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Description
Charge recombination lifetime in a silicon wafer is directly proportional to the high energy particle (HEP) irradiation dose of a device. It is widely recognized that the dose of irradiation can be directly related to the density of defects. However, the microscopic picture of defects is highly inhomogeneous with several kinds of defects clustered along the path of HEPs. Electron and hole motion via defect sites thus define the observed recombination process. For this purpose, we perform quantum chemistry calculations for a small cluster of silicon and study the electron density distribution for electron and hole type electronic configurations in for four types of Si defects. In the optimized geometry we study electron and hole wavefunctions, what allows to determine electron and hole hopping amplitudes as well as electron-hole recombination rates. We find that optimized defect geometries show existence of local minima with reduced symmetry. It shows for instance for a vacancy defect, not only a position of vacancy in the lattice but also the displacement of the surrounding atoms determines the charge density distribution. Comparing electron and hole charge densities only few types of defects demonstrate close overlap of the electron and hole densities. Hence, we identify the types of defects responsible for recombination. We can thus conclude that is not the total concentration of whatever defects in the crystal define the electron lifetime, but the concentration of specific recombination centers.
