Vibrational Modes in Si_xGe_1-xAlloys: Temperature and Compositional Dependence

Abstract

We report infrared absorption spectra of crystalline Si xGe1-x alloys with silicon content 0 < x < 1 at room and liquid nitrogen temperature. We covered the spectral range from 375 to 1200 cm -1 that includes the "Si-Ge" and "Si-Si" single-phonon transitions, the continuum of two-phonon processes, and the localized mode of interstitial oxygen. We study the change of vibrational structure and correlation between reference (pure Si and Ge) and alloy spectra. We observed shifts to lower wave numbers by about 1 cm -1 of the two-phonon absorption bands per 1% increase in germanium concentration. Pronounced changes of the vibrational spectra upon lowering the temperature were detected. The Six Ge1 -x alloys rank among the most interesting solid solutions. They represent a continuous series of crystalline materials with gradually varying properties. Vibrational states of the SiGe 1 -x alloys have been studied several times by infrared methods [1][2][3].In this paper, we present infrared absorption spectra of bulk Si x Ge1-x alloy for 0 < x < 1 in the spectral range 375-1200 cm -1 . As far as we know, it is for the first time that the local vibrations of the interstitial oxygen in the alloys are reported. The measurements have been performed on crystalline and polycrystalline samples grown by Czochralski method. The series of 19 samples covered the whole compositional range from Si to Ge, being mostly polycrystalline. Transmittance spectra have been measured with a FTIR spectrometer Bruker IFS55, the spectral resolution was 1 cm -1 and a number of scans was typically about 100. The transmission Τ of an optically thick slab of thickness d is [4] where K = 4πνk is the absorption coefficient and R = [(n -1) is the reflectivity of a semi-infinite sample with complex refractive index n+ik; n is the real part of the refractive index and k is the extinction coefficient. The approximations are valid for n » k. The real part of the refractive index of the Si x Ge 1 -x alloy has been computed by (899)

Infrared vibrations of interstitial oxygen in silicon-rich SiGe alloys

“…The background lattice absorption, due to the multiplephonon as well as single-phonon (''Si-Si'' and ''Si-Ge'', allowed due to the loss of symmetry in the alloys) transitions, has been reported and analyzed in our previous work [5]. Here, we focus on the infrared absorption due to the antisymmetric stretching vibration of O i causing the ''1106 cm À1 '' room temperature band in Si, for the Si-rich alloys.…”

Ge-rich graded SiGe waveguides and interferometers from 5 to 11 µm wavelength range

“…Si-O-Si absorption peak (λ = 9.2 µm): The increase of propagation losses around 9.2 µm wavelength, reaching 4 dB/cm in TM polarization and 4.6 dB/cm in TE polarization is due to optical absorption from the Si-O-Si vibration of interstitial oxygen (Si:O i ) [24]. As commonly seen in SiGe alloys, the wavelength of this absorption peak is slightly shifted in comparison with the 1106 cm −1 (λ=9.04 µm) absorption peak of Si-O-Si vibration mode in pure Si layers, possibly due to the expansion of Si-Si bonds [25,26] in the SiGe matrix. Secondary ion mass spectrometry (SIMS) characterization has been performed to corroborate the analysis.…”