Ji-Jung Kai

LOW-RESISTANCE OHMIC CONTACTS TO P-TYPE GAN ACHIEVED BY THE OXIDATION OF NI/AU FILMS

A contact has been developed to achieve a low specific contact resistance to p-type GaN. The contact consisted of a bi-layer Ni/Au film deposited on p-type GaN followed by heat treatment in air to transform the metallic Ni into NiO along with an amorphous Ni–Ga–O phase and large Au grains. A specific contact resistance as low as 4.0×10−6u200aΩu200acm2 was obtained at 500u200a°C. This low value was obtained by the optimization of Ni/Au film thickness and heat treatment temperatures. Below about 400u200a°C, Ni was not completely oxidized. On the other hand, at temperatures higher than about 600u200a°C, the specific contact resistance increased because the NiO detached from p-GaN and the amount of amorphous Ni–Ga–O phase formed was more than that of the sample annealed at 500u200a°C. The mechanism of obtaining low-resistance ohmic contacts for the oxidized Ni/Au films was explained with a model using energy band diagrams of the Au/p-NiO/p-GaN structure.

Enhancement of ferromagnetic properties in Zn1−xCoxO by additional Cu doping

The high-temperature ferromagnetism in Co-doped ZnO samples fabricated by the standard solid-state reaction method is reported. Additional Cu doping into bulk Zn0.98Co0.02O is essential to achieve room-temperature ferromagnetism. Structure and composition analyses revealed that cobalt is incorporated into the lattice structure, forming a solid solution instead of precipitates. In the case of Zn0.97Cu0.01Co0.02O, the coercive field measured by a vibrating sample magnetometer at room temperature is 60Oe. The implication of the effect of Cu doping in bulk Zn0.98Co0.02O is also discussed.The high-temperature ferromagnetism in Co-doped ZnO samples fabricated by the standard solid-state reaction method is reported. Additional Cu doping into bulk Zn0.98Co0.02O is essential to achieve room-temperature ferromagnetism. Structure and composition analyses revealed that cobalt is incorporated into the lattice structure, forming a solid solution instead of precipitates. In the case of Zn0.97Cu0.01Co0.02O, the coercive field measured by a vibrating sample magnetometer at room temperature is 60Oe. The implication of the effect of Cu doping in bulk Zn0.98Co0.02O is also discussed.

MICROSTRUCTURAL INVESTIGATION OF OXIDIZED NI/AU OHMIC CONTACT TO P-TYPE GAN

The microstructure of oxidized Ni/Au films on p-GaN was examined to elucidate the formation of a low resistance ohmic contact to p-GaN with a field-emission gun transmission electron microscope in conjunction with composition analyses. The p-GaN/Ni/Au samples were heat treated at 500u200a°C in air mainly composed of a mixture of crystalline NiO, Au, and amorphous Ni–Ga–O phases. Small voids adjacent to the p-GaN film were also observed. The as-deposited Au film converted into discontinuous islands containing small amounts of Ni that connect with p-GaN. NiO formed a continuous film at the surface that covers the Au islands and the amorphous Ni–Ga–O phases. Moreover, NiO partially contacts p-GaN as well as Au islands and the amorphous Ni–Ga–O phase. The orientation relationship of the crystalline NiO, Au-rich islands, and p-GaN film was identified as NiO(111)//Au(111)//GaN(0002) and NiO[110]//Au[110]//GaN[1120]. The results suggested that Ni atoms diffuse through the Au layer onto the surface and react with...

