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 (1 1 1) 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 forward bias 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 (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/T²) 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 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 (R_i) were obtained from forward and reverse bias I-V characteristics by using Ohm's law for each temperature level.