THERMAL-STRESS-INDUCED DISLOCATIONS IN GESI/SI HETEROSTRUCTURES

Ge-rich GeSi layers were grown on (111) Si substrates by liquid phase epitaxy (LPE) at 820-degrees-C. On cooling to room temperature, the thermal expansion mismatch between GeSi and Si generates thermal stresses. X-ray diffraction indicates that these stresses partly relax by plastic strain of the GeSi layer. At the GeSi/Si interface, transmission electron microscopy (TEM) reveals dislocation networks with a mesh width that corresponds quantitatively to the measured plastic strain. Cross-sectional and stereo TEM images indicate that the thermal stress-induced dislocations nucleate as half-loops at the surface of the GeSi layer. The density of thermal stress-induced dislocations was successively increased by exposing the as-grown heterostructures to additional heating/cooling cycles. TEM analysis of the dislocation configuration at increasing dislocation density reveals a transition from a trigonal ''pseudo''-network to a hexagonal network. The hexagonal network is shown to be the more stable configuration, and possible dislocation reactions leading to its formation are discussed.