Study of Charge Carrier Transport in GaN Sensors
Capacitor and Schottky diode sensors were fabricated on GaN material grown by hydride vapor phase epitaxy and metal-organic chemical vapor deposition techniques using plasma etching and metal deposition. The operational characteristics of these devices have been investigated by profiling current transients and by comparing the experimental regimes of the perpendicular and parallel injection of excess carrier domains. Profiling of the carrier injection location allows for the separation of the bipolar and the monopolar charge drift components. Carrier mobility values attributed to the hydride vapor phase epitaxy (HVPE) GaN material have been estimated as μe = 1000 ± 200 cm2/Vs for electrons, and μh = 400 ± 80 cm2/Vs for holes, respectively. Current transients under injection of the localized and bulk packets of excess carriers have been examined in order to determine the surface charge formation and polarization effects.
Implantable real-time dosimetric probe using GaN as scintillation material
Abstract This paper presents an implantable real-time dosimetric probe using near-tissue-equivalent GaN as a scintillator and an optical fiber for radioluminescence (RL) collection and transmission. Heavily-doped GaN was preferred because of enhanced light output with dominant near-band-edge UV emission and minimized yellow luminescence (YL) contribution. For encapsulating the small-volume GaN bulk at the end of the coupled fiber, a polymer cladding fiber was chosen and a core cavity was formed by a chemical-etching and tip-cleavage process. For testing the realized dosimetric probe, a probe-readout system was designed with UV-narrow-band selection and photodetection. Real-time measurements showed detected radiation pulses with fluctuations, but integrating the detected pulses exhibited a dose response curve steadily increasing, with less than 2% reproducibility errors between runs for a dose of more than 50 cGy. The GaN and fiber contributions to the averaged output signal were evaluated by different field size measurements. The fiber contribution was significant because of its much larger irradiated volume compared to that of GaN in the probe. On the other hand, the GaN contribution per volume was much larger than the fiber: 186 times for 6 MV photons and 89 times for 18 MV photons. The probe was also irradiated by receiving a 18 MeV 300 Gy electron beam dose. The GaN contribution was unchanged, while the fiber contribution was increased by 20%. This further demonstrates the need to subtract the fiber contribution for accurate measurements.
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.