Single crystalline bulk and epitaxially grown gallium oxide (β–Ga2O3) was irradiated by 0.6 and 1.9 MeV protons to doses ranging from 5 × 109 to 6 × 1014 cm−2 in order to study the impact on charge carrier concentration and electrically active defects. Samples irradiated to doses at or above 2 × 1013 cm−2 showed a complete removal of free charge carriers in their as-irradiated state, whereas little or no influence was observed below doses of 6 × 1012 cm−2. From measurements at elevated temperatures, a thermally activated recovery process is seen for the charge carriers, where the activation energy for recovery follow a second-order kinetics with an activation energy of ∼1.2 eV. Combining the experimental results with hybrid functional calculations, we propose that the charge carrier removal can be explained by Fermi-level pinning far from the conduction band minimum (CBM) due to gallium interstitials (Gai), vacancies (VGa), and antisites (GaO), while migration and subsequent passivation of VGa via hydrogen-derived or VO defects may be responsible for the recovery. Following the recovery, deep level transient spectroscopy (DLTS) reveals generation of two deep levels, with energy positions around 0.75 and 1.4 eV below the CBM. Of these two levels, the latter is observed to disappear after the initial DLTS measurements, while the concentration of the former increases. We discuss candidate possibilities and suggest that the origins of these levels are more likely due to a defect complex than an isolated point defect.

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We have used depth-resolved cathodoluminescence, positron annihilation, and surface photovoltage spectroscopies to determine the energy levels of Zn vacancies and vacancy clusters in bulk ZnO crystals. Doppler broadening-measured transformation of Zn vacancies to vacancy clusters with annealing shifts defect energies significantly lower in the ZnO band gap. Zn and corresponding O vacancy-related depth distributions provide a consistent explanation of depth-dependent resistivity and carrier-concentration changes induced by ion implantation. DOI: 10.1103/PhysRevB.81.081201 PACS number͑s͒: 72.40.ϩw, 71.55.Gs, 78.60.Hk ZnO is a leading candidate for next generation optoelectronic materials because of its large band gap, high exciton binding energy, thermochemical stability, and environmental compatibility. 1,2 High quality single-crystal bulk ZnO wafers grown by various methods are commercially available 3 and ZnO thin-film growth has attracted intense interest. 4 However, despite nearly sixty years of research, several fundamental issues surrounding ZnO remain unresolved. Chief among these have been the difficulty of p-type doping and the role of compensating native defects. 5,6 Oxygen vacancies ͑V O ͒, V O complexes, Zn interstitial-related complexes, and residual impurities such as hydrogen and aluminum are all believed to be shallow donors in ZnO, while Zn vacancies ͑V Zn ͒ and their complexes are considered to be acceptors. 7,8 Although their impact on carrier compensation is recognized, the physical nature of the donors and acceptors dominating carrier densities in ZnO is unresolved. Thus it remains a challenge to correlate the commonly observed 1.9-2.1 eV "red" and 2.3-2.5 eV "green" luminescence emissions with specific native defects. 9 These and other emissions vary widely in ZnO bulk or thin films grown by various methods. [10][11][12][13][14] Previous optical absorption, photoluminescence, electron paramagnetic resonance, and depth-resolved cathodoluminescence spectroscopy ͑DRCLS͒ ͑Ref. 15͒ studies indicate a correlation between the "green" optical transition and O vacancies ͑V O ͒. 10,16 Still controversial, however, is how such visible emissions correlate with the energetics of Zn/O vacancies, interstitials, and their complexes overall. This work clearly identifies the physical nature of the defects dominating optical features of this widely studied semiconductor and, in turn, these defects provide a consistent explanation for ZnO's effective free-carrier densities on a local scale.Contemporary theoretical approaches are also limited in addressing ZnO defect energetics due to major uncertainties, most notably, the "band-gap problem" within densityfunctional methods. 17 Calculations of such basic ZnO defect properties as formation energy and energy-level relative to band edges vary considerably with different approximations. 5,18-21 Therefore, the determination of energy levels of native point defects and energetics of Zn vacancies versus their clusters provides a method to evaluate methods for calcu...

A center from the family of "fourfold coordinated ͑FFC͒ defects", previously predicted theoretically, has been experimentally identified in crystalline silicon. It is shown that the trivacancy ͑V 3 ͒ in Si is a bistable center in the neutral charge state, with a FFC configuration lower in energy than the ͑110͒ planar one. V 3 in the planar configuration gives rise to two acceptor levels at 0.36 and 0.46 eV below the conduction band edge ͑E c ͒ in the gap, while in the FFC configuration it has trigonal symmetry and an acceptor level at E c − 0.075 eV. From annealing experiments in oxygen-rich samples, we also conclude that O atoms are efficient traps for mobile V 3 centers. Their interaction results in the formation of V 3 O complexes with the first and second acceptor levels at E c − 0.46 eV and E c − 0.34 eV. The overall picture, including structural details, relative stability, and electrical levels, is accompanied and supported by ab initio modeling studies.

Electronic structure and band characteristics for zinc monochalcogenides with zinc-blende-and wurtzite-type structures are studied by first-principles density-functional-theory calculations with different approximations. It is shown that the local-density approximation underestimates the band gap and energy splitting between the states at the top of the valence band, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field-splitting energy. The spin-orbit-coupling energy is found to be overestimated for both variants of ZnO, underestimated for ZnS with wurtzitetype structure, and more or less correct for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at the top of the valence band is found to be anomalous for both variants of ZnO, but is normal for the other zinc monochalcogenides considered. It is shown that the Zn-3d electrons and their interference with the O-2p electrons are responsible for the anomalous order. The effective masses of the electrons at the conduction-band minimum and of the holes at the valence-band maximum have been calculated and show that the holes are much heavier than the conduction-band electrons in agreement with experimental findings. The calculations, moreover, indicate that the effective masses of the holes are much more anisotropic than the electrons. The typical errors in the calculated band gaps and related parameters for ZnO originate from strong Coulomb correlations, which are found to be highly significant for this compound. The local-density-approximation with multiorbital mean-field Hubbard potential approach is found to correct the strong correlation of the Zn-3d electrons, and thus to improve the agreement between the experimentally established location of the Zn-3d levels and that derived from pure LDA calculations.