Influence of carrier freeze-out on SiC Schottky junction admittance

Abstract

Incomplete impurity ionization is investigated in the case of nitrogen donors and aluminum and boron acceptors in 4H and 6H SiC. We calculate the degree of ionization for these impurities residing on different lattice sites in a broad temperature range and for different impurity concentrations. It is shown that the degree of carrier freeze-out is significant in heavily N-doped 6H SiC and in Al- and B-doped SiC. Using the general Schottky junction admittance model we calculate the temperature and frequency dependencies of the junction admittance in the case when impurity ionization by the applied ac bias is present. It is shown that admittance frequency dispersion may be significant at room temperature in the case of N- and B-doped SiC. Finally, we calculate the Schottky junction capacitance as a function of the applied dc bias and simulate the doping profile, using the capacitance-voltage data. The calculated profile is shown to deviate from the actual impurity concentration profile if the impurity ionization time constant is comparable with the ac bias period, which is so for N-and B-doped SiC with certain values of the impurity activation energy and capture cross-section.