Effect of shallow traps on electron–hole competition in semi-insulating photorefractive materials

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Abstract
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 ξ 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.