P-type oxide semiconductor and semiconductor device having pyrochlore structure
11239322 · 2022-02-01
Assignee
Inventors
- Naoto Kikuchi (Ibaraki, JP)
- Yoshihiro Aiura (Ibaraki, JP)
- Akane Samizo (Ibaraki, JP)
- Shintarou Ikeda (Ibaraki, JP)
Cpc classification
C04B2235/96
CHEMISTRY; METALLURGY
C04B2235/3286
CHEMISTRY; METALLURGY
H01L29/22
ELECTRICITY
C04B2235/3244
CHEMISTRY; METALLURGY
C04B2235/3256
CHEMISTRY; METALLURGY
C04B2235/6586
CHEMISTRY; METALLURGY
H01L29/04
ELECTRICITY
C04B2235/3293
CHEMISTRY; METALLURGY
C04B2235/76
CHEMISTRY; METALLURGY
C04B2235/3251
CHEMISTRY; METALLURGY
C01G35/006
CHEMISTRY; METALLURGY
C04B2235/3258
CHEMISTRY; METALLURGY
C01G19/006
CHEMISTRY; METALLURGY
C04B2235/3262
CHEMISTRY; METALLURGY
C04B2235/3232
CHEMISTRY; METALLURGY
C04B35/495
CHEMISTRY; METALLURGY
C04B2235/3239
CHEMISTRY; METALLURGY
C01G33/006
CHEMISTRY; METALLURGY
C01P2002/72
CHEMISTRY; METALLURGY
H01L29/7869
ELECTRICITY
International classification
H01L29/00
ELECTRICITY
H01L29/22
ELECTRICITY
H01L29/786
ELECTRICITY
Abstract
Provided are an oxide semiconductor excellent in transparency, mobility, and weatherability, etc., and a semiconductor device having the oxide semiconductor, a p-type semiconductor being realizable in the oxide semiconductor. The oxide semiconductor consists of a composite oxide, which has a crystal structure including a pyrochlore structure, containing at least one or more kinds of elements selected from Nb and Ta, and containing Sn element, and its holes become charge carriers by the condition that Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) which is a ratio of Sn.sup.4+ to a total amount of Sn in the composite oxide is 0.124≤Sn.sup.4+/(Sn.sup.2++Sn.sup.4+)≤0.148.
Claims
1. A p-type oxide semiconductor comprising: a composite oxide having a crystal structure, containing at least one or more kinds of elements selected from Nb and Ta, and containing Sn element, the crystal structure including a pyrochlore structure, wherein Sn.sup.4+/(Sn.sup.2++Sn.sup.4+), which is a ratio of Sn.sup.4+ to a total amount of Sn in the composite oxide, is 0.13≤Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) 0.148.
2. The p-type oxide semiconductor according to claim 1, wherein the composite oxide represented by Sn.sub.2Nb.sub.2O.sub.7 or Sn.sub.2Ta.sub.2O.sub.7.
3. The p-type oxide semiconductor according to claim 1, wherein at least one or more elements selected from a group consisting of W, Zr, V, Mn, Ti, Ga, Hf, and Mo is added as an additive element.
4. The p-type oxide semiconductor according to claim 3, wherein the additive element is 0.001 atm % or more and 10 atom % or less in total of an additive element.
5. The p-type oxide semiconductor according to claim 1, wherein Sn/(Nb+Ta) is in a range of 1±0.02.
6. A semiconductor device including the p-type oxide semiconductor according to claim 1.
7. The p-type oxide semiconductor according to claim 2, wherein at least one or more elements selected from a group consisting of W, Zr, V, Mn, Ti, Ga, Hf, and Mo is added as an additive element.
8. The p-type oxide semiconductor according to claim 2, wherein Sn/(Nb+Ta) is in a range of 1±0.02.
9. The p-type oxide semiconductor according to claim 3, wherein Sn/(Nb+Ta) is in a range of 1±0.02.
10. The p-type oxide semiconductor according to claim 4, wherein Sn/(Nb+Ta) is in a range of 1±0.02.
11. The p-type oxide semiconductor according to claim 7, wherein Sn /(Nb+Ta) is in a range of 1±0.02.
12. A semiconductor device including the p-type oxide semiconductor according to claim 2.
13. A semiconductor device including the p-type oxide semiconductor according to claim 3.
14. A semiconductor device including the p-type oxide semiconductor according to claim 4.
15. A semiconductor device including the p-type oxide semiconductor according to claim 5.
16. A semiconductor device including the p-type oxide semiconductor according to claim 7.
17. A semiconductor device including the p-type oxide semiconductor according to claim 8.
18. A semiconductor device including the p-type oxide semiconductor according to claim 9.
19. A semiconductor device including the p-type oxide semiconductor according to claim 10.
20. A semiconductor device including the p-type oxide semiconductor according to claim 11.
Description
BRIEF DESCRIPTIONS OF THE DRAWINGS
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
DETAILED DESCRIPTION
(9) Embodiments of the present invention will be described below.
