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A tremendous development in the field of imaging radiation detectors has taken place in the last decade.
Conventional X-ray film has been replaced by digital X-ray imaging systems in a number of ways. Such systems mainly
consist of silicon charge coupled devices (CCDs) where incident photons create electron-hole pairs in the thin silicon
absorption layer near the surface. In contrast to visible light, which is absorbed within a 2 µm layer of silicon, the
penetration of X-ray is much deeper due to higher photon energy. This disadvantage is often circumvented by the use of
a scintillator absorption layer. Due to scattering of the low energy fluorescence photons, resolution and contrast of the
X-ray images decrease. In order to eliminate these disadvantages, hybrid detectors consisting of direct converting
semiconductors and readout electronics parts are fabricated.
For this configuration, it is advantageous that both parts can be optimized separately and different materials can
be used. Because of the well developed technology, the readout chip is fabricated out of silicon. As absorbing material,
silicon is less suitable. In a silicon substrate of 500 µm thickness, only 15% of a 30 keV radiation is absorbed and
converted into charges. In order to increase the absorption, materials with a higher atomic mass have to be used. Several
compound semiconductors can be used for this purpose. One of them is GaAs, which is available as high quality semi-insulating
wafer material.
For detector optimization, GaAs wafers from several manufacturers with different properties were investigated.
Test structures with Schottky and PIN diodes were fabricated. The I/V curves of the diodes, the spectral response from 5
up to 150 keV, the carrier concentration, and the carrier mobility were measured and compared. A survey of the results
and the criteria for material selection resulting from these measurements will be provided in the paper.
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