The characteristics of nucleation and growth process and defect morphology at initial stage of GaAs films on Si (001) substrate tilted toward the [001] direction were investigated by high-resolution transmission electron microscopy (HRTEM) in cross-sectional view and conventional transmission electron microscopy (TEM) in plan-view. Amorphous GaAs films deposited by molecular beam epitaxy (MBE) technique at low temperature were crystallized by subsequent annealing inducing solid phase epitaxial (SPE) growth of the GaAs layer.
Unlike the results of conventional MBE process under comparable condition, cross-sectional ([110] and [1T0]) HRTEM migrographs have shown that three-dimensional crystalline islands were nucleated on terraces as well as at substrate steps and their size and spacing were nearly the same in the flat and the step-rich surfaces. It was observed that the average lateral dimension and height of islands which were formed after annealing at 300℃ for 10 minutes were $^-14$ nm and $^-9$ nm respectively, and the average island spacing was $^-12$㎚. These differences are explained by the different experimental parameters in SPE process.
The interfacial defects which are known to be inevitable in heteroepitaxy such as stacking faults (and/or microtwins) and misfit dislocations were characterized in HRTEM images. It was observed that many stacking faults and a few misfit dislocations were found to be preferentially generated on the tilted step-rich interface. The critical thickness for dislocation generation was dependent on the ratio of height/lateral dimension. The degree of misfit accommodation by these interfacial defects in the two cross-section were calculated and it was shown that asymmetry in misfit accommodation clearly exists. The reason why this asymmetry in defect morphology appears were discussed. By comparing the defect morphologies at this initial stage and after considerable (>1㎛) epi-layer growth, the evolution of defect structure at interface during epi-layer growth were discussed via dislocation reactions. Finally, observed layer tilt (corresponding crystallographic directions are different between the substrate and the epi-layer) were interpreted by the asymmetry in defect morphology.
