Effects of clustering on the properties of defects in neutron irradiated silicon

Abstract

We have exposed silicon bipolar transistors to fast neutrons and characterized the properties of the resulting defects using capacitance-based spectroscopy of the n-type collector. We have performed low-temperature electron capture measurement of the divacancy (=/−) and vacancy-oxygen (−/0) defects after the samples were annealed from 350–500 K. We show from a simple rate equation analysis that one can define an unambiguous test for cluster-induced reductions of defect level occupation due to slow capture. This allows easy identification of deep level transient spectroscopy (DLTS) levels where the capture is inhibited due to band bending. Our measurements show extremely long, temperature-dependent capture times for the doubly charged state of the divacancy. We have modeled the capture dynamics as a function of annealing using a simple electrostatic band-bending approach coupled with a realistic simulation of the cluster size and shape distribution as estimated from computer simulation of the damage cascades. We find that our simulation of neutron damage combined with electrostatic modeling of the capture data, with only a limited number of adjustable parameters, fits the measured data very well. Our annealing studies indicate, however, that isolated divacancies (those with visible DLTS signals from two acceptor states) comprise only about 30% of the charge in the defect cluster.