Photodarkening and dopant segregation in Cu-doped β-Ga2O3 Czochralski single crystals

beta-Ga2O3 has demonstrated insulating properties with Mg, Fe, and Zn acceptor doping. Here we investigate Cu doping (0.25 at.%) in bulk Czochralski (CZ) and vertical gradient freeze (VGF) beta-Ga2O3, with significant Cu incorporation, even with the expected Cu evaporation. Representative crystals were assessed for orientation, purity, optical, and electrical properties. The solubility and electronic behavior of Cu dopants are consistent with measured concentrations of 1 x 1018-1 x 1019 atoms/cm3 and electrical measurements that show high resistivities of 109-1010 omega center dot cm. Segregation and precipitation of Cu species in part of the VGF material was determined to be Cu2O by analysis with Raman spectroscopy, photoluminescence microscopy, energy-dispersive X-ray spectroscopy, and laser ablation inductively coupled plasma mass spectrometry. With sufficient Cu concentration, beta-Ga2O3 crystals excited with deep ultraviolet light photodarken rapidly and exhibit decreased resistivity. This darkened state remains at room temperature for several days before decaying.