We performed Raman spectroscopy on the Si1-x-yGexCy and Si1-xGex epitaxial layers on Si substrates grown by UHV-CVD. The Ge content of SiGeC layers were fixed at 21% and C content was controlled from 0.2% to 1.7% by varying C source gas flow rate. SiGeC layers were grown at 400 °C and 420 °C, and SiGe samples were grown at 450 °C with Ge content from 20 % to 23 % for comparison purpose. From the amount of substitutional C along with the total C content, it was identified that C incorporates into substitutional sites in samples grown at 400 °C and samples grown at 420 °C with total C content lower than 0.8%. Incorporation of C into interstitial sites occurs at 420 °C and at total C contents higher than 0.8%.
In Raman scattering, we observed the strong Si-Si and Si-Ge vibrational modes. When all C incorporated into substitutional sites, both Si-Si and Si-Ge mode frequencies showed consistent and linear reduction. In the contrary, with the increase of the interstitial C content, the frequencies of both modes moves toward higher frequency, showing overall V-shaped behavior along with the total C content. The Si-Si and Si-Ge mode frequency shifts can be accounted by the sum of two factors, strain shift effect and compositional shift effect. We calculated the compositional shift effect of C atoms, and it was found that the interstitial C induces larger compositional shift than the substitutional C. Because the strain status remains unchanged during the incorporation of the interstitial C, this large compositional shift compensates the frequency reduction caused by the strain shift effect and produces the increase in the mode frequencies. Ab initio calculation within the local density approximation (LDA) was utilized to clarify the pathway of the large compositional shift. The interstitial C was set at the position surrounded by Si atoms, and the simulated Si-Si mode frequency showed excellent agreement with experimental results. From these results, the short and stiff bonds formed between Si and the interstitial atom was responsible for the large composition shift and the reversed behavior of the mode frequencies. |