Semi insulating (SI) gallium arsenide (GaAs) devices operating as a reverse biased Schottky diode offer an attractive choice as radiation detector at room temperature both in high energy physics experiments and as x-ray image sensors. However, SI GaAs devices contain a high concentration of traps, which decreases the charge collection efficiency (cce), and affects the energy resolution of such detectors working as nuclear spectrometers In this paper we present a detailed investigation of the spatial uniformity of the charge collection efficiency carried out by analysing ion beam induced charge (IBIC) space maps obtained by scanning a focused 2 MeV proton microbeam on a SI n-GaAs Schottky diode. The microbeam irradiated both the front (Schottky) and back (ohmic) contacts in order to evaluate the transport properties of both electrons and holes generated by ionisation. The IBIC space maps show a clear non-uniformity of the cce. The poor energy resolution previously observed in such detectors working as alpha particle spectrometers is ascribed to the presence of two different "phases" in the material, which produce two distinct collection efficiency spectra. Such "phases" show different behaviour as a function of the applied bias voltage which is most likely due to the different electric field dependence of the relevant capture cross sections of the trapping centres for both charge carriers.
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