2, National Institute of Advanced Industrial Science and Technology, Tsukuba, , Japan
Development of p-type transparent conducting oxides (TCOs) is required for fabricating transparent devices based on p-n junction. However, it is difficult to realize the p-type TCOs with high hole mobility, because their valence band maximum (VBM) is composed of localized O 2p orbital. It is essential for high hole mobility to reduce the effective mass of holes by delocalizing VBM. SnO2 is a typical n-type TCO where conduction band minimum (CBM) is composed of delocalized Sn 5s orbital. Considering high electron mobility of SnO2, Sn2+ oxides with their VBM composed of Sn 5s orbital is expected to have high hole mobility. This approach had been demonstrated by SnO epitaxial thin films with the hole mobility of 2.4 cm2/Vs. But SnO has indirect band gap of 0.7 eV, showing non-negligible absorption of visible light. Moreover, high mobility is incompatible with high carrier density because the hole carriers are generated by Sn vacancies, which leads to destroy the carrier pathway. Thus, we consider the other Sn2+ oxides as new p-type TCOs, and succeeded in preparation of p-type Sn2(Nb2-xTax)O7 (x=0-2) by solid state reaction. The band gap of Sn2(Nb2-xTax)O7 is variable in the range from 2.4 eV to 3.0 eV by changing x. The mobility and density of hole carriers were estimated to be 0.28 to 1.9 [× 10-1 cm2/Vs], and 0.20 to 1.40 [× 1018 /cm3], respectively. The lower mobility of our samples compared with SnO epitaxial thin film was considered to be due to the low relative density (~60 %) of the samples. Since no report on the p-type Sn2M2O7 (M=Nb ,Ta) was found so far, it is considered that generation of hole carriers is difficult. Therefore, we also discussed the mechanism of hole carriers. Sn2M2O7 has its crystal structure of pyrochlore, which is composed of Sn2O tetrahedra and M2O6 ochtahedra. It is well known to have three kinds of defect; the vacancy of Sn (VSn-2), the substitution of Sn4+ in M5+ site (SnM-1), and the vacancy of O (VO+2). From mutual relationship between the defect content and the electrical properties, we concluded that the hole carriers were generated by SnM-1 in the M2O6 octahedra. SnM-1 acting as the carrier generation center in M2O6 was separated spatially from Sn2O tetrahedra acting as the hole carrier pathway. The separation is effective for reducing ionized impurity scattering, which is the advantage for realizing high hole mobility. Sn2(Nb2-xTax)O7 was considered to be a new candidate for p-type TCOs.