Kazuki Wakita

Resonant Raman scattering and luminescence in CuInS2 crystals

Resonant Raman scattering and luminescence have been examined for CuInS2 crystals grown by the traveling heater method (THM) and the iodine vapor transport method (IT). Resonant Raman spectra of CuInS2 have been observed, and the spectra show seven single-phonon peaks and one two-phonon peak. Among them, three single-phonon modes have been found in the low-Raman-shift region because of resonant enhancement of phonon modes. The enhancement of these phonon modes is caused by incoming resonance mediated by bound excitons on the THM crystal, while it is attributed to outgoing resonance due to intermediate states of free excitons on the IT crystal.

Refractive indices of layered semiconductor ferroelectrics TlInS2, TlGaS2, and TlGaSe2 from ellipsometric measurements limited to only layer-plane surfaces

The ternary layered Tl-contained semiconductor ferroelectrics TlInS2, TlGaS2, and TlGaSe2, exhibiting an incommensurate phase and a relaxor state with temperature, were approached down to 0.76eV at room temperature by spectroscopic phase modulated ellipsometry in a region below the energy gap of each material. By using both coherent and incoherent reflection modes on only the layer-plane surfaces, the refractive indices in the parallel- and perpendicular-to-the-layers directions of the light propagation were determined. Sellmeier single-oscillator form was used to fit the model data to the experimental intensities. The discrepancy between the model and experimental data was shown to be negligible. Besides, a self-consistent picture of the obtained refraction indices was emerging after the discussion based on the available data for the studied materials. The applied experimental approach was thus found to be quite effective for layered materials, and the obtained refractive indices might be used for databa...

Photoluminescence excitation spectra of CuInS2 crystals

The photoluminescence excitation (PLE) spectra of CuInS2 crystals grown by the traveling heater method (THM) and the iodine vapor transport method (IT) have been examined. THM crystals show significant structure with many peaks and dips in the PLE spectra, whereas IT crystals exhibits two clear peaks at two free exciton energies in the spectrum. The peak and dip structures of PLE spectra at the free exciton energy depend on the monitoring PL peak, the kind of crystals, and temperatures. A proposed relaxation process of excited electron–hole pairs explains peak and dip structures of PLE spectra at the free exciton energies.

Electrophotographic Properties of RF Glow Discharge-Produced Amorphous Si: H Film

Amorphous silicon film with extremely high dark resistivity, 1013–1014 ohm-cm, at room temperature is prepared for the first time by glow discharge decomposition of SiH4 with suitable doping by B2H6 and a slight content of oxygen. The a-Si: H film exhibits an excellent photoconductive sensitivity in visible wavelength range. Room temperature electron and hole drift mobilities are found to be almost the same in a-Si: H film deposited at 200°C of the substrate temperature and 10-3 B2H6/SiH4 doping gas ratio. These unique properties of a-Si: H films are utilized for bi-chargable monolayer electrophotographic photoreceptor, and positive and negative reproduced images have been firstly demonstrated by utilizing a-Si: H deposited drum.

Luminescence of Mixed-Mode Exciton-Polariton in CuGaS2

The luminescence spectra of the mixed-mode exciton-polariton in CuGaS2 are studied at 8.6 K in conjunction with the absorption spectra. By measuring emitted photons as a function of propagation direction with respect to the z axis of the crystal, it is possible to vary the magnitude of a longitudinal-exciton and extraordinary-photon coupling. The effects of this coupling on the resonance energy and oscillator strength of the extraordinary exciton-polariton are established for the emission line of the Γ4 free exciton which exhibits a doublet structure. In contrast, the emission spectra due to the ordinary photons show one weak, sharp line in the same intrinsic exciton region as the Γ4 line. The polarization dependence of the emission intensity is greatly reduced for this sharp line. The behavior is interpreted as arising due to the optically forbidden exciton having Γ3 symmetry. The resonant exciton transitions to the lower exciton-polariton branch and the forbidden state are observed in the extraordinary and ordinary absorption spectra, respectively.

