Shin-ichi Nakashima

Raman Investigation of SiC Polytypes

It has been recognized that Raman scattering spectroscopy is a powerful tool to characterize SiC crystals non-destructively. We review recent significant developments in the use of Raman scattering to study structural and electronic properties of SiC crystals. The areas to be discussed in the first part include polytype identification, evaluation of stacking disorder and ion-implantation damages, and stress evaluation. The Raman scattering by electronic transitions is discussed in the second part of this article. We concentrate on the plasmon LO-phonon coupled modes whose spectral profiles are used to evaluate the carrier concentration and mobility. Anisotropic electronic properties of α-SiC and characteristics of heavily doped crystals are discussed. Semiconductor-to-metal transition and Fano interference effect are also treated.

Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs.

Terahertz radiation was generated with several designs of photoconductive antennas (three dipoles, a bow tie, and a coplanar strip line) fabricated on low-temperature-grown (LT) GaAs and semi-insulating (SI) GaAs, and the emission properties of the photoconductive antennas were compared with each other. The radiation spectrum of each antenna was characterized with the photoconductive sampling technique. The total radiation power was also measured by a bolometer for comparison of the relative radiation power. The radiation spectra of the LT-GaAs-based and SI-GaAs-based photoconductive antennas of the same design showed no significant difference. The pump-power dependencies of the radiation power showed saturation for higher pump intensities, which was more serious in SI-GaAs-based antennas than in LT-GaAs-based antennas. We attributed the origin of the saturation to the field screening of the photocarriers.

Raman scattering from anisotropic LO‐phonon–plasmon–coupled mode in n‐type 4H– and 6H–SiC

LO‐phonon–plasmon–coupled modes in n‐type 4H– and 6H–SiC single crystals with free‐carrier concentrations of 1016–1018 cm−3 have been measured by Raman scattering at room temperature. The axial‐type mode for which plasma oscillation and atomic displacement are parallel to the c axis, and the planar‐type mode for which these oscillations lie in the c plane, have been individually observed. From a line‐shape analysis of the observed spectra, the plasmon frequency, carrier damping, and phonon damping have been deduced. These quantities have large differences between the axial‐ and planar‐type mode in 6H–SiC, indicating its large crystal anisotropy. On the contrary, 4H–SiC shows small anisotropy. The longitudinal and transverse effective mass components of the electron have been determined from the plasmon frequency using carrier densities derived from Hall measurements. The deduced values are m∥=1.4m0 and m⊥=0.35m0 for 6H–SiC, and m∥=0.48m0 and m⊥=0.30m0 for 4H–SiC. The carrier mobility obtained from the ana...

Ultrashort Electromagnetic Pulse Radiation from YBCO Thin Films Excited by Femtosecond Optical Pulse

We have observed ultrashort electromagnetic pulse radiation from YBa2Cu3O7-δ thin-film dipole antennas. The supercurrent transient is created by the excitation of the supercarriers into quasiparticles with a femtosecond laser pulse, and freely propagated electromagnetic pulses are measured and characterized. A pulse with 0.5 ps full width at half-maximum was obtained, containing frequency components up to 2.0 THz. A femtosecond time-resolved characterization of the spectra revealed that they strongly depend on the excitation conditions, and the quasiparticle recombination time becomes longer with increase in the excitation intensity. It is also observed that the radiation power increases in proportion to the square of both the bias current and the laser power in the region of weak excitation, which is consistent with the classical theory based on a two-fluid model. In the region of strong excitation, deviation from the classical theory was observed.

Determination of crystallographic orientations in silicon films by Raman-microprobe polarization measurements

Local crystallographic orientations of semiconductors have been determined by a scanning Raman microprobe combined with polarization measurements. We present a method of Raman scattering determination of crystallographic orientations in silicon crystals which allows rapid determination with an accuracy of ±2°. It is found that surface morphology affects the Raman polarization analysis. For laser‐annealed silicon layers capped with silicon nitride films, unpolarized scattered light is superimposed on polarized scattered light, because incident light beams entering rough surfaces are directed into random orientations and produce the unpolarized scattered light. The degree of the surface roughness is estimated by assuming that the rough surface consists of periodically arranged spherical segments.

Raman studies on phonon modes in cubic AlGaN alloy

Cubic AlxGa1−xN alloy layers have been successfully grown for x=0−1 by gas-source molecular beam epitaxy on cubic-SiC/Si substrates, and the compositional dependence of the transverse-optic (TO) and longitudinal-optic (LO) phonon modes has been studied by Raman scattering. The LO-mode frequency of mixed crystals shows a systematic variation from the pure cubic AlN phase to the cubic GaN phase (one-mode type). On the contrary, there are two branches for the TO mode varying slowly in frequency with the composition (two-mode type). This behavior is explained within a random-element-isodisplacement model including the effect of polarization field. Our result indicates a strong polarization field acting on the cation-nitrogen bonds.

Electronic properties in p-type GaN studied by Raman scattering

Raman spectra from p-type GaN have been systematically studied in the hole density range of 5×1016–1×1018 cm−3. Contrary to the case of n-type samples, spectral profiles of the LO-phonon-plasmon coupled mode in p-type show no remarkable change with the hole density. Thus, precise evaluation of electrical transport parameters such as carrier density and mobility from the coupled mode profile is difficult. However, a continuum band has been observed in the low-frequency range of the spectra, becoming intense with the increase of the hole density. This band has been attributed to the inter-valence-band transition of holes, and the intensity can be used as a good measure of the hole density.

Electronic properties in doped GaN studied by Raman scattering

Raman scattering spectra from n- and p-type hexagonal GaN epitaxial layers have been measured in the carrier density range of 10 16 10 18 cm -3 . In n-type samples spectral line shape of the LO-phonon plasmon coupled mode changes sensitively with the carrier density. It is shown that lateral distributions of carrier density and mobility in epitaxial layers can be obtained by analyzing the coupled-mode line shape. In p-type samples, on the other hand, the coupled mode shows little variation with the hole density. However, a continuum band. appearing in the low-frequency region. grows in intensity with the increase of hole density. This band arises from the electronic excitations by inter-valence-band transitions, and causes Fano interference with an overlapping phonon band. Intensity of the continuum band, as well as the interference feature. could be used as a measure of hole density.

Surface Structure of Ion-Implanted Silica Glass

The structure of silica glass implanted with O+, B+ and Ag+ at an acceleration energy of the order of MeV has been investigated by infrared reflection and Raman scattering spectroscopies. It is found that the Si–O–Si bond angle decreases with ion implantation. Three-fold siloxane rings are formed with ion implantation. It is deduced that compaction of silica glass caused by ion implantation is attributed to the decrease in the Si–O–Si bond angle and the formation of small siloxane rings. The modification of the silica glass structure caused by ion implantation is similar to that caused by neutron irradiation.

Residual Strain in Single Crystalline Germanium Islands on Insulator

Residual strain in single crystalline germanium islands, recrystallized by zone melting, on SiO2 substrates has been investigated by Raman microprobe measurements with a spatial resolution of 5 µm. It was found that strain of 3.0×10-3–6.0×10-3, corresponding to local tensile stress of 2.3 to 4.6 kbar, remained in single crystalline germanium islands after zone melting recrystallization.