Abstract:
To provide an oxide semiconductor with a novel structure. Such an oxide semiconductor is composed of an aggregation of a plurality of InGaZnO 4  crystals each of which is larger than or equal to 1 nm and smaller than or equal to 3 nm, and in the oxide semiconductor, the plurality of InGaZnO 4  crystals have no orientation. Alternatively, such an oxide semiconductor is such that a diffraction pattern like a halo pattern is observed by electron diffraction measurement performed by using an electron beam with a probe diameter larger than or equal to 300 nm, and that a diffraction pattern having a plurality of spots arranged circularly is observed by electron diffraction measurement performed by using an electron beam with a probe diameter larger than or equal to 1 nm and smaller than or equal to 30 nm.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oxide semiconductor. 
     2. Description of the Related Art 
     The research on a structure of In—Ga—Zn oxide started with the synthesis of the 
     In—Ga—Zn oxide by Kimizuka, Nakamura, et al. from National Institute for Research in Inorganic Materials. They reported that an In—Ga—Zn oxide had a homologous structure and was represented by a composition formula, InGaO 3 (ZnO) m  (m is a natural number) (see Patent Document 1 to Patent Document 6, and Non-Patent Document 1 to Non-Patent Document 4). 
     After that, a report about a transistor including amorphous In—Ga—Zn oxide was released (see Non-Patent Document 5). Since then, various research institutions and companies have actively researched and developed applied technology of In—Ga—Zn oxide. 
     Patent Documents 
     
         
         [Patent Document 1] Japanese Published Patent Application No. S63-210022 
         [Patent Document 2] Japanese Published Patent Application No. S63-210023 
         [Patent Document 3] Japanese Published Patent Application No. S63-210024 
         [Patent Document 4] Japanese Published Patent Application No. S63-215519 
         [Patent Document 5] Japanese Published Patent Application No. S63-239117 
         [Patent Document 6] Japanese Published Patent Application No. S63-265818 
       
    
     Non-Patent Documents 
     
         
         [Non-Patent Document 1] N. Kimizuka, and T. Mohri,  J. Solid State Chem.,  vol. 60, 1985, pp. 382-384 
         [Non-Patent Document 2] N. Kimizuka, et al.,  J. Solid State Chem.,  vol. 116, 1995, pp. 170-178 
         [Non-Patent Document 3] M. Nakamura,  NIRIM NEWSLETTER,  vol. 150, 1995, pp. 1-4 
         [Non-Patent Document 4] M. Nakamura, et al.,  J. Solid State Chem.,  vol. 93, 1991, pp. 298-315 
         [Non-Patent Document 5] K. Nomura, et al.,  Nature,  vol. 432, 2004, pp. 488-492 
       
    
     SUMMARY OF THE INVENTION 
     An object is to provide an oxide semiconductor with a novel structure. 
     An oxide semiconductor according to one embodiment of the present invention is composed of aggregation of a plurality of crystals of InGaZnO 4  (also referred to as InGaZnO 4  crystals) each of which is larger than or equal to 1 nm and smaller than or equal to 3 nm The plurality of InGaZnO 4  crystals have no orientation. 
     Further, an oxide semiconductor according to one embodiment of the present invention shows a diffraction pattern, like a halo pattern (diffraction pattern having no spot), that is observed when the oxide semiconductor is subjected to selected-area electron diffraction measurement with a probe diameter setting to larger than or equal to 300 nm. The oxide semiconductor shows a diffraction pattern having a plurality of spots circularly arranged when the oxide semiconductor is observed by electron diffraction measurement with a probe diameter setting to larger than or equal to 1 nm and smaller than or equal to 30 nm. 
     An oxide semiconductor having the following structure is referred to as a nanocrystalline oxide semiconductor (nc-OS). It is composed of aggregation of a plurality of crystals each of which is larger than or equal to 1 nm and smaller than or equal to 3 nm (the crystal is also referred to as nanocrystal (nc)), and the plurality of crystals have no orientation. 
     Although an nc-OS is sometimes difficult to distinguish from an amorphous oxide semiconductor, the nc-OS has characteristics described below and thus can be regarded as an oxide semiconductor having a novel structure. 
     Just for reference, an amorphous oxide semiconductor is described. 
     The amorphous oxide semiconductor is such an oxide semiconductor having disordered atomic arrangement and no crystal part. For example, the amorphous oxide semiconductor does not have a specific state as in quartz. 
