Sung-Jae Joo

Prediction of the band structures of Bi2Te3-related binary and Sb/Se-doped ternary thermoelectric materials

Density functional calculations are performed to study the band structures of Bi2Te3-related binary (Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3) and Sb/Se-doped ternary compounds [(Bi1−xSbx)2Te3 and Bi2(Te1−ySey)3]. The band gap was found to be increased by Sb doping and to be monotonically increased by Se doping. In ternary compounds, the change in the conduction band structure is more significant as compared to the change in the valence band structure. The band degeneracy of the valence band maximum is maintained at 6 in binaries and ternaries. However, when going from Bi2Te3 to Sb2Te3 (Bi2Se3), the degeneracy of the conduction band minimum is reduced from 6 to 2(1). Based on the results for the band structures, we suggest suitable stoichiometries of ternary compounds for high thermoelectric performance.

Silicon carbide-based hydrogen gas sensors for high-temperature applications.

We investigated SiC-based hydrogen gas sensors with metal-insulator-semiconductor (MIS) structure for high temperature process monitoring and leak detection applications in fields such as the automotive, chemical and petroleum industries. In this work, a thin tantalum oxide (Ta2O5) layer was exploited with the purpose of sensitivity improvement, because tantalum oxide has good stability at high temperature with high permeability for hydrogen gas. Silicon carbide (SiC) was used as a substrate for high-temperature applications. We fabricated Pd/Ta2O5/SiC-based hydrogen gas sensors, and the dependence of their I-V characteristics and capacitance response properties on hydrogen concentrations were analyzed in the temperature range from room temperature to 500 °C. According to the results, our sensor shows promising performance for hydrogen gas detection at high temperatures.

Anti-reflective nano- and micro-structures on 4H-SiC for photodiodes

In this study, nano-scale honeycomb-shaped structures with anti-reflection properties were successfully formed on SiC. The surface of 4H-SiC wafer after a conventional photolithography process was etched by inductively coupled plasma. We demonstrate that the reflection characteristic of the fabricated photodiodes has significantly reduced by 55% compared with the reference devices. As a result, the optical response Iillumination/Idark of the 4H-SiC photodiodes were enhanced up to 178%, which can be ascribed primarily to the improved light trapping in the proposed nano-scale texturing.

Enhanced thermoelectric properties and development of nanotwins in Na-doped Bi0.5Sb1.5Te3 alloy

We found that Na is a good source to develop twin structures in the Bi-Te system, such as Ag as noted in a previous study. The twin boundaries had a considerable influence on reductions of the lattice thermal conductivity due to phonon scattering by the nano-ordered layers and on reductions of the electrical resistivity owing to the defects generated by the substitution of Na into the cation sites. Here, we report the enhanced thermoelectric properties of a Na-doped p-type Bi0.5Sb1.5Te3 alloy. Measurements show that the electrical resistivity and the Seebeck coefficient decrease with Na doping due to an increase in the free carrier (hole) concentration and that the lattice thermal conductivity decreases with Na doping. The achieved maximum ZT value was 1.20 at 423 K, which is approximately 20% higher than that of Bi0.5Sb1.5Te3 under the same fabrication conditions. These results were achievable by controlling the morphology of the twin structure and the carrier concentration by means of Na doping.

Defects responsible for abnormal n-type conductivity in Ag-excess doped PbTe thermoelectrics

Density functional calculations have been performed to investigate the role of Ag defects in PbTe thermoelectric materials. Ag-defects can be either donor, acceptor, or isovalent neutral defect. When Ag is heavily doped in PbTe, the neutral (Ag-Ag) dimer defect at Pb-site is formed and the environment changes to the Pb-rich/Te-poor condition. Under Pb-rich condition, the ionized Ag-interstitial defect (Ag I +) becomes the major donor. The formation energy of Ag I + is smaller than other native and Ag-related defects. Also it is found that Ag I + is an effective dopant. There is no additional impurity state near the band gap and the conduction band minimum. The charge state of Ag I + defect is maintained even when the Fermi level is located above the conduction band minimum. The diffusion constant of Ag I + is calculated based on the temperature dependent Fermi level, formation energy, and migration energy. When T > 550 K, the diffusion length of Ag within a few minutes is comparable to the grain size of the polycrystalline PbTe, implying that Ag is dissolved into PbTe and this donor defect is distributed over the whole lattice in Ag-excess doped polycrystalline PbTe. The predicted solubility of Ag I + well explains the increased electron carrier concentration and electrical conductivity reported in Ag-excess doped polycrystalline PbTe at T = 450–750 K [Pei et al., Adv. Energy Mater. 1, 291 (2011)]. In addition, we suggest that this abnormal doping behavior is also found for Au-doped PbTe.

