Effect of isothermal annealing and visible-light illumination on the AC-impedance behavior of undoped selenium thin films

Abstract

Abstract The effect of post-deposition isothermal annealing (30 °C ⩽ T A ⩽ 70 °C) and visible-light illumination on the complex AC-impedance of undoped selenium thin films deposited at the substrate temperatures T S = 30, 50, 70 °C has been studied in the frequency range 0.2–12 kHz. The AC-impedance of amorphous selenium (a-Se) films ( T S , T A Z ″( ω )– ω curves, irrespective of illumination. This behavior was ascribed to a dominant charge-carrier trapping effect of bulk/surface charged defects usually present in a-Se. On the other hand, the measured Z ″( ω )– Z ′( ω ) diagrams of illuminated polycrystalline Se samples (50 °C ⩽ T S , T A ⩽ 70 °C) exhibited almost full semicircles, whereas their Z ″( ω )– ω curves revealed prominent loss peaks at well-defined frequencies. As the annealing temperature or light intensity is increased the loci of the points determined by intersections of these semicircles with the Z ′-axis at the low-frequency side shift greatly towards the origin, while the loss-peak positions shift to higher frequencies. These experimental findings were explained in terms of a significant increase in electrical conductivity of selenium films due to thermally-induced crystallization at temperatures beyond glass-transformation region of undoped selenium and to creation of electron–hole pairs by visible-light illumination.