Photoluminescence of CVD grown CdS epilayers on CdTe substrates

Photoluminescence (PL) measurements of high structural quality CdS epitaxial layers grown on{111}A CdTe substrates by chemical vapour deposition (CVD) are reported. Low temperature PL spectra show intense excitonic emissions due to both free and bound exciton recombinations, together with their longitudinal optical phonon replicas. At high excitation intensity a strong biexciton emission dominates the spectra. Also, the presence of a weak donor-acceptor pair recombination band around 520 nm suggests that the impurity content in the present samples is not high. A broad emission band peaked at about 590.0 nm is identified as due to excitons bound to isoelectronic substitutional Te atoms on sulfur lattice sites. The dependence of this emission on the growth temperature and epilayer thickness, clearly indicates that Te atoms diffuse from the underlying CdTe substrates into the CdS layers at the relatively high temperatures (570–700°C) used for the CVD growth, giving rise to the autodoping of the CdS epitaxial layers. This is the first time that such Te diffusion is evidenced.