Fabrication of relaxed Si1−xGex layers on Si substrates by rapid thermal chemical vapor deposition

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Chemical vapor deposition has been applied to the fabrication of a relaxed SiGe buffer on (100) Si substrates. Our structures consist of a 0%–x% graded layer and a uniform Si1−xGex capping layer, with x between 32% and 52%. First, the variation of the threading dislocation density with grading rate has been determined. Our results clearly show an abrupt transition between two domains: steep gradings correspond to a high dislocation density, and smooth gradings to low dislocation densities. Second, the surface morphology has been studied at different steps of the buffer fabrication. In addition to the classical 〈110〉 crosshatch morphology, we report for the first time 〈100〉-oriented undulations. This new morphology, only found on steep gradings, is attributed to a Stranski–Krastanov growth mode caused by large surface strains. Finally, we propose a new picture of the role of the grading: a steep grading, with a large surface strain, induces very undulated growths; in this case, small dislocation loops and a high density of threading dislocation are generated. Our calculations allow us to derive a simple criterion: grading rates lower than 137% Ge/μm will guarantee a relaxation of the low mismatched type. Following this, very low threading dislocation densities (103 cm−2) were indeed achieved.