Optical Spectroscopy and Growth of Germanium Silicide Alloys

The structural and electronic properties of GeSi alloys, prepared by several novel growth schemes, are analyzed by optical spectroscopy. GeSi films are grown by thermal and excimer-laser assisted chemical vapor deposition (CVD) and analyzed by Raman scattering. The effect of temperature on optical phonons in bulk GeSi alloys, grown by zone refining, is studied by using Raman spectroscopy. Finally, spectroscopic ellipsometry (1.48 to 5.16 eV) is used to determine the optical dielectric function in GeSi films grown by CVD and by supersonic chemical beam deposition (SCB). The second derivative dielectric function spectra is fit by parabolic critical point lineshapes to determine the critical point energies as a function of alloy composition. Raman scattering is used in the composition and strain analyses of these films. In the investigation of thermal and laser-assisted CVD of GeSi films, thin films are deposited by 193-nm photolysis of GeH_4/Si_2H _6 gas mixtures using an ArF laser. For substrate temperatures below 350^circ C, deposition occurs only with the laser present, while for temperatures above 400^circ C, film growth is little influenced by laser photolysis and resembles conventional CVD. Film stoichiometry is measured by Auger analysis and Raman spectroscopy, with both methods leading to the same film composition. Analysis of the film composition for different reactant gas mixtures indicates that there is strong cross chemistry between silicon and germanium-bearing species in the gas phase. First-order Raman scattering of polycrystalline Ge_{rm 1-x}Si _{rm x} alloys is studied from T = 300-900 K. The Raman shifts of the three optical phonons are modeled by considering thermal expansion and coupling to two phonons. The shift of the Si-Si mode near 500 cm^{-1} has a temperature dependence similar to that of c-Si for x > 0.7, but has a slower temperature variation for more Ge -rich alloys. The Ge-Ge mode near 300 cm^ {-1} has a temperature dependence similar to that of the optical phonon mode in c-Ge for all compositions studied. The Raman shift of the Ge-Si local mode has a dependence in between that of the Ge-Ge and Si-Si modes. Also, the Raman linewidths of the Si-Si and Ge-Ge modes have a temperature dependence similar to that of c-Si and c -Ge, respectively. Two-channel spectroscopic polarization modulation ellipsometry is used to determine the dielectric functions of Ge_{rm 1-x}Si _{rm x} alloys, and the critical point transition energies and associated parameters are determined by fitting the second derivative spectra with parabolic critical point lineshapes. The accuracy of the optical functions determined from the data is improved by correcting for the optical thickness of the overlayer, which is ascertained from the experimental ellipsometry parameters at long wavelengths. After correcting for residual strain, E_1(x) is fit with a quadratic dependence on x. Using this dependence, ellipsometry is a convenient method for determining the alloy composition.