(10) The inventors of the present invention have: paid attention to, in an oxide composite with a pyrochlore structure, an influence on semiconductor characteristics exerted in accordance with a ratio [Sn.sup.4+/(Sn.sup.2++Sn.sup.4+)] of Sn.sup.4+to the total amount of Sn in an oxide composite; researched and developed the oxide composite; and obtained an oxide semiconductor having excellent semiconductor characteristics and p-type semiconductor characteristics.
(11) An oxide semiconductor according to an embodiment of the present invention is a semiconductor: enhancing ionicity of bonding by forming a double oxide with B.sub.2O.sub.5 (B=Nb, Ta) with respect to SnO having a small band gap, a top of whose valence band is composed of a 5s orbital of Sn; mainly having a pyrochlore structure as a crystal structure about composition formula A2B.sub.2O.sub.7 (A=Sn, B=Nb, Ta) that realizes a band gap made wide; and indicating that a ratio Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) of Sn.sup.4+ to the total amount of Sn is 0.124≤Sn.sup.4+/(Sn.sup.2++Sn.sup.4+)≤0.148.
(12) Additionally, the oxide semiconductor according to the embodiment of the present invention is found to be a semiconductor that: is described in a simplified manner by a general formula of A.sub.2B.sub.20.sub.7 (A=Sn, B=Nb, Ta) as a stoichiometric composition; and has a structural defect in which a part of Sn.sup.2+ contained to forms holes serving as p-type charge carriers is oxidized to Sn.sup.4+ and a part of an B.sup.5+ site is substituted, such a structural defect being represented as “Sn′.sub.B” by the Kroger-Vink notation that is a notation of a structural defect.
(13) As explained in M. A. Subramanian, G. Aravamudan, G. V. Subba Rao, Progress in Solid State Chemistry 15, 55 (1983), a compound represented by A.sub.2B.sub.2O.sub.7 (A=Sn, B=Nb, Ta) in a general chemical formula becomes A.sub.2-p(B.sub.2-qSn.sub.q)O.sub.7-p-0.5q if two structural defects are represented. A pyrochlore structure is maintained within a range of p, q=0.1 to 0.48. However, those two structural defects are respectively represented as “V″.sub.A” and “Sn′.sub.B” by the Kroger-Vink notation, and both become structural defects with negative charges. Therefore, the generation of those defects is considered to become a center of a structural defect that leads to the generation of holes in any case.
(14) Here, the ratio Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) of Sn.sup.4+ to the total Sn amount of (Sn.sup.2++Sn.sup.4+) indicates that the defect “Sn′.sub.B”, in which the B.sup.5+ site is substituted for Sn.sup.4+, is reflected as long as precipitation of SnO.sub.2 is not recognized.
(15) No oxide composite of a pyrochlore structure containing Sn has conventionally exhibited p-type semiconductor characteristics including M. A. Subramanian, G. Aravamudan, G. V. Subba Rao, Progress in Solid State Chemistry 15, 55 (1983). This is considered as follows: conditions for generating an appropriate amount of Sn.sup.4+, a defect Sn′.sub.B substituting a B site, and a defect V″.sub.sn lacking Sn.sup.2+ have not been found and, additionally thereto, the generations of a minus monovalent defect Sn′.sub.B and a minus divalent defect V″.sub.sn simultaneously brings the generation of plus divalent oxygen deficiency V**.sub.O, so that expression of p-type conduction due to the generation of holes cannot be obtained for charge compensation. Amounts of generations of the structural defects “Sn′.sub.B”, “V″.sub.sn”, and “V**.sub.O” considered here are considered to depend on temperature and atmosphere gas conditions at a time of producing the oxide composite. In the present invention, appropriately controlling the temperature and atmosphere gas conditions at a time of sample preparation brings a discovery of the optimum condition that “Sn′.sub.B” is generated and “V**.sub.O” is hardly generated. Such a discovery is considered to lead to the expression of the p-type semiconductor characteristics. The p-type semiconductor characteristics are expressed also in the form of bulk or thin film.
