Masayuki Takashiri

Effect of grain size on thermoelectric properties of n-type nanocrystalline bismuth-telluride based thin films

bismuth-telluride-based thin films with the average grain size of 60 nm by a flash-evaporation method and shown the re- duction of thermal conductivity. 12 To further reduce the thermal conductivity of the nano- crystalline bismuth-telluride-based thin films and investigate the effect of grain size on thermoelectric properties, we pre- pare the nanocrystalline thin films by the flash-evaporation method with an improved annealing condition. The grain size of the thin films is estimated using an x-ray diffraction XRD and atomic force microscopy AFM. The thermo- electric properties, in terms of the electric conductivity, the Seebeck coefficient, and the thermal conductivity, are mea- sured at room temperature. The measured thermal conductiv- ity is divided into the lattice thermal conductivity l and electronic thermal conductivity e. Then we investigate the grain size dependence of the lattice thermal conductivity of the nanocrystalline thin films using a simplified phonon transport model.

Thermoelectric properties of n-type nanocrystalline bismuth-telluride-based thin films deposited by flash evaporation

The thermal conductivity of n-type nanocrystalline bismuth-telluride-based thin films (Bi2.0Te2.7Se0.3) is investigated by a differential 3ω method at room temperature. The nanocrystalline thin films are grown on a glass substrate by a flash evaporation method, followed by hydrogen annealing at 250 °C. The structure of the thin films is studied by means of atomic force microscopy, x-ray diffraction, and energy-dispersive x-ray spectroscopy. The thin films exhibit an average grain size of 60 nm and a cross-plane thermal conductivity of 0.8 W∕m K. The in-plane electrical conductivity and in-plane Seebeck coefficient are also investigated. Assuming that the in-plane thermal conductivity of the thin films is identical to that of the cross-plane direction, the in-plane figure of merit of the thin films is estimated to be ZT=0.7. As compared with a sintered bulk sample with average grain size of 30 μm and nearly the same composition as the thin films, the nanocrystalline thin films show approximately a 50% redu...

Combined effect of nanoscale grain size and porosity on lattice thermal conductivity of bismuth-telluride-based bulk alloys

Here, we investigate the combined effect of the nanoscale crystal grains and porosity on the lattice thermal conductivity of bismuth-telluride-based bulk alloys using both experimental studies and modeling. The fabricated bulk alloys exhibit average grain sizes of 30 < d < 60 nm and porosities of 12% < Φ < 18%. The total thermal conductivities were measured using a laser flash method at room temperature, and they were in the range 0.24 to 0.74 W/m/K. To gain insight into the phonon transport in the nanocrystalline and nanoporous bulk alloys, we estimate the lattice thermal conductivities and compare them with those obtained from a simplified phonon transport model that accounts for the grain size effect in combination with the Maxwell-Garnett model for the porosity effect. The results of this combined model are consistent with the experimental results, and it shows that the grain size effect in the nanoscale regime accounts for a significant portion of the reduction in lattice thermal conductivity.

Structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin film

The structure and thermoelectric properties of boron doped nanocrystalline Si0.8Ge0.2 thin films are investigated for potential application in microthermoelectric devices. Nanocrystalline Si0.8Ge0.2 thin films are grown by low-pressure chemical vapor deposition on a sandwich of Si3N4∕SiO2∕Si3N4 films deposited on a Si (100) substrate. The Si0.8Ge0.2 film is doped with boron by ion implantation. The structure of the thin film is studied by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the film has column-shaped crystal grains ∼100nm in diameter oriented along the thickness of the film. The electrical conductivity and Seebeck coefficient are measured in the temperature range between 80–300 and 130–300K, respectively. The thermal conductivity is measured at room temperature by a 3ω method. As compared with bulk silicon-germanium and microcrystalline film alloys of nearly the same Si∕Ge ratio and doping concentrations, the Si0.8Ge0.2 nanocrystallin...

Enhanced thermoelectric properties of phase-separating bismuth selenium telluride thin films via a two-step method

A two-step method that combines homogeneous electron beam (EB) irradiation and thermal annealing has been developed to enhance the thermoelectric properties of nanocrystalline bismuth selenium telluride thin films. The thin films, prepared using a flash evaporation method, were treated with EB irradiation in a N2 atmosphere at room temperature and an acceleration voltage of 0.17 MeV. Thermal annealing was performed under Ar/H2 (5%) at 300 °C for 60 min. X-ray diffraction was used to determine that compositional phase separation between bismuth telluride and bismuth selenium telluride developed in the thin films exposed to higher EB doses and thermal annealing. We propose that the phase separation was induced by fluctuations in the distribution of selenium atoms after EB irradiation, followed by the migration of selenium atoms to more stable sites during thermal annealing. As a result, thin film crystallinity improved and mobility was significantly enhanced. This indicates that the phase separation resulti...

