A COMPARISON OF THE CRITICAL THICKNESS FOR MBE GROWN LT-GAAS DETERMINED BY IN-SITU ELLIPSOMETRY AND TRANSMISSION ELECTRON-MICROSCOPY

The growth of low temperature (LT) GaAs by molecular beam epitaxy has been studied using ellipsometry. Different regimes of growth were obse rved in the data, depending on film thickness. Epitaxial growth of pse udomorphic LT-GaAs occurred immediately above the substrate, followed by a layer with changing dielectric properties. This upper layer can b e modeled as a two-phase region consisting of epitaxial LT GaAs and sm all grained, polycrystalline GaAs, which increases in volume fraction with increasing layer thickness. For sufficiently thick LT layers, cro ss-sectional transmission electron microscopy analysis showed pyramida l defects that were composed primarily of highly twinned regions. The ellipsometry data showed a deviation from the homogeneous growth model at a thickness less than the thickness at which the pyramidal defects nucleated in all samples.