REFLECTION HIGH-ENERGY ELECTRON-DIFFRACTION SCANNING-TUNNELING-MICROSCOPY STUDY OF INAS GROWTH ON THE 3 LOW-INDEX ORIENTATIONS OF GAAS - 2-DIMENSIONAL VERSUS 3-DIMENSIONAL GROWTH AND STRAIN RELAXATION

It is generally believed that strain relaxation in semiconductor heter ostructures having a significant misfit (>2%) occurs by the formation of coherent three-dimensional islands, following the growth of one or two continuous two-dimensional monolayers in a manifestation of the St ranski-Krastanov (SK) growth mode. For the InAs-GaAs system, for which the misfit is approximate to 7.2%, we have shown that this is a very special case, at least during growth by molecular beam epitaxy, as it occurs on only one of the low index orientations, the (001), and then only under As-rich growth conditions. On,(110) and (111) surfaces, gro wth is always two-dimensional layer by layer and strain is relieved by the formation of misfit dislocations. Even when three-dimensional isl ands are formed on (001) substrates, the process is much more complex than the conventional SK mechanism would imply. We have used a combina tion of in situ reflection high-energy electron diffraction (RHEED) an d scanning tunneling microscopy (STM) and ex situ transmission electro n microscopy to follow the initial growth processes and strain relaxat ion mechanisms of InAs on GaAs (001), (110), and (111)A surfaces. RHEE D enables us to establish the symmetry of the wetting layer, whether g rowth is two- or three-dimensional and the external crystallography of any three-dimensional islands. STM images obtained by rapid quenching from the growth temperature show how growth is initiated, provide qua ntitative data cin island formation (number density and volume), and i ndicate dislocation formation via strain field morphology effects. Det ailed results for all three low index surfaces are presented, together with possible reasons for the major differences between them. (C) 199 8 American Vacuum Society.