WO2017204202A1 - Oxide semiconductor - Google Patents

Abstract

Provided is an oxide semiconductor having excellent transparency, mobility, and weather resistance with which it is possible to realize a p-type semiconductor in an oxide semiconductor. An oxide semiconductor was realized by an oxide semiconductor having a pyrochlore structure including Sn and Ta and a compositional ratio Sn/Ta of 0.60 ≤ Sn/Ta < 1.0. The oxide semiconductor has a wide gap of about 3.0 eV, and is therefore a p-type semiconductor having transparency in the visible light region and high mobility. When the amount of Sn is small with respect to the stoichiometric composition of the compositional formula Sn₂Ta₂O₇, i.e., when Sn/Ta < 1, a p-type can be realized by generation of a structural defect V''_Sn, and a pyrochlore structure is obtained when the compositional ratio Sn/Ta is at least 0.60.

Description

Oxide semiconductor

The present invention relates to an oxide semiconductor composed of an oxide composite, and more particularly to an oxide semiconductor capable of realizing p-type semiconductor characteristics.

Conventionally, transparent conductor materials and transparent semiconductor materials having high transparency in the visible light region and high electrical conductivity are known as oxide composites, and are widely used for transparent electrodes and the like. For example, for transparent semiconductors, In ₂ O ₃ , ZnO, SnO ₂ , and Sn-added In ₂ O ₃ , impurities added to these base materials, Al-added ZnO, Ga-added ZnO, Sb-added SnO ₂ , and F-added SnO are used. _{2 and the} like are known, but these are all n-type semiconductors in which electrons serve as charge carriers. On the other hand, semiconductors include p-type semiconductors that use holes as charge carriers. If n-type and p-type semiconductors that are transparent in the visible light region are prepared, forming a pn junction makes it possible to produce a diode, transistor, solar cell, or the like that is transparent in the visible light region.

Cu ₂ O, NiO, and the like are already known as p-type semiconductors, but are not transparent because they absorb light in the visible light region and have strong coloration. Since 1990, research and development of transparent p-type conductors has been promoted, and several new transparent p-type conductors have been reported. For example, an oxide represented by a chemical formula of ABO ₂ having a delafossite structure (A = at least one of Cu or Ag, B = at least one of Al, Ga, In, Sc, Y, Cr, Rh, or La) A complex, an oxychalcogenide compound represented by a chemical formula of LnCuOCh (at least one of Ln = lanthanoid element or Y, Ch = S, Se, or Te), zinc oxide represented by a chemical formula of ZnO, etc. is there. However, a compound having a delafossite structure has low hole mobility. In addition, the oxychalcogenide compound has a considerably high mobility and hole concentration, but is oxidized in the air atmosphere, so that the characteristic deterioration is remarkable. In addition, since zinc oxide is an n-type semiconductor that originally uses electrons as charge carriers, it is necessary to reduce the concentration of structural defects that generate electrons to the limit and introduce structural defects that express p-type semiconductor characteristics such as nitrogen. There is. For this reason, it is difficult to produce zinc oxide having p-type semiconductor characteristics and poor reproducibility due to the difficulty in generating n-type structural defects accompanying the introduction of p-type structural defects and reducing the concentration of n-type structural defects. Therefore, it is difficult to realize a transparent p-type semiconductor suitable for an electronic device.

An oxide semiconductor is expected as a semiconductor material resistant to an oxidation reaction in an atmosphere containing oxygen. However, it is difficult to realize p-type conductivity with an oxide. This is because in the oxide, electrons at the upper end of the valence band are localized on oxygen ions. In the delafossite compound, a metal d-orbital component is introduced at the upper end of the valence band, and in the oxychalcogenide compound, a p-orbital component of a chalcogen element is introduced at the upper end of the valence band. The presence is reduced. In addition, if an s orbital of a metal element having an electron orbit radius larger than the d orbital or p orbital is introduced at the upper end portion of the valence band, the localization of electrons at the upper end portion of the valence band is lowered and high mobility is achieved. Can be expected. In addition, the isotropic spherical structure of the s orbitals is expected to suppress a decrease in mobility with respect to the disorder of the crystal structure that causes variations in bond angles and bond distances. Based on this idea, the production of a p-channel transistor has been reported for tin oxide (SnO) in which a tin s orbital component is introduced at the upper end of the valence band (see, for example, Patent Document 1). In addition, it is known that SnO has a band gap of 0.7 eV, which is smaller than that of the visible light region, so that the visible light region is strongly colored, and transparency in the visible light region cannot be ensured.

