It was recalled that a Li atom at a substitutional Zn site (LiZn) had a rather shallow acceptor level (about 0.1eV). However, the LiZn was not stable and the Li atoms tended to migrate to an interstitial site in p-type material. A previous simple calculation had predicted that the compensating native defects could be decreased by above-bandgap photo-irradiation. The basic concept was that compensation in p-type materials occurred because the effective formation energy of a donor defect decreased, with approach of the Fermi level to the valence band, due to an increase in the energy which was gained upon electron transfer from the donor level to the Fermi level. Thus, compensation could be prevented if the quasi-Fermi level was moved closer to the conduction band by means of photo-irradiation. Here, the model was extended by taking account of the site change of Li atoms and it was shown that the concentration of interstitial Li could be decreased. That is, the Li acceptor could be stabilized by photo-irradiation. Defect concentrations under irradiation were calculated by considering steady-state electron statistics. The interstitial Li concentration was much larger than that of LiZn in the equilibrium state, but most Li atoms occupied a Zn site when the concentration of photo-excited excess carriers was increased to 1012/cm3. The site change mechanism was such that an increase in the carrier excess caused an increase in the occupation probability of the interstitial Li donor level and thus in the concentration of neutral interstitial Li. Consequently, the LiZn concentration increased because of the reaction, Lii + VZn  LiZn, where VZn and LiZn were a neutral Zn vacancy and a neutral Li acceptor, respectively. If the lifetime of the excess carriers was about 1ns, unfocussed irradiation from a mercury lamp could create 1012/cm3 of excess carriers near to the Zn surface. It was concluded that the present results indicated that the Li acceptors tended to be stable in the active regions of light-emitting and laser diodes.

M.Ichimura, T.Wada, S.Fujita, S.Fujita: Materials Science Forum, 1993, 117-118, 525-6