Hiroshi Harima

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.

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...

Optical control of coherent optical phonons in bismuth films

Interference of impulsively excited coherent phonons in semimetals has been studied by using a double‐pulse pump–probe technique. Enhancement of the oscillation amplitude of an A1g mode is observed when the separation time of the double‐pulse is matched to the period of the phonon oscillation, and a cancellation is observed when the separation time is adjusted to half the period of the phonon oscillation. The amplitude after the second pulse shows a sinusoidal dependence as a function of the separation time, and this dependence is explained in terms of a superposition of two coherent phonon oscillations. In addition, not only the A1g mode but also an Eg mode have been observed by electro‐optic sampling.

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.

Detection of stacking faults in 6H-SiC by Raman scattering

Raman spectra of 6H-SiC crystals including stacking faults have been examined for the c face using backscattering geometry. The intensity of the transverse optical phonon band at 796 cm−1, which corresponds to the phonon mode at the Γ point in 3C-SiC, is sensitive to the stacking faults. We found that the intensity of this band depends on the stacking fault density. This is explained based on the bond polarizability model. The spatial distribution of the stacking faults is studied by Raman image measurement.

Spectroscopic Characterization of Low-Temperature Grown GaAs Epitaxial Films

Femtosecond time-resolved reflectance and Raman scattering studies have been made on GaAs epitaxial layers grown at temperatures between 200 and 300° C and subsequently annealed. A subpicosecond carrier lifetime (∼0.25 ps) has been measured for a sample grown at 250° C and annealed at 600° C. Raman measurements using a back scattering geometry show a strong TO phonon band for samples grown at 200° C and 250° C, while it is absent for samples grown at 275 and 300° C. The band width of the LO band increases with decreasing growth temperature. A phonon band corresponding to As precipitates is also observed at 200 cm-1 for samples grown at 200 and 250° C. A strong correlation is found between the measured carrier lifetime and Raman profile.

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.

Measurement of Absolute Densities and Spatial Distributions of Si and SiH in an RF-Discharge Silane Plasma for the Chemical Vapor Deposition of a-Si:H Films

Laser-induced fluorescence (LIF) spectroscopy has been applied to the detection of Si and SiH radicals in a silane plasma used in the chemical vapor deposition of a-Si:H films. Simple methods have been established for the calibration of the absolute densities. From the measured spatial distributions of those densities, the flux onto the substrate surface has been deduced. Based on these measurements, the contribution of Si and SiH radicals to the film growth is discussed in relation with the deposition rate, and it is concluded that, in quantity, these radicals do not contribute significantly to the deposition but may affect the quality of the films.

Chiral sum frequency spectroscopy of thin films of porphyrin J-aggregates.

Thin films of chiral porphyrin J-aggregates have been studied by vibrationally and electronically doubly resonant sum frequency generation (SFG) spectroscopy. It was revealed that the chiral supramolecular structures of porphyrin aggregates in solutions were retained in the thin film samples, and their chirality was determined by using chiral vibrational SFG spectroscopy. Electronic resonance profiles of some vibrational bands in achiral and chiral SFG were different from each other, and both were distinct from electronic absorption spectra. To account for these peculiar profiles, we have proposed interference effects of Raman tensor components in achiral and chiral SFG susceptibilities, which is analogous to that of resonance Raman scattering.

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.