Oxide semiconductor films and thin-film transistors

Abstract

This disclosure aims to provide a crystalline oxide semiconductor film with high carrier mobility. [Solution] An oxide semiconductor film according to one aspect of the present disclosure is an oxide semiconductor film disposed on a substrate, containing In as an element, having a grain boundary line density of 27 / μm or less, and the grain boundary line density is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries.

Description

[Technical Field]

[0001] This disclosure relates to oxide semiconductor films and thin-film transistors. [Background technology]

[0002] In recent years, flat panel displays such as liquid crystal displays (LCDs) and organic electro-luminescence displays (OLEDs) have been widely used in smartphones, tablets, laptop computers, monitors, and televisions. Thin-film transistors (TFTs) are used to drive each pixel in flat panel displays.

[0003] Conventionally, amorphous oxide semiconductor films have been widely used as oxide semiconductor films in thin-film transistors. However, in oxide semiconductor films with an amorphous structure, carriers are formed by electrons generated by oxygen vacancies, which tends to result in high carrier density. Therefore, when amorphous oxide semiconductor films are used in thin-film transistors with small channel lengths and widths, the electrical properties may not be stable.

[0004] From this perspective, crystalline oxide semiconductor films, which tend to have lower carrier densities compared to amorphous oxide semiconductor films, are attracting attention today (see Patent Documents 1-3). However, crystalline oxide semiconductor films have the challenge of being difficult to achieve high-speed carrier mobility. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2017 / 017966 [Patent Document 2] International Publication No. 2018 / 143073 [Patent Document 3] Japanese Patent Publication No. 2012-253315 [Overview of the project] [Problems that the invention aims to solve]

[0006] Patent Document 1 describes a crystalline oxide semiconductor film mainly composed of indium oxide, formed by deposition and heating without introducing impurities such as water, and containing surface crystal grains having a single crystal orientation. Patent Document 1 also describes including a trivalent metal element other than indium in a concentration of more than 8 atomic percent and up to 17 atomic percent relative to the total metal content.

[0007] Patent Document 2 describes a crystalline oxide semiconductor film mainly composed of indium oxide, containing surface crystal grains having a single crystal orientation, and having a band gap of 3.90 eV or more. Patent Document 2 describes forming a protective film on an oxide semiconductor film without performing heat treatment in an oxidizing atmosphere on the oxide semiconductor film. Patent Document 2 describes that by not performing heat treatment before forming the protective film, the carrier density of the oxide semiconductor film can be increased compared to when heat treatment is performed, and the band gap can be made 3.90 eV or more. Furthermore, Patent Document 2 describes that it is preferable to include trivalent metal elements other than indium in amounts of more than 7 atomic percent and 15 atomic percent or less relative to the total metal content.

[0008] Patent Document 3 describes a crystalline oxide layer laminated on an insulating layer, wherein the carrier density of the oxide layer is 10 18 / cm 3The following describes a laminated structure in which the average crystal grain size is 1 μm or more, and the crystals of the oxide layer are arranged columnarly on the surface of the insulating layer. Patent Document 3 describes that by forming an amorphous oxide thin film on an insulating layer and then heating and crystallizing it, the direction of crystal arrangement and crystal grain size can be made uniform. Patent Document 3 describes that the average crystal grain size in the oxide layer is determined by performing azimuthal angle mapping of EBSD and analyzing the region surrounded by components with an azimuthal difference of 15° or more as crystal grains. Furthermore, Patent Document 3 describes that indium oxide, Ga-doped indium oxide, Al-doped indium oxide, Zn-doped indium oxide, and Sn-doped indium oxide are preferred as materials constituting the oxide layer.

[0009] Patent documents 1 and 2 describe how carrier density can be stabilized by including surface crystal particles having a single crystal orientation. Furthermore, patent document 3 describes how field-effect mobility can be improved and TFTs with good S values ​​can be reproducibly formed by controlling the crystal arrangement direction and particle size.

