  Gallium oxide (b-Ga2O3) is a wide-bandgap compound semiconductor with a
bandgap of ~ 4.9 eV that is currently considered promising for a wide range of
applications ranging from transparent conducting electrodes to UV
optoelectronic devices and power electronics. However, all of these
applications require a reliable and precise control of electrical and optical
properties of the material, which can be largely affected by impurities, such
as transition metals commonly present during the growth. In this work we employ
electron paramagnetic resonance (EPR) spectroscopy to obtain EPR signatures of
the 3d-transition metals Co2+ and Cu2+ in b-Ga2O3 bulk crystals and powders
that were unknown so far. Furthermore, we show that Co2+ and Cu2+ both
preferentially reside on the octahedral gallium lattice site.
