Misfit dislocations in low-temperature grown Ge/Si heterostructures

Abstract

We have investigated the initial stages of two-dimensional epitaxial growth of Ge on (001)Si substrates using pulsed laser deposition. The Ge films grow epitaxially above 170°C and switched from two- to three-dimensional growth mode above 350°C. The evolution of the dislocation network as a function of film thickness was studied by high-resolution transmission electron microscopy (TEM) in the films grown at 300°C and at relatively high growth rates to suppress the three-dimensional nucleation and obtain low rate of inelastic misfit relaxation. The relaxation process begins with generation of 60° slip dislocations with a strongly non-equilibrium inhomogeneous 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° dislocations which demonstrate a distinct tendency of pairing of parallel dislocations from different glide planes. As the thickness of the film increases, 60° dislocations convert into 90° pure edge dislocations by the reaction of parallel 60° misfit segments. TEM analysis shows a fraction of 60° pairs of misfit dislocations from different glide planes but with parallel screw components which cannot react directly. Force interactions of possible geometrical configurations of the dislocations are discussed to account for nucleation and formation of the closely spaced pairs. Relatively low portion of planar defects in the thinner films (4 and 8 nm thick) was observed, most of them are growth-related and nucleate predominantly at the amorphous pockets or contaminations near the interface.