(16) An oxide of the present invention, which has the crystal structure including the pyrochlore structure, only has to mainly include a crystal structure of a pyrochlore structure. Incidentally, the term “mainly” refers to, for example, more than 50 weight % of the whole. Of course, the entire crystal structure is preferably composed of a pyrochlore structure. However, the oxide can be permitted to be composed of a crystal structure slightly having a structure other than the pyrochlore structure, and it is preferable that the crystal structure is substantially composed of the pyrochlore structures. The term “substantially” means, for example, 80 weight % or more.
(17) Exemplified as a semiconductor device according to the present invention is a pn junction element in which a pn junction is formed by a p-type semiconductor of the present embodiment and an n-type semiconductor. Given as the n-type semiconductors suitable for the pn junction element are: In.sub.2O.sub.3, ZnO, and SnO.sub.2; Sn-added In.sub.2O.sub.3, Al-added ZnO, Ga-added ZnO, Sb-added SnO.sub.2, and F-added SnO.sub.2 in which impurities are added to those base materials; and the like. Particularly, ZnO is preferable in that it has a feature capable of production from an insulator to a semiconductor due to easiness of control of carrier concentration and, additionally thereto, is capable of easy etching in patterning and has no problem of using a rare raw material.
First Embodiment
(18) In the present embodiment, explained will be an oxide semiconductor essentially containing Sn, containing one or more elements selected from Nb and Ta, and including an oxide composite that has a crystal structure including a pyrochlore structure. Examined in an oxide composite having a pyrochlore structure consisting of Sn, Nb, Ta, and oxygen have been characteristics corresponding to a ratio Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) of Sn.sup.4+ to an amount of (Sn.sup.2++Sn.sup.4+). As described below, when Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) satisfies 0.124≤Sn.sup.4+/(Sn.sup.2++Sn.sup.4+)≤0.148, p-type semiconductor characteristics in which holes are used as charge carriers lead to expression.
(19) [Production of Oxide Composite With Crystal Structure Essentially Containing Sn, Containing One or More Elements of Nb and Ta, and Including Pyrochlore Structure]
(20) SnO powder (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.5% purity), Nb.sub.2O.sub.5 powder (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.9% purity), and Ta.sub.2O.sub.5 powder (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.9% purity) were weighed, and the weighed powders were placed in a mortar made of agate and were wet-mixed for about 1 hour while ethanol (Wako Pure Chemical Corporation, special grade) was added thereto. At this time, SnO, Nb.sub.2O.sub.5 and Ta.sub.2O.sub.5 were mixed so that a ratio (Sn/(Nb+Ta)) of Sn to (Nb+Ta) was 1.00 in atomic ratio. Further, Nb and Ta were mixed so that a ratio of Nb and Ta meets Nb:Ta=2:0, 1.5:0.5, and/or 0:2. In other words, this is identical to a case of x=0, 0.5, and/or 2 in general chemical formula Sn.sub.2Nb.sub.2-xTa.sub.xO.sub.7. Table 1 collectively shows amounts of weighed reagents in preparing samples.
(21) TABLE-US-00001 TABLE 1 x SnO Nb.sub.2O.sub.s Ta.sub.2O.sub.5 0 2.707 g 2.661 g 0 0.5 2.707 g 1.905 g 1.106 g 2 2.707 g 0 4.423 g
(22) Thereafter, the mixture was left overnight at room temperature to dry the ethanol, and was divided into 6 powder groups approximately equal to one another. Each power group was uniaxially pressurized (15 mm in diameter, 170 MPa) to produce six disk-shaped green compacts. The green compacts were placed on an alumina boat, put in an electric furnace having an alumina core tube whose diameter is 50 mm and whose length is 800 mm, and were temporarily calcined at 1173 K for 4 hours while nitrogen gas was caused to flow therein at a flow rate of 150 ml/minute. The temporarily calcined green compacts were crushed in the agate mortar, were mixed with ethanol after addition of a polyvinyl alcohol aqueous solution as a binder to a sample by 2 wt. %, and were left overnight at room temperature for drying. Then, the dried samples were sifted so that each of their particle size is adjusted to 212 μm or less, and were subjected to hydrostatic molding (285 MPa) after their uniaxially pressurization (15 mm in diameter, 170 MPa). Consequently, compacts each having a diameter of about 15 mm and a thickness of about 1.2 mm were produced. The obtained compacts were placed on an alumina boat, and were subjected to main calcination within a range of 853 K to 1173 K for 4 hours while nitrogen gas is caused flow (flow rate: 50 ml/min, but 200 ml/min only in sample number 8). Main calcination temperature of each sample is shown in Table 2 described later. Additionally, the sample numbers 1-3 in Table 2 are samples in which x=0 in Sn.sub.2Nb.sub.2-xTa.sub.xO.sub.7, the sample numbers 4-6 are samples in which x=0.5 therein, and the sample numbers 7-9 are samples in which x=2 therein.