Erratum to: Structural and Thermoelectric Properties of Nanocrystalline Bismuth Telluride Thin Films Under Compressive and Tensile Strain

1.—Department of Materials Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan. 2.—Department of Mechanical and Control Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata-ku, Kitakyushu 804-8550, Japan. 3.—Department of Mechanical Engineering, College of Engineering, Nihon University, 1 Nakagawara, Tokusada, Tamuramachi, Koriyama, Fukushima 963-8642, Japan. 4.—e-mail: takashiri@tokai-u.jp

Comparison of crystal growth and thermoelectric properties of n-type Bi-Se-Te and p-type Bi-Sb-Te nanocrystalline thin films: Effects of homogeneous irradiation with an electron beam

The effects of homogenous electron beam (EB) irradiation on the crystal growth and thermoelectric properties of n-type Bi-Se-Te and p-type Bi-Sb-Te thin films were investigated. Both types of thin films were prepared by flash evaporation, after which homogeneous EB irradiation was performed at an acceleration voltage of 0.17 MeV. For the n-type thin films, nanodots with a diameter of less than 10 nm were observed on the surface of rice-like nanostructures, and crystallization and crystal orientation were improved by EB irradiation. The resulting enhancement of mobility led to increased electrical conductivity and thermoelectric power factor for the n-type thin films. In contrast, the crystallization and crystal orientation of the p-type thin films were not influenced by EB irradiation. The carrier concentration increased and mobility decreased with increased EB irradiation dose, possibly because of the generation of defects. As a result, the thermoelectric power factor of p-type thin films was not improve...

Structural, optical, and transport properties of nanocrystalline bismuth telluride thin films treated with homogeneous electron beam irradiation and thermal annealing

We investigated the effects of homogeneous electron beam (EB) irradiation and thermal annealing treatments on the structural, optical, and transport properties of bismuth telluride thin films. Bismuth telluride thin films were prepared by an RF magnetron sputtering method at room temperature. After deposition, the films were treated with homogeneous EB irradiation, thermal annealing, or a combination of both the treatments (two-step treatment). We employed Williamson-Hall analysis for separating the strain contribution from the crystallite domain contribution in the x-ray diffraction data of the films. We found that strain was induced in the thin films by EB irradiation and was relieved by thermal annealing. The crystal orientation along c-axis was significantly enhanced by the two-step treatment. Scanning electron microscopy indicated the melting and aggregation of nano-sized grains on the film surface by the two-step treatment. Optical analysis indicated that the interband transition of all the thin films was possibly of the indirect type, and that thermal annealing and two-step treatment methods increased the band gap of the films due to relaxation of the strain. Thermoelectric performance was significantly improved by the two-step treatment. The power factor reached a value of 17.2 μW (cm(-1) K(-2)), approximately 10 times higher than that of the as-deposited thin films. We conclude that improving the crystal orientation and relaxing the strain resulted in enhanced thermoelectric performance.

Hollow electrode enhanced RF glow plasma for the fast deposition of microcrystalline silicon

A hollow electrode enhanced RF glow plasma excitation technique has been newly developed. In this technique, the reactor is divided into a capacitively-coupled RF glow discharge space and a processing space by counter electrode, which has a hollow structure and is placed between RF electrode and substrate. Hollow electrode discharge is induced inside this hollow structure. The application of this discharge type for semiconductor processing is studied in the case of plasma enhanced chemical vapor deposition of hydrogenated microcrystalline silicon (μc-Si) thin films. It is found that high crystallinity, photo sensitivity and maximum deposition rate of 4.9 nm/s can be achieved at the plasma excitation frequency of 13.56 MHz. Properties of this plasma are investigated by observation of the plasma emission pattern viewed through the window, optical emission spectral analysis and the plasma potential. Plasmas generated inside the new apparatus is compared to those of normal capacitively-coupled RF plasmas. In our apparatus, high intensity plasma emission is observed near and inside the hollow structure attached to the counter electrode. The spectral and plasma potential analysis showed that both the intensity of SiH* and the plasma potential of our plasma are higher than that of normal capacitively-coupled plasma.

Thermal conductivity reduction mechanisms in superlattices

The large thermal conductivity reduction observed in superlattices has led to the reports of significantly increased thermoelectric figure-of-merit by several groups. The mechanisms of thermal conductivity reduction in superlattices were scrutinized from different angles over the last decade. This paper summarizes our current understanding on the heat conduction mechanisms in superlattices. Among several potential mechanisms, the interface scattering, particularly diffuse interface scattering, plays the most important role. Conclusions drawn from the studies on superlattices have implications to other nanostructured thermoelectric materials.