REFLECTION HIGH-ENERGY ELECTRON-DIFFRACTION (RHEED) INTENSITY OSCILLATIONS - GROWTH MODES AND GROWTH-RATES - A CRITIQUE

The origin of and diffraction effects associated with reflection high energy electron diffraction (RHEED) intensity oscillations which occur during layer-by-layer growth of epitaxial thin films of III-V compoun ds by molecular beam epitaxy (MBE) are explained. It is shown that on (001) oriented substrates the period of the oscillations is in general a direct measure of the film growth rate which corresponds to the gro up III element flux. There are, however, exceptions to this simple con cept including growth under group III rich-conditions, vicinal plane g rowth and growth from pulsed beams; each is considered. On non-(001) l ow index orientations, the RHEED oscillation period only provides a me asure of the growth rate over a very limited range of conditions. The fundamental reason appears to be the more restricted reactivity betwee n the group III and V elements, so the oscillations are induced by the group V element, not the group III, which is quite different from (00 1) surfaces, at least for conventional growth conditions. Finally, gro wth modes and strain relaxation differences between (001) and (110)-ba sed growth of InAs on GaAs are illustrated. It is shown that there is no real relationship between strain and growth mode and it is suggeste d that adatom mobility is the essential parameter which determines gro wth mode. In more general terms, it appears that kinetic factors rathe r than equilibrium considerations are responsible for the growth mode. Models based on purely equilibrium concepts are therefore unlikely to have general validity.