Hiromasa T. Kaibe

Doping Effects on Thermoelectric Properties of Higher Manganese Silicides (HMSs, MnSi1.74) and Characterization of Thermoelectric Generating Module using p-Type (Al, Ge and Mo)-doped HMSs and n-Type Mg2Si0.4Sn0.6 Legs

The effect of Al Doping on the thermoelectric properties of higher manganese silicides, MnSi1.74 (HMSs) parallel and perpendicular to the c-axis was investigated. It was found that Al doping increased electrical conductivity and decreased thermoelectric power parallel to the c-axis, because Al doping of the Si site increased carrier density according to the valence control rule. Al doping was also effective for lowering thermal conductivity and raising the figure of merit parallel to the c-axis. After optimizing the Al content (y=0.0035), the thermoelectric properties of (Al, Ge and Mo)-doped HMSs parallel and perpendicular to the c-axis were evaluated and compared to those of nondoped and Al-doped HMSs. Two types of thermoelectric module consisting of n-type Mg2Si0.4Sn0.6 and p-type (Al, Ge and Mo)-doped HMSs legs parallel (or perpendicular) to the c-axis were fabricated, and their thermoelectric performances were compared to each other. The discrepancy between the measured maximum output power and the value estimated from the thermoelectric data of constituent materials was well explained by the resistance of a porous Al electrode. The measured heat flux of the module was also compared to the estimated heat flux. The maximum energy conversion efficiency of the module parallel to the c-axis was clearly higher than that perpendicular to the c-axis, because of a higher output power and a lower heat flux, and was higher than 7% at ΔT=520 K (303–823 K).

Development of Transient Measurement Method for investigating Thermoelectric Properties in High Temperature Region

The measurement equipment based on a transient measurement method was fabricated to characterize Seebeck coefficient, resistivity, and thermal conductivity simultaneously from room temperature to 573 K. The rate of ambient heat flow was reduced by employing a thermal anchor and a thermal reflector at elevated temperatures, and a model for suitable corrections for compensating remaining errors in data evaluation was studied. Using n-type Bi2Te3-based samples, the comparison of thermoelectric properties with the results obtained using another commercial conventional measurement equipment was performed and showed a very good agreement.

Dominant Deep Level in Annealed Low-Temperature GaAs Layers Grown by Molecular Beam Epitaxy

Deep levels in the annealed low temperature molecular beam epitaxial (LT-MBE) GaAs layer were characterized by using the capacitance-deep-level transient spectroscopy (DLTS) technique in combination with a unique sample structure. We have fabricated the samples by inserting the LT-GaAs layer between two n-type semiconductive layers grown at normal substrate temperatures, like a sandwich. DLTS measurements revealed that one electron trap dominates the annealed LT-MBE GaAs layer. This dominant electron trap was very similar to the so-called EL3 level. By changing growth parameters of LT-GaAs layers, we found that the trap concentration of this EL3-like level was strongly related to Si doping and excess arsenic.

Low-Temperature Fluorination of GaAs Surface by CF4 Plasma

The GaAs surface was fluorinated at low temperatures in CF4 plasma. The constitution and the composition of the formed films as functions of the substrate temperature and exposure time were analyzed by X-ray photoelectron spectroscopy. The fluoride films whose thickness increased with the substrate temperature consisted of gallium oxyfluoride and free As. We have also examined the influence of the surface treatment of GaAs substrates as well as the capping layer on the fluoride films. The sulfur treatment delayed the initiation of fluorination, and a-Si:H capping layer prevented free As from evaporating out.

Morphological and electrical characterization of Al/Ni/ n -InP contacts with tapered insertion Ni-layer

The addition of a thin Ni layer has improved the surface morphology of Al/n-InP contacts which show an enhanced Schottky barrier height (SBH) after rapid thermal annealing. In order to determine the optimum thickness of the insertion Ni layer, we have fabricated a unique sample in which the thickness of the insertion Ni layer tapered off in space. The improvement of the surface morphol-ogy as well as the SBH enhancement were realized by inserting 35 nm Ni layer annealed at rather low temperatures (around 450°C). The solid-phase reaction between Ni and InP might play an important role in the low-temperature formation of A1P which was responsible for the SBH enhancement.

