Observation of hardening during relaxation of InGaAs on GaAs

The relaxation of Si-doped In_0.04Ga_0.96As epitaxial layers on (001) GaAs has been studied by in situ high-resolution x-ray diffraction during molecular beam epitaxy growth. By use of a novel technique to measure the wafer curvature with very high sensitivity, we have identified three regions in the relaxation process as a function of epilayer thickness. For thickness below that for multiplication of the slow A(g) misfit dislocations, the behaviour is effectively elastic, there being no detectable change in curvature at the critical thickness for dislocation nucleation. Beyond the thickness at which the A(g) dislocations multiply there is total or substantial relaxation, with very little increase in elastic strain as a function of thickness in this region corresponding to stage 1 of work-hardening in cubic materials. At much greater thickness, corresponding to stage 2 of bulk work-hardening, only partial relaxation occurs as the lattice hardens to the creation of misfit dislocations. Substantial elastic strain is observed to be locked into highly mismatched epilayers many times thicker than the critical thickness.