EVALUATION OF ACTIVE LAYER PROPERTIES AND CHARGE COLLECTION EFFICIENCY OF GAAS PARTICLE DETECTORS

According to Ramo's theorem the charge collection efficiency of a part icle detector is mainly influenced by the field distribution between t he contacts of a Schottky diode. In semi-insulating GaAs material a sp ace charge layer is formed due to deep levels needed for the compensat ion of accepters. In this paper the deep levels and their influence on the distribution of the electric field is studied experimentally by d ifferent methods of electrical characterization. It is found that the electrical active concentration of the midgap donor of similar to 10(1 5) cm(-3) at an energy of 0.67 eV below the conduction band is only ab out one tenth of its total concentration of similar to 10(16) cm(-3) a s measured by infrared absorption. The Schottky barrier leakage curren t is found to be responsible for the variation of the electrically act ive deep centers and it therefore influences the charge collection eff iciency (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 e quation the electrical field distribution can be calculated through th e coupling of the quasi-Fermi levels and the compensation mechanism. T he 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.