[0010] Meanwhile, the inventors diligently studied how to achieve high-speed carrier mobility in crystalline oxide semiconductor films. To improve carrier mobility in crystalline oxide semiconductor films, it is important to reduce grain boundaries that act as barriers to electron movement and to create crystals with low electron scattering factors. The inventors found that, considering that electrons move along an electric field in a semiconductor, it is important to reduce the density of boundaries (including unclosed grain boundaries) per unit area than the size of the crystal grains (the area of ​​the crystal surrounded by closed grain boundaries), and that it is important to define boundaries with an orientation difference of 2° or more as grain boundaries using EBSD analysis. Patent document 3 describes performing EBSD azimuth mapping and analyzing the region surrounded by components with an orientation difference of 15° or more as crystal grains. However, the crystal grains described in reference document 3 do not represent the density of boundaries per unit area. Furthermore, according to the inventors' research results, it was found that even if the orientation difference is less than 15°, boundaries with an orientation difference of 2° or more have an adverse effect on carrier mobility.

[0011] This disclosure is made in light of these circumstances and aims to provide a crystalline oxide semiconductor film with high carrier mobility. [Means for solving the problem]

[0012] An oxide semiconductor film according to one aspect of this disclosure is an oxide semiconductor film disposed on a substrate, containing In as an element, having a grain boundary line density of 27 / μm or less, and the grain boundary line density is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries. [Effects of the Invention]

[0013] An oxide semiconductor film according to one aspect of this disclosure can increase carrier mobility. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a schematic cross-sectional view showing a thin-film transistor according to one embodiment of the present disclosure. [Figure 2] Figure 2 is a graph showing the relationship between the grain boundary line density analyzed by EBSD analysis and the carrier mobility. [Figure 3] Figure 3 is a graph showing the measurement results of the static characteristics (Id-Vg characteristics) of the thin film transistor of No.1. [Figure 4] Figure 4 is a SEM secondary electron image of the oxide semiconductor films of No.1, No.3, No.4, and No.6. [Figure 5] Figure 5 is a SEM secondary electron image of the oxide semiconductor films of No.1, No.7, and No.8. [Figure 6] Figure 6 is an example of the measurement results of the grain boundary line density by EBSD analysis in No.1. [Figure 7] Figure 7 is an example of the measurement results of the grain boundary line density using the SEM secondary electron image in No.1 (left: secondary electron image (original image), right: image analysis result).

Mode for Carrying Out the Invention

[0015] [Explanation of Embodiments of the Present Disclosure] First, the embodiments of the present disclosure will be listed and explained.

[0016] (1) The oxide semiconductor film according to one aspect of the present disclosure is an oxide semiconductor film disposed on a substrate, containing In as an element, having a grain boundary line density of 27 / μm or less, and the grain boundary line density is a value obtained by determining a boundary having an orientation difference of 2° or more as a grain boundary by EBSD analysis.

[0017] According to the findings of the present inventors, the carrier mobility in a crystalline oxide semiconductor film correlates with the grain boundary line density obtained by determining a boundary having an orientation difference of 2° or more as a grain boundary by EBSD (Electron BackScatter Diffraction Pattern) analysis. The oxide semiconductor film contains In as an element and has a grain boundary line density of 27 / μm or less, and by the fact that the grain boundary line density is a value obtained by determining a boundary having an orientation difference of 2° or more as a grain boundary by EBSD analysis, the carrier mobility can be increased.

[0018] (2) In the above (1), the oxide semiconductor film has a carrier surface density of 6 × 10 12 cm -2 The following configuration is preferable. This configuration allows for stable driving of thin-film transistors.

[0019] (3) In (1) or (2) above, the oxide semiconductor film is preferably of average thickness of 10 nm or more and 45 nm or less. With this configuration, the grain boundary line density can be easily controlled within a desired range. Furthermore, with this configuration, it is easy to reduce the carrier surface density of the oxide semiconductor film.

[0020] (4) A thin-film transistor according to another aspect of the present disclosure comprises an oxide semiconductor film as described in any of (1) to (3) above.

[0021] Because the thin-film transistor is equipped with the oxide semiconductor film, it has high carrier mobility.

