Strain relaxation in (100) and (311) GaP∕GaAs thin films

The nature of strain relaxation in GaP films grown on (100), (311)A, and (311)B GaAs by molecular beam epitaxy has been studied by transmission electron microscopy and atomic force microscopy. It is found that (100) GaP∕GaAs films develop surface undulations with twinning and cracking. (311)A GaAs provides good growth orientation for GaP films, producing flat surfaces and crack-free films. Similarly, (311)B GaP∕GaAs films do not develop cracks, however, the surface is rough. The anisotropy of cracking observed in GaP films has been discussed in terms of a lattice trapping theory. Twinning is an effective form of stress relaxation in GaP films. The features of the dislocation structure produced during relaxation have been discussed in terms of energy considerations. The data suggest that the equilibrium position of misfit dislocations is located in the softer substrate where the total energy of the system is a minimum. During relaxation, those dislocations which acquire larger kinetic energy can move over the small energy well and penetrate deeper into the substrate, in agreement with transmission electron microscope observations.