Influence of Thermal and Fast Neutron Irradiation on dc Electrical Performances of AlGaN/GaN Transistors
The influence of thermal neutron irradiation and fast neutron irradiation on the electrical properties of AlGaN/GaN HEMTs is investigated. An increase in the drain current and a decrease in the access resistances are observed when devices are irradiated with a thermalized neutrons fluence of 4.3 × 1010 neutrons.cm-2 while no evolution is observed with the same fluence of fast neutrons. However, the same phenomenon is observed when the fast neutron fluence is higher (1.8 × 1012 neutrons.cm-2). AlGaN/GaN heterojunctions are analyzed by gamma spectroscopy after thermalized or fast neutron irradiations to understand the physical mechanisms induced by irradiations. In fact, we have shown that the improvement of electrical properties of devices after thermal neutrons irradiation is linked to a Ga-Ge transmutation effect. Moreover, the evolution of the drain current and access resistance when the AlGaN/GaN heterojunctions are irradiated by fast neutrons can be induced by N vacancies creation and/or a change of the strain state of the layers and/or Ga-Ge transmutation effect.
Lattice disorder in neutron irradiated GaN: Nuclear reaction analysis and Rutherford backscattering studies
Abstract Disorders of N and Ga lattice in GaN, introduced by fast neutron irradiation with a fluence of 6.7 × 10 18  cm −2 , are investigated by nuclear reaction analysis (NRA) using 14 N(d, p) 15 N reaction with 2.6 MeV D 2 + ions and by Rutherford backscattering (RBS) with 1.5 MeV 4 He + ions. The 〈0 0 0 1〉 aligned NRA yield measured in as-irradiated GaN slightly increases compared with that of un-irradiated crystals, indicating that primary knock-on (PKO) produced by the neutrons results in ∼7.2 × 10 2 displaced N atoms. The slight increase in the aligned RBS yield for as-irradiated samples relative to that of un-irradiated ones indicates that the ∼1.8 × 10 2 displaced Ga atoms are produced by PKO. The displacement of N atoms is four times larger than that of Ga atoms, reflecting the lighter weight of N than Ga, although both the displacements are recovered by annealing over 1000 °C. The displacement of Ga atoms in GaN is two times smaller than that in GaP (∼3.0 × 10 2 atoms/PKO), showing the stronger bonds in GaN than GaP.
Effect of Calcium Glycerophosphate on Demineralization in an in vitro Biofilm Model
The aim was to investigate the anti-caries properties of calcium glycerophosphate (CaGP) using an in vitro bacterial flow cell model. Four flow cells, inoculated from a chemostat containing a seven-organism bacterial consortium, were pulsed with sucrose twice daily, to provide an acidic challenge and pH-cycling conditions. Blocks of enamel and dentine were mounted in each flow cell. In a study on the effect of CaGP concentration, CaGP was pulsed into three of the flow cells, at the same time as the sucrose, to give concentrations of 0.10, 0.25 and 0.50%. Water was pulsed into the fourth flow cell with the sucrose. Microradiography revealed a significant dose response of decreasing demineralization as CaGP concentration increased. Reductions at 0.25 and 0.5% were significant when compared to the control. A second study investigated the effect of timing of CaGP pulsing, relative to sucrose, on enamel and dentine demineralization. CaGP (flow cell concentration 0.2%), was pulsed 1 h before, during or 1 h after the sucrose pulse; a water control was employed. In enamel, pulsing CaGP before the sucrose reduced demineralization significantly compared to concurrent pulsing, which in turn gave a significant reduction compared to pulsing after sucrose, which did not reduce demineralization significantly compared to the water control. In dentine, CaGP reduced demineralization significantly only when pulsed before the sucrose. The findings suggest that in vivo, the anti-caries potential of CaGP may be greater if it is applied before a cariogenic challenge.