Formation of high concentrations of isolated Zn vacancies and evidence for their acceptor levels in ZnO

Abstract

Research is described here that is directed toward obtaining p-type ZnO crystals either by doping or by creating native defects. Theoretically, zinc vacancies are shallow acceptors that should allow for p-type conduction. Bulk ZnO crystals can be grown by chemical vapor transport (CVT), melt solidification, and hydrothermal methods. Here we have explored annealing processes with the goal of creating zinc vacancies in bulk ZnO crystals. Positron annihilation spectra reveal the reproducible formation of high concentrations (> 10(20) cm(-3)) of isolated zinc vacancies (V-Zn) in oxygen-annealed, CVT-grown ZnO crystals in the similar to 100-150 nm near-surface. Melt- and hydrothermally grown samples, in contrast, show insignificant levels of zinc vacancy creation. Photoluminescence (PL) emission spectra indicate a V-Zn(0/-1) acceptor level at similar to 155-165 meV; red PL (similar to 1.7 eV) emission, related to the V-Zn(-1/-2) level, was also observed. Infrared absorption spectroscopy reveals the presence of a zinc vacancy complex with a hole binding energy range of 420-450 meV - and with a continuum suggesting a p-type region. XPS measurements support the deficiency of Zn after oxygen annealing the CVT-grown ZnO single crystal. (C) 2017 Elsevier B.V. All rights reserved.