Effect of Crystal Structure on Hole Carrier Generation in Wide-gap P-type Tin-Niobate
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2018.687
Effect of Crystal Structure on Hole Carrier Generation in Wide-gap P-type Tin-Niobate Akane Samizo1, Naoto Kikuchi1,2,*, and Keishi Nishio1
1 Department of Materials Science and Technology, Tokyo University of Science, Katsushika, 125-8585, Japan
2
Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8568, Japan
* [email protected]
Abstract
Sn2Nb2O7 and SnNb2O6 are promising candidates for wide-gap p-type conducting oxides with high mobility, because their valence-band maximum are composed of Sn 5s orbital with large spatial spreading and isotropic nature. Though hole carriers were generated by Sn4+ substitutional defects on Nb5+ site (Sn’Nb) in the substructure of Nb2O6 octahedra in both tin niobates, the generation efficiency of hole carriers in SnNb2O6 was larger than that of Sn2Nb2O7. From the variation in bond length of Nb-O in the Nb2O6 octahedra calculated by Rietveld analysis, the difference in carrier generation efficiency of two tin niobates was examined. The bond length of Nb-O in p-type Sn2Nb2O7 with large amounts of Sn’Nb was smaller than that of n-type Sn2Nb2O7 with small amounts of Sn’Nb. The holes generated by Sn’Nb were considered to be captured by the negative charge of oxygen anions consisting of the Nb2O6 octahedra, resulting in low carrier generation efficiency. In SnNb2O6 showing higher efficiency, Nb-O was 8.6 % larger than that of p-type Sn2Nb2O7. It is considered that the large Nb-O bond length provide the preferred environment for the generation of positive holes, resulting in the higher carrier generation efficiency.
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INTRODUCTION Transparent conducting oxides (TCOs) are widely used as transparent electrodes in liquid crystal displays and solar cells. However, p-type TCOs have not been developed as n-type counterparts such as ITO (In2-xSnxO3) and ZnO. In order to fabricate transparent devices based on p-n junctions, the development of p-type TCOs is required. The reason for the difficulty in development of p-type TCOs is low hole mobility due to the localized valence band maximum (VBM) originating from O 2p orbital [1]. In order to realize p-type TCOs with high hole mobility, delocalization of VBM by strong hybridization of O 2p orbital with metal orbital is indispensable. The oxides of metal cations with ns2 electronic configurations (n: the principal quantum number) such as Pb2+, Sn2+ (hereafter referred as s-orbital-based oxides) have recently attracted attention as new candidates for p-type TCOs [1, 2]. The large spatial spreading and isotropic nature of metal s orbitals delocalize the VBM effectively; then, low effective mass of holes can be expected [3]. This approach has been d
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