The temperature-dependent hole concentration p͑T͒ and hole mobility p ͑T͒ are obtained in p-type 4H-SiC epilayers with several Al-doping densities. From p͑T͒, the densities and energy levels of acceptors are determined by the graphical peak analysis method (free carrier concentration spectroscopy: FCCS) without any assumptions regarding the acceptor species. In the heavily Al-doped case, the excited states of acceptors affect p͑T͒ because the Fermi level is located between the valence band maximum and the acceptor level (i.e., the ground state level of the acceptor), indicating that a distribution function for acceptors, which includes the influence of excited states of acceptors, should be required. Here, FCCS can determine acceptor densities and acceptor levels using any distribution function (e.g., the Fermi-Dirac distributing function or the distribution function including the influence of excited states). Two types of acceptor species are detected in the lightly Al-doped epilayers, while only one type of acceptor species is found in the heavily Al-doped epilayer. Some of the parameters required to simulate electric characteristics of 4H-SiC power electronic devices are obtained; (1) the dependence of each acceptor level on a total acceptor density and (2) the dependence of the hole mobility on temperature and total impurity density.

Dependence of acceptor levels and hole mobility on acceptor density and temperature in Al-doped p -type 4H-SiC epilayersIn order to obtain some of the parameters required to simulate the electric characteristics of silicon carbide (SiC) power electronic devices in a wide temperature range from startup temperatures ͑ഛ30°C͒ to steady-operation temperatures ͑ജ200°C͒, we discuss the dependence of the two donor levels on the total donor density ͑N D ͒ as well as the dependence of the electron mobility on the total impurity density ͑N imp ͒ and operating temperature ͑T͒ in the n-type 4H-SiC. The temperature-dependent electron concentration n͑T͒ and electron mobility n ͑T͒ in the n-type 4H-SiC epilayers with several nitrogen-doping densities are obtained from the Hall-effect measurements. By the graphical peak analysis method (free carrier concentration spectroscopy: FCCS) without any assumptions regarding the donor species, the two types of donor species are detected from n͑T͒. Moreover, the energy level and density of each donor species are determined by the FCCS. Using these results, we obtain the parameters with which the dependence of each donor level on N D can be simulated. Using n ͑T͒ at T Ͼ 250 K, moreover, we obtain the parameters with which the dependence of the electron mobility on N imp and T can be simulated.

From the temperature dependence of the hole concentration p(T) in a lightly Al-doped 4H-SiC epilayer, an Al acceptor with ∼200 meV and an unknown defect with ∼370 meV are found. By irradiation with 4.6 MeV electrons, the Al acceptor density is reduced, while the unknown defect density is almost unchanged. This indicates that the substitutional Al atoms in 4H-SiC are displaced by irradiation or that the bonds between the substitutional Al atom and the nearest neighbor atom are broken by irradiation. Moreover, the reduction in p(T) with irradiation arises from the decrease of the Al acceptors, not from the formation of hole traps or donor-like defects.

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 cm CZ-Si, the density of each acceptor species is reduced by irradiation, and finally the conversion occurs with 10 17 cm À2 fluence of 1 MeV electrons or with 2:5 Â 10 14 cm À2 fluence of 10 MeV protons. In 10 cm MCZ-Si and 10 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 þ /p/p þ Si solar cells and upcoming III-V tandem solar cells on n þ /p Si substrates.

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