Site-Selective Oxygen Vacancy Formation Derived from the Characteristic Crystal Structures of Sn–Nb Complex Oxides

Akane Samizo,Kyoko K Bando,Makoto Minohara,Naoto Kikuchi,Keishi Nishio,Yoshihiro Aiura,Ko Mibu

doi:10.1021/acs.jpcc.0c11423

Abstract

Divalent tin oxides have attracted considerable attention as novel p-type oxide semiconductors, which are essential for realizing future oxide electronic devices. Recently, p-type Sn2Nb2O7 and SnNb2O6 were developed; however, an enhanced hole mobility by reducing defect concentrations is required for practical use. In this work, we investigated the correlation between the formation of oxygen vacancy (VO··), which may reduce the hole-generation efficiency and hole mobility, and the crystal structure in Sn–Nb complex oxides. Extended X-ray absorption fine structure spectroscopy and a Rietveld analysis of X-ray diffraction data revealed the preferential formation of VO·· at the O site bonded to the Sn ions in both the tin niobates. Moreover, a larger amount of VO·· around the Sn ions was found in the p-type Sn2Nb2O7 than in the p-type SnNb2O6, indicating the effect of VO·· on the low hole-generation efficiency. To elucidate the dependence of the formation of VO·· on the crystal structure, we evaluated the Sn–O bond strength based on the bond valence sum and Debye temperature. The differences in the bond strengths of the two Sn–Nb complex oxides are correlated through the steric hindrance of Sn2+ with an asymmetric electron density distribution. This suggests the importance of the material design with a focus on the local structure around the Sn ions to prevent the formation of VO·· in p-type Sn2+ oxides.

Highlights

Oxide semiconductors with a high electrical conductivity and transparency have attracted significant attention for applications in several technologies, such as thin-film solar cells and touch screens
One of the intrinsic problems in the development of p-type oxide semiconductors is the low hole mobility due to a flat valence band maximum composed of an O 2p orbital.[3,4]
We investigated the formation of VO·· in SnNb2O6 and Sn2Nb2O7 through extended X-ray absorption fine structure (EXAFS) spectroscopy

Summary

■ INTRODUCTION

Oxide semiconductors with a high electrical conductivity and transparency have attracted significant attention for applications in several technologies, such as thin-film solar cells and touch screens. Considering that the Nb atoms are surrounded by O2 only (see the right panel in Figure 1), we can conclude that the decrease in intensity of the Sn−O peak in the FT of the Sn K-edge EXAFS spectra is originated by the formation of an oxygen vacancy at the O1 site, not at the O2 site. In these analyses, we assumed that only Nj and Rij are changed from p- to n-type samples. For SnNb2O6 with a low symmetric crystal structure, we assumed the following models for analysis

The lattice should expand isotropically when the defect is formed

■ ACKNOWLEDGMENTS

■ REFERENCES