An investigation was made of the initial stages of 2-dimensional epitaxial growth of Ge on (001)Si substrates, using pulsed laser deposition. It was found that the Ge films grew epitaxially at temperatures above 170C, but changed from 2- to 3-dimensional growth at temperatures above 350C. The evolution of the dislocation network as a function of film thickness was studied by means of the high-resolution transmission electron microscopy of films which had been grown at 300C and at relatively high growth rates in order to suppress 3-dimensional nucleation and to obtain a low rate of inelastic misfit relaxation. The relaxation process began with the generation of 60° slip dislocations with a strongly non-equilibrium inhomogeneous distribution which seemed to be controlled by additional stresses that resulted from random surface undulations. When the film was only 8nm thick, 90% of the misfit stress was already relaxed mainly by 60° dislocations which exhibited a clear tendency to the pairing of parallel dislocations from different glide planes. As the thickness of the film increased, the 60° dislocations changed into 90° pure edge dislocations via the reaction of parallel 60° misfit segments. A transmission electron microscopic analysis revealed a fraction of 60° pairs of misfit dislocations from different glide planes, but with parallel screw components which could not react directly. The interactions of possible geometrical configurations of the dislocations were considered in order to account for the nucleation and deformation of the closely spaced pairs. A relatively low fraction of planar defects was observed in thinner films (4 and 8nm thick). Most of them were growth-related, and nucleated predominantly at amorphous pockets or contamination near to the interface.

S.Oktyabrsky, H.Wu, R.D.Vispute, J.Narayan: Philosophical Magazine A, 1995, 71[3], 537-51