Si Substrate Suitable for Radiation-Resistant Space Solar Cells

Irradiating group-III (B, Al, Ga)-doped Czochralski (CZ)-grown Si substrates as well as B-doped magnetic Czochralski (MCZ)-grown and floating-zone (FZ)-grown Si substrates with 10 MeV protons or 1 MeV electrons, we investigate both the reduction in the hole concentration and the conversion of p type to n type using Hall-effect measurements. In all the 10 Ω cm CZ-Si, the density of each acceptor species is reduced by irradiation, and finally the conversion occurs with 1017 cm-2 fluence of 1 MeV electrons or with 2.5×1014 cm-2 fluence of 10 MeV protons. In 10 Ω cm MCZ-Si and 10 Ω cm FZ-Si, on the other hand, the conversion does not occur under the same conditions. Moreover, the reduction in the hole concentration for the FZ-Si is much less than the others. From these results, it is elucidated that the conversion as well as the reduction in the hole concentration is strongly dependent on the concentration of oxygen in Si, not on the type of acceptor species in Si. Therefore, the p-type FZ-Si substrate is appropriate for radiation-resistant space solar cells such as n+/p/p+ Si solar cells and upcoming III–V tandem solar cells on n+/p Si substrates.