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

According to Ramoʹs 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 ∼1015 cm−3 at an energy of 0.67 eV below the conduction band is only about one tenth of its total concentration of ∼1616 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.