EFFECT OF SHALLOW TRAPS ON ELECTRON-HOLE COMPETITION IN SEMIINSULATING PHOTOREFRACTIVE MATERIALS

A model of the photorefractive effect is presented that includes a shallow-trap level (hole or electron trap) as well as a species of deep traps with electron and hole excitation. It is shown that, if the shallow traps can trap a significant density of charge carriers, the electron-hole competition factor xi and the photorefractive grating can switch signs as a function of intensity. This effect may be realized in BaTiO3 crystals, in which the photorefractive effect is strongly affected by electron-hole competition as well as by shallow traps. As in the case of the single-charge-carrier model, it is shown that each of the charge gratings in the shallow and deep levels consists of in-phase gratings and a screening grating. It is shown that part of the screening gratings remains nonzero at infinitly large grating spacings.