(23) [Percentage of Sn Valence and Electric Characteristics]
(24) Identification of crystal structures of the obtained samples were performed by using an X-ray diffractometer (PANalytical, X′ Pert Pro MRD). A composition ratio of (Sn/(Nb+Ta)) after the calcination was estimated by using a wavelength dispersion type fluorescence X-ray analyzer (Rigaku, ZSX). Electric characteristics of the samples were evaluated by using a Hall effect measuring device (TOYO Corporation, Resitest 8310) through a Van der Pauw placement after a sample in which gold electrodes were deposited on four corners of a circular sample was prepared. Seebeck coefficients of the samples were evaluated by using a thermoelectric characteristic evaluation device (ADVANCE RICO, inc., ZEM-3). The measurements of the X-ray diffraction, fluorescence X-ray, and the Hall effect measurements were performed at 300 K, and the Seebeck measurements were performed at 323 K. The estimation of Sn contents of Sn.sup.4+ and Sn.sup.2+ in each sample was made by a Mossbauer spectroscopy through a transmission method. Measurements were made at two points of room temperature (300 K) and liquid nitrogen temperature (78 K). The obtained .sup.119Sn Mossbauer spectrum was subjected to least squares fitting using the Lorentz curve, and integrated intensity of absorption peaks, that is, integrated absorption intensity was found. Here, since a tetravalent Sn site and a bivalent Sn site are different in temperature dependence of recoil-free fraction f (Debye temperatures are different), there are tendencies to underestimate an amount of divalent Sn and to overestimate an amount of tetravalent Sn if a value of a ratio of peak integrated absorption intensity of data at relatively high temperature (room temperature) is used as it is. Therefore, f was corrected by: fitting, by high-temperature approximation of the Debye model, the temperature dependence of absolute integrated absorption intensity for each site; and finding the Debye temperature for each site. Specifically, High-temperature Approximation Equation (1) of the Debye model was applied to the temperature dependency of the normalized integrated absorption intensity, and the Debye temperature was determined for each sample and each site.
a. In f=−(6 E.sub.R/k.sub.Bθ.sub.D.sup.2)T (1)
b. E.sub.R=E.sub.γ.sup.2/2 Mc.sup.2 (2)
c. A=const.×f (3) i. f: Recoil-free fraction ii. T: Measurement temperature iii. θ.sub.D: Debye temperature iv. k.sub.B: Boltzmann constant v. E.sub.R: Recoil energy vi. E.sub.γ: Mossbauer gamma ray energy (23.87 keV) vii. M: Recoil nuclear mass (118.90331 u) viii. c: Light velocity ix. A: Integrated absorption intensity
(25) The obtained Debye temperatures were used to find the recoil-free fraction at each temperature by Equation (1) and correct the integrated absorption intensity and, then, an integrated intensity ratio of tetravalent and divalent Sn sites was found. A value obtained from this ratio was set as a quantitative value independent of temperature.
(26) Each of
(27) Additionally, from results of a composition analysis by a fluorescence X-ray, each of the samples corresponding to x=0, 0.5, 2 in Sn.sub.2Nb.sub.2-xTa.sub.xO.sub.7, that is, the sample numbers 1-9 shows that all of Sn/(Nb.sub.2-xTa.sub.x) is 1.0 identical to a value of its charged amount. Namely, Sn in a main calcination temperature range (853 to 1173 K (580 to 900° C.)) is considered to be hardly evaporated.
(28) Each of
(29) Here, a ratio Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) of Sn.sup.4+ to the total Sn amount (Sn.sup.2++Sn.sup.4+) is found from .sup.119Sn Mossbauer spectrum. As described above, when the ratio is simply obtained from integrated absorption intensity ratios of these peaks, there are tendencies to underestimate the amount of divalent Sn and overestimate the amount of tetravalent Sn. Therefore, the integrated absorption intensity is corrected. The sample number 1 will be concretely described as an example.