Microstructural analysis of neutron-irradiated martensitic steels☆

Abstract Four martensitic steels for fusion applications were examined by transmission electron microscopy after irradiation in the Fast Flux Test Facility (FFTF) at 420°C to 7.8 X 10 26 n/m 2 ( E > 0.1 MeV), about 35 dpa. There were two commercial steels, 9Cr-IMoVNb and 12Cr-1MoVW, and two experimental reduced-activation steels, 9Cr-2WV and 9Cr-2WVTa. Before irradiation, the tempered martensite microstructures of the four steels contained a high dislocation density, and the major precipitate was M 23 C 6 carbide, with few MC carbides. Irradiation caused minor changes in these precipitates. Voids were found in all irradiated specimens, but swelling remained below 1%, with the 9Cr-1MoVNb having the highest void density. Although the 12Cr-IMoVW steel showed the best swelling resistance, it also contained the highest density of radiation-induced new phases, which were identified as chi-phase and possibly α′. Radiation-induced chi-phase was also observed in the 9Cr-1MoVNb steel. The two reduced-activation steels showed very stable behavior under irradiation: a high density of dislocation loops replaced the original high dislocation density; moderate void swelling occurred, and no new phase formed. The differences in microstructural evolution of the steels can explain some of the mechanical properties observations made in these steels.

Oxidized Ni/Pt and Ni/Au ohmic contacts to p-type GaN

The Ni/Pt and Ni/Au contacts to p-GaN heat treated in air are investigated to study the effect of the metallic phase of high work function on specific contact resistance (ρc). The current–voltage measurement suggests that there is an optimal microstructure for the oxidized Ni/Au contact to obtain a minimum ρc of 4×10−6u200aΩu200acm2. Such low ohmic contact resistance could be related to the formation of the NiO and the specific microstructure. However, high ρc of about 2–5×10−2u200aΩu200acm2 are obtained for the oxidized Ni/Pt contacts, which is attributed to the different microstructure from that of the oxidized Ni/Pt contact. The NiO phase of the oxidized Ni/Pt contact is separated from p-GaN by a continuous face-centered-cubic Pt–Ni–Ga film. Therefore, the oxidized Ni/Pt contact behaves just like a common metal contact, but not a metal/NiO composite contact. Comparison of the two oxidized contacts implies that the previously proposed model based on metal/p-NiO/p-GaN heterojunction is reliable to explain the low ρc to ...

Aging embrittlement and lattice image analysis in a Fe-Cr-Ni duplex stainless steel aged at 400°C

Abstract Aging embrittlement, normally observed in ferritic stainless steels, was found in a Fe-Cr-Ni duplex stainless steel thermally aged at 400°C for a long time. The ferrite content and composition of the duplex stainless steel was changed by varying the solution annealing temperature in order to investigate the effect of ferrite phase on the aging embrittlement. The material was aged at 400°C up to 10 000 h. Aging embrittlement was characterized by microhardness and other mechanical tests. The results show that the aging embrittlement of the duplex stainless steel was attributed to the degradation in ferrite phase. Microstructural studies in the ferrite phase indicated that spinodal decomposition and G-phase precipitation led to the degradation. High resolution electron microscopy (HREM) was utilized to analyze the lattice images of G-phase and ferrite matrix. The extent of embrittlement was found to be strongly dependent on the ferrite content and the composition in ferrite. The Ni content in ferrite seems to play an important role in aging embrittlement by promoting G-phase precipitation. As Ni content increased in the ferrite, the degradation of the material aged at 400°C became more severe.

Oxidation of Ta diffusion barrier layer for Cu metallization in thermal annealing

Abstract This work examines the thermal stability of Ta barrier layer for Cu metallization with the effect of oxygen. The Cu/Ta/SiO 2 /Si films were annealed at temperatures ranging from 400 to 600°C under various vacuum conditions. Transmission electron microscopy has been performed to characterize the microstructure of the films after annealing. The results show that an amorphous interlayer of oxide between Cu and Ta can be formed at 400°C in a vacuum of 10 −2 mbar. X-Ray energy dispersive spectroscopy and electron energy loss spectroscopy confirm that this interlayer is tantalum oxide. This interlayer transformed into a crystalline phase of Ta–Cu oxide at 600°C. In addition, formation of tantalum oxide interlayer is more thermodynamically favorable than that of copper oxide layer at the Cu/Ta interface. Growth of the amorphous interlayer is atmosphere-dependent, as evidenced by the thickness of tantalum oxide being decreased with better vacuum or argon gas. This observation suggests that the oxidation source may arise from the annealing atmosphere rather than from interior SiO 2 . Furthermore, it has been observed that oxygen diffuses along grain boundaries in copper films to cause tantalum oxidation.

