Dislocation glide and blocking kinetics in compositionally graded SiGe/Si

The effects of growth temperature, substrate offcut, and dislocation pileup formation on threading dislocation density (TDD) in compositionally graded SiGe buffers are explored. To investigate dislocation glide kinetics in th ese structures, a series of identical samples graded to 30% Ge were grown a t temperatures between 650 and 900 degreesC on (001)-, (001) offcut 6 degre es towards an in-plane [110]-, and (001) offcut 6 degrees towards an in-pla ne [100]-oriented Si substrates. The field threading dislocation density (f ield TDD) in the on-axis samples varied exponentially with temperature, fro m 3.7x10(6) cm(-2) at 650 degreesC to 9.3x10(4) cm(-2) at 900 degreesC. The activation energy for dislocation glide in this series, calculated from th e evolution of field TDD with growth temperature, was 1.38 eV, much lower t han the expected value for this composition. This deviation indicates that strain accumulating during the grading process at low growth temperatures i s forcing further dislocation nucleation, resulting in a deviation from pur e glide-limited relaxation. The TDD of samples grown on offcut substrates e xhibited a more complicated temperature dependence, likely because films gr own on offcut substrates have an increased tendency towards saturation in d islocation reduction reactions at high temperature. Dislocation reduction p rocesses were further explored by initiating compositional grading up to 15 % Ge at 650 degreesC and continuing the grade to 30% Ge at 900 degreesC. Th e low temperature portion of this growth provided an excess concentration o f threading dislocations which could subsequently be annihilated during the high temperature portion of the growth, enabling a comparison of reduction rates for different substrate offcuts. Combining these results with thread ing dislocation densities in a variety of other samples, a complete picture of strain relaxation kinetics in compositionally graded SiGe/Si emerges. G enerally, strain relaxation in these structures is limited by dislocation g lide, and threading dislocation densities are independent of final Ge conte nt. However, we theorize that dislocation pileup formation inhibits the str ain relaxation process and is therefore accompanied by a rise in field thre ading dislocation density. Based on these results, we now have a predictive model for TDD in compositionally graded SiGe/Si over a wide range of growt h conditions. (C) 2001 American Institute of Physics.