Metallicity at interphase boundaries due to polar catastrophe induced by charge density discontinuity

Arwa Albar,Udo Schwingenschlögl,Hassan Ali Tahini

doi:10.1038/am.2017.236

Arwa Albar, Udo Schwingenschlögl + Show 1 more

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https://doi.org/10.1038/am.2017.236

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Abstract

The electronic properties of interphase boundaries are of basic importance for most materials, particularly when those properties deviate strongly from the bulk behavior. We introduce a mechanism that can result in metallicity at stoichiometric interphase boundaries between semiconductors based on the idea of polar catastrophe, which is usually considered only in the context of heterostructures. To this end, we perform ab initio calculations within density functional theory to investigate the electronic states at stoichiometric SnO/SnO2 (110) interphase boundaries. In this system, one would not expect polar catastrophe to have a role according to state-of-the-art theory because the interface lacks formal charge discontinuity. However, we observe the formation of a hole gas between the semiconductors SnO and SnO2. To explain these findings, we provide a generalized theory based on the idea that the charge density discontinuity between SnO and SnO2, a consequence of lattice mismatch, drives a polar catastrophe scenario. As a result, SnO/SnO2 (110) interphase boundaries can develop metallicity depending on the grain size. The concept of metallicity due to polar catastrophe induced by charge density discontinuity is of general validity and applies to many interphase boundaries with lattice mismatch.

Highlights

Tin oxide has two stoichiometric phases, the monoxide (SnO) and the dioxide (SnO2), the latter being frequently used in gas sensors, oxidation catalysts and solar cell electrodes, for example.[1]
We provide a generalized theory based on the idea that the charge density discontinuity between SnO and SnO2, a consequence of lattice mismatch, drives a polar catastrophe scenario
The two component semiconductors SnO and SnO2 do not give rise to a formal charge discontinuity in the (110) direction, we demonstrate the possibility of forming a hole gas at interphase boundaries and explain the observations by a generalization of the polar catastrophe model in terms of charge density discontinuity

Summary

Introduction

Tin oxide has two stoichiometric phases, the monoxide (SnO) and the dioxide (SnO2), the latter being frequently used in gas sensors, oxidation catalysts and solar cell electrodes, for example.[1]. An important example is the appearance of a quantum gas at the interface between the wide-band-gap semiconductors LaAlO3 and SrTiO36 as well as at other n-type interfaces,[7,8,9,10,11] whereas an insulating character has been reported for the p-type LaAlO3/SrTiO3,6 LaVO3/ SrTiO312 and NaNbO3/SrTiO313 interfaces, likely due to O vacancies that neutralize hole carries.[14,15] such defects form spontaneously at low O partial pressure.[16] At an n-type interface, the additional charge from O vacancies, on the other hand, enhances the electron density.[17]

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