PRECIPITATION OF BETA-SIC IN SI1-YCY ALLOYS

The infrared modes of annealed Si1-yCy alloys were studied experimenta lly and theoretically. The alloys were grown on Si(100) substrates by solid-source molecular beam epitaxy and were characterized by Fourier transform infrared spectroscopy. At annealing temperatures above 850 d egrees C, the localized vibrational mode of substitutional C around 60 5 cm(-1) diminished in intensity while another mode due to incoherent silicon carbide precipitates appeared at 810 cm(-1). For lower process ing temperatures, a peak around 725 cm(-1) has been tentatively attrib uted to a C-rich phase, which is a precursor to SiC precipitation. The oretical calculations based on the anharmonic Keating model predict th at small (1 nm) 3C-SiC coherent precipitates may actually produce a mo de at 725 cm(-1). This mode occurs if the bonds gradually vary in leng th between the C-rich region and the host lattice. On the other hand, if the bonds are abruptly distorted at the edges of the precipitate, i t becomes elastically isolated from the host lattice, and the 810 cm(- 1) mode appears. This study yields a picture of the thermal stability of dilute SiC alloys, which is important for the high-temperature proc essing steps necessary for device applications. Moreover, the coherent precipitation may provide a controllable way to form self-assembled 3 C-SiC quantum dots into silicon germanium carbon alloys. (C) 1998 Amer ican Institute of Physics. [S0021-8979(98)08219-X]