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Double-Beam Optically Compensated Fourier Transform Spectrometer (FTS) And Its Application To The Vibrational Spectra Of Amorphous And Crystalline Si And Ge

A double-beam optically compensated Fourier transform spectrometer is described. The sensitivity of the instrument is superior by a factor of 5 to 10 compared to the results obtained with a normal single-beam FTS using the same detector and measuring time. The instrument was used for the investigation of far-infrared vibrational spectra of amorphous Si and Ge and the crystalline alloy. The studies have been made on pure and hydrogenated amorphous Si and Ge, on deuterated and fluorinated amorphous Si, on phosphorus- or boron-doped amorphous Si and crystalline Si-Ge alloys. We found fine structure in the phonon spectra of the pure amorphous materials. By introducing lighter impurities, a resonant mode is also found, both in the amorphous and the crystalline cases. A strong increase of the TO band upon doping has also been found in the amorphous case. These spectra are compared with the vibrational density of states of the crystalline materials.