Energy Dependence of Proton Irradiation Damage in Silicon

Abstract

The energy dependence of radiation damage in silicon for proton energies in the range 1.35 to 130 MeV has been measured by observing the degradation of the bulk minority carrier diffusion length in silicon solar cells. Variability in proton flux determination at four different accelerators was minimized by employing prebombarded solar cells with known minority carrier diffusion lengths as calibrated solid-state ionization chambers. Where beam intensity measurement comparisons with Faraday cups could be made, agreement to better than 5% was obtained. The quantity characterizing the damage rate is the rate of change of the inverse square diffusion length with flux K≡d(1/L2)/dΦ. The 1-Ω-cm p-type silicon degraded, on the average at a rate six times less rapid than 1-Ω-cm n type, independent of energy. Room temperature annealing gave 30% to 50% decrease in K whenever the diffusion length was measured during and after irradiation. The energy variation of K agrees with the variation predicted by Rutherford scattering below 8 MeV, but decreases less rapidly at higher energies. The measured diffusion lengths increased with excess carrier density n from 2% per decade at n=109cm−3 to 20% per decade at n=1014cm−3. The reported results, obtained at low excess carrier density, can be used to predict solar cell degradation under conditions of outer space illumination if the appropriate excess carrier density is used. Failure to take into account the diffusion length variation will result in an underestimate of the solar cell output of less than 7%.