Multi-wavelength in situ reflectometry at normal incidence has been applied to monitoring metal organic vapour phase epitaxy of GaN films on sapphire (0 0 0 1) substrates. A new quantitative analysis model has been developed to incorporate time-dependent light scattering by a rough surface, and a time-dependent vertical growth rate during growth on a rough surface, into the virtual interface model that has previously been applied to multilayer structures with optically smooth surfaces and interfaces. It is shown that the vertical growth rate increases as the surface roughness decreases in the early stage of high-temperature GaN growth, reaching a limiting value when the surface becomes optically smooth. The time dependence of growth rate is correlated with microscopic crystal growth mechanisms on the rough surface, which involve mass transport on the facets and/or mass exchange between the growing surface and gas-phase ambient. Our optical modelling is supported by direct morphological investigations of films from growths terminated at various stages, using atomic force microscopy. High-temperature optical constants of GaN layers extracted from the simulations are well matched to literature values.
- optical modelling