T.Arai

Low-resistance Ta/Ti ohmic contacts for p-type GaN

Although extensive efforts have continued to develop Ohmic contacts for p-type GaN, which have specific contact resistance (ρc) lower than that (ρc∼10−2 Ω cm2) of conventional Ni/Au contacts, to the best of our knowledge no breakthrough has been reported in open literature. We demonstrated that bilayered Ta/Ti contacts have a ρc value of around 3×10−5 Ω cm2 for p-type GaN with a hole concentration of 7×1017 cm−3. This contact has resistance low enough to manufacture blue laser diodes, but deterioration of the ρc value during room-temperature storage is the key issue.

Development of Pt-based ohmic contact materials for p-type GaN

The stabilities of the electrical properties and microstructures of Pt, PtAu, NiAu, and TaTi ohmic contacts after contact formation were studied. The Pt and PtAu contacts annealed in an O2 and N2 mixed gas ambient had the specific contact resistance (ρc) of high-10−3 Ω cm2 and strong adhesion to the GaN substrates. The NiAu contact annealed in the partial O2 ambient had poor adhesion to the GaN, although the ρc value of low-10−3 Ω cm2 was obtained. The TaTi contact had the lowest ρc values of less than 10−4 Ω cm2. After contact formation, the Pt and NiAu ohmic contacts prepared by annealing in the partial O2 ambient showed the excellent electrical and microstructural stabilities during room temperature storage and current injection. However, the contact resistance of the TaTi contact prepared by annealing at 800 °C increased during room temperature storage, and the mechanical failure of the contact was also observed after injecting current as low as 4 kA/cm2. From the present experiments, it was concluded...

Influences of cracking of coated superconducting layer on voltage-current curve, critical current, and n-value in DyBCO-coated conductor pulled in tension

Influences of cracking of coating layer under applied tensile strain on V(voltage)-I(current) curve, critical current, and n-value of DyBa2Cu3O7−δ coated conductor were studied experimentally and analytically. The experimentally measured variations in V-I curve, critical current, and n-value with increasing applied strain and the correlation of n-value to critical current were described well by the partial crack-current shunting model of Fang et al. Also, the variations in the ratio of shunting current to overall critical current and the ratio of voltage developed in the cracked region to overall voltage with extension of crack, and the variation in critical current with the ratio of noncracked area to overall cross-sectional area of superconducting layer were revealed.

Formation and deterioration mechanisms of low-resistance TaTi ohmic contacts for p-GaN

We demonstrated in our previous paper [Appl. Phys. Lett. 74, 275 (1999)] that TaTi contacts annealed at temperature of 800 °C provided a specific contact resistance (ρc) of around 3×10−5 Ω cm2 for p-GaN epilayers with hole concentration of 7.0×1017 cm−3. The reduction of the contact resistances was believed to be due to reactivation of Mg atoms doped in the GaN epilayer by removing H. In the present article, diffusion behavior of hydrogen atoms in the p-GaN was extensively studied by secondary-ion mass spectroscopy and correlated with the electrical properties of the TaTi contacts. It was found that formation of the low-resistance TaTi ohmic contacts was well explained by diffusion behavior of hydrogen in the p-GaN. Although the deterioration rates of the TaTi contacts during low temperature storage after ohmic contact formation were coincident with the diffusion rates of hydrogen atoms in the GaN, the deterioration mechanism could not be explained simply by hydrogen release from the TaTi contacts into th...

Production of a novel tryptophan analog, β-1-indazole-L-alanine with tryptophan synthase of Escherichia coli

The tryptophan synthase α2β2 complex from Escherichia coli has been found to catalyze the β‐replacement reaction of L‐serine with indazole, an indole analog which has a nitrogen atom at the 2‐position (pyrazole ring). The reaction product was isolated and identified as β‐indazolealanine by mass spectrometric, elemental and NMR analyses. Careful assignment of 1H‐ and 13C‐signals with several NMR techniques revealed that the β‐carbon of the product alanine moiety was bound to the 1‐N‐position of the indazole ring. This is the first example of the β‐replacement reaction catalyzed by tryptophan synthase occurring at any other position than the 3‐position of indole analogs.

Development of novel PET probes targeting phosphatidylinositol 3-kinase (PI3K) in tumors

Phosphatidylinositol 3-kinase (PI3K) activity and protein expression levels are often increased in tumor regions. Since PI3K plays a crucial role in regulating cell growth and proliferation, inhibiting PI3K-dependent pathways could be a promising approach for cancer treatment. In clinical practice, however, evaluation of PI3K expression levels is limited to immunohistochemistry of patient samples, which requires invasive biopsies. Here we report the synthesis of three candidate compounds, FMTA-1, 2 and 3, and evaluate their capacity to detect PI3K expression levels with positron emission tomography (PET). Among the three candidates, FMTA-2 showed a lower IC50 value for PI3K. (18)F Radiolabeling of FMTA-2 to produce [(18)F]FMTA-2 was accomplished and its capacity for detecting PI3K expression levels was evaluated in vitro and in vivo. Cell uptake of [(18)F]FMTA-2 correlated well with cellular PI3K expression levels, and was suppressed by the ATP-competitive PI3K inhibitor ZSTK474. In an in vivo experiment using tumor-transplanted model mice, a higher signal-to-noise ratio (S/N) was seen with [(18)F]FMTA-2 in animals transplanted with DMS114 cells (expressing high PI3K levels) relative to DU145 cells (expressing low PI3K levels). However, in vivo pharmacokinetics of [(18)F]FMTA-2 was undesirable and the absolute amount of this compound that accumulated at the tumor region was low. To the best of our knowledge, this study represents the first trial of a PET tracer for detecting PI3K. Although further improvement of the probe is required prior to clinical application, these results should encourage future work.