(30)
(31) Integrated absorption intensity A at the measurement temperature of 78 K was 0.031550 from the peak due to Sn.sup.4+, and was 0.138330 from the peak due to Sn.sup.2+. Integrated absorption intensity A at the measurement temperature of 300 K was 0.018353 from the peak due to Sn.sup.4+ and was 0.037422 from the peak due to Sn.sup.2+. The inclination (ΔlnA/ΔT) (−0.00244 at Sn.sup.4+ and −0.00589 at Sn.sup.2+) was obtained from these integrated absorption intensities, and the Debye temperature θ.sub.D (271 K at Sn.sup.4+, and 174 K at Sn.sup.2+) was obtained from these inclinations. From these Debye temperatures θ.sub.D, Equation (1) was used to find recoil-free fraction fat 78 K (0.8267 at Sn.sup.4+ and 0.6317 at Sn.sup.2+), and recoil-free fraction f at 300 K (0.4809 at Sn.sup.4+ and 0.1709 at Sn.sup.2+) was obtained. Next, the corrected integrated absorption intensity Acorr when it is assumed that the recoil-free fraction ratio f is equal to 1 (f=1) was found so that Sn.sup.4+ is 0.0382 and Sn.sup.2+ is 0.2190 at both of the measurement temperatures of 78K and 300K. Then, the corrected integrated absorption intensity A.sub.corr was calculated as a quantitative value independent of the measurement temperature. When this is expressed as an intensity ratio (sum is 1), the expression was calculated so that Sn.sup.4+ is 0.148, Sn.sup.2+ is 0.852, and Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) is 0.148.
(32) Table 2 collectively shows Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) and electric measurement results (specific resistance, concentration of charge carriers, mobility, and Seebeck coefficient), which are found from .sup.119Sn Mossbauer spectra, about samples different from x of Sn.sub.2Nb.sub.2-xTa.sub.xO.sub.7 and main calcination temperature.
(33) TABLE-US-00002 TABLE 2 Production Conditions Measurement Results Main Concentration Calcination Specific of Charge Seebeck Sample Temperature Sn.sup.4+/ Resistance Carrier Mobility Coefficients Nos. x (K) (Sn.sup.2+ + Sn.sup.4+) (Ωcm) (cm.sup.−3) (cm.sup.2V.sup.−1s.sup.−1) (VK.sup.−1) 1 0 1023 0.148 1.0 × 10.sup.2 2.5 × 10.sup.17 2.4 × 10.sup.−1 +5.6 × 10.sup.−6 2 0 1073 0.130 4.4 × 10.sup.4 2.1 × 10.sup.15 6.7 × 10.sup.−2 +2.2 × 10.sup.−3 3 0 1173 0.089 — — — — 4 0.5 1023 0.145 2.3 × 10.sup.2 2.0 × 10.sup.17 1.4 × 10.sup.−1 −6.5 × 10.sup.−6 5 0.5 853 0.141 1.3 × 10.sup.4 5.1 × 10.sup.13 1.7 × 10.sup.1 −4.1 × 10.sup.−4 6 0.5 1173 0.092 — — — — 7 2 1023 0.125 2.1 × 10.sup.2 1.4 × 10.sup.18 2.8 × 10.sup.−2 +8.5 × 10.sup.−6 8 2 1023 0.124 2.5 × 10.sup.4 8.5 × 10.sup.14 3.1 × 10.sup.−1 +1.1 × 10.sup.−5 9 2 1173 0.093 — — — —
(34) In Table 2, the sample numbers 1-3 correspond to x=0 in Sn.sub.2Nb.sub.2-xTa.sub.xO.sub.7, that is Sn.sub.2Nb.sub.2O.sub.7, but their electric characteristics like the samples (sample numbers 1-2) showing p-type conduction and the sample (sample number 3) showing insulating properties are found to be different on main calcination temperatures. Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) in these samples is found indicating that each of the samples (sample numbers 1-2) showing the p-type conduction has a value higher than that of the sample (sample number 3) showing the insulating properties. Further, the samples (sample numbers 1-2) showing the p-type conduction are found indicating that the sample (sample number 1) with high concentration of charge carriers has a higher value of Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) than that of the sample (sample number 2) with low concentration thereof. Namely, concentration of the holes which are charge carriers exhibiting p-type depends on a ratio of Sn.sup.4+, and shows high concentration of the holes when the ratio of Sn.sup.4+ is high. In other words, this means to have some relationship with the amount of structural defect Sn'B that produces the holes. Further, almost the same tendencies appear also in the samples (sample numbers 4-6) corresponding to x=0.5 in Sn.sub.2Nb.sub.2-xTa.sub.xO.sub.7 and the samples (sample numbers 7-9) corresponding to x=2 therein.
