Effects of sodium ions on trapping and transport of electrons at the SiO₂/4H-SiC interface

Capacitance-voltage (C-V) and Deep-Level-Transient Spectroscopy (DLTS) measurements were performed on Metal-Oxide-Semiconductor (MOS) capacitors fabricated on 4H-SiC with the SiO₂ layer grown by Sodium-Enhanced Oxidation. This technique has yielded 4H-SiC MOS transistors with record channel mobility, although with poor bias stability. The effects of the mobile positive charge on the C-V characteristics and DLTS spectra were investigated by applying a sequence of positive and negative bias-temperature stresses, which drifted the sodium ions toward and away from the SiO₂/4H-SiC interface, respectively. Analytical modeling of the C-V curves shows that the drift of sodium ions in the SiO₂ layer during the voltage sweep can explain the temperature dependence of the C-V curves. The effects of lateral fluctuations of the surface potential (due to a non-uniform charge distribution) on the inversion layer mobility of MOS transistors are discussed within a two-dimensional percolation model.