Thermal Stabilization of Porous Silicon: A Key Step for High-Quality SiGe Epitaxy

This study focuses on the epitaxial growth of virtual silicon-germanium (SiGe) substrates on porous silicon (PSi). Epitaxy was performed on different types of PSi substrates, with or without prior thermal annealing. Morphological and structural investigations by atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) show that epitaxial SiGe films grown on double-layer PSi substrates, annealed at 1000°C, exhibit significantly higher crystalline quality than those grown on unannealed PSi substrates or PSi substrates annealed at temperatures lower than or equal to 900°C. This improvement is attributed to the beneficial effects of 1000°C annealing, which leads to stress relaxation, internal microstructure stabilization and significant improvement of PSi surface morphology. In contrast, direct growth of SiGe on unannealed PSi, even at moderate temperature (~ 400°C), induces structural degradation of the porous buffer, leading to a very high dislocation density in the epitaxially grown SiGe films. A well-optimized thermal treatment of PSi substrates promotes the growth of high-quality virtual SiGe substrates on PSi that is both efficient and economically viable for the development of SiGe-based photovoltaic cells.