Toru Miyakawa

Temperature and porosity dependence of the thermoelectric properties of SiC/Ag sintered materials

We have studied the thermoelectric properties of a SiC-based thermoelectric semiconductor with Ag and polysilastylene (PSS) as a dopant and as a sintering additive, respectively. Ag is an effective dopant to decrease the electrical resistivity of the SiC-based p -type thermoelectric semiconductor. It introduces carrier (hole) concentration 10 3 −10 4 times larger than the case of Al-doped SiC with the typical doping concentration. PSS can control the sample density, which is one of the important factors in decreasing the electrical resistivity and thermal conductivity of the sintered samples. The figure of merit of the sample with Ag 2.0 wt% and PSS 0.1 wt% was estimated to reach 4 × 10 −4 K −1 at 700 °C. This value implies that the SiC/Ag system is one of the promising thermoelectric materials for a high-temperature region.

Band Gap of Porous Silicon Estimated by Photoacoustic Spectra

The conditions under which photoacoustic (PA) measurements of porous Si samples can yield reliable results on the widening of their band gaps are studied. The enhancement of the PA signals observed from porous Si samples is stronger than those from the powdered ones. Although this enhancement makes the observation of both the magnitude and phase of the PA signals easy, it also causes an apparent shift in the observed gap of the samples with a thin porous Si layer on the Si substrate. A simplified two layer model is examined, including the effect of scattering to separate this effect. The results of the samples with a thicker porous Si layer reveal the real shift of the band gap.

Photoacoustic Spectra for Porous Silicon Using Piezoelectric Transducer and Microphone.

Photoacoustic (PA) spectra are measured for porous Si (PS) using piezoelectric detectors (PPT) and compared with the microphone PA (MPA) spectra to examine the nonradiative properties of PS that has complicated nanostructures. Three peaks (peaks 1 and 2, and a small peak at 600 nm) are observed in samples 2 and 3 with porosities p=60 and 70%, respectively, while only a single peak (peak 1) is observed in sample 1 with p=30%. Peak 2 is related to the band gap of the Si substrate. In contrast, peak 1 seems to be due to the absorption change at the band gap unique to the PS. This band gap shifts to higher energy for PS samples with higher porosity. We propose that this band gap is defined by the largest size nanocrystals, where PS has nanocrystallite size distribution. The small peak at 600 nm is related to the PL peak and it is observed only in PPT spectra. These results suggest that different mechanisms are operating in PPT spectra and MPA ones for the enhancement of the PA spectra.

Photoacoustic Spectrum and Surface Morphology of Porous Silicon.

Both the photoacoustic (PA) and the photoluminescence (PL) spectra were studied in porous Si. The surface morphology was investigated using atomic force microscopy (AFM). The PA signal intensity was increased as porosity increased. The enhancement of the PA signal intensity with an increase in the surface area was observed in samples whose porosity was above 60%. These surface effects were observed in AFM images. We found a reciprocal correlation between the PA spectrum and the PL spectrum in the same sample. We also observed the reduction of the red spectrum in the PA spectra of the samples which showed a peak at 600 nm in the PL spectrum.

Size Effects on Photoacoustic Spectra for GaAs Fine Powder

The photoacoustic (PA) spectra of GaAs fine powder whose particle size was of the micrometer order were measured. The PA spectra show a broad peak and this peak shifts to the high-energy region as the sample particle size decreases. The band gap shift can be observed for the large-particle-size samples in the PA spectra. On the other hand, the PA peak shift for the small-particle samples is caused by the light penetration effect causing the decrease of the PA signals, where the particle size is smaller than the light absorption length. Although careful elucidation of the band gap shift in the fine powder samples from the PA measurement is required, the light absorption spectra above the band gap energy can be estimated.

Amplitude and Phase Signals of the Photothermal Deflection Images

Noncontact photothermal deflection (PTD) techniques are applied to the nondestructive photothermal detection of buried structures. The data are analyzed to take account of the finite radius of the excitation laser. The calculated PTD signals and the experimental data are compared. The modulation frequency dependence of both the PTD amplitude and phase signals has been used to estimate the thermal diffusivity. The phase spectra are more sensitive to the buried structure in the deep interior than the amplitude signals and are able to show the sharp edge at the corner of a buried structure. The case where the laser radius is larger than the buried structure is also investigated using fine cross pattern samples.

Effect of electric field on deep centers in Si∶V studied by spectral analysis of capacitance transients

The effect of bias on the nonexponentiality of transient capacitance waveforms due to V1, V2 and V3 centers in vanadium-dopedn-Si is analyzed using Spectral Analysis of DLTS (SADLTS). The emission rate spectrum broadens with the maximum valueλmax of emission rate with increasing the reverse bias. A less sensitive field dependence of the emission rate spectrum of V1 compared to V2 and V3 centers suggests differences in the nature of these centers. The results of simulation based on the Poole-Frenkel model for the field enhanced emission rate combined with the field-distribution model inside a Schottky diode serve to check the reliability of analysis by code CONTIN and reproduce almost quantitatively the field effect on the emission rate spectrum of V2 centers when an effective dielectric constant ɛS=23.4 is used. From the bias dependence of the peak height in the emission rate spectrum, the profile of the V3 center concentration is deduced.