Substitutional carbon in Si1-xGex

Local vibrational modes of carbon impurities in relaxed Si1-xGex have been studied with infrared absorption spectroscopy in the composition range 0.05 less than or equal to x less than or equal to 0.50. Carbon modes with frequencies in the range 512-600 cm(-1) are: observed in C-13(+)-implanted Si1-xGex after annealing at 550 degrees C. Measurements on samples coimplanted with C-12(+) and C-13(+) show that these modes originate from defects containing a single carbon atom and From the variation of the mode Frequencies with composition x, the modes are assigned to substitutional carbon in Si1-xGex. Based on the frequencies obtained from a simple vibrational model, the observed modes are assigned to specific combinations of the four Si and Ge neighbors to the carbon. The intensities of the modes indicate that the combination of the four neighbors deviates from a random distribution. Ab initio local-density-functional cluster theory has been applied to calculate the structure and the local mode frequencies of substitutional carbon with n Ge and 4 - n Si neighbors in a Si and a Ge cluster. The calculated frequencies are similar to 9% higher than those observed, but the ordering and the splitting of the mode frequencies agree with our assignments. [S0163-1829(99)01243-6].