Microstructure of thin layers of MBE-grown GaAs on Si substrates

ABSTRACTThe ability to grow high quality epitaxial GaAs films directly on Si substrates has recently been demonstrated by Molecular Beam Epitaxy. Many of the most successful growth techniques include initial growth at low temperatures and slow growth rate to nucleate the compound on the elemental semiconductor with subsequent normal growth under conditions used for homoepitaxial GaAs MBE. The mechanisms for the success of this method are still poorly understood. This work focusses on a study of the structural evolution of this low temperature initial layer and conventionally grown overlayer. Thin films in the 5 to 300 nm range were used to evaluate the effect of the temperature and thickness on the growth process. Thicker films, described elsewhere, were used to evaluate the crystalline and optical quality of potential device layers.The structure of the thin buffer layers was studied as a function of both temperature and thickness. The crystalline quality of these layers was found to be quite poor, with substantial disorder at the GaAs/Si interface. The quality improved slowly as a function of thickness. The effect of the conventional overgrowth on this initil layer was also investigated. It was found that the quality of the low tumperature layer improved as normal overgrowth continued, as measured by ion channeling aligned yields. After 200 nm of overgrowth, ion channeling aligned yields from the material at the interface had declined substantially. This indicates that substantial regrowth and annealing of defects occurs as growth continues. These observations are further confirmed by TEM.