Charge-exchange processes in titanium-doped sapphire crystals. I. Charge-exchange energies and titanium-bound excitons

The photoionization of ${\mathrm{Ti}}^{3+}$ in sapphire α-${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ was studied by means of one-step and two-step photoconductivity measurements. An absorption band of ${\mathrm{Ti}}^{3+}$ at 2700 Å was shown to be due to a ${\mathrm{Ti}}^{3+}$ localized exciton. One-photon photoconductivity begins at the high-energy side of this band and thus the photoionization threshold can be located at 4.71±0.07 eV. The two-photon photoconductivity spectrum is nearly coincident with the excited state absorption and is shifted 0.6 eV to higher energy than the one-photon spectrum, due to the effect of the Jahn-Teller distortion of the ${}_{}{}^2{}_{}{}^{}$E initial state. The origin of the transition due to valence band electron capture by ${\mathrm{Ti}}^{4+}$ is located at 4.17 eV. These electron and hole ionization energies are compared to values obtained by Born cycle calculations. Their sum is 0.5 eV less than the band gap of ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$.