It was recalled that P was one of several dopants which electronically compensated the native deep donor that was responsible for the yellow coloration of this oxide. Low-temperature optical absorption measurements of a series of Czochralski-grown P-doped crystals showed that between about 0.1 and 0.15at%P was required in order to remove the yellow coloration completely. The absorption cut-off in the fully compensated P-doped sample was at 3.2eV whereas, in compensated Al-doped and Ga-doped samples, the cut-off was at 3.35eV. Excitation at 10 to 15K, using near-bandedge light, produced photochromic absorption bands. In lightly doped (partially bleached) samples, these bands were identical to those which were observed in undoped material. A new spectrum was observed in fully bleached samples. Its major contribution was a band that was centered near to 1.8eV, with a weaker absorption in the blue-green range. By comparing it with the spectra which were observed in undoped and Al-doped material, before and after photo-excitation, it was deduced that the 1.8eV band was due to the [PO4]- center and that the broad 2.45eV band which was observed in Al-doped and Ga-doped material was due to the [BiO4]0 center.

J.S.McCullough, A.Harmon, J.J.Martin, M.T.Harris, J.J.Larkin: Journal of Applied Physics, 1995, 78[3], 2010-4