Carrier Arcing Atomic Atomic temperature ature ness thin density (cm.sup.2/ occur- ratio ratio (° C.) (° C.) (nm) film (×10.sup.17 cm.sup.−3) V .Math. s) rence Comparative Example 1 0.15 0 Room temperature 300 50 In.sub.2O.sub.3 Not measured Not measured No Comparative Example 2 0.1 0.01 Room temperature 300 50 In.sub.2O.sub.3 Not measured Not measured No Comparative Example 3 0.15 0.00005 Room temperature 300 50 Amorphous 87 23.5 No Example 1 0.15 0.0001 Room temperature 300 50 Amorphous 13.8 21.5 No Example 2 0.15 0.01 Room temperature 300 50 Amorphous 7.6 18.9 No Example 3 0.15 0.2 Room temperature 300 Not measured Amorphous 1.0 15.2 No Example 4 0.15 0.24 Room temperature 300 50 Amorphous 0.8 14.2 No Comparative Example 4 0.15 0.25 Room temperature 300 50 Amorphous 0.7 9.8 Yes Example 5 0.2 0.0001 Room temperature 350 50 Amorphous 10.4 16.1 No Example 6 0.2 0.01 Room temperature 350 50 Amorphous 5.7 16.2 No Example 7 0.2 0.2 Room temperature 350 50 Amorphous 0.8 15.9 No Example 8 0.2 0.24 Room temperature 350 50 Amorphous 0.6 10.7 No Comparative Example 5 0.2 0.25 Room temperature 350 50 Amorphous 0.5 7.4 Yes Example 9 0.25 0.05 Room temperature 400 50 Amorphous 3.8 15.1 No Example 10 0.45 0.0001 Room temperature 500 50 Amorphous 0.3 11.7 No Example 11 0.45 0.01 Room temperature 500 50 Amorphous 0.2 15.1 No Example 12 0.45 0.2 Room temperature 500 50 Amorphous 0.1 15.4 No Example 13 0.45 0.24 Room temperature 500 50 Amorphous 0.1 10.8 No Comparative Example 6 0.45 0.25 Room temperature 500 50 Amorphous 0.1 7.9 Yes Comparative Example 7 0.45 0.25 200 400 50 Amorphous 0.1 7.8 Yes Example 14 0.49 0.0001 200 400 50 Amorphous 0.3 11.0 No Example 15 0.49 0.01 200 400 50 Amorphous 0.2 10.5 No Example 16 0.49 0.2 200 400 50 Amorphous 0.1 10.3 No Example 17 0.49 0.24 200 400 50 Amorphous 0.1 10.2 No Comparative Example 8 0.5 0.25 200 400 50 Amorphous 0.1 7.5 Yes