Spatial dependence of electron concentration and effect of localized conductivity in n-type silicon wafers due to neutron damage




Quiroz, Crystal Baker

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Image sensors in radioactive environments are especially sensitive to radiation damage due to their internal circuitry and direct exposure to radiative particles. Displacement damage from neutron interactions creates non-uniformity in the crystal lattice of semiconductors that comprise the devices. Defects, dislocations, and dopant atoms create new electron traps within the bandgap of the semiconductor. This affects the microscopic and macroscopic properties of the material by decreasing carrier mobility and results in spatial nonhomogeneity of conductivity leading to a decrease in performance or failure of the device. An experimental campaign, along with a two part mathematical model, is proposed that investigates the spatial dependence of carrier concentration on conductivity in an n-type silicon wafer that is exposed to neutron damage. The model determines an expression for the concentration of free electrons that take into account the defects and dislocations in a system after neutron irradiation. This expression is then used to determine the local conductivity changes in the system, and then, the overall conductivity change of the material. This model is compared to electrical properties of n-type silicon (nSi) wafers and charged-coupled devices (CCD) before and after neutron irradiation. Three neutron sources were chosen based on their neutron spectra and similarities to neutron sources utilized at Los Alamos National Laboratory (LANL) and the University of Texas (UT) facilities. A 32 to 39% increase to resistance of nSi wafers was found with the TRIGA MARK II rotary specimen rack (RSR) neutron source, while a negligible difference was found with the TRIGA MARK II BP5 and PuBe sources. A decrease in resistance was found in CCD sensors after irradiation with the TRIGA RSR, with an increase in leakage current as a function of voltage. Comparisons with the mathematical model to experimental and published results indicate further investigations must be accomplished to achieve a more complete model.


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