Point defects in ZnO: an approach from first principles

Abstract

Recent first-principles studies of point defects in ZnO are reviewed with a focus on native defects. Key properties of defects, such as formation energies, donor and acceptor levels, optical transition energies, migration energies and atomic and electronic structure, have been evaluated using various approaches including the local density approximation (LDA) and generalized gradient approximation (GGA) to DFT, LDA + U/GGA + U , hybrid Hartree-Fock density functionals, sX and GW approximation. Results significantly depend on the approximation to exchange correlation, the simulation models for defects and the post-processes to correct shortcomings of the approximation and models. The choice of a proper approach is, therefore, crucial for reliable theoretical predictions. First-principles studies have provided an insight into the energetics and atomic and electronic structures of native point defects and impurities and defect-induced properties of ZnO. Native defects that are relevant to the n-type conductivity and the non-stoichiometry toward the O-deficient side in reduced ZnO have been debated. It is suggested that the O vacancy is responsible for the non-stoichiometry because of its low formation energy under O-poor chemical potential conditions. However, the O vacancy is a very deep donor and cannot be a major source of carrier electrons. The Zn interstitial and anti-site are shallow donors, but these defects are unlikely to form at a high concentration in n-type ZnO under thermal equilibrium. Therefore, the n-type conductivity is attributed to other sources such as residual impurities including H impurities with several atomic configurations, a metastable shallow donor state of the O vacancy, and defect complexes involving the Zn interstitial. Among the native acceptor-type defects, the Zn vacancy is dominant. It is a deep acceptor and cannot produce a high concentration of holes. The O interstitial and anti-site are high in formation energy and/or are electrically inactive and, hence, are unlikely to play essential roles in electrical properties. Overall defect energetics suggests a preference for the native donor-type defects over acceptor-type defects in ZnO. The O vacancy, Zn interstitial and Zn anti-site have very low formation energies when the Fermi level is low. Therefore, these defects are expected to be sources of a strong hole compensation in p-type ZnO. For the n-type doping, the compensation of carrier electrons by the native acceptor-type defects can be mostly suppressed when O-poor chemical potential conditions, i.e. low O partial pressure conditions, are chosen during crystal growth and/or doping.

Structural stability and defect energetics of ZnO from diffusion quantum Monte Carlo

“…For the Zn interstitial , hydrogen interstitial and , DMC predicts the transition levels to be above the CBM (Table 5). Different DFT studies, [37][38][39] as well as experimental results, 36 indicate that these defects are shallow donor with transition levels slightly below the CBM. DMC yields the levels above the CBM likely because of uncorrected image interaction errors.…”

“…[37][38][39] To enable a consistent comparison with our DMC results, we also performed DFT calculations for and the band-gap in ZnO with a 32 atom supercell using LDA, GGA and hybrid functionals. In Figure 4, we compare our DMC and DFT calculations as well as previous DFT results.…”

Effect of RF power on the structural, optical and gas sensing properties of RF-sputtered Al doped ZnO thin films

“…The ZnO is mostly oxygen deficient in nature i. e. oxygen vacancies (V O ) are the majority type of defects [31][32][33][34][35][36][37][38] . Nonetheless, various other point defects are also possible namely oxygen interstitials (O i ), Zn interstitials (Zn i ) and Zn vacancies (Zn v ), which have slightly higher energies of formations.…”

Spectroscopy and control of near-surface defects in conductive thin film ZnO

“…Three species in particular have been widely discussed as candidates for the pervasive n-type nature of ZnO: oxygen vacancies (O v ), zinc interstitials (Zn i ), and atomic hydrogen. 9,11,[21][22][23] Much less is known however regarding the electronic structure and role of point defects and dopants in the near-surface region of ZnO. Only a few studies have investigated the role of surface defects on the electronic structure, and an understanding of the impact of surface and near-surface defects on hybrid organic / inorganic semiconductor interfacial electronic structure and charge transfer is only starting to emerge.…”