Grazing incidence reciprocal space mapping of partially relaxed SiGe films

Epitaxially grown lattice mismatched semiconductor structures are increasingly important for microelectronic and optoelectronic applications. Recently, a great deal of research has been carried out on strain relaxation mechanisms in lattice mismatched epitaxial films. Here, we describe triple-axis x-ray diffraction measurements that were performed to study strain relaxation mechanisms and dislocation formation in Si_1–xGe_x alloys grown on (0 0 1) Si substrates. At low growth temperature (T_g 600°C) and small lattice mismatch (>2%), two different mechanisms of strain relaxation are observed, depending on the growth temperature and the magnitude of the strain. At Higher growth temperatures or larger lattice mismatch, strain relaxation occurs initially by surface roughening. Subsequently, 60° misfit dislocations nucleate in regions of high strain. At smaller lattice mismatch or lower growth temperature, the surface of the film does not roughen and the 60° misfit dislocations are formed primarily by Frank–Read multiplication. Triple-axis x-ray diffraction reciprocal space maps taken at grazing incidence on very thin epitaxial films can easily distinguish between these two mechanisms. Here, the lattice planes perpendicular to the interface are measured, whereas conventional diffractometry looks either at the planes parallel to the wafer surface or at planes having components both parallel and perpendicular to the surface. In the grazing incidence geometry, thickness broadening of the x-ray peak is eliminated, since the film is essentially infinitely thick parallel to the surface.