J.S. Lannin

Raman Scattering of amorphous selenium films

Abstract First order Raman scattering measurements are presented for thin film evaporated a -Se. High temperature films yield polarized and depolarized spectra that are very similar to those of bulk a -Se. Low temperature in situ studied films indicate increased disorder and a considerable reduction in the intensity of the ring attributed 112 cm −1 peak. The Raman spectra of higher temperature films are consistent with a small number of rings and ordered chains that arise from constraints on the dihedral angle. Films deposited at low temperature suggest a reduction in chain order and ring population as might arise from a weakening of constraints on the dihedral angle.

Raman scattering and short range order in amorphous germanium

Abstract Variations in short range order of amorphous Ge films are determined from a combination of depolarized Raman scattering, optical absorption, and previous radial distribution function studies. An estimated minimum value of the bond angle width of Δθ ≅ 9°, obtained for evaporated, anneal stable a-Ge differs significantly from a recent EXAFS estimate. The maximum disorder observed in dc sputtered films deposited at ∼90K indicates that an ∼20% variation in bond angle width is possible in a-Ge. Substantial modification of the Raman spectrum of the low temperature film annealed at 300K also demonstrates that lower temperature structural relaxation processes involve changes in short range order.

Raman scattering and infrared absorption in bulk amorphous red phosphorous

Abstract Raman scattering and infrared absorption measurements in bulk amorphous red P indicate considerable spectral detail for an elemental amorphous solid, including a distinct separation of acoustic and lower lying optic-like bands. The results suggest appreciable structural correlations and weak interlayer-like bonding. Increased covalency of amorphous red P relative to amorphous As is indicated.

Phonon spectra of crystalline iodine

Abstract First and second order Raman scattering is reported in thin film crystalline I. The first order spectra indicate previously unobserved weak intermolecular k = 0 Raman modes which are systematically lower than neutron scattering results. The second order high frequency spectra indicate a narrow overtone band that yields the form of the intramolecular phonon density of states. Lower frequency overtone and combination processes are compared to a theoretical model for the intermolecular phonon spectra.

Raman scattering in amorphous boron

Abstract Raman scattering measurements have been performed at room temperature in thin film evaporated amorphous B and B 0.91 C 0.09 . The amorphous B spectrum indicates density of states features similar to those observed in infrared absorption as well as lower frequency scattering attributed to acoustic plus optic-like modes. The addition of C to amorphous B broadens the spectrum and shifts the lowest frequency peak to higher frequencies. A comparison of the amorphous B and broadened crystalline β-B Raman spectra indicates a number of similarities in peak frequencies that are consistent with similar short range order in both materials.

The specific heat of bulk amorphous arsenic

Abstract The specific heat of bulk amorphous arsenic deposited from the vapor at 415°K has been measured between 1.8 and 20°K. A comparison with compacted amorphous As films deposited at room temperature reveals appreciable differences in the form of C/T 3 . The results indicate a relative increase in the density of low frequency vibrational modes in the bulk amorphous As. The specific heat measurements support x-ray and Raman scattering studies that suggest variations in the local structure of amorphous As prepared under different deposition conditions. A comparison with neutron scattering measurements in bulk amorphous As also indicates an underestimate of the low frequency density of states.

Far infrared absorption in bulk amorphous red phosphorous

Abstract Far infrared absorption measurements have been performed above 30 cm −1 in bulk amorphous red P at 300 K. A number of low frequency spectral features are observed that are not present in the corresponding bulk amorphous As spectrum. Deep minima observed at 163 and 320 cm −1 in α(ω) and in the Raman spectra suggest that the phonon density of states is essentially divided into 3 bands. A calculation of the mean effective charge of the bands indicates weak coupling to the lowest frequency band. A comparison with the results in a -As suggests a less rapid frequency variation in the infrared coupling parameter at low frequencies in a - P .

Optical absorption in sputtered amorphous phosphorus

Abstract Optical absorption measurements are reported on r.f. sputtered films of a -P prepared at variable substrate temperatures. With decreasing substrate temperature the absorption spectra shift to lower frequencies and the energy gap decreases. The form of the spectra are, however, similar both in the band edge and Urbach tail regions. The Urbach slope is found to differ substantially from that of a -As, indicating that factors other than coordination number may influence this parameter. The changes in the absorption spectra are suggestive of a possible role of intermediate range order on the electronic states in a -P.

Raman scattering of Ni and Cr amorphous disilicides

Abstract The Raman spectra and reflectivity of r.f. sputtered amorphous semimetallic a -NiSi 2 and a -CrSi 2 thin films are reported. Substantial differences between the two amorphous disilicide Raman spectra are associated with changes in the phonon density-of-states as a result of variations in Si( p )-transition metal ( d ) interactions. The greater width of the bands in the Cr alloy suggests a less ordered local environment. Neither spectrum indicates distinct a -Si-like bands, demonstrating that significant short range order changes have occured.

Vibrational spectra of crystalline Bi1−xSbx alloys

Abstract Second order Raman scattering is reported at 300 K in crystalline Bi 1− x Sb x alloys. The spectra, which are interpreted in terms of dominant overtone processes, indicate variations in the phonon density of states for the optical bands with composition. The form of the spectra, which include contributions from Bi-Bi, Bi-Sb and Sb-Sb bonds, implies microscopic statistical clustering effects analogous to those in Ge 1− x Si x alloys. A comparison of the spectra with inelastic tunneling results in superconducting amorphous Bi 1− x Sb x indicates covalent bonding contributions in the latter.