MS-9-P-2585 Characterisation of Neutron Irradiated Gallium Arsenide

Space based solar cells are exposed to high doses of radiation from electrons, protons, γ-rays, X-rays and also neutrons [1, 2]. The resulting radiation damage leads to the eventual degradation of these solar cells. It is therefore important to understand and predict the radiation effects on the properties of these materials. GaAs is a well suited material for solar cells deployed in space, due to its intrinsic semiconducting properties [3, 4]. A review of the relevant literature indicates that the effects of neutron irradiation on GaAs have not been studied in detail. The aim of this investigation is the characterisation of the microstructure of GaAs irradiated with a spectrum of fast and thermal neutrons to a dose of 1x10²⁰ neutrons/cm². Irradiation is followed by annealing in the temperature range from 600 to 1000 °C. The evolution of the microstructure was investigated by using conventional and high resolution transmission electron microscopy (TEM and HRTEM). Conventional TEM revealed a high density of dislocation loops in the unannealed neutron irradiated GaAs (Fig. 1). The presence of these small loops indicates that annealing occurred during the neutron irradiation process, facilitating the agglomeration of point defects. The loop diameters increased after annealing at 600 °C (Fig. 2) and 800 °C (Fig. 3). The dislocation loops which have {110} habit planes were found to be of interstitial nature. This finding is in agreement with earlier studies on proton bombarded and 1 MeV electron irradiated GaAs where interstitial loops on {110} planes became visible after annealing at 500 °C [5]. It was found that the dislocation loop density produced by a dose of 1x10²⁰ neutrons/cm² in GaAs, is similar to that produced by a proton dose of ~1x10¹⁶ protons/cm² at their projected range [6]. High resolution (aberration corrected) TEM and STEM of samples annealed at 600 °C confirmed the nature of the dislocation loops (not shown) and also revealed the presence of defects that were not observed before by conventional TEM (Fig. 4). These defects were found to be two-layer nano-twins on the {111} plane. An explanation for the origin of the nano-twins in neutron irradiated and annealed GaAs wil be presented.

References
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