Akio Ushirokawa

Effect of clusters on long‐wavelength optical phonons in Ga1−xInxAs

The phonon spectra of Ga1−xInxAs have been studied by measuring the infrared reflectivity at near‐normal incidence in the 180 ∼400‐cm−1 frequency range. Only one reststrahlen band corresponding to the GaAs‐like mode was observed for x≲0.2, and two bands were observed for the remaining composition. The data were analyzed by using the Kramers‐Kronig relationship and the dependence of the mode frequencies on alloy composition was interpreted on the basis of the modified cluster model including the effect of clusters. The fact that the calculated oscillator strength for the InAs‐like mode is substantially zero in the composition range x≲0.2 explains the reason why phonons of the InAs‐like mode are not detected experimentally for x≲0.2. The clustering parameter representing the effect of clusters is determined by fitting the measured mode frequencies to the calculated compositional dependence and takes the maximum value near the midrange of alloy composition.

Observation of the amorphous‐to‐crystalline transition in silicon by Raman scattering

Raman scattering, electron diffraction, and dark‐conductivity measurements have been made on so‐called a‐Si films deposited at various rf powers by a glow‐discharge technique. These measurements show that an abrupt transition between amorphous and polycrystalline states occurred between 350 and 370 rf voltages and films deposited at high rf voltages are polycrystalline. Dark conductivity of the silicon films changed largely with the amorphous‐to‐crystalline transition. It is also observed that some polycrystallized silicon films exhibited new peaks between 505 and 513 cm−1 in the Raman spectra.

Raman spectra from heat‐treated semi‐insulating GaAs

Raman‐spectra measurements were made on semi‐insulating (SI) GaAs heat‐treated in an atmosphere of H2 or Ar to study the effect of structural changes or strains on a thermal conversion of Si GaAs. Correlations between Raman spectra and electrical changes of SI GaAs indicate that strains in the interface region between SI GaAs and a dielectric film is, at least, one of the origins of the thermal conversion, while the structural changes accompanying the generation of the TO‐phonon line has no effect on the thermal conversion.

Radiation Induced Si?SiO2 Interface States and Positive Charge Buildup of MOS Capacitors Annealed in Nitrogen and in Hydrogen After Metalization

The density of Si–SiO2 interface states (ΔDit) and the density of trapped oxide charges (ΔNot) generated by Co-60 gamma ray irradiation are examined for MOS capacitors with post metalization annealing (PMA) in hydrogen and in nitrogen. A good correlation between |ΔVmg-ΔVFB| and \int Dit dE is found, and it verifies that the charging at the interface states affects the estimation of ΔNot from the VFB shift. The radiation hardness of the MOS structure is sensitive to annealing conditions. PMA is effective in decreasing ΔDit and ΔNot. The PMA effect is discussed in terms of weak bond and interstitial hydrogen atoms. Moreover annealing after irradiation is found to be effective in recovery of radiation damage on MOS devices.

Measuring Methods of Solar Cell Parameters Using Spectrum Response of Short-Circuit Current

Using the spectrum response in which a photon flux F is required to produce the short-circuit current to be constant, new methods for the determination of the solar cell parameters, i.e. the ratio (Dp/Sp) of the diffusion coefficient Dp of minority carriers in the n-type top region to the surface recombination velocity Sp at the front surface, the junction depth xj, the reflectivity R and the diffusion length Ln of minority carriers in the p-type base region, are proposed. It is obtained that Ln has a tendency to decrease with increasing the doping level and that R of black cells has a tendency to decrease with etching time. It is found experimentally that Dp/Sp of flat-surface cells and of black cells has a tendency to increase with doping level and with etching time, respectively.

Extended AC Conductance-Bias Method Considering Semiconductor Conductance and Capacitance in MOS Structure

The influence of the semiconductor conductance Gs and the interface trap conductance Gp on MOS ac conductance is revealed in all bias regions from inversion to accumulation in the high frequency range, for example, 1 kHz-1 MHz. The conductance–bias (G-V) method is extended considerably in order to clarify the interface state characteristics more easily. Under the bias conditions of accumulation and weak inversion, the measured results are successfully explained by the newly-introduced semiconductor conductance Gs. The conductance curve of the interface and oxide traps with respect to the semiconductor capacitance Cs shows a peak in the depletion region. The semiconductor conductance curve with respect to Cs is linear in the accumulation region.

Difference of Characteristics of a-Si: H Solar Cells between under Illumination and Dark

The purpose of this paper is to show that the a-Si: H p-i-n type solar cells become more conductive as the incident light intensity increases. A new method for the determination of the series resistance Rs, the shunt resistance Rsh and the diode factor (n-value) is given from the measured dc current I-the dc voltage V curve under the illumination of a fixed light intensity. Consequently, as the light intensity increases from zero to 140 mW/cm2, it is found that Rs decreases from 47.3 ohm to 0.1 ohm and that Rsh decreases from 2.2×105 ohm to 7×102 ohm in the case of #E7-1 sample as well as the other samples. The increase of the n-value with increasing light intensity suggests that the a-Si: H cells show a tendency toward the property due to the recombination current in the space charge region near the p-i junction.