Thermally stimulated luminescence (TSL) of β-Ga2O3 single crystals doped with Cr3+ and Mg2+ impurities was investigated. Based on the correlation between the Cr3+ concentration and light sum accumulated in the thermoluminescence (TL) glow peak at 285 K, it was concluded that doping of β-Ga2O3 with Cr3+ ions leads to the formation of electron traps manifested in this peak. The activation energy of peak at 285 K is equal to Ec-0.55 eV and close to E1. Thus the Cr3+e− centers can be a candidate for E1. The high-temperature TL glow peak at 385 K (Ec-0.94 eV) is related to oxygen vacancies which are created in gallium oxide doped by Mg2+ ions to compensate for the negative charge formed by the substitution of gallium sites by magnesium ions.The co-doping of β-Ga2O3 crystals with Cr3+ and Mg2+ impurities leads to the appearance of a new TL glow peak at 320 K with an energy close to E*2 (Ec-0.7). It is suggested that this peak is formed by defect complex, in particular, oxygen vacancies with Cr3+ or Fe3+ ions.
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.
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