Yoichi Okamoto

Thermoelectric Properties of Single-Crystalline SiC and Dense Sintered SiC for Self-Cooling Devices

We investigated the thermoelectric properties of 4H-SiC substrates and dense sintered SiC, which are expected to be candidate materials for use in self-cooling devices because of their high Seebeck coefficient, low electrical resistivity, and high thermal conductivity. The carrier concentration of 4H-SiC samples doped with nitrogen is in the range of 1016 to 1019 cm-3. The sintered SiC samples of the α-type and β-type contain less than 1000 ppm of cation impurities and have a relative density higher than 98% with respect to single-crystalline SiC. 4H-SiC with a carrier concentration of 1019 cm-3 has the highest power factor of 2.7 ×10-3 WK-2m-1 and a high thermal conductivity of 260 WK-1m-1 at room temperature. One-dimensional calculations for heat distribution indicate that a Si chip in a self-cooling device, which consists of 4H-SiC with a carrier concentration of 1019 cm-3, could be refrigerated more strongly than one on a copper plate under specific operating conditions.

Annealing Temperature Dependence of Crystallization Process of SiGeAu Thin Film

The large thermoelectric power of SiGeAu thin film depends on the sizes of microcrystals and the crystalline fraction of thin films. We investigate the crystallization process and control method of the microcrystal size and crystalline fraction. In the samples with 7 and 12 at. % Au, the crystallite diameter of Si1-xGex was constant when the annealing temperature was between 573–773 K and increased with 873 K annealing. These results suggest that there are two crystallization processes depending on annealing temperature: metal-induced crystallization (573–773 K) and liquid phase crystallization (873 K). The crystallite diameter increased with increasing Au composition and crystalline fraction increased with increasing annealing temperature. These new findings provide a strong hint to the control of the sizes of microcrystals and the crystalline fraction of thin films.

Thermoelectric Characteristics of Si/Ge Superlattice Thin Films at Temperatures Less Than 300 K

We have studied the thermoelectric characteristics of Si/GeAu superlattice thin films at temperatures ranging from 290 K to 75 K and compared them to those of the SiGeAu alloy thin film. In the annealed Si/GeAu superlattice, the electrical resistivity was lower than that of the unannealed Si/GeAu superlattice at all temperatures. The annealed Si/GeAu superlattice showed a high thermoelectric power of 105 µV/K at 290 K. At temperatures less than 200 K, however, the polarity of the thermoelectric power of the unannealed Si/GeAu superlattice switched from positive to negative and a large negative thermoelectric power of -4.6 mV/K was attained at 80 K. On the other hand, the characteristics of all samples showed no magnetic field effect at all temperatures. To explain the causes of the extremely small variations in the characteristics by the magnetic field effects, we calculated the transport coefficients for the SiGeAu alloy using the two-band parabolic model, and compared them to those of experimentally measured values. When a large amount of acceptor concentration from Au doping and the very low carrier mobility were assumed, similar transport coefficients to the measured ones resulted.

A magnetic study of sintering of ultrafine particles

Sintering of Ni ultrafine particles and Cu ultrafine particles was studied by measuring the ferromagnetism of Ni. It was found that diffusion started at about 100°C and accelerated above 250°C. In a reducing atmosphere of H2, the diffusion started at a lower temperature than in a He atmosphere.

Structural Effects on Thermal Conductivity of SiGeAu Superlattice Thin Films

We have measured the thermal conductivity of thin films and discussed the relationship between the thermal conductivity and structure of thin films. All samples had a very low thermal conductivity compared with a conventional bulk sample. The thermal conductivity changed depending on the sample structure. The thermal conductivity decreased with the existence of the artificial superlattice structure and the phase transition to the amorphous phase.

