FERMI LEVEL SHIFT IN BI12SIO20 VIA PHOTON-INDUCED TRAP LEVEL OCCUPATION

Photon-induced local departures from charge neutrality are the origin of the photorefractive effect. When a periodic photorefractive grating is stored in the material, one can associate with it quasi-Fermi levels having corresponding periodic variations. The hopping band structure is modified across the local variation in charge distribution. A model is presented for the shift of the Fermi level with photon-induced trap occupation in Bi12SiO20. The shift of the Fermi level is dependent on the density of trap sites in the band gap, the radiation-induced occupation density, and the energy levels of the traps. Shifts in the Fermi levels with trap occupation have been calculated, using a simple model. Estimations of the change of density of sites in the hopping band with Fermi level shift for typical photoexcited charge concentration are shown to be consistent with experimental data.