Correlation between electrical conductivity and luminescence properties in β-Ga₂O₃:Cr³⁺ and β-Ga₂O₃:Cr,Mg single crystals

The photoluminescence, excitation, and absorption spectra as well as the electrical conductivity of β-Ga₂O₃:Cr and β-Ga₂O₃:Cr,Mg single crystals were studied. The as-grown β-Ga₂O₃:Cr crystals had a green color, the conductivity at about 10⁻²–10⁻³ Ω⁻¹ cm⁻¹, and a low yield of Cr³⁺ impurity luminescence. Annealing in oxygen atmosphere led to a strong increase in Cr³⁺ red luminescence yield, increase in the resistivity, and changes in the absorption and excitation spectra. Similarly, increases in the Cr³⁺ luminescence yield and resistivity were observed after codoping of β-Ga₂O₃:Cr crystals with magnesium (Mg²⁺). The registered changes in the Cr³⁺ luminescence yield, electrical conductivity, and in the absorption and excitation spectra are considered to be due to the shift in the Fermi level. In the as-grown β-Ga₂O₃:Cr crystals, the Fermi level is located near the bottom of the conduction band, and most chromium ions are in the Cr²⁺ charge state. Annealing in an oxygen atmosphere as well as codoping of the crystals with chromium and magnesium impurities moves the Fermi level toward the middle of the bandgap and recharges the chromium ions to the Cr³⁺ state.