On the mechanism of current-transport in Cu/CdS/SnO 2/In–Ga structures

Abstract

The structural and optical properties of CdS films deposited by evaporation were investigated. X-ray diffraction study showed that CdS films were polycrystalline in nature with zinc-blende structure and a strong (1 1 1) texture. The study has been made on the behavior of Cu/n-CdS thin film junction on SnO 2 coated glass substrate grown using thermal evaporation method. The forward bias current–voltage ( I– V) characteristics of Cu/CdS/SnO 2/In–Ga structures have been investigated in the temperature range of 130–325 K. The semi-logarithmic ln I– V characteristics based on the Thermionic emission (TE) mechanism showed a decrease in the ideality factor ( n) and an increase in the zero-bias barrier height ( Φ Bo) with the increasing temperature. The values of n and Φ Bo change from 8.98 and 0.29 eV (at 130 K) to 3.42 and 0.72 eV (at 325 K), respectively. The conventional Richardson plot of the ln( I o/ T 2) vs q/ kT shows nonlinear behavior. The forward bias current I is found to be proportional to I o( T)exp( AV), where A is the slope of ln( I)– V plot and almost independent of the applied bias voltage and temperature, and I o( T) is relatively a weak function of temperature. These results indicate that the mechanism of charge transport in the SnO 2/CdS/Cu structure in the whole temperature range is performed by tunneling among interface states/traps or dislocations intersecting the space-charge region. In addition, voltage dependent values of resistance ( R i ) were obtained from forward and reverse bias I– V characteristics by using Ohm's law for each temperature level.