ELECTRON AND PROTON IRRADIATION-INDUCED DEGRADATION OF EPITAXIAL INP SOLAR-CELLS

The degradation of epitaxial, shallow homojunction n + p InP solar cel ls under 1 MeV electron and 3 MeV proton irradiation is presented. The data measured under 3 MeV proton irradiation are analyzed in terms of displacement damage dose which is the product of the particle fluence and the calculated non-ionizing energy loss (NIEL)[1]. A characterist ic proton degradation curve is derived from which the cell degradation under any energy proton irradiation can be calculated. The data measu red under 1 MeV electron irradiation is also analyzed in terms of disp lacement damage dose. The electron irradiation-induced degradation is correlated with the proton degradation curve by determining electron t o proton dose ratios for each of the photovoltaic (PV) parameters. A c omparison of the characteristic degradation curves for InP and GaAs/Ge solar cells, which was determined previously, shows InP to be intrins ically more resistant to displacement energy deposition. The base carr ier concentration was measured during the irradiations, and significan t carrier removal was observed. When analyzed as a function of displac ement damage dose, the reduction in carrier concentration under both t he 1 MeV electron and the 3 MeV proton irradiation is shown to follow the same degradation curve. From this common degradation curve, a char acteristic carrier removal rate is calculated for InP under any irradi ation. The junction dark current was also measured during both irradia tions, and the data were fit to a three-term diode dark current equati on. From the fits, the diffusion current is determined as a function o f particle fluence. Changes in the diffusion current under electron an d proton irradiation are shown to correlate in terms of displacement d amage dose in the same way as the cell maximum power. The junction rec ombination current is also determined from the dark current data, and the results show the energy level of the dominant radiation-induced re combination center to be approximately the same in both the electron a nd proton irradiated samples. In addition, the dark current analysis i ndicates that the relative changes in the hole and electron lifetimes are essentially the same under both the electron and the proton irradi ations. Based on these results and the overall correlation between the electron and proton damage, a detailed description of the mechanism o f the radiation response of InP is developed which describes the cell degradation under any particle irradiation.