Hybrid functional investigations of point defects in ZnO and β-Ga2O3

Metal oxide semiconductors are widely recognized as prime materials for future energy technology along the low-carbon development path. Zinc oxide and gallium oxide, studied in the present thesis, promise highly efficient optoelectronic and electricity conversion devices. Point defects can significantly influence the materials properties of semiconductors, and thus have a defining impact on their functionality, both as enablers and barriers. Understanding and controlling defects is therefore pivotal to the technological development of zinc oxide and gallium oxide. The present work explores defects in zinc oxide and gallium oxide using hybrid functional calculations. Recently developed methodology is em-ployed to predict defect properties that can be compared directly with experimental data from electrical and optical defect spectroscopic techniques, aiding in the identification of prominent and technologically relevant defect signatures, and providing new insights into the defect physics. The studied defects include self-trapped holes and polaronic accep-tors, iron and titanium impurities in gallium oxide, and close-associate cation-oxygen divacancies

Broad luminescence from donor-complexed LiZn and NaZn acceptors in ZnO

Zn substitutional lithium ( Li Zn ) and sodium ( Na Zn ) acceptors and their complexes with common donor impurities ( Al Zn , Hi, and HO) in ZnO have been studied using hybrid functional calculations. The results show that the complexes are not exclusively charge neutral, but rather exhibit a thermodynamic ( + / 0 ) transition level close to the valence band maximum. The positive charge states are associated with a polaronic defect state, similar to those of the well-studied charge-neutral isolated acceptors. This incomplete passivation has profound consequences for the optical properties of the complexes. Indeed, electron transitions from the conduction band minimum to the ( + / 0 ) transition level of the complexes result in broad luminescence bands that are blueshifted with respect to those originating from the isolated acceptors. Such complexes are proposed as a potential defect origin of the green luminescence observed at the high-energy side of the orange luminescence band (caused by Li Zn ) in hydrothermally grown ZnO. This prediction is supported by experimental photoluminescence and secondary ion mass spectrometry data on a hydrothermally grown ZnO sample. We have also explored how the parameters controlling the fraction and screening of exchange in the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional influence the results by comparing two parametrization approaches: (i) the conventional one where the exchange fraction is adjusted to reproduce the experimental band gap, and (ii) tuning both parameters in order to also comply with the generalized Koopmans theorem (gKT). Interestingly, these approaches were found to yield similar results

Self-trapped hole and impurity-related broad luminescence in β-Ga2O3

This work explores the luminescence properties of self-trapped holes and impurity-related acceptors using one-dimensional configuration coordinate diagrams derived from hybrid functional calculations. The photoluminescence spectrum of as-grown β-Ga2O3 typically consists of a broad band in the wavelength region from ultraviolet to green and is often dominated by an impurity independent ultraviolet band that is commonly attributed to self-trapped holes. Here, we use the self-trapped hole as a benchmark to evaluate the accuracy of the theoretical defect luminescence spectra and estimate the optical properties of MgGa, BeGa, CaGa, CdGa, ZnGa, LiGa, and NO acceptor impurities, as well as their complexes with hydrogen donors. We also explore VGa acceptors complexed with hydrogen and SiGa donor impurities. The results show that these defects can give rise to broad luminescence bands peaking in the infrared to visible part of the spectrum, making them potential candidates for the defect origin of broad luminescence bands in β-Ga2O3

Primary intrinsic defects and their charge transition levels in beta-Ga2O3

A steady-state photocapacitance (SSPC) setup directly connected to the beamline of a MeV ion implanter is utilized to study primary intrinsic defects in β–Ga2O3 generated by He implantation at cryogenic temperatures (120 K). At low temperatures, the migration of defects is suppressed, and hence the generation of primary intrinsic defects is expected to prevail. SSPC measurements reveal defect-related optical transitions in halide vapor-phase epitaxy (HVPE) -grown β–Ga2O3 thin films with onset energies at 1.3 (T1), 1.7 (T2), 1.9 (T3), 2.6 (T4), 3.7 (T5), and 4.2 eV (T6). T2, T4, T5, and T6 were observed in as-received HVPE-grown β–Ga2O3 thin films, whereby T2 is only sporadically observed. The introduction rates for T3, T4, as well as T6 indicate an origin related to primary intrinsic defects. Notably, T1 and T3 are only observed after He implantation at cryogenic temperatures. Hybrid-functional calculations were performed to estimate the optical absorption cross-section spectra for the gallium (Gai) and oxygen (Oi) interstitials as well as the corresponding vacancies (VGa and VO, respectively), and compared with the measured onsets for optical absorption found by SSPC measurements. Indeed, we propose T3 to be associated with Ga(+/+2)i and/or V(−3/−2)GaI, while T4 is suggested to be related to V(0/+)OK (K=I, II, III) and/or V(−3/−2)GaII. Additionally, several further charge-state transition levels associated with VGaI and VGaII may contribute to T4 and T6. We further studied the kinetics of the defects created with He implantation by exposing the sample to room temperature. The kinetics observed for T3 and T4 further support the proposed assignments of the corresponding defect signatures

