Hajime Kasahara

Raman scattering from gas-evaporated silicon small particles

Abstract Raman scattering measurements are reported on silicon small particles prepared by gas-evaporation technique. The crystalline structure is also observed for the sample having 70 A particles in average size. Four resolved component modes with Gaussian distribution function are identified with the three usual modes (LA, TO and allowed-TO) and a new surface mode. The surface mode of silicon particle, whose relative integrated intensity decrease with an increase of the particle size, is presented for the first time.

Probing the Crystallinity of Evaporated Silicon Films by Raman Scattering

Changes in the Raman spectra of evaporated a-Si films resulting from thermal annealing are reported in terms of the peak frequency and linewidth of a TO-like mode and the volume fraction of crystallinity ρ, where ρ is determined from the integrated intensities of resolved modes corresponding to a crystalline component and an amorphous-like component. The peak frequency and the linewidth show abrupt changes as the annealing temperature Ta rises, while ρ varies slowly with Ta, indicating a continuous crystallization with a constant rate, from Ta <400^°C. Moreover, the activation energy of the crystallization can be calculated by fitting ρ to the Avrami formula and is found to be the activation energy for viscous flow (0.35 eV). The crystalline TO mode of the highly-crystallized film is also compared to the results of the theoretical calculation reported by Richter et al.

The Annealing Behavior of Si–F Bonding Structure of Amorphous Si–F Films

The infrared transmittance and reflectance spectra of amorphous fluorinated silicon films (a-Si: F) have been measured as a function of a ratio of SiF 4 /(SiF 4 +Ar) partial pressure and annealing temperature. The absorption bands were assigned: 1,015 cm -1 (SiF 4 stretching), 965 cm -1 (Si–F 3 stretching), 920 cm -1 (Si–F 2 stretching), 870 cm -1 ((Si–F 2 ) 2 stretching), 825 cm -1 (Si–F stretching), 600 cm -1 (Si–C stretching or Si 2LA mode), 515 cm -1 (Si TO mode), 380 cm -1 (SiF 4 bending), and 300 cm -1 (Si LA mode). SiF 4 molecules incorporated into a-Si: F films are observed in samples prepared with high SiF 4 partial pressures, but in samples prepared lower SiF 4 partial pressures dangling bonds seem to be saturated by fluorine atoms. The transition from Si–F, Si–F 2 , and Si–F 3 groups to SiF 4 molecules occurs above annealing temperatures (600°C). The total fluorine content is constant until crystallization, and a-Si: F films are found to be more stable than a-Si: H films.

Raman Study of Silicon Small Particles

The particle-size dependence of Raman spectra of gas-evaporated silicon small particles, whose size distribution is well described by a log-normal distribution function, is reported in connection with their electron diffraction patterns. Although a diamond structure is confirmed by the Debye-Scherrer rings for the samples with an average particle size from 70 to 220 A, the Raman spectra are rather complex; the spectra are resolved into a new surface mode and three known modes (LA, TO and allowed-TO ones) in a range of 250 to 600 cm -1 . A disadvantage of the present theoretical studies relating to the finite-size effects of microcrystalline silicon is pointed out.

Raman scattering of SiF4 molecules in amorphous fluorinated silicon

Abstract The Raman spectra of amorphous fluorinated silicon films prepared by the reactive sputtering of Si in a gaseous mixture of SiF4-Ar have been measured. The calculations of the normal vibration frequencies of SiFn bonds using the valency force field model and the central force field model were also made. Comparing the experimental results with these calculations, the SiFn vibrational modes were assigned and the existence of SiF4 molecules was confirmed.

Infrared absorption by coupled surface-phonon-surface-plasmon modes in small GaAs crystals

The infrared transmission spectra of small GaAs crystals (0.5 mu m in average size) are reported. The samples are produced by mechanical grinding down from the bulk crystals with known carrier concentrations from 5.3*10-17 to 2.4*1018 cm-3. The spectrum is constructed from a surface phonon mode and coupled modes between surface phonons and plasmons. The two coupled modes are clearly separated and their frequencies agree very well with the theoretical dispersion curves. On the basis of the theory of an average dielectric function the observed spectra are reproduced from the calculations, in which the particle is assumed to be a sphere composed of a shell with no carriers and a core containing high carrier concentration.

Raman study of thermally annealed silicon small particles and thin films

Abstract A comparative study on thermal annealing of gas-evaporated silicon small particles and evaporated thin films is made using Raman scattering. The Raman spectra of the annealed silicon particles show only small change due to SiO bond on the surface, suggesting the crystalline structure of the particle. On the other hand, the films are crystallized thermally because of their amorphous-like nature; the activation energy of the crystallization can be determined by resolving the Raman spectra into crystalline parts and amorphous-like ones, and is found to be that of viscous flow (0.35 eV).

Incipient crystallization of evaporated germanium film probed by Raman scattering

The crystallization produced by isochronal annealing of evaporated Ge film is analyzed by using Raman spectroscopy. A volume fraction of crystallinity is deduced from the Raman spectrum and is then substituted into the modified Avrami formula. From the analysis, an activation energy for the incipient stage of crystallization is found to be 0.13 eV, which agrees with the activation energy for viscous flow of Ge. The present result is consistent with the result of Si film.

Photoacoustic Spectra in Ternary Semiconducting Compound CdIn2S4

The amplitude and phase of the photoacoustic (PA) signal have been studied in ternary semiconducting compound CdIn2S4. By measurements of the phase difference of variable chopping frequency of the incident light on different thickness of samples, thermal conductivity and thermal diffusivity were given to be 1.3×10-2 cal/gsecK and 2.3×10-2/sec, respectively. The treatment of sample surface has been also studied: the PA amplitude shows gentle slope with a rough surface layer. The phase difference of the PA signal agreed well with a two-layer model by extending the Rosencwaig and Gersho theory.

Lattice Vibrations in CdGa2S4

The far-infrared reflectance spectra of meltgrown single crystal CdGa2S4 have been measured at room temperature. Nine of ten reflectance spectra which were anticipated from the irreducible representation are observed for this material. These reflectance peaks are attributed to the reststrahlen bands due to the ionic bonds between the positive metal ions and negative chalcogen ions. From the reflectance spectra, the transverse and longitudinal optical phonon frequencies and the static dielectric constant are calculated. In this case, ωTO=84 cm-1, 134 cm-1, 166 cm-1, 239 cm-1, 265 cm-1, 310 cm-1, 319 cm-1, 342 cm-1 and 359 cm-1, ωLO=89 cm-1, 136 cm-1, 171 cm-1, 245 cm-1, 267 cm-1, 312 cm-1, 340 cm-1, 348 cm-1 and 384 cm-1, and e0=9.14 for CdGa2S4 single crystal.