Anisotropy of hydrogen plasma effects in bulk n-type β-Ga₂O₃

Bulk n-type β-Ga2O3 samples with orientation (-201) and (010) were exposed to a high density hydrogen plasma at 330 °C for 0.5 h. The effects were radically different for the two orientations. For the (-201) sample, H plasma exposure increased the net surface concentration of shallow donors from 2.7 × 1017 cm-3 to 2.6 × 1018 cm-3, with the shallow donors having an ionization energy close to 20 meV as deduced from the temperature dependence of the series resistance of Ni Schottky diodes. By sharp contrast, H plasma exposure of the (010) sample led to a strong decrease in the net shallow donor density from 3.2 × 1017 cm-3 to below 1015 cm-3 in the top 0.9 μm of the sample and to 3.2 × 1016 cm-3 near the edge of the space charge region at 0 V, with the total width of the region affected by plasma treatment being close to 1.1 μm. For both orientations, we observed a major decrease in the concentration of the dominant E2 traps near Ec-0.82 eV related to Fe acceptors. The deep trap spectra in hydrogenated samples were dominated by the E2∗ traps commonly ascribed to native defects in β-Ga2O3. The peak of these traps with a level near Ec-0.74 eV was masked in the starting samples by the peak of the E2 Fe acceptors present in high concentration, so that E2∗ only broadened the Fe peak on the low temperature side, but could be revealed by the modeling of the spectra. The concentration of the E2∗ center was not strongly affected in the hydrogen-treated samples with orientation (010), but in the (-201) samples, the concentration of the E2∗ peak was greatly enhanced. The results are discussed in conjunction with previous reports on hydrogen plasma treatment of β-Ga2O3 and on obtaining p-type conductivity in the surface layers of β-Ga2O3 crystals annealed in molecular hydrogen at high temperatures [Islam et al., Sci. Rep. 10, 6134 (2020)].