Localized dry-etch substrate thinning for dislocation reduction in heteroepitaxial CdTe/Si(211)a)

Threading dislocations are a significant problem for heteroepitaxial growth of thin films on large lattice-mismatched substrates. In the case of HgCdTe thin films on Si, Ge, or GaAs, the molecular beam epitaxy (MBE) of 10–15-μm-thick CdTe buffer layers has historically played a crucial role in reducing threading dislocation densities to current state-of-the-art levels. In this work, the authors investigate a localized substrate thinning approach and its overall effect on further reducing dislocation densities in the CdTe/Si heteroepitaxial system. In using substrates with regions thinned to thicknesses on the order of the CdTe buffer, the attempt is to reduce the dislocation image force acting from the interface toward the epilayer surface. The authors employ both wet- and dry-etching techniques to create locally back-thinned regions of Si(211) wafers. Localized rather than whole wafer thinning was necessary to maintain sufficient substrate thickness for handling. The opposite sides of the wafers were cleaned using standard techniques prior to CdTe MBE. Scanning electron microscopy and Fourier transform infrared spectroscopy were used to measure epilayer and substrate thicknesses. Using CdTe defect-decoration techniques, a decrease in threading dislocation density by up to 60% has been observed in regions for which the underlying Si substrate was thinned to 2 μm. Results obtained for wet-etch and dry-etch back-thinning approaches suggest that the dislocation-reduction mechanism is not solely based on substrate-thickness induced image forces.Threading dislocations are a significant problem for heteroepitaxial growth of thin films on large lattice-mismatched substrates. In the case of HgCdTe thin films on Si, Ge, or GaAs, the molecular beam epitaxy (MBE) of 10–15-μm-thick CdTe buffer layers has historically played a crucial role in reducing threading dislocation densities to current state-of-the-art levels. In this work, the authors investigate a localized substrate thinning approach and its overall effect on further reducing dislocation densities in the CdTe/Si heteroepitaxial system. In using substrates with regions thinned to thicknesses on the order of the CdTe buffer, the attempt is to reduce the dislocation image force acting from the interface toward the epilayer surface. The authors employ both wet- and dry-etching techniques to create locally back-thinned regions of Si(211) wafers. Localized rather than whole wafer thinning was necessary to maintain sufficient substrate thickness for handling. The opposite sides of the wafers were cle...