EFFECT OF THERMAL ANNEALING ON THE ELECTRICAL PROPERTIES OF CdTe-ZnSe THIN FILM HETEROJUNCTIONS

摘要

Thin film (p)CdTe/(n)ZnSe heterojunctions were fabricated by depositing n-type ZnSe thin films over ptype CdTe thin films on cleaned glass substrates by thermal evaporation technique. The current-voltage characteristics of the prepared junctions were studied at room temperature as well as elevated temperatures. All the junctions exhibited rectifying I-V characteristics with non saturating reverse current. From the current-voltage characteristics, the different junction parameters such as ideality factor, saturation current density, series resistance etc. were measured. The ideality factors (> 2 in dark) were found to decrease with increase in temperature. The saturation current density of the junction at room temperature was found to be 6 nA/cm2 and found to increase with temperature. At room temperature (298K), the built in potential was found to be 0.77eV and showed no temperature dependence in the temperature range from 298K to 333K. The junctions were also studied after a short heat treatment at 373K for 20 minutes in vacuum. The junction parameters were found to change significantly after short heat treatment. The junctions were found to possess a series resistance as high as 16 MΩ for an untreated sample and 9 MΩ for heat treated sample. The photovoltaic effect of the junctions was studied by illuminating the junctions by a source of white light with illumination 50mW/cm2. The junctions showed a photovoltaic effect with fill factor 0.38, open-circuit voltage 470 mV, short-circuit current density 2.1×10-6 A/cm2 for an as prepared sample. The photo-voltaic performance of the junction was found to be improved after short heat treatment. Proper doping and annealing are necessary to reduce the series resistance so as to achieve an ideal solar cell.