Optical and Microwave Properties of Trivalent Chromium inβ-Ga2O3

Optical absorption, fluorescence, and paramagnetic-resonance experiments on single crystals of $\beta$-${\mathrm{Ga}}_2$ ${\mathrm{O}}_3$ doped with chromium were performed from room temperature down to a few degrees Kelvin. All the important low-lying energy levels have been identified from the optical data. The average positions of the three sets of sharp lines (doublet transitions) are almost identical to those of ruby, whereas the two prominent bands (quartet transitions) are broadened and shifted to the red for ${\mathrm{Ga}}_2$ ${\mathrm{O}}_3$. Comparison of the 77°K absorption data with octahedral crystal-field calculations for the $d^3$ configuration yielded the values $B=700\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and $\frac{\mathrm{Dq}}{B}=2.35\mathrm{}$. Fluorescence is observed from the ${}_{}{}^2{}_{}{}^{}E$ to ${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ transition with a room-temperature lifetime of 0.23 msec. At 77°K these emission peaks occur at 6885 and 6956 Å. The fluorescence efficiency of crystals grown from different starting materials varied considerably, but the factors that give rise to this variation are only partly understood. The spin Hamiltonian parameters for the ${}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ ground state were determined from $X$-band paramagnetic-resonance measurements and are: $D=-14.01\mathrm{}$ kMc/sec, $E=+6.14\mathrm{}$ kMc/sec, $g_x=1.978\mathrm{}$, $g_y=1.978\mathrm{}$, and $g_z=1.983\mathrm{}$ for the z axis along [010]. The zero-field ground-state splitting was observed at a frequency of 35.18 kMc/sec. Calculations for the angular spectrum using this spin Hamiltonian are in excellent agreement with the $X$-band measurements. The possibility of using ${\mathrm{Cr}}^{3+}$ in ${\mathrm{Ga}}_2$ ${\mathrm{O}}_3$ as the active material in an optical or microwave maser is briefly discussed.