Transition-metal ions in β-Ga2O3 crystals: Identification of Ni acceptors

Abstract

Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band. Both Ni2+ (3d8) and Ni3+ (3d7) acceptors are present in the as-grown crystal. Also present are unintentional Ir3+ (5d6) and Ir4+ (5d5) donors. The neutral Ni3+ acceptors have a low-spin S = 1/2 ground state and are easily monitored with electron paramagnetic resonance (EPR). Principal values of the g matrix for these acceptors are 2.131, 2.138, and 2.233. Although paramagnetic, the singly ionized Ni2+ acceptors are not seen with EPR at X band (9.4 GHz). The Ir4+ donors are monitored with EPR and with infrared absorption spectroscopy. Exposing the Ni-doped β-Ga2O3 crystal to 275 nm light at room temperature increases the concentration of Ni3+ ions and reduces the concentration of Ir4+ ions as electrons move from the acceptors to the donors. After illumination, heating the crystal above 375 °C restores the initial concentrations of the Ni3+ and Ir4+ ions. Broad optical absorption bands peaking near 303 and 442 nm are attributed to the Ni3+ acceptors.