There are the following publicly known documents relating to oxides having a pyrochlore structure.

It has been reported that a metal oxide having a pyrochlore structure represented by a composition formula Sn ₂ Ta ₂ O ₇ is composed of a 5s component of Sn at the upper end of the valence band (see Non-Patent Document 1).

In a study on the structure of a compound described as Sn ₂ Ta ₂ O ₇ with a simple composition formula, (1) a part of the divalent Sn site is missing, and (2) one of the pentavalent Ta sites. It is known that a part of Sn which has been oxidized to a tetravalent part is substituted. If these two structural defects are expressed, it becomes Sn _2-x (Ta _2-y Sn _y ) O _7-x-0.5y . Further, it is described that the pyrochlore structure is maintained in the range of x, y = 0.1-0.48 (Non-patent Document 2).

Further, it has been reported that an oxide having a pyrochlore structure containing Sn decomposes an organic substance when irradiated with light and acts as a photocatalyst (see Patent Documents 2 and 3 and Non-Patent Document 3). Patent Document 2 reports a photocatalyst of an oxide complex composed of Sn ₂ Nb ₂ O ₇ (oxide semiconductor) and titanium oxide. In Patent Document 2, the photocatalyst is composed of an oxide composite having a junction made of different kinds of oxide semiconductors having different energy levels of electrons at the bottom of the conduction band and electrons at the top of the valence band. In Patent Document 2, Ta is listed as an option similar to Nb. Patent Document 3 discloses ABO _{4 + x} (where −0.25 ≦ x ≦ 0.5, A ions are Sn elements, B ions are one or more elements selected from Nb and Ta), and fluorite. Pyrochlore-related structure in which oxygen vacancies are regularly present and oxygen vacancies in the pyrochlore structure where cations are regularly arranged are filled with oxygen, either α-PbO ₂ -related structures or rutile-related structures A photocatalyst having a structure has been reported. Non-Patent Document 3 reports that Sn ₂ Nb ₂ O ₇ having a pyrochlore structure does not develop a photocatalyst, whereas Sn ₂ Ta ₂ O ₇ having the same pyrochlore structure has a weak photocatalytic activity.

International Publication No. 2010/010802 JP 2003-117407 A JP 2004-344733 A

Conventionally, it has been difficult to realize a transparent p-type semiconductor suitable for an electronic device in an oxide semiconductor resistant to an oxidation reaction in an atmosphere containing oxygen. In particular, if n-type and p-type semiconductors that are transparent in the visible light region are aligned, a pn junction can be formed and a transparent semiconductor device is expected, but it is difficult to realize.

The present invention is intended to solve these problems, and an object of the present invention is to provide a novel oxide semiconductor that has low light absorption in the visible light region and can realize high charge carrier mobility. And It is another object of the present invention to provide an oxide semiconductor that exhibits p-type semiconductor characteristics.

The present invention has the following features in order to achieve the above object.

The present invention relates to an oxide semiconductor, and is composed of an oxide composite having a crystal structure including Sn and Ta and including at least a pyrochlore structure, and a composition ratio Sn / Ta is 0.60 ≦ Sn / Ta <1.0. It is characterized by being. In addition, the oxide semiconductor of the present invention is characterized in that holes serve as charge carriers.

According to the present invention, a transparent and high mobility semiconductor having a wide gap can be realized in an oxide semiconductor. In addition, a p-type oxide semiconductor was realized by the oxide semiconductor of the present invention. According to the present invention, when Sn is less than the stoichiometric composition of the composition formula Sn ₂ Ta ₂ O ₇ , that is, when Sn / Ta <1, the p-type is realized by generating the structural defect V ″ _Sn. When the composition ratio Sn / Ta is 0.60 or more, it takes a crystal structure including at least a pyrochlore structure.

The oxide semiconductor of the present invention is composed of a 5s component of Sn at the upper end of the valence band. As a result, since the s orbit has an isotropic spherical shape with a large orbit radius, there is an effect that the localization of electrons is lowered and high mobility can be realized even for structural disturbance.

Since the semiconductor of the present invention is made of an oxide and has weather resistance, an electronic device having excellent weather resistance can be realized by forming a pn junction of the p-type oxide semiconductor and the n-type oxide semiconductor of the present invention. .

