  Beta-Ga2O3 is an ultra-wide bandgap semiconductor with emerging applications
in power electronics. The introduction of acceptor dopants yields
semi-insulating substrates necessary for thin-film devices. In the present
work, exposure of Cu-doped Ga2O3 to UV light > 4 eV is shown to cause large,
persistent photo-induced darkening at room temperature. Electron paramagnetic
resonance spectroscopy indicates that light exposure converts Cu2+ to Cu3+, a
rare oxidation state that is responsible for the optical absorption. The
photodarkening is accompanied by the appearance of O-H vibrational modes in the
infrared spectrum. Hybrid function calculations show that Cu acceptors can
favorably complex with hydrogen donors incorporated as interstitial (Hi) or
substitutional (H_O) defects. When Cu_Ga-H_O complexes absorb light, hydrogen
is released, contributing to the observed Cu3+ species and O-H modes.
