LOW-TEMPERATURE EPITAXIAL-GROWTH OF GE-RICH GE-SI-C ALLOYS - MICROSTRUCTURE, RAMAN STUDIES, AND OPTICAL-PROPERTIES

Low-temperature (similar to 200 degrees C) molecular beam epitaxy of G e-rich Ge1-x-ySiyCx alloys grown on Si(100) have been investigated by in situ reflection high-energy electron diffraction, ex situ x-ray dif fraction, transmission electron microscopy, Raman scattering, and elli psometry. The Si contents were either similar to 20 or similar to 40 a t % and the C concentrations were nominally varied from zero up to sim ilar to 8 at %. Selected samples were annealed in an Ar ambient at 750 degrees C to evaluate the stability of the thin films. With increasin g C concentration, the epitaxial growth mode changes from two-dimensio nal (2D) layer growth to 3D island growth. Under the growth conditions studied, the GeSiC films have a tendency to form planar defects, whos e density increases with increasing C and Si concentrations. The x-ray diffraction data show that the lattice parameter decreases with incre asing C concentration. It is estimated that a maximum of similar to 2- 3 at % C is substitutionally incorporated into these films. Raman spec tra of the alloy films show that the effects of C on the strong Ge-Ge and Ge-Si local modes are far less than the effects due to Si. We are unable to observe any systematic change in the Ge-Ge mode, whereas the Ge-Si mode appears to shift to lower frequency with the small additio n of C. Ge1-x-ySiyCx films formed by annealing Ge1-xCx films on Si are also discussed. Spectroscopic ellipsometry determinations of the film 's optical constants show that the primary effect of C is to reduce th e strength of the E-1 critical point feature. (C) 1998 American Instit ute of Physics. [S0021-8979(98)02316-0].