MISFIT DISLOCATIONS IN LOW-TEMPERATURE-GROWN GE SI HETEROSTRUCTURES

We have investigated the initial stages of two-dimensional epitaxial g rowth of Ge on (001)Si substrates using pulsed laser deposition. The G e films grow epitaxially above 170-degrees-C and switched from two- to three-dimensional growth mode above 350-degrees-C. The evolution of t he dislocation network as a function of film thickness was studied by high-resolution transmission electron microscopy (TEM) thickness was s tudied by high-resolution transmission electron microscopy (TEM) in th e films grown at 300-degrees-C and at relatively high growth rates to suppress the three-dimensional nucleation and obtain low rate of inela stic misfit relaxation. The relaxation process begins with generation of 60-degrees slip dislocations with a strongly non-equilibrium inhomo geneous distribution which seems to be controlled by additional stress resulting from random surface undulations. When the film is just 8 nm thick, 90% of misfit stress is already relaxed by mostly 60-degrees d islocations which demonstrate a distinct tendency of pairing of parall el dislocations from different glide planes. As the thickness of the f ilm increases, 60-degrees dislocations convert into 90-degrees pure ed ge dislocations by the reaction of parallel 60-degrees misfit segments . TEM analysis shows a fraction of 60-degrees pairs of misfit dislocat ions from different glide planes but with parallel screw components wh ich cannot react directly. Force interactions of possible geometrical configurations of the dislocations are discussed to account for nuclea tion and formation of the closely spaced pairs. Relatively low portion of planar defects in the thinner films (4 and 8 nm thick) was observe d, most of them are growth-related and nucleate predominantly at the a morphous pockets or contaminations near the interface.