Structural and optical study of irradiation effect in GaN epilayers induced by 308 MeV Xe ions

Wurtzite GaN epilayers irradiated at room temperature with 308 MeV ¹²⁹Xe³⁵⁺ ions to fluences of 1 × 10¹³ and 3 × 10¹³ cm⁻² have been studied by contact mode atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), micro-Raman scattering and photoluminescence (PL) spectroscopy. The AFM images showed that the surface of GaN films was etched efficiently due to the Xe ion irradiation. The initial step-terrace structure on GaN surface was eliminated completely at a fluence of 3 × 10¹³ cm⁻². HRXRD and Raman results indicated that the Xe ion irradiation led to a homogenous lattice expansion throughout the entire ∼3 μm-thick GaN films. The lattice expansion as well as the biaxial compressive stress of the films was increasing with the increase of ion fluence. PL measurements showed that a dominant yellow luminescence band in the as-grown GaN films disappeared, but a blue and a green luminescence bands were produced after irradiation. Based on these results, the strong electronic excitation effect of 308 MeV Xe ions in GaN is discussed.     

Irradiation Behaviors of Nuclear Grade Graphite in Commercial Reactor, (III) Mechanical Properties

Bending and compressive strengths, and Young's modulus were measured for Pechiney nuclear grade graphite irradiated in the temperature range 220～400°C in the environment of CO₂ in a commercial reactor, up to the neutron fluence 6.2 × 10¹⁹ and 2.2 × 10²⁰n/cm² (E>0.85 MeV), respectively.

All of them increased owing to neutron irradiation, and the changes in both strengths were almost similar in the whole range of irradiation temperature, however the changes in Young's modulus depended on irradiation temperature.

It was clarified in the present experiment that both strengths were related with Young's modulus and the relation could be expressed by the formula σ=kE ⁿ, where σ and E are strength and Young's modulus, respectively, and n is constant which has different value for bending or compressive strength and also for their measured direction.