Microstructure and Electronic Structure of Strain-Relaxed SiGe Films

Strain-relaxed step-graded Si_1-xGe_x/Si structures having low densities of threading dislocations are used as buffer layers for modulation-doped field effect transistors (FETs). Uniform composition
Si_1-xGe_x layers grown on step-graded structures are ideal for studying the microstructure and electronic structure of SiGe films. Here we discuss the results of x-ray microdiffraction experiments which reveal the presence of local tilted regions on the order of 10 µm in dimension originating from the network of misfit dislocations which relieve the strain in these structures. We also discuss results of low temperature photoluminescence experiments which reveal new information on the electronic state associated with threading dislocations in these films. Specifically, we have observed the anti-crossing of the D1 and D2 states upon thermal annealing of the samples indicating that D2 cannot be a phonon replica of D1 as proposed earlier. This study highlights the importance of local strain at dislocations on these electronic states. Polyexcitons confined in local potential wells due to alloy composition fluctuations in step-graded buffer layers have also been observed with low temperature photoluminescence. The presence of only a singly type of polyexciton in a given sample is a manifestation of the indistinguishability of quantum particles having zero or integral spin.