Further, the oxide semiconductor of the present invention has at least a pyrochlore structure containing Sn and Ta having a band gap of 3.0 eV, and when Ta ₂ O ₅ coexists slightly, the band gap of Ta ₂ O ₅ Is 4.0-4.5 eV, a wide gap can be realized and high transparency is obtained in the visible light region.

The figure which shows the X-ray-diffraction pattern in case analysis value (Sn / Ta) _after of 0.60, 0.71, 0.81, and 0.998 of an oxide composite_body | complex in 1st Embodiment. is there. It is a figure which shows the change of the specific resistance with respect to the analysis composition value (Sn / Ta) _after in 1st Embodiment. It is a figure which shows the change of the density | concentration of the charge carrier with respect to the analysis composition value (Sn / Ta) _after in 1st Embodiment.

Embodiments of the present invention will be described below.

The present inventor conducted research and development by paying attention to the fact that semiconductor characteristics are influenced by the composition ratio of Sn / Ta in an oxide composite having a pyrochlore structure, and has excellent semiconductor characteristics. Thus, an oxide semiconductor having semiconductor characteristics has been obtained.

In the oxide semiconductor according to the embodiment of the present invention, SnO having a small bandgap in which the upper end portion of the valence band is composed of 5s orbitals of Sn is bonded to ions by forming a double oxide with Ta ₂ O _5. In the composition formula Sn ₂ Ta ₂ O ₇ which has improved the property and widened the band gap, the crystal structure has at least a pyrochlore structure, and the Sn / Ta composition ratio Sn / Ta is 0.60 ≦ It is a semiconductor with Sn / Ta <1.0.

In addition, the oxide semiconductor according to the embodiment of the present invention is Sn ₂ Ta ₂ O ₇ having a small composition with respect to the stoichiometric composition ratio so as to form holes that are p-type charge carriers. A semiconductor having a structural defect represented by “V ″ _Sn ” in the Kroger-Vink notation which is a notation of a structural defect.

The mechanism by which p-type semiconductor characteristics are manifested can be considered as follows.

As described in the description of Non-Patent Document 2, in a compound described as Sn ₂ Ta ₂ O ₇ with a simple composition formula, if two structural defects are represented, Sn _2-x (Ta _2-y Sn _y ) O _7-x-0.5y . The pyrochlore structure is maintained in the range of x, y = 0.1-0.48.

By the way, these two structural defects are V ″ _Sn and Sn ′ _Ta , respectively, according to the Crager-Bink notation, and both are structural defects having negative charges. Therefore, when these defects are generated, it is considered that any of them becomes a center of a structural defect that causes generation of holes. That is, less than the stoichiometric composition, that is, Sn / Ta <1 indicates a structural defect represented by V ″ _Sn .

No oxide composite having a pyrochlore structure has developed p-type semiconductor characteristics, including Non-Patent Document 2 above. It generates a -2 valence defect V '' _Sn simultaneously generates +2 oxygen vacancy V ^{· ·} _O, so resulting in charge compensation, the expression of p-type conduction is not obtained due to holes generated Therefore, it is considered. The amount of V ″ _Sn and V ^·· _O produced is considered to depend on the temperature and the atmospheric gas conditions when the oxide composite is produced. In the present invention, the generation of V ″ _Sn is controlled by changing the composition ratio Sn / Ta at the time of sample preparation, while V ^·· _O appropriately controls the atmospheric gas conditions. As a result, it is considered that charge compensation due to simultaneous generation of V ″ _Sn and V ^·· _O is suppressed, leading to the development of p-type semiconductor characteristics. The p-type semiconductor characteristics are manifested in a bulk form or a thin film form.

As an n-type semiconductor suitable for forming a pn junction with the p-type semiconductor of this embodiment, In ₂ O ₃ , ZnO, SnO ₂ , Sn-doped In ₂ O ₃ in which impurities are added to these base materials, Al adding ZnO, Ga added ZnO, Sb added _SnO 2, F added SnO _2. In particular, ZnO is preferable from the standpoints that, in addition to the feature that it is possible to fabricate from an insulator to a semiconductor due to easy control of the carrier concentration, there is no problem of etching and easiness of patterning and the scarcity of raw materials.

