Rutherford backscattering study of sputtered CdTe/CdS bilayers

Rutherford backscattering (RBS) and x-ray photoelectron spectroscopy (XPS) were used to investigate interdiffusion and surface properties of CdS/CdTe bilayers. The films were grown by radio-frequency sputtering and received postdeposition heat treatments similar to the ones employed in CdTe solar cell fabrication. It is found that a ${\mathrm{CdCl}}_{\mathrm{2}}$ anneal strongly enhances both the diffusion of S into the CdTe layer and the surface oxidation. The diffusion of S in CdTe in this process can be described by a constant surface source model with the diffusivity given by ${\text{D=3.2×10}}^{\text{−5}} \exp \text{(−1.2\hspace{0.05em}}\mathrm{eV}\mathrm{/kT) }{\mathrm{cm}}^{\mathrm{2}}{\mathrm{ s}}^{\mathrm{-1}}$ in the temperature range studied. The change in the chemical composition of the surface following the ${\mathrm{CdCl}}_{\mathrm{2}}$ anneal was analyzed by XPS showing that Te oxides and residual compounds containing Cl were present which could account for the straggling in the RBS spectrum. An HCl etch completely removes oxides and residues leaving a smooth surface.