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

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

A model for the electric field distribution beneath the Schottky contact in semi-insulating (SI) GaAs particle detectors is developed. The model is based on a field-enhanced electron capture of the EL2-defect. The influence of the compensation mechanism in SI-GaAs on the field distribution, leakage current density and charge collection properties of the detectors will be discussed. The detailed understanding allows then a device optimization.

Formation of a quasi-neutral region in Schottky diodes based on semi-insulating GaAs and the influence of the compensation mechanism on the particle detector performance

Semi-insulating (SI) GaAs is now being reconsidered as a promising material for radiation detectors, mostly due to greatly improved quality of the material. In this paper we shall describe the properties of the state-of-the-art SI GaAs crystals grown by LEC method as relevant for such applications. Specifically, we shall concentrate on the assessment of the spectra and density of residual impurities, on the measurements of deep levels spectra and on studying the impact of these centers on material parameters. The spectra of deep centers as studied by PICTS and DLTS indicate that, in LEC crystals grown from stoichiometric melts, besides the most prominent EL2 center, other electron and hole traps are usually observed. By comparing the DLTS and PICTS results we show that PICTS often tends to overestimate the relative contributions of traps shallower than EL2. Two approaches to decreasing EL2 concentration in LEC grown crystals have been proposed in literature: to grow crystals from Ga-rich melts and to anneal the material at high temperatures. We have studied the changes in deep level spectra and transport parameters for both cases. In annealing experiments the possibility of thick samples modification by arsenic diffusion was also investigated.

Semi-insulating LEC GaAs as a material for radiation detectors: materials science issues

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

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