Zinc-Vacancy–Donor Complex: A Crucial Compensating Acceptor in ZnO

Abstract

Transparent conductive oxides (TCOs) are currently employed in a wide variety of applications ranging from light emission and light harvesting to touch screens. At the moment, the most commonly used TCO is indium tin oxide, but a steep increase of the indium price in recent years, partly because of a limited abundance, has urged a widespread search for alternative materials. One of the most promising candidates is ZnO [1–3], since it is transparent to visible light, nontoxic, widely abundant, and cheap. Device applications of ZnO require reliable and precise control of its electrical and optical properties, which can be largely affected by intrinsic defects and impurities. Here, the key point defects to be considered include zinc vacancies (VZn), zinc interstitials (Zni), oxygen vacancies (VO), and oxygen interstitials (Oi). Among them, VZn is probably the most relevant defect, since it has the lowest formation energy among native point defects in n-type ZnO [1] and is commonly found in bulk and nanostructured materials [4–8]. VZn is also suggested to be the origin of the observed n-type doping limit in ZnO [9,10] by forming complexes with donors leading to their compensation [11–13]. Therefore, it is crucial to understand the formation of intrinsic defects, especially VZn, and their interaction with extrinsically important impurities such as shallow dopants in ZnO, which remains far from complete. In this study, we use electron irradiation with variable energies to generate point defects either solely on the O sublattice (when the irradiation energy Eirr ¼ 0.45–0.8 MeV) or on both Zn and O sublattices (when