A summary is presented of the present status of several compound semiconductor radiation detectors, including detectors fabricated from GaAs, HgI2, CdTe, Cd1−xZnxTe and PbI2

Room-temperature compound semiconductor radiation detectors

We present room temperature measurements on 200 /spl mu/m thick GaAs pixel detectors, which were hybridized to silicon readout circuits. The whole detector array contains 320/spl times/240 square shaped pixel with a pitch of 38 /spl mu/m and is based on semi-insulating liquid-encapsulated Czochralski (LEC) GaAs material. After fabricating and dicing, the detector chips were indium bump flip chip bonded to CMOS readout circuits based on charge integration and finally evaluated. This readout chip was originally designed for the readout of flip chip bonded infrared detectors, but appears to be suitable for X-ray applications as well. A bias voltage between 50 V and 100 V was sufficient to operate the detector at room temperature. The detector array did respond to X-ray radiation by an increase in current due to production of electron hole pairs by the ionization processes. Images of various objects and slit patterns were acquired by using a standard X-ray source for dental imaging. The new X-ray hybrid detector was analyzed with respect to its imaging properties. Due to the high absorption coefficient for X-rays in GaAs and the small pixel size, the sensor shows a high modulation transfer function up to the Nyquist frequency.

X-ray imaging using a 320/spl times/240 hybrid GaAs pixel detector

X-ray imaging using a 320×240 hybrid GaAs pixel detector

Semi-insulating GaAs crystals grown by liquid encapsulated Czochralski technique from Ga-rich melts were evaluated as a possible material for radiation detectors with a high active layer thickness. The density of deep traps, particularly the midgap EL2 donors pinning the Fermi level, was measured by various techniques in conducting and semi-insulating samples. For EL2 traps, a direct evidence of their partial neutralization in the space charge region of reverse biased Schottky diodes due to nonequilibrium capture of electrons is presented for the first time. It is shown that the density of EL2 centers decreases with decreased As composition of the melt very gradually, especially for post-growth annealed samples. Subsequently, if one aims to decrease the EL2 density to such an extent that it would make a serious impact on the depletion layer width in GaAs-based detectors one has to grow semi-insulating GaAs crystals from melts with As composition below about 43% which poses a problem for the preservation of high resistivity of the material due to the relatively high concentration of compensating acceptors.

Study of GaAs as a material for solar neutrino detectors

Hybrid pixel detectors for medical X-ray imaging applications with Si and GaAs sensors are compared. Both sensor types are bump-bonded onto the MPEC single photon counting pixel read out chip. The image homogeneity and count rate of the two assemblies are compared. For the GaAs assembly the active layer thickness and charge collection efficiency have been measured.

Comparison of hybrid pixel detectors with Si and GaAs sensors

According to Ramo's theorem the charge collection efficiency of a particle detector is mainly influenced by the field distribution between the contacts of a Schottky diode. In semi-insulating GaAs material a space charge layer is formed due to deep levels needed for the compensation of acceptors. In this paper the deep levels and their influence on the distribution of the electric field is studied experimentally by different methods of electrical characterization. It is found that the electrical active concentration of the midgap donor of ∼10 15 cm −3 at an energy of 0.67 eV below the conduction band is only about one tenth of its total concentration of ∼16 16 cm −3 as measured by infrared absorption. The Schottky barrier leakage current is found to be responsible for the variation of the electrically active deep centers and it therefore influences the charge collection efficiency (c.c.e.). The c.c.e. turns out to be inversely proportional to the active concentration of deep centers. These results are supported by our modelling of the c.c.e.: Using the transport and the Poisson equation the electrical field distribution can be calculated through the coupling of the quasi-Fermi levels and the compensation mechanism. The model calculations of charge collection efficiencies for both alpha particles and protons are confirmed by the experimental results. The work is performed within the framework of the RD8 project.

Evaluation of active layer properties and charge collection efficiency of GaAs particle detectors

We report on test beam measurements of GaAs detectors. The detectors differ in pitch (44, 200, 375 and 1000 μm) and thickness ( d = 500 and 200 μm). Results on the charge collection, the signal to noise ratio and the charge division were obtained with a 5 GeV/c pion beam at the CERN-PS. Position dependent measurements of cross-talk between strips and the charge collection efficiency (CCE) were performed at the 1.5 MeV He + beam of the Freiburg Van de Graaff facility.

/pdf/beam-tests-of-gaas-strip-detectors-4ej9efgiol.pdf

Beam tests of GaAs strip detectors

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Evaluation of active layer properties and charge collection efficiency of GaAs particle detectors

Beam tests of GaAs strip detectors