Seinosuke Onari

Low-temperature crystallization of hydrogenated amorphous silicon induced by nickel silicide formation

We have investigated the silicidation process of the Ni/a-Si:H system on a fused silica substrate mainly by in situ electric resistance measurement. A rise of the resistance is observed in the temperature range of 480–510°C of the resistance curve at a heating rate of 2 K/min. The results of the RBS measurements and the XRD measurements show that the rise of the resistance originates from the diffusion of Ni from the NiSi2 layer to a-Si:H film, and that the a-Si:H crystallizes during the diffusion. Noting that the crystallization temperature of a-Si:H film is higher than 700°C, the present crystallization occurs at a much lower temperature. It is suggested that this low-temperature crystallization is induced by heterogeneous nucleation, where the NiSi2 becomes the nucleus for Si crystallization.

Initial Stage of the Interfacial Reaction between Nickel and Hydrogenated Amorphous Silicon

The initial stage of the interfacial reaction between Ni and hydrogenated amorphous silicon has been studied mainly by in situ electrical resistance measurement. The change of the resistance in this system induced by the annealing at a constant heating rate shows a sudden drop, which corresponds to the amorphous-to-crystalline transformation of the Ni–Si intermixing layer. In situ resistance measurements for various intermixing layers in the initial stage demonstrate that the crystallization temperature becomes lower with the increase of the amount of Ni contained in the layer. The result means that the thermal stability of the intermixing layer decreases with its growth. It is suggested that the crystallization occurs when the amount of Ni contained in the intermixing layer reaches the critical thickness, which depends on the temperature.

Structures and optical properties of silicon nanocrystallites prepared by pulsed-laser ablation in inert background gas

Silicon (Si) nanocrystallites have been synthesized using pulsed-laser ablation in inert background gases, for studying the structures and optical properties as one of the quantum confinement effects. We extracted a process condition where well-dispersed Si nanocrystallites devoid of droplets and debris are prepared, by varying excitation laser conditions. Furthermore, we investigate the influence of the inert background gas pressures on transition from amorphous-like thin films to nanocrystallites. It was clarified that there is a processing window of the inert background gas pressure in which the quantum confinement effects for carriers and phonons become apparent.

Raman Scattering in Amorphous As-Se System

Raman spectra of amorphous AsxSe1-x system (0≤x≤0.5) were measured and the spectra were interpreted with the molecular model of Lucovsky and Martin. In the Se-rich region the observed Raman spectra are well reproduced by the superposition of the Raman intensity of pure Se, As2Se3 and As2Se4 in proportion to x. In the As-rich region the Raman spectrum at x=0.45 is well described by the average of those at x=0.4 and 0.5. The amorphous state of AsxSe1-x for 0<x<0.4 is shown to be composed of a Se8 ring, Sen chain, AsSe3 unit and As2Se4 unit. In the As-rich region, the local structure change due to the substitution of As and Se is small.

Annealing effects on structures and optical properties of silicon nanostructured films prepared by pulsed-laser ablation in inert background gas

We studied the annealing effects on the structures and optical properties of silicon (Si) nanostructured films. The Si nanostructured films were synthesized by pulsed-laser ablation in inert background gas. It was found that the Si nanostructured films partially include an amorphous-like structure under the as-deposited condition. After annealing in nitrogen gas, the crystallinities of the Si nanoparticles recovered, and four visible photoluminescence (PL) bands (1.7, 2.2, 2.7, and 3.1 eV) appeared at room temperature. Furthermore, upon subsequent annealing in oxygen gas, strong quantum confinement effects for both phonons and carriers of the Si nanoparticles appeared, and the relative intensity of the 2.7 eV blue band increased. We fabricated electroluminescent (EL) diodes with active layers of annealed Si nanostructured films, and verified visible EL spectra at room temperature, which exhibited the same trends as the PL spectra. A possible explanation for the increase of relative intensity of the 2.7 eV...

