On the origin of red luminescence from iron-doped beta-Ga2O3 bulk crystals

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+.