(35)
(36) Meanwhile, in T. Birchall, A. W. Sleight, J. Solid State Chemistry 13, 118-130 (1975) described as the conventional technique, Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) in the nonstoichiometric composition is 0.20 or 0.212, and shows that its value is higher than that (0.116 or 0.120 disclosed in D. J. Stewart, O. Knop, R. E. Meads and W. G. Parker, Canadian J. Chemistry 51, 1041-1049 (1973)) in the stoichiometric composition. Thus, how excess Sn is distributed to Sn.sup.2+ and Sn.sup.4+ was calculated in the nonstoichiometric composition. Consequently, from each compositional formula of the stoichiometric composition and the nonstoichiometric composition and the value of Sn.sup.4+/(Sn.sup.2++Sn.sup.4+), the excess Sn was found to distribute 67% to Sn.sup.4+ and 33% to Sn.sup.2+ in Sn.sup.2+.sub.1.52Nb.sup.5+.sub.1.59Sn.sup.4+.sub.0.41O.sub.6.32 (Sn Nb=1.21), and 40% to Sn.sup.4+ and 60% to Sn.sup.2+ in Sn.sup.2+.sub.1.76Ta.sup.5+.sub.1.56Sn.sup.4+.sub.0.44O.sub.6.54 (Sn /Ta=1.41). That is, the excess Sn can be considered to be approximately equally distributed to Sn.sup.4+ and Sn.sup.2+. When A.sub.2B.sub.2O.sub.7 (A=Sn, B=Nb, Ta) satisfies A/B=1.02, that is, when an excess amount of Sn becomes 2%, the excess amount was calculated and Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) was found to be overestimated within a range of 5.8 to 6.0% in the stoichiometric composition. Also, when A.sub.2B.sub.2O.sub.7 (A=Sn, B=Nb, Ta) satisfies A/B=0.98, that is, when a shortage of Sn becomes 2%, the shortage was calculated and Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) was found to be underestimated in a range of 6.7 to 7.8% in the stoichiometric composition. Therefore, even if A/B in A.sub.2B.sub.2O.sub.7 (A=Sn, B=Nb, Ta) deviates from the stoichiometric composition by about 2%, an influence on Sn.sup.4+ is considered to be at most 8% or less in percentage as long as A.sub.2B.sub.2O.sub.7 has a pyrochlore structure. This leads to obtaining almost the same effects as those of the stoichiometric composition, that is, those of A/B=1.
(37) Thus, A.sub.2B.sub.2O.sub.7 (A=Sn, B=Nb, Ta) in a general chemical formula exhibits p-type conduction in 0.124 Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) 0.148 even if Sn/(Nb+Ta) deviates from the stoichiometric composition by 1 to about ±2%, that is, as long as Sn/(Nb+Ta) is within a range of 1±0.02. Incidentally, Sn in Sn/(Nb+Ta) is used as a meaning of (Sn.sup.2++Sn.sup.4+).
Second Embodiment
(38) Explained in the present embodiment will be a case of the oxide composite of the first embodiment to which a trace of element other than Sn, Ta, and Nb is added. Even if the trace of elements is added, the added oxide composite exhibits p-type semiconductor characteristics similarly to those of the first embodiment when its crystal structure is a pyrochlore structure.
(39) [Production of Oxide Composite With Crystal Structure that Includes Pyrochlore Structure Elementally Containing Sn, Containing One or More of Nb or Ta, and Having Other Elements as Additive Elements]
(40) Similarly to the production of the oxide composite in the first embodiment, SnO powder (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.5% purity) and Ta.sub.2O.sub.5 powder (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.9% purity) were weighed; any one of V.sub.2O.sub.5 (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.9% purity) of 3 atom %, WO.sub.2 powder (Wako Pure Chemical Corporation, 99.9% purity) of 3 atom %, and Mn.sub.2O.sub.3 (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.9% purity) of 3 atom % was added to the weighed powders; and the added powders were placed in a mortar made of agate and were wet-mixed for about 1 hour while ethanol (Wako Pure Chemical Corporation, special grade) was added thereto.
(41) Thereafter, the mixture was left overnight at room temperature to dry the ethanol, and was divided into 6 powder groups approximately equal to one another. Each power group was uniaxially pressurized (15 mm in diameter, 170 MPa) to produce six disk-shaped green compacts. The green compacts were placed on an alumina boat, put in an electric furnace having an alumina core tube whose diameter is 50 mm and whose length is 800 mm, and were temporarily calcined at 1173 K for 4 hours while nitrogen gas was caused to flow therein at a flow rate of 150 ml/minute. The temporarily calcined green compacts were crushed in the agate mortar, were mixed with ethanol after addition of a polyvinyl alcohol aqueous solution as a binder to a sample by 2 wt.%, and were left overnight at room temperature for drying. Then, the dried samples were sifted so that each of their particle size is adjusted to 212 μm or less, and were subjected to hydrostatic molding (285 MPa) after their uniaxially pressurization (15 mm in diameter, 170 MPa). Consequently, compacts each having a diameter of about 15 mm and a thickness of about 1.2 mm were produced. The obtained compacts were placed on an alumina boat, and were subjected to main calcination at 1023 K for 4 hours while nitrogen gas is caused flow (flow rate: 50 ml/min).
