Atomic force microscopy investigation of dislocation structures and deformation characteristics in neutron-irradiated silicon detectors

The structure, microhardness and deformation characteristics of silicon detectors were investigated following a neutron irradiation, using optical and Atomic Force (AFM) microscopes. Electron Beam induced Current (EBIC) of a Scanning Electron Microscope (SEM) was used for a direct investigation of the influence of the neutron irradiation on a p-n junction space charge region. This neutron irradiation resulted in a deterioration in the electronic performance of silicon detectors. The results of these investigations had an important contribution to the understanding of silicon damage processes created by neutron irradiation. The studies have shown that in the interval of neutron fluences (Φ)9.9×10¹⁰≤Φ≤3.12×10¹⁵ n/cm² the damage was accumulative (from small point defects to high-defects accumulations). Abrupt changes in the microstructure, electrical and mechanical properties appeared practically at the same fluence value of Φ≥10⁴ n/cm². This fact demonstrates on the strong relation between the properties of the damaged silicon semiconductor and the silicon detector as an electronic device. Microscopy studies have shown that with the increase of Φ, different types of defects such as dislocation loops and their tangling appear. These dislocations are displaced in specific regions, due to the process of vacancies and interstitials, which increase their cumulation. The strongly damaged regions were imaged in the microscope as black (B) islands and the less damaged regions were imaged as white (W) islands. Microhardness measurements to the damaged detectors have shown the same islands in the damaged regions. Using an AFM it was found that the W regions contain smaller number of dislocation loops and a large amount of single point defects with their cumulations.