Computational identification of Ga-vacancy related electron paramagnetic resonance centers in β-Ga2O3

A combined experimental/theoretical study of the electron paramagnetic resonance (EPR) centers in irradiated β-Ga2O3 is presented. Four EPR spectra, two S = 1/2 and two S = 1, are observed after high-energy proton or electron irradiation. Three of them have been reported before in neutron irradiated samples. One of the S = 1/2 spectra (EPR1) can be observed at room temperature and below and is characterized by the spin Hamiltonian parameters gb = 2.0313, gc = 2.0079, and ga* = 2.0025 and a quasi-isotropic hyperfine interaction with two equivalent Ga neighbors of ∼14 G on 69Ga and correspondingly ∼18 G on 71Ga in their natural abundances. The second (EPR2) is observed after photoexcitation (with a threshold of 2.8 eV) at low temperature and is characterized by gb = 2.0064, gc = 2.0464, and ga* = 2.0024 and a quasi-isotropic hyperfine interaction with two equivalent Ga neighbors of 10 G (for 69Ga). A spin S = 1 spectrum with a similar g-tensor and a 50% reduced hyperfine splitting accompanies each of these, which is indicative of a defect of two weakly coupled S = 1/2 centers. Density functional theory calculations of the magnetic resonance fingerprint (g-tensor and hyperfine interaction) of a wide variety of native defect models and their complexes are carried out to identify these EPR centers in terms of specific defect configurations. The EPR1 center is proposed to correspond to a complex of two tetrahedral VGa1 with an interstitial Ga in between them and oriented in a specific direction in the crystal. This model was previously shown to have lower energy than the simple tetrahedral Ga vacancy and has a 2−/3− transition level higher than other VGa related models, which would explain why the other ones are already in their diamagnetic 3− state and are thus not observed if the Fermi level is pinned approximately at this level. The EPR2 spectra (S = 1/2 as well as the related S = 1) are proposed to correspond to the octahedral VGa2 in which the spin is located on an oxygen off the defect’s mirror plane and has a tilted spin density. Models based on self-trapped holes and oxygen interstitials are ruled out because they would have hyperfine interaction with more than two Ga nuclei and because they cannot support a corresponding S = 1 center.