Dislocation density and structure in Si1-xGex buffer layers deposited by LEPECVD

Fully relaxed, linearly graded Si1-xGex virtual substrates (VSs) have been grown by low-energy plasma-enhanced chemical vapour deposition (LEPECVD) in the concentration range between x=0.4 and x=1. Despite extraordinary growth rates of the order of 10 nm/s in LEPECVD, the key factors leading to low concentrations of threading dislocations turn out to be essentially the same as those previously established in molecular beam epitaxy (MBE): 1) low grading rates, preferably below 10%/?m, 2) decrease of the substrate temperature with the increasing Ge content, in order to limit the surface mobility. For Ge concentrations below x=0.5 the threading dislocation density (obtained by etch pit counting from AFM images) was found to be as low as 7 × 1055 cm-2. HRTEM inspection of the dislocation structure in LEPECVD-grown VSs has been carried out in order to obtain additional information on optimising growth parameters and post-growth annealing.