Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

This work explores the Zn vacancy in ZnO using hybrid density functional theory calculations. The Zn vacancy is predicted to be an exceedingly deep polaronic acceptor that can bind a localized hole on each of the four nearest-neighbor O ions. The hole localization is accompanied by a distinct outward relaxation of the O ions, which leads to lower symmetry and reduced formation energy. Notably, we find that initial symmetry-breaking is required to capture this effect, which might explain the absence of polaronic hole localization in some previous hybrid density functional studies. We present a simple model to rationalize our findings with regard to the approximately equidistant thermodynamic charge-state transition levels. Furthermore, by employing a one-dimensional configuration coordinate model with parameters obtained from the hybrid density functional theory calculations, luminescence lineshapes were calculated. The results show that the isolated Zn vacancy is unlikely to be the origin of the commonly observed luminescence in the visible part of the emission spectrum from n-type material, but rather the luminescence in the infrared region.

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.

Self-trapped hole and impurity-related broad luminescence in β-Ga 2 O 3

Vacancy-mediated migration of Al in single-crystal zinc oxide (ZnO) is investigated using secondary-ion mass spectrometry (SIMS) combined with hybrid density-functional theory (DFT) calculations. A thin film of Al-doped ZnO is deposited by sputtering onto the single-crystal bulk material and heat treated at temperatures in the range of 900°C-1300°C. The migration of Al is found to be Zn-vacancy mediated. In order to elucidate the physical processes involved, an alternative model based on reactive diffusion is developed. The model includes the time evolution of the concentration of Al atoms on the Zn site (Al Zn ), Zn vacancies (v Zn ), and a complex between the two, where the influence of the charge state of v Zn on its formation energy is incorporated through the free carrier concentration. The modeling results exhibit close agreement with the experimental data and the Al Zn v Zn complex is found to diffuse with an activation energy of 2.6 eV and a preexponential factor of 4 × 10 −2 cm 2 s −1 . The model is supported by the results from hybrid DFT calculations combined with thermodynamical modeling, which also suggest that a complex between Al Zn and v Zn is promoted in n-doped material. The charge state of this complex is effectively −1, and it thus acts as a compensating acceptor, limiting full utilization of the shallow Al Zn donor. Furthermore, the DFT calculations also predict a high formation energy for both substitutional Al on the O site (Al O ) and interstitial Al (Al i ), and are therefore of minor importance for Al migration in ZnO. The close coupling between the hybrid DFT calculations and the developed diffusion model enable benchmarking of the accuracy of several parameters extracted from the DFT calculations. Furthermore, since the diffusion model hinges strongly on defect concentrations, it couples directly to results from measurements by other experimental techniques than those used in this paper and provides an opportunity for independent verification of the estimated values by future studies.