(42) [Additive Elements and Electric Characteristics]
(43) Table 3 collectively shows electric measurement results (specific resistance, concentration of charge carriers, mobility, and Seebeck coefficient) about samples in each of which 3 atom % of each of V.sub.2O.sub.3, WO.sub.2, and Mn.sub.2O.sub.3 is added as an additive compound.
(44) TABLE-US-00003 TABLE 3 Measurement Results Concen- Specific tration Resis- of Charge Seebeck Sample Additive tance Carriers Mobility Coefficients Nos. Amount (Ω cm) (cm.sup.−3) (cm.sup.2V.sup.−1s.sup.−1) (VK.sup.−1) 10 V.sub.2O.sub.3 2.8 × 10.sup.2 6.0 × 10.sup.17 4.1 × 10.sup.−2 +4.6 × 10.sup.−6 11 WO.sub.2 2.3 × 10.sup.1 3.5 × 10.sup.18 8.1 × 10.sup.−2 +3.6 × 10.sup.−6 12 Mn.sub.2O.sub.3 3.0 × 10.sup.2 3.1 × 10.sup.17 6.7 × 10.sup.−2 +4.5 × 10.sup.−6
(45) In Table 3, Sn.sub.2Ta.sub.2O.sub.7, to which 3.0 atom % of each of V.sub.2O.sub.3, WO.sub.2, and Mn.sub.2O.sub.3 was added, was found to be p-type semiconductor since its Seebeck coefficient took a positive value in each sample. Further, in each sample of the sample numbers 10-12 in which each additive element was 3 atom %, only a peak due to Sn.sub.2Ta.sub.2O.sub.7 was observed from X-ray diffraction measurements, and a different phase due to an additive compound was not observed.
(46) [Additive Amount and Electric Characteristics of Additive Element]
(47) Explained will be an example of examining an additive amount of additive elements. Its production process was performed in the same manner as the production process of the above-mentioned oxide composite except that the above-described Ta.sub.2O.sub.5 powder was replaced by the Nb.sub.2O.sub.5 powder and that the additive compound was changed to various additive amounts of WO.sub.2 powder. That is, similarly to the production process of the oxide complex in the first embodiment, SnO powder (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.5% purity) and Nb.sub.2O.sub.5 powder (High purity Materials KOJUNDO CHEMICAL LABORATORY CO. LTD, 99.9% purity) were weighed; WO.sub.2 powder (Wako Pure Chemical Corporation, special grade) of 1.5 to 10 atom % was added to the weighed powders; and the added powders were placed in a mortar made of agate and were wet-mixed for about 1 hour while ethanol (Wako Pure Chemical Corporation, special grade) was added thereto. Thereafter, the oxide composite was produced by using almost the same steps as those of the above-mentioned production method.
(48) Table 4 collectively shows electric measurement results (specific resistance, concentration of charge carriers, mobility, Seebeck coefficient) of a sample to which WO.sub.2 is added as an additive compound and in which an additive amount to be added is 0.5 to 10 atomic %.
(49) TABLE-US-00004 TABLE 4 Measurement Results Concen- Specific tration Additive Resis- of Charge Seebeck Sample Amount tance Carriers Mobility Coefficients Nos. (atm %) (Ω cm) (cm.sup.−3) (cm.sup.2V.sup.−1s.sup.−1) (VK.sup.−1) 13 0.5 6.5 × 10.sup.1 5.3 × 10.sup.17 2.4 × 10.sup.−1 +8.8 × 10.sup.−6 14 1.5 3.4 × 10.sup.1 1.7 × 10.sup.18 1.1 × 10.sup.−1 +8.8 × 10.sup.−6 15 5 1.9 × 10.sup.1 4.1 × 10.sup.18 9.0 × 10.sup.−2 +9.0 × 10.sup.−6 16 10 3.3 × 10.sup.1 3.2 × 10.sup.18 6.0 × 10.sup.−2 +6.3 × 10.sup.−6
(50) In Table 4, Sn.sub.2Nb.sub.2O.sub.7, to which 1.5 to 10 atom % of WO.sub.2 were added, was found to be p-type semiconductor since its Seebeck coefficient took a positive value in each sample. Further, in each of the samples of sample numbers 13-16 in which each additive element is 0.5 to 10 atm %, a peak due to Sn.sub.2Ta.sub.2O.sub.7 was observed from X-ray diffraction measurements in any case. It is understood from this that preferable characteristics are obtained when the additive element is 10 atom % or less. Additionally, some different phases due to W are slightly observed in the samples (sample numbers 15-16) in which the additive elements are 5 atm % and 10 atm %, but no different phases are observed in the samples (sample numbers 13-14) in which the additive elements are 0.5 atm % and 1.5 atm %. Understood from this is that more preferable characteristics are obtained when the additive element is 3 atom % or less. However, if the additive element is 10 atm % or less, the p-type semiconductor is found to be realizable even when any different phases due to the additive element are slightly observed.
