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, w
hich decreases the charge collection efficiency (cce), and affects the ener
gy resolution of such detectors working as nuclear spectrometers. In this p
aper we present a detailed investigation of the spatial uniformity of the c
ce carried out by analysing ion beam induced charge (IBIC) space maps obtai
ned by scanning a focused 2 MeV proton microbeam on a SI n-GaAs Schottky di
ode. The microbeam irradiated both the front (Schottky) and back (ohmic) co
ntacts in order to evaluate the transport properties of both electrons and
holes generated by ionisation, The IBIC space maps show a clear non-uniform
ity of the cce. The poor energy resolution previously observed in such dete
ctors working as alpha particle spectrometers is ascribed to the presence o
f two different "phases" in the material, which produce two distinct collec
tion efficiency spectra. Such ''phases" show different behaviour as a funct
ion of the applied bias voltage which is most likely due to the different e
lectric field dependence of the relevant capture cross sections of the trap
ping centres for both charge carriers. (C) 1999 Elsevier Science B.V. All r
ights reserved.