1D-TlInSe2: Band Structure, Dielectric Function and Nanorods

Linear combination of atomic orbitals (LCAO) analysis of the electronic band states has been completed for one-dimensional (1D) TlInSe2 having rod-like ground state shape of bulky crystal. The total scenario of the occurrence of the band states from the atomic states has been established. According to this scenario, in dipole approximation the optical transitions at band gap (point T of Brillouin zone) are either entirely forbidden or allowed for T2-T10 transitions in e⊥c configuration provided that either initial or terminate state has T2 symmetry and both are Se-like. As a whole, the obtained results on the electronic spectrum, including dielectric function, are applicable to all obtained 1D-TlInSe2 nanorods which were as thin as 30–50 nm in cross-section, and apparently preserved tetragonal crystal structure of bulky material. The thermal instabilities developing already in bulky samples of 1D-TlInSe2 are considered to be an ultimate source of the nanoparticles emerging in plenty during nanorods preparation. The nanoplates of a chemically similar but 2D material, TlInS2, are demonstrated for comparison to show the absence of nanoparticles in that case. A possibility of nanoparticle preparation using laser excited coherent phonon trains in the nanorods of 1D-TlInSe2 is figured out.

Optical properties of CuAlSe2

We have determined the complex dielectric tensor components of the chalcopyrite semiconductor CuAlSe2 in the energy range between 1.4 and 5.2 eV, at room temperature, using spectroscopic ellipsometry. We present results obtained on two single crystals grown by the traveling-heater method using In solvent. Values of refractive indices n, extinction coefficients k and normal-incidence reflectivity R in the two independent polarizations are reported. The structures observed in the energy region studied are analyzed by fitting the second-derivative spectra d2e(ω)/dω2 to analytic critical-point line shapes. The obtained energies are assigned to certain electronic interband transitions by comparison with existing band structure calculations.

Temperature-Dependent Angle-Resolved Photoemission Spectra of TlInSe2: Manifestations of Incommensurate and Commensurate Phases

The energy band structures of the valence band of the TlInSe2 material with a quasi one-dimensional chain structure and a giant Seebeck coefficient at temperatures below 413 K were investigated by angle-resolved photoemission spectroscopy at 280 and 50 K. The obtained energy bands and the calculated band structure were found to correspond well in main futures. A detailed analysis using both experimental and theoretical data was performed and the obtained results were evident of the presence of the incommensurate phase linked to a certain point on the A line of the Brillouin zone of TlInSe2. A self-consistent picture of the phase transition in TlInSe2 was drawn after discussion using available data for this material. According to this picture, TlInSe2 at 50 K is already a commensurate material while the incommensurate-commensurate phase transition occurs between 280 and 50 K.

Time-resolved photoluminescence studies of free excitons in CuInS2 crystals

The time-resolved photoluminescence of free excitons in bulk single-crystal CuInS2 grown by the traveling heater method is examined. The decay of free exciton emission exhibits a double exponential curve at low temperature. The decay-time constant of the fast component increases monotonically with excitation density, whereas that of the slow component appears to be independent of excitation density. From this result, the fast and slow components are attributed to nonradiative and radiative recombination processes, respectively. The radiative lifetime of free excitons is estimated to be 320±30 ps at 10 K.

Effect of annealing on photoluminescence spectra and film structure in a-SiNx:H

The change of photoluminescence (PL) spectra and structure in hydrogenated amorphous silicon-nitrogen alloy films deposited by glow-discharge decomposition of SiH4 and NH3 with various mixture ratios has been investigated as a function of annealing temperature. The PL spectra have a single broad band in the visible range and its peak energy shifts toward lower energy for NH3/SiH4≤6.0 and higher energy for NH3/SiH4≥6.8 by annealing at 600°C. The PL-peak energy varies linearly with optical band gap. Annealing at temperatures higher than 400°C breaks Si–H and N–H bonds to form Si–N bonds for all samples. The Si–Si bond concentration decreases with annealing for NH3/SiH4≥7.5, while it is considered to increase for NH3/SiH4=5. The change observed in the PL-peak energy and the optical band gap with annealing is associated with the variation in the concentration not only of Si–N but also of Si–Si bonds.