(First embodiment)
In this embodiment, an oxide semiconductor including an oxide composite including Sn and Ta and having at least a pyrochlore structure will be described. In the oxide composite having a pyrochlore structure composed of Sn, Ta and oxygen, the characteristics corresponding to the composition ratio Sn / Ta were examined. As shown below, the composition ratio Sn / Ta is 0.81 ≦ Sn / Ta ≦ 1.0, indicating a single phase of the pyrochlore structure, and 0.60 ≦ Sn / Ta <0.81, and the pyrochlore structure and Ta ₂ O ₅ Shows a coexisting crystal structure. Here, Ta ₂ O ₅ coexists in the crystal structure by a maximum of 25%. Further, p-type semiconductor characteristics using holes as charge carriers are exhibited at 0.60 ≦ Sn / Ta <1.0.

Incidentally, it has been conventionally known that the band gap of Ta ₂ O ₅ exhibits 4.0 to 4.5 eV. In the oxide semiconductor embodiment of the present invention, at least has the pyrochlore structure containing Sn and Ta which is the band gap showing the 3.0 eV, if further slightly coexist Ta ₂ O ₅ is the wide gap Ta ₂ Since it contains O ₅ , a wide gap can be realized even if it is not a single phase with a pyrochlore structure, and it has high transparency in the visible light region.

[Production of oxide composite having pyrochlore structure containing Sn and Ta]
Example 1
Weighed SnO powder (purity 99.5%, Purity Chemical Laboratory Co., Ltd.) and Ta ₂ O ₅ (purity 99.9% Purity Chemical Laboratory, Inc.) into an agate mortar and added ethanol (Japanese Wet mixing was carried out for about 1 hour while adding Kogyo Pharmaceutical Co., Ltd. special grade). At this time, SnO and Ta ₂ O ₅ were mixed so that the ratio of Sn to Ta (Sn / Ta) was 0.60, 0.70, 0.80, and 1.00 in terms of atomic ratio. This charged composition value is hereinafter referred to as “(Sn / Ta) _before ”. Table 1 summarizes the amount of reagent weighed during sample preparation.

Thereafter, the mixture was allowed to stand overnight at room temperature to dry the ethanol, and the powder divided into approximately six equal parts was uniaxially pressed (diameter: 15 mm, 170 MPa) to produce six disk-shaped green compacts. The green compact was placed on an alumina boat, placed in an electric furnace having an alumina furnace tube having a diameter of 50 mm and a length of 800 mm, and pre-baked at 900 ° C. for 4 hours while flowing a nitrogen gas at a flow rate of 150 ml / min. The calcined green compact was crushed in an agate mortar, and an aqueous polyvinyl alcohol solution as a binder was 2 wt. % And mixed with ethanol and left to dry overnight at room temperature. Thereafter, the particle size was adjusted to 212 μm or less by sieving, and uniaxial pressing (diameter: 15 mm, 170 MPa) followed by hydrostatic pressing (285 MPa) to produce a molded body having a diameter of about 15 mm and a thickness of about 1.2 mm. The obtained molded body was placed on an alumina boat and subjected to main firing at 750 ° C. for 4 hours while flowing nitrogen gas (flow rate: 50 ml / min). As shown in Table 2 to be described later, Sample Nos. 1 to 4 have a charge composition value ((Sn / Ta) _before ) of 0.60, 0.70, 0.80, and 1.00, respectively, and nitrogen during firing. This is a sample prepared under the condition of a gas (N ₂ gas) flow rate of 50 ml / min.

[Analytical composition value and electrical property of oxide composite having pyrochlore structure containing Sn and Ta]
The crystal structure of the sample obtained in Example 1 was identified by an X-ray diffractometer (Panalytic X'Pert Pro MRD). For the estimation of the Sn / Ta composition ratio after firing, a wavelength dispersive X-ray fluorescence analyzer (Rigaku ZSX) was used. The analytical composition value after firing is expressed as “(Sn / Ta) _after ”. The evaluation of the electrical characteristics of the sample was performed using a Hall effect measuring device (Toyo Technica Resitest 8310) with a van der Pau arrangement prepared by depositing gold electrodes on the four corners of a circular sample. The Seebeck coefficient of the sample was evaluated using a thermoelectric property evaluation apparatus (Advanced Riko ZEM-3). All measurements were performed at room temperature.