[0022] In this disclosure, "grain boundary line density" means a value measured by either (1) or (2) below. (1) Perform EBSD analysis, remove unmeasurable points and measurement points with low data reliability, derive adjacent analysis points with an orientation difference of 2° or more from the obtained measurement points, and sum the lengths of the edges shared by the analysis points (here, "edge" includes not only closed boundaries (boundaries that can be traced around) but also open boundaries (boundaries that cannot be traced around)). Then, calculate Σ(length of edges shared by analysis points with an orientation difference of 2° or more) / Σ(analysis points), and the obtained value is taken as the grain boundary line density. (2) Set observation conditions so that contrast due to differences in crystal orientation (channeling contrast) can be obtained in the SEM (Scanning Electron Microscope) reflection image, and then acquire the SEM reflection image. The obtained image is color-coded in black and white based on Otsu's binarization method. The number of pixels corresponding to the white edge and black edge of the painted boundary is measured, and the total number of these pixels is divided by 2 to obtain the boundary pixel count. The length density is obtained by dividing the obtained boundary pixel count by the total number of pixels in the image. This length density correlates with the grain boundary line density obtained in (1) and can be converted back to the grain boundary line density in (1) by linear approximation (see Figures 6 and 7). "Average thickness" means the average value of the thickness of any 5 points.

[0023] [Details of the embodiments of this disclosure] The embodiments of this disclosure will be described in detail below, with reference to the drawings as appropriate. It should be noted that, regarding the numerical values ​​described herein, only one of the upper or lower limits may be adopted, or the upper and lower limits may be combined in any way. Furthermore, Figure 1 is schematic and may not correspond to the actual dimensions, shape, etc.

[0024] <Oxide semiconductor film> An oxide semiconductor film according to one aspect of this disclosure is disposed on a substrate. More specifically, the oxide semiconductor film is disposed on a substrate in a thin-film transistor. The oxide semiconductor film is polycrystalline. The oxide semiconductor film contains indium (In) as an element. The oxide semiconductor film has a grain boundary line density of 27 / μm or less, and the above grain boundary line density is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries. Note that "disposed on a substrate" includes not only configurations in which the film is directly disposed on the substrate, but also configurations in which it is disposed on the substrate via other layers (configurations in which it is indirectly disposed on the substrate).

[0025] To improve carrier mobility in crystalline oxide semiconductor films, it is desirable to suppress electron scattering within the oxide semiconductor film. Specifically, it is desirable to suppress scattering within crystal grains, such as phonon scattering, scattering due to defects such as oxygen vacancies (defect scattering), and scattering due to impurities such as dopants (impurity scattering), as well as scattering at crystal grain boundaries.

[0026] In this regard, Patent Documents 1 and 2 state that it is preferable to include 7 atomic percent or more of Al (aluminum), Ga (gallium), etc., in order to reduce scattering at grain boundaries. However, it is anticipated that adding large amounts of other elements in this way will increase the likelihood of impurity scattering. Furthermore, Patent Document 3 describes a technique for determining the average grain size using EBSD analysis, where boundaries with an orientation difference of 15° or more are considered grain boundaries. However, it is difficult to sufficiently increase carrier mobility using this technique.

[0027] Taking the above-mentioned disadvantages into consideration, the inventors diligently studied how to increase the proportion of In in the composition while simultaneously improving the carrier mobility.

[0028] The oxide semiconductor film in question contains In as an element, has a grain boundary line density of 27 / μm or less, and this grain boundary line density is determined by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries, thereby increasing carrier mobility. More specifically, even if the area of ​​the crystal grains is the same, the closer the shape of the crystal grain is to a circle, the shorter the length of the grain boundary will be, and the further the shape deviates from a circle, the longer the length of the grain boundary will be. Furthermore, since electrons involved in conduction are pulled by the electric field, it is presumed that they cannot bypass boundaries (finite line segments) with different orientations. From this perspective, it is considered that carrier mobility can be increased by setting the grain boundary line density, determined by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries, to 27 / μm or less. In addition, since the oxide semiconductor film is polycrystalline, it is easy to keep the carrier density low.

[0029] (In) In (in) is an element that contributes to improving electrical conductivity. The higher the In content, the better the conductivity and carrier mobility of the oxide semiconductor film. The lower limit of the In content in all metal elements may be 90 atomic%, 92 atomic%, or 95 atomic%, from the viewpoint of sufficiently increasing carrier mobility. The upper limit of the In content in all metal elements may be 100 atomic%, or 98 atomic%,.

