The effects of growth temperature, substrate off-cut, and dislocation pile-up formation on threading dislocation density in compositionally graded SiGe buffers were explored. To investigate dislocation glide kinetics in these structures, a series of identical samples graded to 30%Ge were grown at between 650 and 900C on (001)-, (001) off-cut 6° towards an in-plane <110>-, and (001) off-cut 6° towards an in-plane <100>-oriented Si substrates. The field threading dislocation density in the on-axis samples varied exponentially with temperature; from 3.7 x 106/cm2 at 650C to 9.3 x 104/cm2 at 900C. The activation energy for dislocation glide in this series, calculated from the evolution of field threading dislocation density with growth temperature, was 1.38eV, much lower than the expected value for this composition. This deviation indicated that strain accumulating during the grading process at low growth temperatures was forcing further dislocation nucleation, resulting in a deviation from pure glide-limited relaxation. The TDD of samples grown on off-cut substrates exhibited a more complicated temperature dependence, likely because films grown on off-cut substrates have an increased tendency towards saturation in dislocation reduction reactions at high temperature. Dislocation reduction processes were further explored by initiating compositional grading up to 15% Ge at 650C and continuing the grade to 30% Ge at 900C. The low temperature portion of this growth provided an excess concentration of threading dislocations which could subsequently be annihilated during the high temperature portion of the growth, enabling a comparison of reduction rates for different substrate off cuts. Combining these results with threading dislocation densities in a variety of other samples, a complete picture of strain relaxation kinetics in compositionally graded SiGe/Si emerges. Generally, strain relaxation in these structures was limited by dislocation glide, and threading dislocation densities were independent of final Ge content. However, it was proposed that dislocation pile-up formation inhibits the strain relaxation process and was therefore accompanied by a rise in field threading dislocation density. Based on these results, a predictive model was developed for TDD in compositionally graded SiGe/Si over a wide range of growth conditions.

Dislocation Glide and Blocking Kinetics in Compositionally Graded SiGe/Si. C.W.Leitz, M.T.Currie, A.Y.Kim, J.Lai, E.Robbins, E.A.Fitzgerald, M.T.Bulsara: Journal of Applied Physics, 2001, 90[6], 2730-6