Gallium-mediated homoepitaxial growth of silicon at low temperatures

The present work reports on the molecular-beam homoepitaxial growth of silicon with and without gallium in a low-temperature regime (200-500 degrees C). The growth mechanisms were investigated by reflection high-energy electron diffraction (RHEED) and by high-resolution electron microscopy (HREM). The progressive appearance of the surface roughness was observed in situ by the continuous change in the RHEED pattern and was quantified with the damping coefficient of the RHEED oscillations. This coefficient was also used to determine the critical temperature of transition from the two-dimensional (2D) nucleation to step-flow growth regime. HREM cross-sectional images showed that the surface kinetic roughening observed at temperatures below 300 degrees C was associated with a high density of periodically repeated stacking faults (ABCBCA-type fault). HREM comparison of layers grown with and without gallium showed that the adsorption of gallium greatly reduced the defect density and the roughness of the epitaxial layer. The Ga surfactant-mediated epitaxy thus enables us to produce epitaxial layers of high-perfection structure, defectless and with homogeneous thickness. Up to 20-nm-thick silicon epitaxial layers were grown at 300 degrees C on gallium-activated Si(111) with a perfect planeity (at the atomic scale) and crystallinity (no crystalline defect was observed at the microscopic level). Thanks to systematic measurements of the oscillation damping, we also show that Ga does not modify the 2D nucleation-step-how temperature transition. After analysis of different existing kinetic models, we conclude that the adsorption of gallium does not change the diffusion kinetics of silicon adatoms on the silicon substrate. Therefore, thermodynamic considerations would be invoked to explain the role of gallium on the reduction of defects and surface roughness.