Electrical and thermal properties for p-type /spl beta/-FeSi/sub 2/ with Mn and Al double doping

p-type /spl beta/-FeSi/sub 2/ doped with Mn and Al was prepared by hot pressing in order to examine the effect of double doping effect on electrical resistivity and thermal conductivity. The Hall coefficient as a function of temperature strongly suggested the existence of double valence bands, which become more pronounced with increasing Mn content. At low temperature around several ten K, an anomalous Hall effect has been observed. It becomes more pronounced and the transition temperature from anomalous non-linear Hall voltage as a function of magnetic induction to normal behavior shifts to higher temperature with increasing Mn content. Thermal conductivity /spl kappa/ for Fe/sub 0.936/Mn/sub 0.064/Al/sub 0.015/Si/sub 1.988/ was 0.048 W/cmK at 900 K, which is 13% lower than that of Fe/sub 0.915/Mn/sub 0.085/Si/sub 2/. Thus, it has been shown that alloying effect by double doping is considerably useful for reduction of K.

DLTS Study on Annealed Low-Temperature GaAs Layers with An n-I(LT)-n Structure Grown by MBE

Deep levels in the annealed low-temperature molecular beam epitaxial (LT-MBE) GaAs layer were successfully characterized by using the capacitance deep-level transient spectroscopy (C-DLTS) as well as photocapacitance quenching technique in combination with a unique sample structure. In this work, we have fabricated the samples by inserting the LT-GaAs layer into two n-type semi-conductive layers, like a sandwich ( n -LT- n structure), grown at normal substrate temperatures. DLTS measurements have revealed that one electron trap dominates the annealed LT-MBE GaAs. The dominant electron trap was very similar to the so-called EL3 level. Moreover, we found the midgap level appeared upon 800-900°C RTA, although no midgap level was detected in the as-grown n -LT- n sample (annealed at 620°C) and confirmed with photoquenching measurements that it is the EL2 level.

Microanalysis of inhomogeneous reaction and Schottky-barrier change of Al/n-InP contacts upon rapid thermal annealing

Although metal contacts to oxidized n-InP show relatively high Schottky-barrier height, they will not be stable upon thermal annealing since the interfacial reaction might dissociate the oxides. The authors present the change in electrical properties of Al/n-InP contacts upon the rapid thermal annealing in relation to the interfacial reaction. In a certain range of the annealing temperature, a rectifying contact was obtained with a high Schottky-barrier height close to 0.7 eV, for which compositional and electrical microanalysis were carried out by means of the electron-probe microanalysis and the scanning internal-photoemission microscopy. The results are discussed.<<ETX>>

Temperature dependence of the figure-of-merit of Ag/sub 0.208/Sb/sub 0.275/Te/sub 0.517/

The p-type Ag/sub 0.208/Sb/sub 0.275/Te/sub 0.517/ boule was unidirectionally grown using a Bridgman furnace and subsequently rapidly cooled by Ar to form a Widmannstatten structure of a high temperature phase AgSbTe/sub 2/, which could be desirable for power generation. Though the boule looked a homogeneous Widmannstatten structure, XRD patterns revealed that some precipitates of Ag/sub 2/Te and Sb/sub 2/Te/sub 3/ were contained in the boule and increased in volume in the growth direction. The figure-of-merit Z of the p-type Ag/sub 0.208/Sb/sub 0.275/Te/sub 0.517/ boule have been evaluated in the temperature range from 300 to 700 K. The maximum figure-of-merit Z/sub max/ was different in the portions of the boule. The value of 2.0/spl times/10/sup -1//K was at 620 K for the former half portion of the boule and that of 1.7/spl times/10/sup -3//K was at 585 K for the latter half portion. The Ag/sub 0.208/Sb/sub 0.275/Te/sub 0.517/ boule with less precipitates showed higher Z at higher temperatures.

Temperature dependence of thermal conductivity for Mg/sub x/Si/sub 1-x/Ge/sub x/ solid solution

The thermal conductivity of n-type Mg/sub 2/Si/sub 0.6/Ge/sub 0.4/ doped with 8000 ppm of Sb was measured over the temperature range from 80 to 350 K. The measurement was carried out by a comparative static method using transparent quartz as a reference specimen. The carrier concentration at 300 K was 6.25/spl times/10/sup 25/ m/sup -3/ determined from Hall measurement. The Hall coefficient was almost constant over the whole temperature range, which indicates the degenerate state of electrons. The /spl kappa/ was separated into two components of phonon /spl kappa//sub ph/ and electron /spl kappa//sub el/ on the basis of Wiedemann-Franz law. Assuming that the Lorenz number is 2.44/spl times/10/sup -8/ V/sup 2//K/sup 2/ which is a value for a metal and using values of the electrical conductivity /spl sigma/, it was found that /spl kappa//sub ph/ has a temperature dependence proportional to T/sup -1/2/ and that point defects related to the mass difference between Si and Ge is predominant for the phonon conduction.