     In an image obtained with a transmission electron microscope (TEM) (also referred to as a TEM image), a crystal part in the amorphous oxide semiconductor cannot be found. 
     When the amorphous oxide semiconductor is subjected to structural analysis by an out-of-plane method with an XRD apparatus, a peak which shows a crystal plane does not appear. A halo pattern is observed in an electron diffraction pattern of the amorphous oxide semiconductor. 
     The amorphous oxide semiconductor contains impurities such as hydrogen at a high concentration. In addition, the amorphous oxide semiconductor is an oxide semiconductor with a high density of defect states. 
     The oxide semiconductor having a high impurity concentration and a high density of defect states has many carrier traps or many carrier generation sources. 
     Next, an nc-OS is described. 
     The nc-OS has a periodic atomic order in a microscopic region. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS. Accordingly, there is no orientation on the whole nc-OS. Thus, in some cases, the nc-OS cannot be distinguished from an amorphous oxide semiconductor depending on an analysis method. For example, in a TEM image of the nc-OS, a crystal cannot occasionally be observed clearly, which is similar to the case of an amorphous oxide semiconductor. Further, in a TEM image of the nc-OS, a boundary between crystals, i.e., grain boundary, cannot be clearly observed occasionally. Furthermore, when the nc-OS is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a larger diameter than a diameter of a crystal part, a peak indicating a crystal plane does not appear, which is similar to the case of an amorphous oxide semiconductor. 
     When the nc-OS is subjected to electron diffraction measurement using an electron beam with a probe diameter (e.g., 50 nm or larger) larger than a diameter of a crystal part, a halo pattern is occasionally observed. Meanwhile, when electron diffraction measurement is performed on the nc-OS by using an electron beam with a probe diameter (e.g., larger than or equal to 1 nm and smaller than or equal to 30 nm) that is almost equivalent to or smaller than a diameter of a crystal part (the diffraction measurement is also referred to as nanobeam electron diffraction measurement), spots can be observed. Further, in a nanobeam electron diffraction pattern of the nc-OS, a region with high luminance in a circular (ring) shape is shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots are observed in a ring-like region in some cases. 
     Thus, the nc-OS is an oxide semiconductor that has high regularity as compared to an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an amorphous oxide semiconductor. 
     An object is to provide an oxide semiconductor with a novel structure that can be used for a transistor or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show a planar TEM image and electron diffraction patterns of an In—Ga—Zn oxide including nanocrystal. 
         FIG. 2  shows a nanobeam electron diffraction pattern of a quartz substrate. 
         FIG. 3  shows nanobeam electron diffraction patterns of an In—Ga—Zn oxide including nanocrystal. 
         FIGS. 4A to 4C  show structural analysis results of an In—Ga—Zn oxide including nanocrystal. 
         FIG. 5  shows structural analysis results of In—Ga—Zn oxides including nanocrystal. 
         FIG. 6  shows structural models used for calculation. 
         FIG. 7  shows structural analysis results obtained by calculation of crystalline InGaZnO 4 . 
         FIG. 8  shows structural analysis results obtained by calculation of amorphous InGaZnO 4 . 
         FIG. 9  shows half widths at half maximum of first peaks of electron diffraction luminance profiles. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will be described below in detail with reference to drawings. 
       FIG. 1A  is a TEM image of an In—Ga—Zn oxide including nanocrystal, as an example of an nc-OS, observed from a planar surface side (the image is also referred to as planar TEM image). 
     The TEM image was observed with use of a Hitachi H-9000NAR transmission electron microscope by setting the accelerating voltage to 300 kV and the camera length to 500 mm As a shooting medium, a film was used. 
     It is difficult to clearly observe crystallinity of the In—Ga—Zn oxide including nanocrystal (also referred to as nanocrystalline In—Ga—Zn oxide) in  FIG. 1A . 
       FIG. 1B  shows electron diffraction patterns. One of them is an electron diffraction pattern which was observed when an electron beam with a probe diameter of 300 nm was incident on a cross section of Sample A that was the nanocrystalline In—Ga—Zn oxide thinned to approximately 50 nm (such electron diffraction is also referred to as selected area electron diffraction (SAED)). The other electron diffraction patterns were obtained by measuring nanobeam electron diffraction patterns of Sample A under conditions where probe diameters of electron beams were 30 nm, 20 nm, 10 nm, and 1 nm. 