[0030] In this oxide semiconductor film, elements other than In may be O (oxygen) and unavoidable impurities. This configuration allows for the maximum utilization of the high carrier mobility based on In2O3.

[0031] (Inevitable impurities) The above-mentioned unavoidable impurities may be present due to raw materials, materials, manufacturing equipment, etc. Examples of these unavoidable impurities include Al, Pb, Si, Fe, Ni, Ti, Mg, Cr, Zr, W, Ta, and rare earth elements. The content of unavoidable impurities in the oxide semiconductor film is preferably 1% by mass or less for each element, and more preferably 500 ppm by mass or less.

[0032] (Other elements) On the other hand, the oxide semiconductor film may also actively contain elements other than In (hereinafter also referred to as "other elements") within a certain range. Examples of these other elements include B (boron), Fe (iron), Al, and Ga. Other elements that may also be included are Zn, Mg, La, Nb, Hf, Gd, Dy, Ce, Nd, Ti, V, Mo, Mn, Co, Ni, Si, Bi, Ge, Zr, Ta, W, Ag, and Ru. The content of each of these elements in the total metallic elements can be set within a range that does not result in an insufficient In content, and the upper limit may be, for example, 7 atomic percent, 5 atomic percent, or 4.5 atomic percent.

[0033] As mentioned above, the oxide semiconductor film is polycrystalline. Being polycrystalline allows for a low carrier density. More specifically, indium oxide is known to readily increase in carriers due to oxygen vacancies, hydrogen, etc. If the carrier density increases too much, the oxide semiconductor film behaves like a conductor, preventing the transistor from remaining in the OFF state. In this regard, crystallizing the indium oxide is thought to reduce the likelihood of vacancies forming within the film. As a result, it is possible to suppress excessive increases in carrier density, enabling stable transistor switching.

[0034] The upper limit of the carrier surface density of the oxide semiconductor film is set at 1 × 10⁻¹⁶, from the viewpoint of enabling the oxide semiconductor film to function well as a switching element in a thin-film transistor. 13 cm -2 Preferably, 6 × 10 12 cm -2is more preferable. In order to drive the thin film transistor, it is necessary to deplete the channel region in the OFF state. In order to deplete it, since it is necessary to push out the carriers contained in the channel region by an electric field, it is presumed that the absolute amount of carriers (carrier surface density) affects. Therefore, by setting the carrier surface density to be not more than the above upper limit, the thin film transistor can be stably driven. On the other hand, as the lower limit of the carrier surface density, for example, 1×10 11 cm -2 can be set. Note that the carrier surface density [cm -2 means a value obtained by integrating the carrier density [cm -3 in the thickness direction. In the present disclosure, the carrier surface density can be obtained by carrier density × average thickness.

[0035] The oxide semiconductor film has a large In content as described above. Therefore, although the oxide semiconductor film is polycrystalline, the carrier surface density tends to be large. On the other hand, by controlling the average thickness of the oxide semiconductor film, the carrier mobility and the carrier surface density can be more reliably controlled.

[0036] In addition, in order to improve the carrier mobility in the oxide semiconductor film, it is desirable that the grain boundary line density obtained by using a boundary having an orientation difference of 2° or more by EBSD analysis as a grain boundary is low. In this regard, when the average thickness of the oxide semiconductor film is large, crystallization occurs during film formation, and even if heat treatment is performed at about 400° C. or lower thereafter, the crystals at the time of film formation are maintained, so that crystal grains are difficult to grow and the grain boundary line density is difficult to decrease. As a result, a large number of grain boundaries that hinder the movement of electrons are generated, and the carrier mobility is difficult to increase. Furthermore, when crystallization occurs during film formation, patterning with a chemical solution becomes difficult, and the manufacturing of the thin film transistor itself becomes difficult.

[0037] On the contrary, the oxide semiconductor film is likely to maintain an amorphous state immediately after film formation by reducing the average thickness. Therefore, it becomes easy to grow crystals in the in-plane direction at the timing of crystallization by subsequent heat treatment.

[0038] The upper limit of the average thickness of the oxide semiconductor film is preferably 45 nm, more preferably 40 nm, even more preferably 35 nm, and may also be 30 nm or 25 nm, from the viewpoint of suppressing the carrier plane density and easily increasing the grain boundary line density. On the other hand, the lower limit of the average thickness may be, for example, 10 nm or 15 nm.

