So-called split Ga vacancies, where a next-nearest-neighbor Ga atom relaxes strongly creating a structure with two "half-Ga vacancies" are the main defect observed with positron annihilation in almost all as-grown β-Ga2O3, irrespective of doping or conduction type. Unrelaxed Ga vacancies are observed in some samples subjected to high-energy particle irradiation. It also appears to n-type doping with Si promotes the formation of unrelaxed Ga vacancies. In spite of the high concentrations of the Ga vacancy related defects in the as-grown materials, the electrical compensation appears not to be directly affected by the vacancies.
Positron annihilation spectroscopy has been applied to study the vacancy-type defects in a wide range of β-Ga2O3 bulk crystals and thin films. The experimental data show that all studied samples contain high concentrations of Ga vacancies in a split vacancy configuration. These split Ga vacancies are observed also with other experimental techniques, and theoretical calculations predict that their formation energies are lower than those of other vacancy configurations. The exact structure of the split Ga vacancies appears to vary across samples.
We have applied positron annihilation spectroscopy to study a wide range of β-Ga2O3 bulk crystals and thin films with various doping levels. The Doppler broadening of the 511 keV positron-electron annihilation line exhibits colossal anisotropy compared to other three-dimensional crystalline semiconductors. State-of-the-art theoretical calculations of the positron characteristics in the β-Ga2O3 lattice reveal that the positron state is effectively 1-dimensional, giving rise to strong anisotropy. Strongly relaxed split Ga vacancies are found to exhibit even stronger anisotropy and to dominate the positron annihilation signals in almost all experiments. The evidence leads to the conclusion that split Ga vacancies are abundant, with concentration of 1018 cm-3 or more, in β-Ga2O3 samples irrespective of conductivity.
We have applied positron annihilation spectroscopy to study vacancy-type defects in unintentionally doped and Si and Sn doped β-Ga2O3 homoepitaxial thin films grown by metal-organic chemical vapor deposition (MOCVD). We detect Ga vacancy related defects at high concentrations in semi-insulating and highly resistive material, while conductive (ntype) material exhibits very low Ga vacancy concentrations. These findings show that Ga vacancies can act as efficient electrical compensators for n-type conductivity, but their concentrations can be suppressed by controlling the growth environment, leading to efficient n-type doping. We also note the strong anisotropy of the positron annihilation signals and give recommendation for presenting positron data obtained in β-Ga2O3.
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