Relevance of Thermal Mismatch in Large-Area Composite Substrates for HgCdTe Heteroepitaxy

It is well known that the large lattice mismatch (>14%) associated with CdTe/Si, CdTe/Ge, and CdTe/GaAs composite substrates, is a great contributor to large dislocation densities and other defects that limit the performance of HgCdTe-based infrared detectors. Though thermal expansion mismatch is another possible contributor to material defects, little work has been done towards documenting and understanding its effects in these systems. Here, we perform studies to determine the relative contributions of lattice and thermal mismatch to CdTe film characteristics, including dislocation density and residual stress. Unannealed and thermally cycled films are characterized using x-ray diffraction, defect decoration, and Nomarski and transmission electron microscopy. For CdTe/Si, the residual stress is consistently observed to be tensile, while for CdTe/Ge and CdTe/GaAs, a compressive residual film stress is measured. We show based on theoretically predicted stress levels that the experimental measurements imply the dominance of thermal mismatch in the residual stress characteristics.