DEVELOPMENTS IN GAAS PIXEL DETECTORS FOR X-RAY-IMAGING

Position sensitive hybrid pixel detectors have been fabricated by bump bonding silicon or bulk grown semiinsulating gallium arsenide pixel d etectors to CMOS read-out chips. Their performance as X-ray imaging se nsors, in the energy range of 10-70 keV, was evaluated in terms of spa tial 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 corre sponding modulation transfer function (MTF) and noise equivalent passb and (N-e) evaluated. A value of 5.7 line pairs per mm (1p/mm) was foun d 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 wi th 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 (L P-VPE). Hall measurements on initial samples gave free carrier concent ration of 1-8x10(11) cm(-3) From the C-V dependence of a reverse-biase d Schottky diode this material, however, a space charge density of 2x1 0(13) cm(-3) was measured. The observed temperature and frequency depe ndency of the capacitance is characteristic of the presence of deep le vels and so the material is believed Co have a small degree of compens ation due to these levels. The measured charge collection efficiency d etermined (c.c.e.) for 60 keV gamma rays showed an increase with rever se bias, reaching a plateau value of 93% for 100V. The limitations of present detectors are discussed and possible future developments indic ated.