Study of Surface Effects in Thick Lithium Drifted Silicon Radiation Detectors

Leakage current in lithium drifted detectors is a very sensitive function of surface condition. In this paper some attempts at gaining a better understanding of the relevant surface phenomena are reported. N-type conduction layers were found which extended from the lithium-rich side across the depleted material to an inversion layer on the p-type material, at the other end of the detector. Regions of high electric field exist in the vicinity of the i-p boundary. By measuring surface potentials and solving Laplace´s equation in the depletion region, one can compute electric field configurations as well as net charge concentration at the surface. Collection probability for pairs generated in the surface layer was measured and found to be independent of bias above VB /m=simeq/ 1 volt. A model for the generation of surface currents is proposed. Measurements indicate that most of the surface current is created in the region of high field. Finally, the number of occupied surface states is computed and found to be, at low bias, of the order of 1/104 times the number of atoms at the Si surface.