[0039] As described above, in this disclosure, the grain boundary line density of the oxide semiconductor film is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries. Because the oxide semiconductor film has a high In content, the nucleus density increases during crystallization, and grain boundary scattering tends to occur easily. In this regard, the oxide semiconductor film can form crystals with a low scattering factor by having a grain boundary line density of 27 / μm or less, obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries. Furthermore, according to the inventors' findings, boundaries with an orientation difference of less than 2° do not substantially affect carrier mobility. As an upper limit for the grain boundary line density, from the viewpoint of reducing grain boundaries that act as barriers to electron transfer, 26 / μm or less is preferred, and 22 / μm or less is more preferred. On the other hand, the lower limit for the grain boundary line density is not particularly limited, but for example, it may be 2 / μm, preferably 2.5 / μm, or more preferably 3 / μm.

[0040] The crystalline form of the oxide semiconductor film may be faceted or radial (dendrite-like).

[0041] <Method for manufacturing oxide semiconductor films> The oxide semiconductor film is deposited, for example, by a sputtering method using a sputtering target. The sputtering target may also be formed using an oxide sintered body. After deposition, the oxide semiconductor film is patterned by photolithography or the like. The oxide semiconductor film is then heat-treated before or after patterning to form a polycrystalline structure. This heat treatment may be an annealing treatment performed after patterning to improve the film quality. The temperature of the heat treatment is not particularly limited, as long as it is a temperature at which almost the entire film crystallizes, but for example, it may be between 300°C and 450°C. The heat treatment may be performed, for example, in an atmospheric environment.

[0042] <Oxide sintered body> An oxide sintered body according to one aspect of the present disclosure contains In as an element. More specifically, the oxide sintered body has the same chemical composition as the oxide semiconductor film. The oxide sintered body may be formed, for example, through a step of weighing raw material powder (weighing step S1), a step of drying and granulating the raw material powder weighed in weighing step S1 (drying and granulation step S2), a step of molding the granulated powder obtained in drying and granulation step S2 (molding step S3), and a step of sintering the molded powder obtained in molding step S3 (sintering step S4).

[0043] <Sputtering target> A sputtering target according to one aspect of this disclosure contains In as an element. More specifically, the sputtering target has the same chemical composition as the oxide semiconductor film. The shape of the target in the sputtering target can be appropriately set according to the shape and structure of the sputtering apparatus. For example, the shape of the target can be a rectangular plate, a circular plate, a cylindrical plate, etc.

[0044] In the sputtering method, a mixed gas of argon and oxygen (Ar / O2 mixed gas) can be used as the carrier gas. While a mixed gas containing hydrogen in addition to argon and oxygen (Ar / O2 / H2 mixed gas) can also be used, it is preferable to use a general Ar / O2 mixed gas due to the complexity of the apparatus configuration. The stage temperature during film deposition can be, for example, within the range of room temperature to 200°C.

[0045] <Thin-film transistor> An example of a thin-film transistor comprising the oxide semiconductor film is shown in Figure 1. Figure 1 shows a bottom-gate type thin-film transistor 10, and more specifically, a bottom-gate type thin-film transistor with an etch-stop structure. However, the thin-film transistor in this disclosure is not limited to the structure shown in Figure 1. For example, in the case of a bottom-gate type, the thin-film transistor may have a back-channel etch structure. Furthermore, the thin-film transistor may be, for example, a top-gate type, or a double-gate type from the viewpoint of the number of gates. In the case of a top-gate type, the thin-film transistor may have, for example, a coplanar structure.

[0046] Because the thin-film transistor 10 is equipped with the oxide semiconductor film, it has high carrier mobility. Furthermore, because the thin-film transistor 10 is equipped with the oxide semiconductor film, it is easy to keep the carrier density low.