     For measurement of selected area electron diffraction, a Hitachi H-9000NAR transmission electron microscope was used under conditions where the accelerating voltage was 300 kV and the camera length was 500 mm. Further, for measurement of nanobeam electron diffraction, a Hitachi HF-2000 field-emission transmission electron microscope was used under conditions where the accelerating voltage was 200 kV and the camera length was 400 mm. As a shooting medium, a film was used. 
     According to  FIG. 1B , in the case of the selected area electron diffraction (probe diameter of 300 nm) of Sample A, a spot is not clearly observed, and an electron diffraction pattern like a blur halo pattern is observed. On the other hand, in the case of the nanobeam electron diffraction (probe diameters of 30 nm, 20 nm, 10 nm, and 1 nm) of Sample A, electron diffraction patterns having spots are observed. The number of spots is increased as the probe diameter is reduced. 
     For comparison, nanobeam electron diffraction of quartz in an amorphous state was observed using a nanobeam with a probe diameter of 1 nm As a result, an electron diffraction pattern that is a halo pattern shown in  FIG. 2  was observed. Thus, the fact of the electron diffraction pattern having spots observed by the nanobeam electron diffraction measurement is one of proofs that Sample A is an aggregation of nanocrystals. 
     Furthermore, for more detailed structural analysis, nanobeam electron diffraction was measured in the following manner: an electron beam with a probe diameter of 1 nm was incident on a cross section of Sample B of a nanocrystalline In—Ga—Zn oxide thinned to several nanometers (approximately 5 nm or less). As a result, electron diffraction patterns having spots which indicate crystallinity and are shown in  FIG. 3  were observed in four different portions. Note that as a shooting medium, films were used. 
     According to  FIG. 3 , diffraction patterns showing crystallinity were obtained from Sample B, but orientation along a crystal plane in a specific direction was not observed. 
     As described above, though an nc-OS is not distinguished from an amorphous oxide semiconductor in some cases depending on an analysis method, an exact analysis makes it possible to distinguish the nc-OS and the amorphous oxide semiconductor. Further, it is found that a microscopic region in the nc-OS has a periodic atomic order. Thus, the nc-OS is an oxide semiconductor that has high regularity as compared to an amorphous oxide semiconductor. Therefore, the nc-OS is likely to have a lower density of defect states than an amorphous oxide semiconductor. 
     EXAMPLE 1 
     In this example, detailed structural analysis of an In—Ga—Zn oxide was conducted with calculation. 
     First, a nanobeam electron diffraction pattern of a nanocrystalline In—Ga—Zn oxide was obtained. 
       FIG. 4A  shows an electron diffraction pattern that was obtained in the following manner: an electron beam with a probe diameter of  1  nm was incident on a cross section of Sample 1 of a nanocrystalline In—Ga—Zn oxide thinned to approximately 50 nm. Note that as a shooting medium, an imaging plate was used. 
       FIG. 4B  shows an electron diffraction pattern that was averaged in the uniform magnitude of a scattering vector |q| rotating the electron diffraction pattern shown in  FIG. 4A  with a center of the pattern as an axis.  FIG. 4C  is a graph showing an electron diffraction luminance profile along a dashed-dotted line shown in  FIG. 4B , where the horizontal axis indicates the magnitude of scattering vector |q|[nm −1 ], and the vertical axis indicates the diffraction intensity [arbitrary unit]. Note that in  FIG. 4C , a transmitted wave in the vicinity of the center in  FIG. 4B  is not taken into consideration for easy understanding. 
     In addition, a nanobeam with a probe diameter of 1 nm was incident on cross sections of Sample 2 to Sample 7 each of which was a nanocrystalline In—Ga—Zn oxide thinned to approximately 50 nm, whereby nanobeam electron diffraction patterns were obtained. Then, the obtained electron diffraction patterns were averaged in the uniform magnitude of a scattering vector |q| by rotating the diffraction patterns with centers thereof as an axis. Profiles of electron diffraction luminance are shown in graphs where the horizontal axes indicate the magnitude of scattering vector |q|[nm −1 ] and the vertical axes indicate the diffraction intensity [arbitrary unit] (see  FIG. 5 ), like the case of Sample 1. 
