Configurable Compliant Substrates for SiGe Nanomembrane Fabrication

The aim of this work is to give quantitative guides for the fabrication of strain-engineered SiGe epitaxial nanomembranes using a compliant substrate. We theoretically determine the effect of the elastic properties (softness and strain level) of a compliant substrate on the morphological evolution of an epilayer. The experimental system under investigation is the SiGe on Si(001) model system, which develops an Asaro–Tiller–Grinfel’d (ATG) growth instability or misfit dislocations for large epitaxial stresses. The compliant substrate is a porous silicon layer whose softness and strain level can be adjusted by varying the density of pores and the annealing conditions. The softness and strain level of the compliant substrate are analyzed independently. We show that the softness of the compliant substrate produces a significant enhancement of the growth instability that normally develops in SiGe/Si systems. We rationalize the counterintuitive and commonly misunderstood instability enhancement by considering the elastic energy in the substrate after deposition of an epilayer. A fundamental result to mention is that in the experimentally relevant system consisting of Si substrate/compliant pseudosubstrate/Si buffer layer/epitaxial layer, a major parameter that controls the instability development is the thickness of the Si buffer layer. For a soft compliant substrate (typically with a Young’s modulus ten times smaller than silicon), a thin buffer layer (20 nm thick) suppresses the compliant effect. This is an important result for experimental studies, which commonly neglect the influence of the buffer layer. We quantify this effect and give an experimental proof, which confirms the theoretical result. We then consider the effect of the strain level of the compliant substrate. We give evidence that, in standard experimental situations, a tensilely strained compliant substrate could kinetically inhibit the development of the instability and could also delay the nucleation of misfit dislocations. The theoretical results are confirmed by exemplary experimental results using a tensilely strained porous silicon substrate obtained by annealing in specific conditions (HT-PSi). Both the inhibition of the ATG growth instability and the delay of the nucleation of misfit dislocations are evidenced for different SiGe alloy concentrations in good agreement with theoretical models. These results strongly highlight the importance of the HT-PSi as a configurable compliant substrate not only for the fabrication of SiGe nanomembranes totally flat and free of dislocation but also for the heterogeneous growth of various systems on silicon. They also give a basic understanding of the elastic mechanisms and the universal rules for producing generic configurable compliant substrates