[0047] The thin-film transistor 10 has a structure in which a substrate 1, a gate electrode 2, a gate insulating film 3, the oxide semiconductor film 4, a protective layer 5 (ESL protective film), source and drain electrodes 6 (source electrode 6a and drain electrode 6b), and an insulating film 7 (passivation insulating film) are stacked in this order. The thin-film transistor 10 also includes a conductive film 8 which is disposed on the insulating film 7 and connected to the drain electrode 6b via a contact hole 7a provided in the insulating film 7. In the thin-film transistor 10, the oxide semiconductor film 4 is indirectly disposed on the substrate 1 via the gate electrode 2 and the gate insulating film 3.

[0048] (substrate) The substrate 1 is not particularly limited, but examples include transparent substrates such as glass substrates and silicone resin substrates. The glass used for the glass substrate is not particularly limited, but examples include alkali-free glass, high-strain point glass, and soda-lime glass. In addition, a metal substrate such as a stainless steel thin film or a resin substrate such as polyethylene terephthalate (PET) film can also be used as the substrate 1.

[0049] (gate electrode) The gate electrode 2 is conductive. The thin film constituting the gate electrode 2 is not particularly limited, but examples include an Al alloy or an Al alloy with thin films or alloy films of Mo, Cu, Ti laminated on its surface, or a Cu alloy with thin films or alloy films of Mo, Ti laminated on its surface.

[0050] (gate insulating film) The gate insulating film 3 is laminated on the substrate 1 so as to cover the gate electrode 2. The thin film constituting the gate insulating film 3 is not particularly limited, but examples include silicon oxide film (SiO2 film), silicon nitride film (SiN film), silicon oxynitride film, and metal oxide films such as Al2O3 and Y2O3. Furthermore, the gate insulating film 3 may be a single-layer structure of these thin films, or it may be a multilayer structure in which two or more thin films are laminated.

[0051] (Oxide semiconductor film) The structure of the oxide semiconductor film 4 is as described above.

[0052] (protective layer) The protective layer 5 prevents damage to the oxide semiconductor film 4 when the source and drain electrodes 6 are formed by etching. The thin film constituting the protective layer 5 is not particularly limited, but an example is a silicon oxide film.

[0053] (Source and drain electrodes) The source and drain electrodes 6 cover a portion of the gate insulating film 3 and the protective layer 5, and are electrically connected to the oxide semiconductor film 4 at both ends of the channel of the oxide semiconductor film 4. A drain current of the thin-film transistor 10 flows between the source electrode 6a and the drain electrode 6b, depending on the voltage between the gate electrode 2 and the source electrode 6a, and the voltage between the source electrode 6a and the drain electrode 6b. The thin films constituting the source electrode 6a and the drain electrode 6b are not particularly limited as long as they are conductive, and for example, a thin film similar to that of the gate electrode 2 can be used.

[0054] (Insulating film) The insulating film 7 covers the gate electrode 2, the gate insulating film 3, the oxide semiconductor film 4, the protective film 5, and the source / drain electrodes 6, preventing degradation of the characteristics of the thin-film transistor 10. The thin film constituting the insulating film 7 is not particularly limited, but a silicon nitride film can be used because its sheet resistance can be relatively easily controlled by its hydrogen content. Alternatively, the insulating film 7 may have a two-layer structure, for example, a silicon oxide film and a silicon nitride film.

[0055] A contact hole 7a is formed in the insulating film 7, leading to the drain electrode 6b. The contact hole 7a penetrates the insulating film 7 in the thickness direction.

[0056] (Conductive film) The conductive film 8 is filled into the contact hole 7a. The conductive film 8 constitutes wiring for acquiring drain current. The conductive film 8 is not particularly limited, and a thin film similar to that of the gate electrode 2 can be used. Among these, a transparent conductive film suitable for display applications is preferred. Examples of such transparent conductive films include ITO films, ZnO films, and IZO films.

[0057] [Other embodiments] The above embodiments do not limit the configuration of the present invention. Therefore, the above embodiments allow for the omission, substitution, or addition of components of each part of the above embodiments based on the description herein and common technical knowledge, and all such omissions, substitutions, or additions should be interpreted as falling within the scope of the present invention. [Examples]

[0058] The present disclosure will be described in detail below based on examples, but the present disclosure should not be construed as being limited based on the description of these examples.

