Strain relaxation and related phenomena in gallium nitrogen arsenide and gallium phosphide films on gallium arsenide substrates

GaNAs and GaP films grown on (100) GaAs, as well as GaP films grown on (311)A and (311)B GaAs by molecular beam epitaxy, have been investigated by transmission electron microscopy (TEM), atomic force microscopy (AFM), and optical microscopy. The investigation focuses on structural defects induced by lattice mismatch between an epitaxial film and a substrate.; For (100) GaNAs/GaAs and GaP/GaAs, it is found that the nature of the defects produced is directly related to the nature of the surface developed during growth of these compound films. In general, when the surface of the film is smooth, strain relaxation occurred by cracking; while in films with a rough surface, relaxation occurred by twinning or by formation of perfect dislocations. The results are discussed in the context of available models for strain relaxation in epitaxial films.; The stress distribution in GaP films grown on (100) GaAs substrates with (311) faceting surfaces is calculated by the finite element method (FEM). The FEM makes it possible to both calculate stress distribution in a film with finite thickness and arbitrary surface morphology and study 3-D stress distributions in these systems. The results indicate that stress concentration is located at valley positions and the concentration region is very narrow near the surface. The stress concentration is significant for nucleation of dislocations from the surface.; A crack healing phenomenon occurring during epitaxial growth of GaP films on (100) GaAs substrates is studied. The process is driven by the decrease of the surface energy of the cracked film, balanced by the strain energy. The results indicate that the fundamental mechanism operating during healing is the deposition and diffusion of Ga and P atoms onto the crack surface in the GaP lattice, combined with the self-diffusion of GaAs within the crack tip in the GaAs substrate. This process in not fully completed in the GaP/GaAs system and unhealed crack tips located in the GaAs substrate lattice always remain in the structure. Development of cracks and subsequent crack healing during the film growth leads to the decrease of the residual stress in the film; the new cracks are formed at the equilibrium spacing, which increases as the film thickness increases. A new expression for predicting the evolution of the crack spacing with the film thickness in epitaxial films has been proposed.; (311)A GaP/GaAs provides good growth orientation with flat surface and without cracking. (311)B GaP/GaAs also does not crack but in this case the surface is rough. The terminating positions of gliding dislocations are found to be correlated to the Schmidt factors and dislocation densities. Calculations have been carried out to show how a dislocation changes the system energy while moving from a film surface into the substrate. The results showed that there is an equilibrium position in the softer substrate at which the system has a minimum energy, but the energy-well is very shallow. A dislocation acquiring a certain kinetic energy from large driving force can move over the shallow energy-well and penetrate deeply into the substrate. Anisotropy of cracking found in GaP films is also discussed in the light of lattice trapping theory.