Insights into the electronic structure of tin(ii) pyrochlore oxides with 5s2 lone pair states as transparent p-type oxide semiconductors

Abstract

Developing transparent p-type oxide semiconductors has been a long-standing subject of interest for optoelectronic devices, but it has been hindered by the strongly localized valence band (VB) structure intrinsic to metal oxides. Sn²⁺ oxides represented by SnO are proposed as promising p-type semiconductors since the Sn 5s² state could help to alleviate the carrier localization at the VB. In this work, using a combination of X-ray spectroscopy and density functional theory calculations, we explore the electronic structures of Sn²⁺-based Sn₂Nb₂O₇ and Sn₂Ta₂O₇ pyrochlores as wide bandgap p-type oxide semiconductors. Our results show that Sn₂Nb₂O₇ and Sn₂Ta₂O₇ have large optical bandgaps of 2.8 eV and 3.4 eV, respectively, and better chemical stability over SnO. Both the experimental and theoretical calculations verified the presence of Sn 5s² states at the top of the VB of Sn₂Nb₂O₇ and Sn₂Ta₂O₇, and the Sn 5s² states increase the VB dispersion and result in lower hole effective masses of 2.09 m_e and 2.23 m_e for Sn₂Nb₂O₇ and Sn₂Ta₂O₇, respectively, but work less effectively than that for SnO. The different VB features originate from the varied Sn–O interactions influenced by crystal structures. The lattice distortions in SnO allow hybridization between Sn 5p orbitals with occupied (Sn 5s–O 2p)* states, forming asymmetrically distributed electronic states with enhanced dispersion. However, in Sn₂Nb₂O₇ and Sn₂Ta₂O₇, these interactions are forbidden by their cubic symmetry and lead to less-dispersive electronic states. Increasing lattice distortions in Sn₂Nb₂O₇ and Sn₂Ta₂O₇ would be necessary to achieve higher hole mobilities. Our findings elucidate the microscopic origins of the optoelectronic properties of tin (II) pyrochlore oxides, highlighting the significant role of synergistic valence band modulation and crystal structural design in advancing high-performance p-type oxide semiconductors.