FIG. 1 shows changes in the X-ray diffraction pattern of the Sn ₂ Ta ₂ O ₇ sample in Example 1 by (Sn / Ta) _after . The horizontal axis in FIG. 1 is the diffraction angle 2Θ with respect to the incident angle Θ using CuKα rays. In the X-ray diffraction pattern when (Sn / Ta) _after is 0.81 or 0.998, a peak attributed to Sn ₂ Ta ₂ O ₇ having a pyrochlore structure belonging to a cubic system (shown by a black circle (222)) (400) (440) (622) etc.) only appears. On the other hand, when (Sn / Ta) _after is 0.60 or 0.71, in addition to the peak attributed to Sn ₂ Ta ₂ O ₇ having a pyrochlore structure, it is attributed to Ta ₂ O ₅ having an orthorhombic structure. A weak peak was observed. From this, the sample having an analytical composition value of 0.60 ≦ (Sn / Ta) _after <1.00 has a crystal structure containing at least Sn ₂ Ta ₂ O ₇ having a pyrochlore structure belonging to a cubic system. I understand that.

Table 2 shows the analytical composition value (Sn / Ta) _after and electrical measurement results (specific resistance, concentration of charge carrier, mobility, Seebeck coefficient) for samples prepared by changing the charged composition value ((Sn / Ta) _before ). )

FIG. 2 shows a change in specific resistance with respect to (Sn / Ta) _after . In FIG. 2, the nitrogen flow rate at the time of firing in Table 2 is 50 ml / min. The case of producing under the conditions (white circle) is shown. The error bar in the figure indicates the standard deviation σ. FIG. 2 shows specific resistance values in Table 2 in the range of (Sn / Ta) _after of about 0.60 to about 1.0 (specifically 0.998). As can be seen from FIG. 2, the specific resistance of the sample is (Sn / Ta) _after in this range from 5 × 10 ¹ to 4 × 10 ² Ωcm, and both exhibit electrical conductivity.

FIG. 3 shows changes in the concentration of the charge carrier with respect to (Sn / Ta) _after . Also in FIG. 3, the error bar indicates the standard deviation σ. As shown in FIG. 3, when the (Sn / Ta) _after is in the range of about 0.60 to about 1.0 (specifically 0.998), the concentration of the charge carrier increases as (Sn / Ta) _after decreases. It can be seen that it has increased. This is presumably because Sn in the pyrochlore structure was deficient and Sn vacancies V ″ _Sn were generated by the decrease in Sn / Ta. Since Sn vacancies V ″ _Sn are defects that generate carrier holes, it is considered that the concentration of holes, which are charge carriers of the p-type semiconductor, increased as V ″ _Sn increased.

The Seebeck coefficients obtained by the thermoelectric property evaluation apparatus shown in Table 2 all showed positive values. This indicates that all of sample numbers 1 to 4 in Example 1 have holes as charge carriers.

As described above, in the compound represented by Sn ₂ Ta ₂ O _{7 in} a simple composition formula based on the identification of the crystal phase by X-ray diffraction and the evaluation of the electrical properties by Hall effect measurement and thermoelectric property measurement, the composition ratio Sn / Ta It is clear that when p is 0.60 ≦ Sn / Ta <1.0, it exhibits a p-type semiconductor characteristic having a crystal structure including at least a pyrochlore structure and using holes as charge carriers.

Although the case of a bulk composite was illustrated by the above-described oxide composite manufacturing method, the same p-type characteristics can be obtained even in a thin film form. Thin-film oxide semiconductors can be produced by sputtering, heating, electron beam deposition, ion plating, and other oxide film production methods such as spin coating and spray coating using solutions as starting materials. Can be manufactured by technology.

In addition, the example shown by the said embodiment etc. was described in order to make invention easy to understand, and is not limited to this form.

Since the oxide semiconductor of the present invention can realize a p-type semiconductor, a pn junction can be realized by an n-type semiconductor and a p-type semiconductor that are transparent in the visible light region, and widely used in devices such as a transmissive display and a transparent transistor. Can be industrially useful.

Claims (2)

1. An oxide semiconductor comprising an oxide composite containing Sn and Ta and having a crystal structure including at least a pyrochlore structure, wherein the composition ratio Sn / Ta is 0.60 ≦ Sn / Ta <1.0 .
2. 2. The oxide semiconductor according to claim 1, wherein holes serve as charge carriers.
   
   

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