The interaction between lithium acceptors and gallium donors in zinc oxide

Diffusion of lithium (Li) in uniformly gallium (Ga)-doped monocrystalline bulk Zinc Oxide (ZnO) is studied over a wide temperature range (500 − 1150◦C) and is demonstrated to be dictated by the distribution of Ga. Below 800◦C, the indiffusion of Li from a Li-doped ZnO sputtered film into a n + ZnO bulk yields an abrupt and compensated Li-doped box region with the Li concentration matching the free-electron concentration, in accordance with several previous experimental and theoretical reports. However, experimental observations of Li-diffusion at higher temperatures have not previously been reported. In this study we give a detailed description of a dissociative diffusion mechanism for Li up to 1150◦C. By employing a reaction-diffusion model that accounts for the presence of both Li and Ga, a dissociation energy of 4.6 eV with an attempt frequency of 5×1015s −1 is obtained from the experimental Li diffusion data. This is in excellent agreement with theoretical results for the dissociation of (LiZnGaZn) 0 (4.8 eV) into Li+ i and (GaZnVZn) −, and strongly suggest that this neutral and stable acceptor-donor pair prevails in Li- and Ga-doped ZnO

Multistability of isolated and hydrogenated Ga-O divacancies in beta-Ga2O3

This work systematically explores 19 unique configurations of the close-associate Ga–O divacancies (VGaVO) in β−Ga2O3, including their complexes with H impurities, using hybrid functional calculations. Interestingly, most configurations are found to retain the negative-U behavior of VO, as they exhibit a thermodynamic (−/3−) charge-state transition level energetically located in the upper part of the band gap, where the 3− charge state is associated with the formation of a Ga–Ga dimer. The energy positions of the thermodynamic (−/3−) charge-state transition levels divide the divacancy configurations into three different groups, which can be understood from the three possible Ga–Ga dimerizations resulting from the tetrahedral and octahedral Ga sites. The relative formation energies of the different divacancy configurations, and hence the electrical activity of the divacancies, is found to depend on the Fermi-level position, and the energy barriers for transformation between different divacancy configurations are explored from nudged elastic band calculations. Hydrogenation of the divacancies is found to either passivate their negative-U charge-state transition levels or shift them down in Fermi level position, depending on whether the H resides at VO or forms an O–H bond at VGa, respectively. Finally, the divacancy is discussed as a potential origin of the so-called E∗2 center previously observed by deep-level transient spectroscopy

Formation and control of the E2* center in implanted b-Ga2O3 by reverse-bias and zero-bias annealing

Deep-level transient spectroscopy measurements are conducted on β-Ga2O3 thin-films implanted with helium and hydrogen (H) to study the formation of the defect level E2 lowast (EA = 0.71 eV) during heat treatments under an applied reverse-bias voltage (reverse-bias annealing). The formation of E2 lowast during reverse-bias annealing is a thermally-activated process exhibiting an activation energy of around 1.0 eV to 1.3 eV, and applying larger reverse-bias voltages during the heat treatment results in a larger concentration of E2 lowast. In contrast, heat treatments without an applied reverse-bias voltage (zero-bias annealing) can be used to decrease the E2 lowast concentration. The removal of E2 lowast is more pronounced if zero-bias anneals are performed in the presence of H. A scenario for the formation of E2 lowast is proposed, where the main effect of reverse-bias annealing is an effective change in the Fermi-level position within the space-charge region, and where E2 lowast is related to a defect complex involving intrinsic defects that exhibits several different configurations whose relative formation energies depend on the Fermi-level position. One of these configurations gives rise to E2 lowast, and is more likely to form if the Fermi-level position is further away from the conduction band edge. The defect complex related to E2 lowast can become hydrogenated, and the corresponding hydrogenated complex is likely to form when the Fermi level is close to the conduction band edge. Di-vacancy defects formed by oxygen and gallium vacancies (VO-VGa) fulfill several of these requirements, and are proposed as potential candidates for E2 lowast