Magnetoabsorption in Single-Crystal HgCr2Se4

The optical absorption spectra of HgCr 2 Se 4 single crystals in the intrinsic transition region were measured as a function of magnetic field at a number of fixed temperatures. The absorption edge, 0.80 eV, at room temperature shifted to 0.27 eV at liquid helium temperature. The temperature dependence of the absorption edge was nonlinear and remarkable around the Curie temperature. The absorption edge also shifts to the lower energy side with the increasing magnetic field. The field dependence of the edge shift in weaker magnetic field was larger at the Curie temperature than it in the higher temperature region. The temperature and magnetic field dependence of the absorption edge was compared to a calculated spin correlation function in spinels with the nearest neighbour exchange interaction between localized Cr +3 ions. Other kind of exchange interaction would be required to explain the temperature dependence of the absorption edge.

Raman scattering under pressure in ZnGa2Se4

Abstract The Raman scattering spectra of ZnGa 2 Se 4 under pressure were investigated at 300 K up to 18.9 GPa. Two stages were observed in the pressure dependences of Raman bands. Such behavior in accordance with the experimental findings existing in literature and was attributed as arising due to the order–disorder phase transition in the cation sublattice. Using the Harrison–Keatings model of the lattice dynamics modified for the crystals with the tetragonal structure, the bulk modulus B and the mode-Gruneisen parameters Γ i were determined for the first time. It is shown that a better agreement between the experimental and calculated values of Γ i is observed, if one takes into consideration different frequency-pressure behavior for the bond-bending and the bond-stretching parameters, which determine the low- (lower than 140 cm −1 ) and high- (higher than 140 cm −1 ) frequency phonons, respectively.

Photoluminescence of CdS nanoparticles suspended in vacuum and its temperature increase by laser irradiation

We report here photoluminescence of CdS nanoparticles suspended in vacuum. The CdS nanoparticles, whose size distribution was bimodal having peaks at 11.3 and 43.1 nm, were produced by the gas-evaporation method and the nanoparticle beams were formed within the rarefied inert-gas flow. The band-edge emission and surface luminescence of the CdS nanoparticles, whose surfaces were completely free from adsorption, reaction or contamination with their surroundings, were observed. As the excitation intensity increased, the band-edge emission shifted to lower energy and the surface state luminescence shifted to higher energy. These energy shifts in the photoluminescence spectra as functions of excitation intensity were interpreted as an internal temperature rise of the CdS nanoparticles suspended in vacuum, and the internal temperature of the CdS nanoparticles was evaluated by means of photoluminescence. It was found that the nanoparticles suspended in vacuum were heated by much weaker laser irradiation than the bulk surface due to the large surface-to-volume ratio.

Far Infrared and Raman Spectra in (As2S3)1-x(Sb2S3)x Glasses

The far infrared reflection and Raman spectra of (As2S3)1-x(Sb2S3)x glasses (0≤x≤0.6) were measured in the spectral region 60~600 cm-1 at room temperature. Two-mode type behaviors of the dominant modes originating from the local vibrations of AsS3 and SbS3 pyramidal units were observed. The infrared and Raman dominant mode frequencies of g-Sb2S3 are 270 cm-1 and 297 cm-1, respectively. The optical constants were calculated by Kramers-Kronig analysis, and were also analyzed by the classical oscillators model. The effective charges of the AsS3 and SbS3 pyramidal units were calculated. The force constants were calculated using Gordys model, Somayajulus model and also by the general valence force field method. The dependence of the infrared and Raman dominant modes on the composition x shows that hetero-bondings (chemical ordering) and therefore pyramidal units are predominant in this system.

Structural relaxation in the glass transition region of chalcogenide amorphous semiconductors (GeS2)1−Y(Sb2S3)Y

The structural relaxation in the glass transition of the amorphous (GeS2)1−Y(Sb2S3)Y (0.1⩽Y⩽0.9) system has been investigated with heat capacity data obtained from differential scanning calorimetry experiments. The heat capacity curves were analyzed by the Moynihan formulation with a Kohlrausch–Williams–Watts relaxation function φ(t)=exp[−(t/τ0)β] (0<β⩽1) and we used the Narayanaswamy expression for τ0. The values of model parameters, the non-exponentiality, β, the activation energy, Δh, and non-linearity, x, were obtained from the best model-fitting to experimental data. The dimension in the structural relaxation decreased with increasing Y. This tendency is consistent with decreasing network dimensionality with increasing Y. The values of the non-exponentiality, β, were in the range of 0.6 to 0.8 and changed with the composition Y. The change in the network dimensionality with composition Y is responsible for the change in β for the GeS2 rich composition range (Y⩽0.3).