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We describe a class of Si-based semiconductors in the Ge1−xSnx system. Deuterium-stabilized Sn hydrides provide a low-temperature route to a broad range of highly metastable compositions and structures. Perfectly epitaxial diamond-cubic Ge1−xSnx alloys are grown directly on Si(100) and exhibit high thermal stability, superior crystallinity, and crystallographic and optical properties, such as adjustable band gaps and lattice constants. These properties are completely characterized by Rutherford backscattering, low-energy secondary ion mass spectrometry, high-resolution transmission electron microscopy, x-ray diffraction (rocking curves), as well as infrared and Raman spectroscopies and spectroscopic ellipsometry. Ab initio density functional theory simulations are also used to elucidate the structural and spectroscopic behavior.

We study the vibrational spectrum of AlN grown on Si͑111͒. The AlN was deposited using gas-source molecular beam epitaxy. Raman backscattering along the growth c axis and from a cleaved surface perpendicular to the wurtzite c direction allows us to determine the E 2 1 , E 2 2 , A 1 ͑TO͒, A 1 ͑LO͒, and E 1 ͑TO͒ phonon energies. For a 0.8-m-thick AlN layer under a biaxial tensile stress of 0.6 GPa, these are 249.0, 653.6, 607.3, 884.5, and 666.5 cm Ϫ1 , respectively. By combining the Raman and x-ray diffraction studies, the Raman stress factor of AlN is found to be Ϫ6.3Ϯ1.4 cm Ϫ1 /GPa for the E 2 2 phonon. This factor depends on published values of the elastic constants of AlN, as discussed in the text. The zero-stress E 2 2 energy is determined to be 657.4 Ϯ0.2 cm Ϫ1. Fourier-transform infrared reflectance and absorption techniques allow us to measure the E 1 ͑TO͒ and A 1 ͑LO͒ phonon energies. The film thickness ͑from 0.06 to 1.0 m͒ results in great differences in the reflectance spectra, which are well described by a model using damped Lorentzian oscillators taking into account the crystal anisotropy and the film thickness.

We have studied the optical properties (complex dielectric function) of bulk SrTiO3 and thin films on Si and Pt using spectroscopic ellipsometry over a very broad spectral range, starting at 0.03 eV [using Fourier transform infrared (FTIR) ellipsometry] to 8.7 eV. In the bulk crystals, we analyze the interband transitions in the spectra to determine the critical-point parameters. To interpret these transitions, we performed band structure calculations based on ab initio pseudopotentials within the local-density approximation. The dielectric function was also calculated within this framework and compared with our ellipsometry data. In the FTIR ellipsometry data, we notice a strong lattice absorption peak due to oxygen-related vibrations. Two longitudinal optic (LO) phonons were also identified. In SrTiO3 films on Si, the refractive index below the band gap decreases with decreasing thickness because of the increasing influence of the amorphous interfacial layer between the SrTiO3 film and the Si substrate. There is also a decrease in amplitude and an increase in broadening of the critical points with decreasing thickness. In SrTiO3 films on Pt, there is a strong correlation between the crystallinity and texture of the films (mostly aligned with the Pt pseudosubstrate) and the magnitude of the refractive index, the Urbach tail below the bulk band edge, and the critical-point parameters. FTIR reflectance measurements of SrTiO3 on Pt (reflection–absorption spectroscopy) show absorption peaks at the LO phonon energies, a typical manifestation of the Berreman effect for thin insulating films on a metal. The Urbach tail in our ellipsomety data and the broadening of the optical phonons in SrTiO3 on Pt are most likely caused by oxygen vacancy clusters.