Crystal orientation dependence of deep level spectra in proton irradiated bulk β-Ga₂O₃

The effects of 20 MeV proton irradiation with fluences of 5 × 10¹⁴ and 10¹⁵ p/cm² on electrical properties of lightly Sn doped n-type (net donor concentration 3 × 10¹⁷ cm⁻³) bulk β-Ga₂O₃ samples with (010) and (−201) orientation were studied. Proton irradiation decreases the net donor density with a removal rate close to 200 cm⁻¹ for both orientations and similar to the electron removal rates in lightly Si doped β-Ga₂O₃ epilayers. The main deep electron traps introduced in the β-Ga₂O₃ crystals of both orientations are near E_c−0.45 eV, while in Si doped films, the dominant centers were the so-called E2* (E_c−0.75 eV) and E3 (E_c−0.1 eV) traps. Deep acceptor spectra in our bulk -Ga₂O₃(Sn) crystals were dominated by the well-known centers with an optical ionization energy of near 2.3 eV, often attributed to split Ga vacancies. These deep acceptors are present in a higher concentration and are introduced by protons at a higher rate for the (010) orientation. Another important difference between the two orientations is the introduction in the surface region (∼0.1 μm from the surface) of the (010) of a very high density of deep acceptors with a level near E_c−0.27 eV, not observed in high densities in the (−201) orientation or in Si doped epitaxial layers. The presence of these traps gives rise to a very pronounced hysteresis in the low temperature forward current-voltage characteristics of the (010) samples. These results are yet another indication of a significant impact of the orientation of the β-Ga₂O₃ crystals on their properties, in this case, after proton irradiation.