Effects of wet-oxidized 4H-SiC annealed in HNO3/H2O vapour

Purpose – The high density of defects mainly attributed to the presence of silicon oxycarbides, residual C clusters, Si- and C-dangling bonds at or near the SiO2/SiC interface degrades the performance of metal-oxide-semiconductor (MOS) devices. In the effort of further improving the quality and enhancement of the SiC oxides thickness, post-oxidation annealed by a combination of nitric acid (HNO3) and water (H2O) vapor technique on thermally grown wet-oxides is introduced in this work. The paper aims to discuss these issues. Design/methodology/approach – A new technique of post-oxidation annealing (POA) on wet-oxidized n-type 4H-SiC in a combination of HNO3 and H2O vapor at various heating temperatures (70°C, 90°C and 110°C) of HNO3 solution has been introduced in this work. Findings – It has been revealed that the samples annealed in HNO3 + H2O vapour ambient by various heating temperatures of HNO3 solution; particularly at 110°C is able to produce oxide with lower interface-state density and higher break...

Enhanced thermoelectric properties of AgSbTe 2 obtained by controlling heterophases with Ce doping

We report the enhanced thermoelectric properties of Ce-doped AgSbTe2 (AgSb1−xCexTe2) compounds. As the Ce contents increased, the proportion of heterophase Ag2Te in the AgSbTe2 gradually decreased, along with the size of the crystals. The electrical resistivity and Seebeck coefficient were dramatically affected by Ce doping and the lattice thermal conductivity was reduced. The presence of nanostructured Ag2Te heterophases resulted in a greatly enhanced dimensionless figure of merit, ZT of 1.5 at 673 K. These findings highlight the importance of the heterophase and doping control, which determines both electrical and thermal properties.

Fabrication characteristics of 1.2kV SiC JBS diode

The design, process, and the DC and switching characterization of 4H-SiC junction barrier Schottky (JBS) diode from room temperature up to 200degC have been presented. We used a p-type, slightly deep inner ring and a simple p-type stripe in the Schottky region, and used a field limiting ring (FLR) having variable space and width for an edge termination. The fabrication process was also optimized. The JBS diode having Schottky area of 9.8 x 10-3 cm2, and having the space and width of 3 mum for FLR and ring showed the blocking voltage of 1.2 kV at reverse leakage current of 50 uA and the forward current density of 300 A/cm at 3A. We also investigated the operation of SiC JBS diode at temperature up to 200degC. Forward on-state voltage characteristics is 1.8 V(@1A) at room temperature, however, it is slightly increased to 2.4 V(@1A) at 200degC. The recovery characteristics of the fabricated JBS was 7.6 ns at room temperature. This result was much superior to that of Si-FRD.

Electrical Characteristics of Ni/Ti/Al Ohmic Contacts to Al-implanted p-type 4H-SiC

Ni/Ti/Al multilayer system (`/`denotes the deposition sequence) was tested for low-resistance ohmic contact formation to Al-implanted p-type 4H-SiC. Ni 30 nm / Ti 50 nm / Al 300 nm layers were sequentially deposited by e-beam evaporation on the 4H-SiC samples which were implanted with Al (norminal doping concentration

Characterization of Surface Textured Silicon Substrates by SF 6 /O 2 Gas Mixture

The optical losses associated with the reflectance of incident radiation are among the most important factors limiting the efficiency of a solar cell. Therefore, photovoltaic cells normally require special surface structures or materials, which can reduce reflectance. In this study, nano-scale textured structures with anti-reflection properties were successfully formed on silicon. The surface of sicon wafer was etched by the inductively coupled plasma process using the gaseous mixture of . We demonstrate that the reflection characteristic has significantly reduced by ~0% compared with the flat surface. As a result, the power efficiency of the nano-scale textured silicon solar cell were enhanced up to 20%, which can be ascribed primarily to the improved light trapping in the proposed nano-scale texturing.