Controlling the Conductivity in Oxide Semiconductors

Abstract

Controlling the electrical conductivity in oxide thin films and nanostructures is an important step toward their application in electronics and optoelectronics. Despite recent progress in growth and characterization, the causes of the often-observed unintentional conductivity in oxide semiconductors are still under debate. This chapter focuses on the theory of native defects and doping in oxide semiconductors such as ZnO, SnO2, and TiO2, from the perspective of first-principles calculations based on density functional theory, the DFT + U, and hybrid functionals. The possible causes of unintentional n-type conductivity and the prospects of achieving p-type doping are addressed. In the case of ZnO and SnO2, it is found that the unintentional conductivity is not due to oxygen vacancies or cation interstitials, but rather to the incorporation of donor impurities, with hydrogen being a likely candidate. In the case of TiO2, it is found that oxygen vacancies are shallow donors, but their formation energy is low only in extreme oxygen-poor conditions. Although the calculations were aimed at understanding the behavior of defects and impurities in bulk single crystals, the main results and conclusions are expected to be valid for thin films and nanostructures.