Role of self-trapping in luminescence and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi></mml:math>-type conductivity of wide-band-gap oxides

TLDR

The study investigates hole behavior in the valence bands of various wide‑band‑gap oxides such as ZnO, MgO, In₂O₃, Ga₂O₃, Al₂O₃, SnO₂, SiO₂, and TiO₂. Hybrid‑functional calculations reveal that holes in these oxides form energetically favorable self‑trapped small polarons—except in ZnO and SiO₂—explaining observed luminescence peaks and demonstrating that polaron formation blocks p‑type conductivity.

Abstract

We investigate the behavior of holes in the valence band of a range of wide-band-gap oxides including ZnO, MgO, In${}_{2}$O${}_{3}$, Ga${}_{2}$O${}_{3}$, Al${}_{2}$O${}_{3}$, SnO${}_{2}$, SiO${}_{2}$, and TiO${}_{2}$. Based on hybrid functional calculations, we find that, due to the orbital composition of the valence band, holes tend to form localized small polarons with characteristic lattice distortions, even in the absence of defects or impurities. These self-trapped holes (STHs) are energetically more favorable than delocalized, free holes in the valence band in all materials but ZnO and SiO${}_{2}$. Based on calculated optical absorption and emission energies we show that STHs provide an explanation for the luminescence peaks that have been observed in many of these oxides. We demonstrate that polaron formation prohibits $p$-type conductivity in this class of materials.