(51) Although each concrete example of V, W, and Mn has been shown as an additive element, their respective ion radii are 0.064 nm (V.sup.3+), 0.066 nm (W.sup.4+), 0.065 nm (Mn.sup.3+), and are smaller 0% (V.sup.3+), 3.1% (W.sup.4+), and 1.6% (Mn.sup.3+) in difference than ion radii (both 0.064 nm) of Nb.sup.5+ and Ta.sup.5+. Generally, it is said that a difference in size between an ion to be substituted and an ion of an additive element is within 15% to form a substitution type solid solution. Therefore, ion radii of Zr, V, Ga, Hf and Mo are also 0.072 nm (12.5% in difference), 0.061 nm (4.7% in difference), 0.062 nm (3.1% in difference), 0.071 nm (10.9% in difference), and 0.065 nm (1.6% in difference), respectively. As with V, W, and Mn, since the difference therebetween is within 15%, the substitution type solid solution can be formed to a pyrochlore structure. Additionally, any of the structural defects are also considered as defects which generate holes, so that the p-type can be realized even when an almost equal amount of additive elements is added.
(52) As described above, from the identification of the crystal phase by the X-ray diffraction, the evaluations of the electric characteristics by the Hall effect measurement and thermoelectric characteristic measurement, and the results of Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) by the Mossbauer spectrum measurement, a compound represented by Sn.sub.2Nb.sub.2-xTa.sub.xO.sub.7 in a general chemical formula demonstrably has the crystal structure including the pyrochlore structure and expresses the p-type semiconductor characteristics using holes as charge carries when the ratio Sn.sup.4+/(Sn.sup.2++Sn.sup.4+) of Sn.sup.4+ to the total amount of Sn in a substance is 0.124≤Sn.sup.4+/(Sn.sup.2++Sn.sup.4+)≤0.148. Further, the oxide semiconductor of the present invention can demonstrably realize the p-type oxide semiconductor even when having, as an additive element, at least one or more kinds of elements selected from a group composed of W, Zr, V, Mn, Ti, Ga, Hf, and Mo. The p-type oxide semiconductor can be realized even when the amount of the additive elements is 0.001 atom % or more and 10 atom % or less. Additionally, the total amount of the additive elements is preferably 0.001 atom % or more and 10 atom % or less, more preferably 0.001 atom % or more and 3 atom % or less.
(53) Although a case of the bulk-like composite is exemplified by a producing method of the above-mentioned oxide composite, almost the same p-type characteristics are acquired also in thin film form. Thin-film oxide semiconductors can be produced by oxide thin-film producing techniques such as a spin coating method and a spray coating method that use a solution as a starting material in addition to vacuum deposition techniques such as a sputtering method, an evaporation method by heating or electron beams, and an ion plating method.
(54) Further, for example, a semiconductor device including the oxide semiconductor of the present invention can vary a band gap of the oxide semiconductor by its composition. Use of such a variable band gap makes it possible to realize a device capable of effectively using light in all of areas where the band gap is in a range of 2.3 to 3.0 eV if a solar cell has a structure in which a pn junction element made of an oxide semiconductor having the largest (widest) band gap is formed on a light-emitting uppermost surface and in which pn-junction elements each made of an oxide semiconductor having a smaller band gap are multilayer-arranged.
(55) Incidentally, the examples shown in the above-described embodiments etc. are described to make the invention easily understood, and so the present invention is not limited to such embodiments.
(56) Since the oxide semiconductor of the present invention can realize the p-type semiconductor, the pn junction can be realized by the transparent n-type and p-type semiconductors in the visible light region, is widely usable in devices such as transmissive type displays and transparent transistors, and is industrially useful.
(57) While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.