Isamu Imai

Optical and Electrical Properties of Tin Oxide Films

The electrical and optical properties of the transparent conducting tin oxide films were studied experimentally. The observed Hall effect and Seebeck effect show that the present specimens are n -type conductor whose carrier density is 10 19 ∼10 20 cm -3 . The optical measurements made it clear that the fundamental energy gap was about 4 ev and the plasma frequency of the conducting carriers lied in the near infrared region. By comparing the observed optical transmission and reflection with the theoretical values which can be calculated with the electrically measured constants, the effective mass of the carriers was determined to be about 1/5 of the free electron mass. It will be discussed in conclusion that, to satisfy the thermoelectric power relation with this small effective mass, we must assume that the Coulomb scattering of impurity ions is the predominant scattering mechanism in the transparent conducting tin oxide films.

Cyclotron Resonance Line Broadening due to Carrier-Carrier Interaction in Germanium

The resonance line broadening due to carrier-carrier interaction in cyclotron resonance spectrum in pure germanium was investigated by increasing the density of photo-generated carriers up to 10 14 cm -3 . Measuring temperature ranged from 1.6°K to 15°K. The carrier density was estimated by combining the plasma shift of resonance line with the area under the absorption curve. While at high temperature the increment of the line width is approximately proportional to the carrier density, it is proportional to the square root of the carrier density at low temperature. It was found that the behavior at high density was well interpreted by the term of the scattering process by another carrier obeying the Brooks-Herring formula with the use of reduced mass of two interacting carriers. On the other hand, the broadening at low temperature for low density is rather inhomogeneous in the sense that the cyclotron frequency is randomly modulated by the Coulomb field due to another carrier.

The Optical Spectrum of HgI2

The optical spectra of tetragonal HgI 2 single crystals were investigated at 4.2 K and 79 K in 2∼6 eV region of photon energy. The main part of the dichroic optical spectra is well described by the optical transition from three p -like valence bands, which are split due to spin orbit interaction and crystal field perturbation, to an s -like conduction band. The optical structures, which seem to be due to exciton series, were observed in the fundamental absorption edge for an ordinary light. The tail part of the absorption edge was well expressed by Urbach Rule.

Line-Broadening of Cyclotron Resonance due to Lattice and Neutral Impurity Scattering in Silicon and Germanium

The relaxation times for electrons in germanium and silicon were measured from the half-width of the cyclotron resonance line at 6 mm wavelength over the temperature range from 1.6° to 20°K for the specimens of different impurity concentration. Above 3°K the reciprocal relaxation times for electrons in high purity specimens are 1/τ 0 =3.6×10 8 T 3/2 sec -1 for germanium and 1/τ 0 =2.6×10 8 T 3/2 sec -1 for silicon, respecitively. These results show that the acoustical lattice scattering is predominant in this region. For impure specimens, deviation from the T 3/2 -dependence was observed at low temperatures. This deviation is due to neutral impurity scattering. The applicability of Erginsoys formula for neutral impurity scattering was examined. It was found that the conventional Erginsoy,s formula corrected for chemical shift agrees fairly well with the experimental results.

Piezooptical Studies of Group 4B Transition Metal Disulfides ZrS2 and HfS2

Piezotransmission spectra near the indirect absorption edge and piezoreflectance spectra in the visible region of group 4B transition metal disulfides ZrS 2 and HfS 2 have been studied by the stress modulation technique at 77 K. The uniaxial stress X was applied along the a axis and the spectra were measured using the polarized light, E // X and E ⊥ X , where E is the electric field of the incident light. The change of the energy levels by the strain is evaluated by comparing the piezo-modulated spectra with the wavelength-modulated spectra. The main peak and the broad structure at higher energy side in the spectra originate from the overlapping of the transitions at and near Γ point and the other structures at lower energy side are due to the transitions at L and M points.

A Study of Interband Scattering of Holes in Germanium

An experimental study of the cyclotron resonance of hot holes in germanium was carried out at 1.6°K making use of 6 mm microwaves. The half widths of the resonance lines for light holes, heavy holes and electrons were observed as functions of the microwave power falling on the specimen. It was found that when a large microwave power fell on the specimen, the increase in the width of the light hole line was nearly the same as that of heavy hole. This fact can reasonably be interpreted, because light and heavy holes are strongly coupled by the interband transition.

Indirect Transitions in the Fundamental Absorption Edges of ZrS2 and HfS2

The wavelength-modulated transmission spectra near the fundamental absorption edges of ZrS 2 and HfS 2 have been studied. The observed spectra are well described by the theory of the indirect transitions with excitonic effect. The structures in the spectra are assigned to the indirect transitions, Γ 2 - → L 1 + and Γ 2 - → M 1 + . From the fitting of the observed spectra to the theoretical curves, the parameters including the band gap energies, the binding energies of the excitons, and the phonon energies at L and M points of ZrS 2 and HfS 2 , are determined.