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Reverse graded high content (x>0.75) Si1-xGex virtual substrates

By V. A. Shah


Silicon germanium alloy layers can be grown epitaxially on a silicon substrate to provide a means of adjusting the lattice parameter of the crystal. Such a platform, known as a virtual substrate, has a number of potential applications. For instance, it allows for subsequent overgrowth of highly strained layers of silicon, or germanium, that could enable very high speed transistors, similarly it could be used as the starting point of a range a silicon-based optoelectronic devices. In this work, a novel adaptation has been made to a recently proposed reverse grading technique to create high Ge composition SiGe virtual substrates. The proposed structures consist of a relaxed, highly defected, pure Ge underlayer on a Si (001) substrate prior to reverse grading where structures have final compositions of Si0.2Ge0.8. Additionally, two grading schemes are studied, reverse linear grading and reverse terrace grading. All buffers are grown by reduced pressure chemical vapour deposition. The relaxation, defect levels and surface roughness of the fabricated buffers have been quantified whilst varying the grading rate.\ud An ideal grading rate has been found where the quality of the buffer is very high, due to the highly defected Ge underlayer and that the buffer relaxes under tensile strain. Outside of this ideal grading rate three dimensional growth, stacking fault formation and crack generation can occur. Cracking of the buffer has been modelled and some conditions where the buffer is stable have been found. This study experimentally investigates this proposed solution and a crack-stable high quality buffer is fabricated. Comparisons have been drawn with other more popular buffer fabrication techniques and it is found that this technique has very competitive qualities

Topics: TK, QC
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