PROTON DAMAGE ANNEALING KINETICS IN SILICON SOLAR CELLS W. E. Horne, I. Arimura and A. C. Day The Boeing Aerospace Company, Seattle, Washington 9812A Proton damage annealing has been postulated as a method for prolonging the life of solar power systems in space. This paper describes a study of such damage annealing. The objectives of the study were to 1) minimize variables and examine fundamental characteristics of proton damage annealing, 2) to make preliminary evaluation of the usefulness of annealing for prolonging space missions, 3) to make a preliminary determination of optimum annealing conditions, and A) to provide a data base for planning more detailed research programs. A preliminary analytical model has been developed to describe the annealing of proton damage as a function of time and temperature in silicon solar cells. The analytical work is supported by data from detailed isochronal and isothermal annealing experiments on 2-Q-cm N/P silicon solar cells after irradiation to various fluences of 1.5 MeV protons. The data indicate that several defect species are created in silicon during the irradiate-anneal process and that each species anneals with its own characteristic time-temperature kinetics. This observation is in general agreement with those of other workers for high energy electron and neutron damage annealing. The relative amount of each species of defect appears to be a function of either the silicon starting material, i.e., low or high dislocation density, or the impurity concentrations such as oxygen, phosphorous, and boron in the silicon. It is found that the annealing process for the cells studied can be described by a model which considers that at room temperature the defects consist mainly of vacancy clusters. In the temperature range 100 to 150°C these clusters begin to break up and release vacancies. Between 150 and 200°C, these released vacancies diffuse throughout the silicon and either pair with interstitial silicon atoms and disappear (anneal) or pair with other impurity atoms creating new defect species (reverse anneal). At still higher temperatures these newly created defects are disassociated and eventually annealed. This model is supported by the isochronal annealing data of figure I. The model can be expressed analytically by the following equations:
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