MONOLAYER GROWTH OSCILLATIONS AND SURFACE-STRUCTURE OF GAAS(001) DURING METALORGANIC VAPOR-PHASE EPITAXY GROWTH

Reflectance anisotropy spectroscopy (RAS) was used to study metalorgan ic vapor phase epitaxial growth of GaAs(001). In previous studies, GaA s(001) surfaces have been identified as c(4x4)-like under pregrowth ar senic stabilized conditions. During growth the RAS spectra showed sign ificant differences with respect to the pregrowth spectra. Moreover, t he RAS signal at a fixed photon energy has been shown to exhibit an os cillatory behavior when growth is initiated. The period of these oscil lations has been identified as the time needed for the growth of exact ly one monolayer. In this study, the dependence of the oscillations (a mplitude and period) on temperature and trimethylgallium (TMGa) partia l pressure is investigated and discussed together with the correspondi ng RAS spectra taken at V/III ratios between 5 and 300. Three differen t types of RAS spectra are identified during growth. Firstly, at high temperatures and low TMGa partial pressures, the surface anisotropy is nearly the same as in the c(4X4)-like pregrowth state, with a minimum at 2.5 eV (type I spectra). This indicates that the surface during gr owth is essentially covered with a double layer of As. Secondly, at lo w temperatures or high TMGa partial pressures a different structure wi th a sharp minimum at 2.65 eV is observed (type III spectra). This mig ht be related to a surface largely covered with TMGa fragments. Finall y, intermediate between these extreme epitaxial conditions, RAS spectr a with a minimum at 2.0 eV and nearly vanishing anisotropy around 2.5 eV are found. This indicates that the As dimers of the pregrowth surfa ce have largely disappeared and a new surface structure, possibly cons isting of Ga dimers, is now present. Oscillations have their maximum a mplitude at the boundary between type II and type III spectra in a p(T MGa)-T diagram. At this boundary the growth rate becomes nonlinear and starts to saturate with increasing pressure of TMGa.