Jiro Nagao

Thermoelectric and transport properties of β-Ag2Se compounds

The Hall coefficient, electrical resistivity, and Seebeck coefficient of n-type specimens of β-Ag2Se, the low temperature polymorph of silver selenide, were measured over the temperature range from 70 to 300 K. The results showed maxima in both Hall coefficient and the electrical resistivity just below the onset of the intrinsic conduction range. This anomaly was qualitatively explained by the deviation of the Coulomb scattering from the usual assumption of independence due to the degenerate nature of the samples. The estimated energy gap for different samples of about 160 meV seems to confirm the existence of the second low temperature phase β2. This second phase is a probable reason for the relatively high thermoelectric figure of merit observed.

Anisotropic Factor of Electrical Conductivity in p-Bi2Te3 Crystals

Temperature variation of electrical conductivity in two crystallographic directions (σ∥ and σ⟂) of p-Bi2Te3 crystals has been measured. The anisotropic factor, σ∥/σ⟂, shows an exponential form involving an activation energy. This can be interpreted as an effect of defects between the layers on the electronic density of states.

Electron tunneling experiments on skutterudite Co1−xFexSb3 semiconductors

Electron tunneling experiments were performed on p-Co1−xFexSb3–Al-oxide–Al junctions for x=0 and x=0.1 at 4.2 K. A U-shaped tunneling conductance curve obtained for polycrystalline p-CoSb3 clearly shows an energy-band gap of ∼50 meV. For p-Co0.9Fe0.1Sb3 skutterudites, a strong zero-bias conductance anomaly is observed. This anomaly may be due to a structural disorder arising from defects such as vacancies and interstitial Fe atoms.

Tunneling Conductance of Al–Al Oxide–p‐Bi2Te3 Junctions

We measured tunneling conductance of Al-Al oxide-p-Bi 2 Te 3 tunnel junctions. A small increase of the tunneling conductance was observed in the energy gap region. The intensity of the increase of tunneling conductance decreased by annealing the p-Bi 2 Te 3 films. Therefore, it is considered that this increase in tunneling conductance is closely related to energy states caused by defects in p-Bi 2 Te 3 films. For the first time, energy states caused by defects are revealed by tunneling experiments in p-Bi 2 Te 3 films.

Electron tunneling experiments on spin glass Ni1−xMnx films

Systematic tunneling measurements have been performed on AlAl oxide/Ni1−xMnx junctions to study the one-particle density of states in spin glass Ni1−xMnx (x = 0.24–0.35) films where the lattice and spin disorders coexist. The strong zero bias conductance anomaly indicating the metal-insulator transition was observed at a critical concentration xc ≈ 0.24. This is the direct evidence that the variation of the tunneling conductance is largely due to the magnetic disorder.

Thermoelectric properties of Bi2(1−x)In2xTe3 ternary samples

Low temperature thermoelectric properties of Bi2(1−x)In2xTe3 ternary samples have been measured in the 50–300 K temperature range. At room temperature, the thermal conductivity decreases with increasing In2Te3 content, reaches a minimum at x=0.2 and then slightly increases with increasing In2Te3 concentration. For x=0.1 and x=0.2 samples, the thermal conductivity varies approximately as 1/T, which may indicate that the defect scattering with phonon is predominant to thermal conduction. Seebeck coefficient and electrical resistivity have been measured and the thermoelectric figure merit calculated. The maximum value obtained is ZT=0.5 around 300 K, a value comparable to those of state-of-the-art thermoelectric semiconductors. From x-ray diffraction measurements, it is found that these samples are composites with In2Te3 particles in Bi2Te3. In this case, the phonons would be scattered by the inclusion particles, depending on their size which is critical for achieving an increase in ZT compare to Bi2Te3. An ...

Ionized cluster beam deposition of antimony and bismuth films

The structure of antimony and bismuth thin films grown on glass or Si(001) substrates by ionized cluster beam (ICB) deposition was characterized. X-ray diffraction measurements were carried out to investigate the structures of the films. The dependence of the crystalline orientation in antimony films grown on the glass substrate on the applied voltage was observed. With increasing applied voltage, the x-ray diffraction peaks from the planes become weak and those from the planes become relatively strong. In bismuth films deposited on the Si(001) substrate at , the dominant crystal face is the (012) plane. These results show that the crystalline orientation of antimony and bismuth films can be controlled well according to the applied voltage and substrates used in the ICB deposition.

Inelastic electron tunneling experiments in Bi1−xSbx alloys

Inelastic electron tunneling experiments have been performed in Bi0.94Sb0.06–Al oxide-Al junctions. A strong peak observed at 1.9 mV in the second derivative tunnel spectra is closely related to the estimated effective Debye temperature, besides intervalley acoustic and optical phonon structures. This structure clearly corresponds to the electron–intravalley acoustic phonon coupling.

Inelastic electron tunneling into p-Bi2Te3 crystals

Inelastic electron tunneling experiments have been performed in p-Bi2Te3–Al oxide-Al junctions. Strong peaks due to electron-acoustic phonon coupling are observed in the second derivative tunnel spectrum. These phonon structures are closely related to the effective Debye temperature. This gives direct evidence that the electron-acoustic phonon scattering plays a dominant role in p-Bi2Te3.

Tunneling through narrow-gap semiconductor Sb2Te3 barrier

Tunnel experiments have been performed on Au/Sb2Te3/Al tunnel junctions to study elastic interelectrode tunneling through the small energy gap of a narrow-gap semiconductor. Tunnel conductance exhibited narrow width conductance peak at zero bias voltage. This behaviour is in accordance with the result of the theoretically calculated tunnel conductance, in which the nonparabolic dispersion relation within the energy gap of the narrow-gap semiconductor used as a tunnel barrier in a metal/narrow-gap semiconductor/metal tunnel structure is included. And some interesting structures are also observed in the conductance curves.