SYNTHESIS OF SI1-YCY ALLOYS BY MOLECULAR-BEAM EPITAXY

We have synthesized pseudomorphic Si1-yCy (y less-than-or-equal-to 0.05) alloys and strained layer superlattices on silicon by molecular beam epitaxy using solid sources for carbon and silicon. The introduction of C into substitutional sites in the silicon lattice is kinetically stabilized by low-temperature growth conditions (500-600-degrees-C) and relatively high Si fluxes, against extremely low C solubility (10(-6) at 1420-degrees-C) and the thermodynamically favored silicon carbide phases. Higher temperature growth leads to an islanded morphology. At lower temperatures, disruption of epitaxy occurs via the formation of highly twinned layers or even amorphous growth. The temperature window for alloy growth is reduced as the C concentration is increased. X-ray diffraction, transmission electron microscopy, secondary ion mass spectroscopy, and Raman spectroscopy confirm the growth of pseudomorphic, tetragonally strained alloy layers with no detectable silicon carbide precipitation. These alloy layers allow for the engineering of Si-based lattice constants less than that of bulk silicon using isovalent elements. They have potential application in band-engineered Si devices and may provide band gaps wider than that of Si, and perhaps, larger band offsets than that provided by the Si1-xGex system.