Developments in GaAs pixel detectors for X-ray imaging

Position sensitive hybrid pixel detectors have been fabricated by bump bonding silicon or bulk grown gallium arsenide pixel detectors to chips. Their performance as X-ray imaging sensors in the energy range of 10-70 keV, was evaluated in terms of spatial resolution. For the GaAs device a fit was made to the line spread function (LSF) obtained from the image of a narrow slit and the corresponding modulation transfer function (MTF) and noise equivalent passband (N_e) evaluated. A value of 5.7 line pairs per mm (1p/mm) was found for the latter, with a modulation of 10% at the Nyquist frequency (N_y). A comparison is also given of the performance of these devices with state-of-the-art scintillator on silicon CCD dental X-ray sensors. In a bid to improve detector performance, thick layers of high quality GaAs have recently been grown by low pressure vapour phase epitaxy (LP-VPE). Hall measurements on initial samples gave free carrier concentration of 1-8×10¹¹ cm^-3. From the C-V dependence of a reverse-biased Schottky diode this material, however, a space charge density of 2×10¹³ cm^-3 was measured. The observed temperature and frequency dependency of the capacitance is characteristic of the presence of deep levels and so the material is believed to have a small degree of compensation due to these levels. The measured charge collection efficiency determined (c.c.e) for 60 keV gamma rays showed an increase with reverse bias, reaching a plateau value of 93% for 100 V. The limitations of present detectors are discussed and possible future developments indicated