The mechanism of the high conduction state in the Ca2+Ge4+:YIG system

Abstract

Semi-insulating epitaxial garnet films of p-type Ca2+Ge4:YIG can be excited to a high conduction state (HCS) under the effects of light and electric field. To clarify the mechanism of the HCS, epitaxial films of Ca2+:YIG, Ge4+:YIG, Ca or Ge excess Ca2+Ge4+:YIG have been investigated. dc I-V characteristics were measured in electric fields up to 105 V/cm, with and without light. Thermopower and deep level transient spectroscopy (DLTS) were measured between 140 and 300 K. At low electric field the conduction is ohmic. As the electric field is increased a square-law dependence is observed and at a critical field, on the order of 105 V/cm and whose value is sensitive to the frequency and intensity of the light, a negative resistance state is formed in Ca:YIG and CaGe:YIG (both p- and n-type) films, and in this region the sample suddenly transforms to the HCS. A very slow transformation to the HCS can also be observed at lower electric fields, on the order of 104 V/cm, by keeping the electric field constant and illuminating the sample with light on the order of 10 mW/cm2 intensity. Thermopower and DLTS data indicate that electrons located in compensated acceptor states are removed in the HCS. The presence of a sufficient number of compensated acceptors and oxygen vacancies serving as repulsive traps is a necessary condition for the formation of the HCS. The transition to this HCS is then proposed to be a process where electrons are transferred to the repulsive traps located below the conduction-band mobility edge. This trapping process lowers the Fermi level toward the valence band.