Optical Absorption and Photoconductivity in the Band Edge of $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$

Optical absorption and photoconductivity have been observed in the ultraviolet in single crystals of nominally pure $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$. At room temperature a steep absorption edge, characteristic of a band-to-band transition, is observed at 2700 Å. The edge is shifted approximately 100 Å toward shorter wavelengths when the temperature is reduced to 77°K. Photoconductivity begins coincident with the absorption edge at 77°K, but could not be detected at room temperature. A model is proposed in which the absorption arises as a result of excitation of an electron from the oxygen $2p$ band to the gallium $4s$ band. Calculations using this model and the Born-Haber cycle are in good agreement with the observed band gap of 4.7 eV. It is suggested that the much smaller band gap of $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$ as compared with sapphire is due to the reduced coordination number of the ions involved in the transition.