  Currently, Fe doping in the ~10^18 cm-3 range is the most widely-available
method for producing semi-insulating single crystalline beta-Ga2O3 substrates.
Red luminescence features have been reported from multiple types of Ga2O3
samples including Fe-doped -Ga2O3, and attributed to Fe or N at O. Herein,
however, we demonstrate that the high-intensity red luminescence from Fe-doped
beta-Ga2O3 commercial substrates consisting of two sharp peaks at 689 nm and
697 nm superimposed on a broader peak centered at 710 nm originates from Cr
impurities present at a concentration near 2 ppm. The red emission exhibits
two-fold symmetry, peaks in intensity for excitation near absorption edge,
seems to compete with Ga2O3 emission at higher excitation energy and appears to
be intensified in the presence of Fe. Based on polarized absorption,
luminescence observations and Tanabe-Sugano diagram analysis, we propose a
resonant energy transfer of photogenerated carriers in beta-Ga2O3 matrix to
octahedrally-coordinated Cr3+ to give red luminescence, possibly also
sensitized by Fe3+.
