Excitation effects in excess Ca-doped epitaxial CaGe:YIG garnets

The effects of light and electric field on the electrical transport properties of p-type CaGe:YIG epitaxial films, having 0.1 at. f.u. excess Ca2+, were investigated from 30 to 340 K by measurements of dc conductivity, space-charge-limited current, and thermally stimulated current decay. The room-temperature conductivity of the films is of the order of 10−6 mho/cm, the mobility of the holes is 0.5 cm2/V s, and the density of charge carriers is 1012/cm3. The low-temperature conductivity of the films can be increased by six orders of magnitude under the effect of light (∼25 mW/cm2) and electric field (≤104 V/cm) at temperatures below 140 K. This photo- and electric-field-excited high conductivity state can be destroyed by thermal excitation at higher temperatures. It is assumed that below 200 K the holes are localized at Ca2+-Fe4+ centers which constitute an impurity band. Electrons excited from the valence band are trapped under the influence of the field at centers, formed probably by oxygen vacancies. The holes left in the valence band contribute to the high conductivity at low temperature.