     Next, as for InGaZnO 4  in a crystal state (crystalline InGaZnO 4 ) and InGaZnO 4  in an amorphous state (amorphous InGaZnO 4 ), calculation was performed. Then, graphs showing the calculation results of crystalline InGaZnO 4  and amorphous InGaZnO 4  were made (see  FIG. 7  and  FIG. 8 ). In the graphs, the horizontal axes indicate the magnitude of scattering vector |q|[nm −1 ] and the vertical axes indicate the diffraction intensity [arbitrary unit]. 
     For the calculation, TEM simulation software jems was used. The calculation mode was set to a mode for calculating powder patterns, and as the fitting function, Gaussian function was used. As the calculation conditions, the accelerating voltage was set to 200 kV, and the camera length was set to 400 mm. 
     For the calculation, InGaZnO 4  structure models shown in  FIG. 6  were used. Note that the structure model of crystalline InGaZnO 4  was obtained from Inorganic Material Database of National Institute for Materials Science (AtomWork, http://crystdb.nims.go.jp). The structure model of amorphous InGaZnO 4  was made by a melt-quench method in classical molecular dynamics calculation. As software for the classical molecular dynamics calculation, “SCIGRESS ME 2.0” was used, and for potential, Born-Mayer-Huggins potential was used. 
     In the calculation, structure factors in each plane (hkl) of the structure models were determined, and the diffraction position and the diffraction intensity were calculated. A shape of a diffraction peak of each plane (hkl) was calculated by fitting using Gaussian function. Note that the sample shape was isotropic powder. The powder size generally relates to the half width at half maximum (HWHM) of the diffraction peak. 
       FIG. 7  shows profiles of electron diffraction luminance of crystalline InGaZnO 4 , which were obtained by the calculation. According to  FIG. 7 , besides a first peak, a plurality of peaks are observed in crystalline InGaZnO 4 . In addition, as the powder size is increased, the width of the first peak becomes narrow. 
       FIG. 8  shows profiles of electron diffraction luminance of amorphous InGaZnO 4 , which were obtained by the calculation. According to  FIG. 8 , only a first peak is clearly observed, and it was difficult to distinguish another peak from the other parts of profile. Note that the case where the powder size is larger than 3.0 nm is not shown because the powder is larger than a cell size of the model. 
     Next, the half widths at half maximum of the first peaks (H 1 ) obtained from the calculation results of the crystalline InGaZnO 4  and amorphous InGaZnO 4  and the half widths at half maximum of the actual measured first peaks (H 1 ) of samples of nanocrystalline In—Ga—Zn oxides (Sample 1 to Sample 7) were compared. The comparison results are shown in  FIG. 9 . 
     According to  FIG. 9 , each calculation value of half widths at half maximum of the first peaks (H 1 ) of amorphous InGaZnO 4  is approximately 1.0 [nm −1 ]. In the case of amorphous InGaZnO 4 , the half widths at half maximum of the first peaks (H 1 ) were equivalent to each other regardless of the powder sizes. 
     According to  FIG. 9 , each calculation value of half widths at half maximum of the first peaks (H 1 ) of crystalline InGaZnO 4  is about in a range of 0.3 [nm −1 ] to 0.6 [nm −1 ]. In the case of crystalline InGaZnO 4 , as the powder size is increased, the periodicity of atomic arrangement becomes high. Thus, the larger that powder size is, the narrower the half widths at half maximum of the first peaks (H 1 ) are. 
     Further, according to  FIG. 9 , each actual measurement value of half widths at half maximum of the first peaks (H 1 ) of nanocrystalline In—Ga—Zn oxides is about in a range of 0.4 [nm −1 ] to 0.6 [nm −1 ]. Thus, it is found that the half width at half maximum of the first peak (H 1 ) of nanocrystalline In—Ga—Zn oxide is narrower than that of the amorphous InGaZnO 4  and is almost equivalent to that of the crystalline InGaZnO 4 . 
     Moreover, the half width at half maximum of the first peak (H 1 ) was compared between the nanocrystalline In—Ga—Zn oxide and the crystalline InGaZnO 4 . The comparison result indicated that the size of nanocrystal was about in a range from 1 nm to 3 nm. 
     This application is based on Japanese Patent Application serial no. 2013-056952 filed with Japan Patent Office on Mar. 19, 2013, the entire contents of which are hereby incorporated by reference.