Physical properties of semiconducting CdF2 crystals doped with In are determined from measurements of the radio-frequency response of a sample with Schottky barriers at frequencies 10 − 10 6 Hz. The dc conductivity, the activation energy of the amphoteric impurity, and the total concentration of the active In ions in CdF2 are found through an equivalent-circuit analysis of the frequency dependencies of the sample complex impedance at temperatures from 20 K to 300 K. Kinetic coefficients determining the thermally induced transitions between the deep and the shallow states of the In impurity and the barrier height between these states are obtained from the time-dependent radio-frequency response after illumination of the material. The results on the low-frequency conductivity in CdF2:In are compared with submillimeter (10
11− 10 12 Hz) measurements and with room-temperature infrared measurements of undoped CdF2. The low-frequency impedance measurements of semiconductor samples with Schottky barriers are shown to be a good tool for investigation of the physical properties of semiconductors.
We consider the mechanisms responsible for the photoinduced change in the optical properties of semiconducting CdF2 crystals with metastable Ga impurities forming DX centers. Unlike the case of compound semiconductors with DX centers (GaAlAs:Si, GaAlAs:Te, CdZnTe:Cl), this change is caused not by free electrons but by a redistribution of electrons between deep and shallow localized states. The resulting modification of the refractive index of the crystal allows writing of persistent holographic gratings at temperatures up to 200 K, high for this class of holographic materials. Holographic characteristics of CdF2:Ga crystals such as refractive index change, sensitivity, and grating decay are described.
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