Strain relief in linearly graded composition buffer layers: A design scheme to grow dislocation-free (<105 cm−2) and unstrained epilayers

The strain relaxation in linearly graded composition InGaAs layers grown on (001) GaAs substrates by molecular beam epitaxy is studied by transmission electron microscopy (TEM) and double crystal x-ray diffraction (DCXRD). The dislocation distribution in these layers does not coincide with the predicted equilibrium dislocation distribution [J. Tersoff, Appl. Phys. Lett. 62, 693 (1993)]. The dislocation density in the dislocation-rich layer thickness is slightly smaller than the equilibrium density. The thickness of the dislocation-rich region is different in the [110] and [11̄0] directions. A good correspondence exists between the TEM and DCXRD strain measurements. The dislocation distribution observed by TEM has made it possible to design a scheme to grow dislocation-free and unstrained top layers on linearly graded composition buffer layers.