Feasibility of Localized Substrate Thinning for Reduced Dislocation Density in CdTe/Si Heterostructures

Abstract

HgCdTe heteroepitaxy on low-cost, large-lattice-mismatched substrates such as Si continue to be plagued by large threading dislocation densities that ultimately reduce the operability of the thermal imaging detector array. Molecular-beam epitaxy (MBE) of 10 lm-to 15 lm-thick CdTe buffer layers has played a crucial role in reducing dislocation densities to current state-ofthe-art levels. Herein, we examine the possibility that growth on locally backthinned substrates could prove advantageous in further reducing dislocation densities in the CdTe/Si heteroepitaxial system. Using defect decoration techniques, a decrease in dislocation (etch-pit) density of up to $42% has been measured in CdTe regions where the underlying Si substrate was chemically back-thinned to $20 lm. A theoretical understanding is proposed, where a substrate-thickness-dependent dislocation image force is a likely cause for the experimentally observed reduction in threading dislocation density. These observations raise the prospect of combining localized substrate thinning with other techniques to further reduce dislocation densities to levels sought for HgCdTe/CdTe/Si and other large-lattice-mismatched systems.

Microstructural Characterization of CdTe(211)B/ZnTe/Si(211) Heterostructures Grown by Molecular Beam Epitaxy

“…4 A further reduction in the CdTe buffer dislocation density is expected to benefit long-wavelength infrared (LWIR) HgCdTe detectors. 5 Close attention must, therefore, be given to controlling the generation and propagation of the defects into the HgCdTe epilayer.…”

Si Wafer Thinning Techniques Compatible With Epitaxy of CdTe Buffer Layers

“…This result was unexpected. As previously stated, Jacobs et al 16 reported 40% EPD reduction for CdTe grown on 20-lm-thick Si obtained by chemical etching. Additionally, calculations made from the image force model developed by Freund and Nix 17 suggested a greater reduction in TDs for Si thicknesses of 5 lm or lower.…”

“…The morphology of back-thinned Si using the DRIE technique described above was compared with Si back-thinned using HF-HNO 3 chemical techniques described in Jacobs et al 16 Optical images of the chemically etched Si show a rough and pitted surface, with large lateral inconsistencies in thickness and isotropic sidewalls. Optical images of the DRIE-thinned Si show a specular surface with distinct vertical sidewalls.…”

“…This effort continues the work performed by Jacobs et al 16 Instead of a wet chemical etch to locally back-thin Si substrates, a dry-etching process is examined with the goal of thinning down to 5 lm. Thinning is followed by growth of a CdTe buffer layer.…”

“…[13][14][15] Additionally, Jacobs et al attempted growth of CdTe buffer layers on back-thinned conformal Si substrates. 16 It was reported that (211) Si substrates were locally backthinned by chemical etching, using a mixture of HF and HNO 3, down to thicknesses as low as 20 lm. CdTe buffer layers were then grown on these substrates using standard methods.…”