[0059] [Fabrication of oxide semiconductor films] A crystalline oxide semiconductor film with an average thickness of 15 nm consisting of an InO film (No. 1), a crystalline oxide semiconductor film with an average thickness of 40 nm consisting of an InO film (No. 2), a crystalline oxide semiconductor film with an average thickness of 100 nm consisting of an InO film (No. 3), a crystalline oxide semiconductor film with an average thickness of 100 nm consisting of an InO film doped with 8.5 atomic percent Al (No. 4), a crystalline oxide semiconductor film with an average thickness of 15 nm consisting of an InO film (No. 5), a crystalline oxide semiconductor film with an average thickness of 100 nm consisting of an InO film doped with 10 atomic percent Ga (No. 6), a crystalline oxide semiconductor film with an average thickness of 23 nm consisting of an InO film doped with 1 atomic percent B (No. 7), and an IBO / IGBO multilayer film (No. 8) in which a crystalline oxide semiconductor film with an average thickness of 15 nm consisting of an InO film doped with 1 atomic percent B is stacked on an IGBO film with an average thickness of 10 nm were arranged in the bottom-gate thin-film transistor shown in Figure 1 by DC magnetron sputtering. Table 1 shows the quality of the obtained oxide semiconductor films. The content [atomic %] of each element in Table 1 was calculated from the composition of the target used for simultaneous discharge during film deposition and the deposition rate of each target. The target composition was determined by XRF (X-ray fluorescence analysis). The configuration of each part and the manufacturing conditions of the oxide semiconductor films are as follows. Crystalline oxide semiconductor films No. 3, 4, and 6 crystallized immediately after deposition. This is presumed to be because the large average thickness resulted in a longer deposition time, causing crystallization due to the rise in substrate temperature during deposition. On the other hand, all crystalline oxide semiconductor films other than those mentioned above were in an amorphous state before heat treatment 1. (Sputtering conditions) Atmosphere gas: Ar / O2 mixed gas (oxygen partial pressure ratio 4%) Deposition pressure: 1 mTorr Stage temperature: Room temperature (no heating) (Heat treatment conditions) Heat treatment 1: Nos. 1 to 4 are heated in air with added steam by bubbling into water (350°C), and Nos. 5 to 8 are annealed in air (350°C). Heat treatment 2: 250°C, 30 minutes (after device fabrication) (common to No. 1 to No. 8)

[0060] <Grain boundary linear density> The crystalline morphology of the oxide semiconductor films No. 1 to No. 8 described above was faceted for No. 1, No. 5, and No. 7, and dendrite for No. 2, No. 3, and No. 8. For No. 4 and No. 6, identification was difficult due to the fineness of the crystal grains, but since a certain degree of grain expansion is necessary for dendrite formation, it is presumed to be faceted. For No. 1, 7, and 8, grain boundary line density was determined by EBSD analysis, and for the others, by SEM reflection imaging, with boundaries having an orientation difference of 2° or more considered as grain boundaries. More specifically, for No. 1, 7, and 8, EBSD analysis was performed, and after excluding unmeasurable points and measurement points with low data reliability, adjacent analysis points with an orientation difference of 2° or more were derived from the obtained measurement points, and the lengths of the edges shared by these analysis points were summed (here, "edge" includes not only closed boundaries but also open boundaries). Then, Σ(length of the edges shared by analysis points with an orientation difference of 2° or more) / Σ(analysis points) was calculated, and the obtained value was taken as the grain boundary line density. Furthermore, for No. 2 to No. 6, the observation conditions were set so that contrast due to differences in crystal orientation (channeling contrast) could be obtained in the SEM reflection image, and then the SEM reflection image was acquired. The obtained image was then colored black and white based on Otsu's binarization method. The number of pixels corresponding to the white edge and black edge of the colored boundary was measured, and the total number of these pixels was divided by 2 to obtain this value as the boundary pixel count. The length density was obtained by dividing the obtained boundary pixel count by the total number of pixels in the image. Then, with the SEM length density on the x axis and the EBSD grain boundary line density on the y axis, y = 2.3433x + 0.4071 was used, and the coefficient of determination R 2 The grain boundary line density was calculated using a linear approximation with =0.8632. The results are shown in Table 1. Figure 2 shows the relationship between grain boundary line density and carrier mobility for No. 1 to No. 8.

