Charge state transition levels of Ni in -Ga₂O₃ crystals from experiment and theory: An attractive candidate for compensation doping

Nickel-doped beta-Ga2O3 crystals were investigated by optical absorption and photoconductivity, revealing Ni-related deep levels. The photoconductivity spectra were fitted using the phenomenological Kopylov and Pikhtin model to identify the energy of the zero-phonon transition (thermal ionization), Franck-Condon shift, and effective phonon energy. The resulting values are compared with the predicted ones by first-principle calculations based on the density functional theory (DFT). An acceptor level (0/-) of 1.9 eV and a donor level (+/0) of 1.1 eV above the valence band minimum are consistently determined for Ni-Ga, which preferentially incorporates on the octahedrally coordinated Ga site. Temperature-dependent resistivity measurements yield a thermal activation energy of similar to 2.0 eV that agrees well with the determined Ni acceptor level. Conclusively, Ni is an eminently suitable candidate for compensation doping for producing semi-insulating beta-Ga2O3 substrates due to the position of the acceptor level (below and close to the mid-bandgap).(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)