SURFACE PROCESSES BEFORE AND DURING GROWTH OF GAAS(001)

GaAs(001) surfaces in both metalorganic vapour phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been studied under As-stabilization as well as during growth by reflectance anisotropy spectroscopy (RAS). During MBE growth simultaneously reflection high-energy electron diffraction (RHEED) measurements were performed. With increasing growth rate, the surfaces transform from a c(4 x 4) to a (2 x 4) in MBE. In MOVPE, with trimethylgallium (TMGa) and AsH3 a transition occurs from a ''c(4 x 4)-like'' behaviour to a surface best described as a mixture of dielectric contributions from Ga-rich ''(1 x 6)-like'' and the As-rich ''c(4 x 4)-like'' regions. Time resolved RAS measurements show oscillations with monolayer periodicity in both epitaxial systems, however, with opposite phase. In MOVPE additionally data with time and spectral resolution are obtained which give insight into the surface modifications during a monolayer growth cycle. A model using an effective medium approach is proposed which relates the RAS oscillations during MBE growth to morphological changes associated with the island formation in the layer-by-layer growth modus. The MOVPE oscillation behaviour is more complex due to the final decomposition steps of trimethylgallium at the surface. In addition to the island formation it has to be included in the model that the surface oscillates between a state ''more c(4 x 4)-like As dimers'' and a state ''more (1 x 6)-like As and Ga dimers''.