[0061] <Carrier density and carrier surface density> For oxide semiconductor films No. 1 to No. 8, the carrier density [cm³] -3 ] and carrier surface density [cm -2was obtained. The carrier surface density was calculated by multiplying the carrier density by the average thickness. The results are shown in Table 1.

[0062] <Evaluation of Static Characteristics> For the thin film transistors of No.1, 3, 4, and 6, the drain current (Id)-gate voltage (Vg) characteristics (Id-Vg characteristics) were measured under the following conditions using a probe and a semiconductor parameter analyzer ("4200SCS" manufactured by Keithley). The measurement results (n = 3) of the static characteristics of No.1 are shown in Figure 3. Since the oxide semiconductor films other than No.1 were crystallized immediately after film formation and thus could not be patterned for TFT formation, the static characteristics were not evaluated. Gate voltage: -30V to 30V (step 0.25V) Source voltage: 0V Drain voltage: 10V Stage temperature: room temperature (23°C)

[0063] <Field Effect Mobility> The field effect mobility μ FE [cm 2 / Vs] is calculated by the following formula (1) in the saturation region (Vg > Vd - Vth) of the above static characteristics, where the gate voltage is Vg [V], the threshold voltage is Vth [V], the drain current is Id [A], the channel length is L [m], the channel width is W [m], and the capacitance of the gate insulating film is C ox [F]. The calculation results are shown in Table 1. FE [Number] <...> <...> <...> <...> <...> <TFT Saturation Mobility> From the above formula (1), the TFT saturation mobility [cm 2 / Vs] was calculated. The TFT saturation mobility was calculated only for No.1. The results are shown in Table 1.

[0065] <Threshold Voltage> The threshold voltage [V] is when the drain current of the transistor is 10 -9 ​The gate voltage that becomes A was set as the value calculated from the above static characteristics. The threshold voltage was calculated only for No.1. The calculation results are shown in Table 1.

[0066] <S value> The S value [V / decade] was calculated as the change amount of the gate voltage required to increase the drain current by one digit from the above static characteristics, and the minimum value was used. The S value was calculated only for No.1. The calculation results are shown in Table 1.

[0067]

Table 1

[0068] As shown in FIG. 3 and Table 1, for the oxide semiconductor film of No.1, since the grain boundary line density obtained by taking the boundary with an orientation difference of 2° or more as the grain boundary by EBSD analysis is 10.2 / μm, the field-effect mobility can be increased. Further, for the oxide semiconductor film of No.1, since the carrier surface density is 3.3×10 12 cm -2 , the TFT saturation mobility is large and it has excellent switching characteristics.

[0069] [Relationship between film thickness and crystallization] FIG. 4 and FIG. 5 show the relationship between the film thickness (average thickness) and crystallization in the oxide semiconductor films of No.1, No.3, No.4 and No.6 to No.8. FIGS. 4 and 5 are SEM secondary electron images of the oxide semiconductor film surface. As shown in FIG. 4, for the oxide semiconductor films of No.3, No.4 and No.6, as the film thickness increases, crystallization occurs immediately after film formation, so it can be seen that it is difficult for crystal grains to grow even after subsequent heat treatment. Therefore, from the viewpoint of improving carrier mobility, control of the film thickness is considered important.

[0070] [Relationship between addition of other elements and carrier density] As shown in FIG. 5, when other elements are added to In, the crystal grains are larger compared to No.1, and it is considered that crystallization immediately after film formation can be suppressed.

Explanation of symbols

[0071] 1 circuit board 2 Grid cells 3 gate insulating film 4. Oxide semiconductor film 5 Protective layer 6 Source and Drain Electrodes 6a Source electrode 6b Drain electrode 7. Insulating film 7a Contact hole 8. Conductive film 10 Thin-film transistors

Claims

1. An oxide semiconductor film disposed on a substrate, It contains In as an element, The grain boundary line density is 27 / μm or less. The above grain boundary line density is an oxide semiconductor film obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are identified as grain boundaries.

2. Carrier area density is 6 x 10 １２ cm －２ The oxide semiconductor film according to claim 1, wherein the following applies:

3. The oxide semiconductor film according to claim 1, wherein the average thickness is 10 nm or more and 45 nm or less.

4. A thin-film transistor comprising an oxide semiconductor film according to any one of claims 1 to 3.

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