Influence of substrate temperature on the processing of CdTe and CdS thin films

The structural and radiative defect states resulting from thin film CdTe/CdS deposited onto different temperature (from ambient to 250 degrees C) glass substrates have been investigated. Samples represent the as-deposited and treated ones. Treatments include annealing at 400 degrees C for 30 min, dipping into a bath of saturated CdCl2 solution (at standard temperature and pressure conditions) for 5 s and finally another cycle of heat treatment, in open air. Measurements were done by three complementary techniques, namely Rutherford backscattering (RBS), photoluminescence (PL) and thermoluminescence (TL), Structural variations were sought by RBS while information about both radiative and trap defect states was determined by both PL and TL techniques. All three methods gave consistent results, which indicate variations in the structure and formation of shallow and deeper defect states that are a function of the preparation conditions. The PL emission spectra of treated samples reveal changes in the maximum intensity correlated with the relative magnitudes of excitonic emissions from the detected asymmetric broad band centred at 1.43 eV, The TL glow curves, on the other hand, indicate the presence of new trapping states detected at 75, 135 and 205 degrees C. Changes in the relative concentration of relatively deeper levels centred at 290 degrees C and 385 degrees C are noted. The newly formed shallow defects were absent prior to treatment and are due to chlorine incorporation, surface region depletion and film species interdiffusion. The trap depth (activation energy) before and after treatment were determined from kinetic analysis using total glow curve deconvolution.