Properties of monoclinic β-Ga2O3 are strongly dependent upon orientation. In the case of optical properties, polarization can reveal differences in optical bandgap and absorptions related to transition metal ions. This phenomenon is known as pleochroism and has been extensively studied in minerology. β-Ga2O3 bulk single crystals doped with Zn, Mn, Cr, or Cu were grown by the Czochralski and vertical gradient freeze methods. Ultraviolet-visible-near infrared spectroscopy and photoluminescence (PL) revealed polarization- and orientation-dependent optical absorptions in β-Ga2O3. Crystals were annealed in reducing and oxidizing environments in an attempt to alter the intensities of absorptions characteristic to the different transition metal oxidation states in a given ligand field. Visible pleochroism was strongest in (001) oriented Mndoped samples as shown by polarized optical microscopy. All samples were electrically insulating, indicative of acceptor doping, aside from Cr-doped samples where Cr acts as a deep donor.
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