Properties of Y1Ba2Cu3O7-x superconducting thin films prepared by reactive evaporation method

The superconducting YBa2Cu3O7-x (YBCO) thin films on MgO(100) substrates were prepared by a reactive evaporation method. Superconducting transition temperature and crystal quality were examined as a function of the substrate temperature (Ts) and the oxygen partial pressure (PO2 ). The as-grown thin films at the substrate temperature of 620°C exhibited a zero resistance temperature (Tczero ) of 83 K with the onset (Tconset ) temperature of 88 K under the PO2 of 2×10-3 Torr. The lattice parameter c was about 11.74 A for the as-grown film with Tczero of 80 K and was shortened to about 11.70 A by the post-annealing at 900°C for 10 min in O2 flow, although Tczero was not changed. This phenomena may be ascribed to the deficiency of oxygen in the CuO2 plane of the YBCO layered perovskite structure.

Electric Current Dependence of a Self-Cooling Device Consisting of Silicon Wafers Connected to a Power MOSFET

A self-cooling device has been developed by combining a commercial n-channel power metal–oxide–semiconductor field-effect transistor (MOSFET) and single-crystalline Sb-doped n-type or B-doped p-type silicon wafers in order to improve the heat removal or cooling quantitatively. The electric current dependence of the temperature distribution in the self-cooling device and the voltage between the source and drain electrodes have been measured to estimate the Peltier heat flux. We found that the average temperature is decreased for a power MOSFET in which an electric current of 50 A flows. In particular, the average temperature of the power MOSFET was decreased by 2.7°C with the n-type Si wafer and by 3.5°C with the p-type Si wafer, although an electric current of 40 A makes little difference. This certainly warrants further work with improved measurement conditions. Nonetheless, the results strongly indicate that such n-type or p-type silicon wafers are candidate materials for use in self-cooling devices.

Characterization of Defects in Semi-Insulating 6H-SiC Substrates Using IR Thermal Imaging Camera

We have detected defects micro-pipes and a cluster of impurities in semi-insulating 6H-SiC substrates using long-wavelength infrared thermal imaging camera (IR-camera) with 8 ~ 14 µm in non-destructive and non-contact. Also we have evaluated the thermal influence of defects on the entire substrates from the observation results of scanning laser microscope (SLM) and light scattering tomography (LST). Through the process, it was certificated that the defects in the substrates could be detected with relatively macroscopic scale (8 uf0b4 6 mm2). Moreover, through a temperature profile processing by a 0.1 K thermal resolution, we estimated thermal behavior of the defect areas in the 6H-SiC precisely. The IR-camera is considered as effective technique for evaluating the defects in the intermediate range between micro and macro scale.

Vanadium-Related Deep Levels in n-Silicon Detected by Junction Capacitance Waveform Analysis

Multiexponential (ME-) and spectral analysis (SA-) Deep Level Transient Spectroscopy (DLTS) are made on vanadium-related deep levels in n-silicon. We resolved three traps with large amplitude and three more traps with comparatively small amplitude, VA (ΔE=0.08 eV, σ=3.2×10-18 cm2), VB (ΔE=0.28 eV, σ=2.6×10-14 cm2) and VC (ΔE=0.24 eV, σ=8.4×10-18 cm2). These vanadium-related levels do not have appreciable broadening of the emission rate spectrum.

Crystallization and reconstructive layer transformation of a-Si/Au multilayer thin films under a strong gravitational field

There were still unclear questions in the new method that fabricate the high quality poly crystalline Si thin film from amorphous Si thin film with lower annealing temperature than conventional Si recrystallization temperature. In that recrystallization process, the recrystallization mechanism was generally explained by the MIC (Metal Induced Crystallization) of Au. In this paper, we have discussed the effects of film structure and strong gravity on recrystallization, by using conventional furnace and high-temperature ultracentrifuge furnace system. The five kinds of samples (two bilayered Si/Au thin films, two multilayered Si/Au thin films and trilayered Si/Au/Si thin film) and found the effects of structure and strong gravity. The best for crystallization was Au/Si multilayered thin film, which is almost finished to crystallize even at 673 K annealing. The strong gravity advanced and retreated the crystallization, depending to thin film structure.