By combining results from positron annihilation and photoluminescence spectroscopy with data from Hall effect measurements, the characteristic deep level emission centered at ∼1.75 eV and exhibiting an activation energy of thermal quenching of 11.5 meV is associated with the zinc vacancy. Further, a strong indication that oxygen interstitials act as a dominating acceptor is derived from the analysis of charge carrier losses induced by electron irradiation with variable energy below and above the threshold for Zn-atom displacement. We also demonstrate that the commonly observed green emission is related to an extrinsic acceptorlike impurity, which may be readily passivated by oxygen vacancies.

Monocrystalline n-type zinc oxide (ZnO) samples prepared by different techniques and containing various amounts of lithium (Li) have been studied by positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry. A distinct PAS signature of negatively charged Li atoms occupying a Zn-site (Li − Zn), socalled substitutional Li, is identified and thus enables a quantitative determination of the content of Li Zn. In hydrothermally grown samples with a total Li concentration of ∼2 × 10 17 cm −3 , Li Zn is found to prevail strongly, with only minor influence, by other possible configurations of Li. Also in melt grown samples doped with Li to a total concentration as high as 1.5 × 10 19 cm −3 , a considerable fraction of the Li atoms (at least 20%) is shown to reside on the Zn-site, but despite the corresponding absolute acceptor concentration of (2-3) × 10 18 cm −3 , the samples did not exhibit any detectable p-type conductivity. The presence of Li Zn is demonstrated to account for the systematic difference in positron lifetime of 10-15 ps between Li-rich and Li-lean ZnO materials as found in the literature, but further work is needed to fully elucidate the role of residual hydrogen impurities and intrinsic open volume defects.

A significant conduction band offset appearing in n‐ZnO/p‐Si heterojunction solar cells is recognized as a serious roadblock to obtain high efficiency solar cells. By alloying with Mg, the conduction band in Zn1–xMgxO can be raised above that of Si, so that the influence of recombination centers at the interface between the two materials is strongly reduced, enabling high efficiency despite recombination velocities as high as 106 cm s−1. By simulating these phenomena we predict an optimal design of a n‐Zn0.8Mg0.2O/p‐Si solar cell resulting in high conversion efficiencies.

Li and H are important electrically active impurities in ZnO and this work presents a detailed experimental and computational study of the behavior of H and Li in ZnO and their effect on its defect structure. We employ AC conductivity measurements as a function of temperature and partial pressure of O2, H2O, and D2O, which is combined with first principles density functional theory (DFT) calculations and thermodynamic modeling (TDM) of finite temperature defect structures in undoped and Li doped ZnO. Undoped ZnO is dominated by protons as hydroxide defects (OHO •), oxygen vacancies (vO ••), and electrons under a large variety of atmospheric conditions, and we also predict from DFT and TDM the substitutional hydride ion (HO •) to dominate concentration-wise under the most reducing conditions at temperatures above 500 °C. The equilibrium concentrations of defects in ZnO are small, and dopants such as Li strongly affect the electrical properties. Experimentally, Li doped ZnO is found to be n-type under all available atmospheric conditions and temperatures, with an n-type conductivity significantly lower than that of as-grown ZnO. The n-type conductivity also increases with decreasing p O2 and with increasing p H2O. The observed electrical properties of Li doped ZnO are attributed to dominance of the ionic defects LiZn /, OHO •, Lii •, vO ••, and the neutral complexes (LiZnOHO)× and (LiZnLii)×. Although Li doping lowers the Fermi level of as-grown ZnO significantly, low formation energy of the ionic donors, and passivation of LiZn / in the form of (LiZnOHO)× and (LiZnLii)×, prevents realization of significant/stable p-type activity in Li doped ZnO under equilibrium conditions.

Diffusion of Li into ZnO from an "infinite" surface source under oxygen-rich conditions is studied using secondary ion mass spectrometry. The Li concentration-versus-depth profiles exhibit a distinct and sharp drop, which evolves in position with temperature and time. The sharp drop is associated with an efficient conversion from highly mobile Li-interstitials (Li i) to practically immobile Li-substitutionals (Li Zn) via a kick-out mechanism. The characteristic concentration level at which Li drops provides a measure of the active donor concentration in the samples at the processing temperature, and gives evidence of residual impurities being responsible for the commonly observed "native" n-type conductivity. These donors are suggested to arise from different impurities, with Al and Si as the prevailing ones in hydrothermal and melt grown material. Further, evidence of electric field effects on Li diffusion profiles is obtained, and they are considered as a main reason for the slow diffusivity obtained in this work (using O-rich conditions) relative to those previously reported in the literature (obtained under Zn-rich conditions).