On the mechanism of current-transport in Cu/CdS/SnO2/In-Ga structures

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 (111) texture. The study has been made on the behavior of Cu/n-CdS thin film junction on SnO2 coated glass substrate grown using thermal evaporation method. The current-voltage (I-V) characteristics of Cu/CdS/SnO2/In-Ga structures have been investigated in the temperature range of 130-325 K. The semi-logarithmic lnI-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 (?Bo) with the increasing temperature. The values of n and ?Bo change from 8,98 and 0,29 eV ( at 130K) to 3,42 and 0,72 eV ( at 325 K), respectively. The conventional Richardson plots of the ln (Io/T2) vs q/kT show nonlinear behavior. The forward bias current I is found to Io(T)exp(AV), where A is the slope of ln(I)-V plot and almost independent of the bias voltage and temperature, and Io(T) is relatively a weak function of temperature. These results indicate that the mechanism of charge transport in the SnO2 /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 (Ri) were obtained from forward and reverse bias I-V characteristics by using Ohm?s Law for each temperature level.