The IGSCC, sensitization, and microstructure study of Alloys 600 and 690☆

Both Alloys 600 and 690 were studied to understand the effect of heat treatment on the sensitization and SCC behavior of these alloys. The microstructural evolution and chromium depletion near the grain boundaries were carefully studied using analytical electron microscopy. The majority of the precipitates formed in Alloy 600 was found to be M7C3 with a hexagonal structure (a0 = 1.398 nm, c0 = 0.45 nm); whereas the carbides found in Alloy 690 were identified as M23C6 with an fcc structure (a0 = 1.06 nm). Modified Huey test performed in boiling 40% HNO3 was used to study the effect of heat treatment and degree of sensitization. Constant load tests and constant extension rate tests were performed in the solution containing sodium thiosulfate to study the SCC resistance of these alloys. The results of the constant load tests for Alloy 600 indicated that the susceptibility to SCC is sensitive to the chromium depletion depth at grain boundary, and the minimum value to prevent SCC failure is approximately 8 wt%. No SCC was observed for Alloy 690 tested using constand load and CERT in the same environments. All tests showed that Alloy 690 has a far better resistance to intergranular attack and SCC than Alloy 600, which is believed due to its high chromium content. It is therefore anticipated that Alloy 690 now a better substitute to Alloy 600 as a steam generator tubing material for pressurized water reactor will also offer a superior corrosion resistance when “sensitized” and in particular if exposed to sulfur containing media such as thiosulfate solutions.

The microstructural evolution of zircaloy-4 subjected to proton irradiation

Abstract Various thermo-mechanically treated Zircaloy-4 specimens which were subjected to 1 MeV H + irradiation at 350° C up to a total dose of 1 dpa were studied by transmission electron microscopy (TEM). It was observed that the proton irradiation has a great effect on the microstructural evolution in this alloy. In β-quenched specimens, the original fine ZrFe 2 (fcc) particles which were decorated along the lamellae boundary were gradually dissolved following the subsequent thermo-mechanical treatment or by the proton irradiation. Radiation-enhanced precipitate coarsening was observed for the Zr(Cr, Fe) 2 (hcp) precipitate phase after irradiation. In β-annealed specimens, the starting phase of the precipitates was identified as a complex structure (hcp and fcc) Laves phase of Zr(Cr, Fe) 2 . Proton irradiation enhanced the phase transformation from the C14 (hcp) phase to the C15 (fcc) phase. A new phase was also occasionally observed in one type of specimens, B6, irradiated to 1 dpa. This phase was identified as Zr 4 Sn (tetragonal) phase. It is concluded that after a long time exposure, the microstructural evolution of Zircaloy-4 cladding material will be dominanted only by irradition.

Four-probe electrical-transport measurements on single indium tin oxide nanowires between 1.5?and?300?K

Single-crystalline indium tin oxide (ITO) nanowires (NWs) were grown by the standard thermal evaporation method. The as-grown NWs were typically 100-300 nm in diameter and a few microm long. Four-probe submicron Ti/Au electrodes on individual NWs were fabricated by the electron-beam lithography technique. The resistivities of several single NWs have been measured from 300 down to 1.5 K. The results indicate that the as-grown ITO NWs are metallic, but disordered. The overall temperature behavior of resistivity can be described by the Bloch-Grüneisen law plus a low-temperature correction due to the scattering of electrons off dynamic point defects. This observation suggests the existence of numerous dynamic point defects in as-grown ITO NWs.