| This dissertation reports a detailed experimental study of the electronic and vibrational properties of Ge1- ySny and Ge1- x-ySixSny alloys. The E 0, E0+Delta0, E 1, E1+Delta1, E'0 , and E2 optical transition energies in the electronic band structure of Ge1- ySny alloys are obtained by spectroscopic ellipsometry. The lowest direct gap E0 of Ge 1-ySny alloy has also been determined by photoreflectance. A detailed compositional study of all the electronic transitions in Ge1-ySn y alloys has been carried out including a detailed comparison with the same transitions in Ge1- xSix alloys. We find that the quadratic deviation (bowing) from a linear compositional dependence of optical transitions in Ge1-ySny alloys is downward as in Ge1-xSi x alloys but is greater in magnitude than the bowing in Ge1-xSix alloys. The bowing in Ge1-ySn y and Ge1-xSi x alloys scale with the atomic size mismatch and electronegativity difference between Si, Ge and alpha-Sn. From our E0 analysis, Ge 1-ySny alloys are predicted to be a direct gap material for Sn concentration as low as 6-11%, which is much smaller than the theoretical estimates of the indirect-direct gap crossover.;A study of vibrational properties of Ge1- ySny alloys shows evidence for Ge-Sn-like and Ge-Ge-like optical modes. In addition to these modes, two disorder-activated modes are observed. A detailed compositional study of the main vibrational modes has been carried out.;Addition of Si to Ge1- ySny alloys allows simultaneous strain and band gap engineering. The electronic and vibrational properties of Ge 1-x-ySixSny alloys have been studied. The critical point energy transitions above E 0 are obtained by spectroscopic ellipsometry. A detailed compositional study has been carried out for the average of E1 and E1+Delta1 transitions, denoted by E1. We show that this dependence can be explained in terms of three bowing parameters---bsiGe, bGeSn, and bSiSn---which scale with the product DeltachiDeltaR of the electronegativity and size mismatches between Si, Ge, and Sn. E0 has been measured by photoreflectance.;The vibrational study of Ge1- x-ySixSny alloys shows the typical vibrations observed in Ge1-xSix alloys. i.e the so called Ge-Ge, Si-Ge and Si-Si modes. The compositional dependence of Ge-Ge and Si-Si vibrations in Ge1- x-ySixSny alloys can be expressed in terms of the corresponding vibrations in the binaries constituting the ternary alloys. A remarkable scaling has been observed in the linear coefficients associated with the total observed Raman shift. The compositional dependence of the Raman frequencies of various vibrations in group-IV binary alloys can be predicted based on a simple model described in terms of bond distortion and mass perturbation. |
