Interface trap properties of thermally oxidized n-type 4H-SiC and 6H-SiC

This work presents detailed investigations of interface traps at the Si-face 4H- and 6H-SiC/SiO2 interfaces using thermally stimulated current (TSC) and capacitance-voltage (C-V) techniques. Using n-type material we focus on the interface traps near the SiC conduction band edge which, in the case of 4H-SiC, severely suppress the effective mobility in n-channel metal-oxide-silicon carbide transistors. Our TSC measurements demonstrate that electron traps at the 4H-SiC/SiO2 interface consist of two groups of trap levels displayed as two distinguishable TSC signatures and differing by their trapping/detrapping behavior. One of them, chargeable at low temperatures, is displayed as a well-defined TSC peak assigned to a trapping level with an activation energy of 0.11 eV. Another is displayed as a wide TSC hump, and its charging mechanism strongly depends on temperature, indicating that these traps are not conventional “fast” interface states but border traps. A near-continuous distribution of activation energies ranging from 0.1 to 0.7 eV is obtained for these traps. The above two groups are observed in differently prepared thermal oxides on Si-face 4H-SiC. We hypothesize that both groups are due to the same interfacial defects but differing by their spatial closeness to the interface: the first is located in immediate proximity to the interface, while the second is composed of defects distributed within the oxycarbide transition layer, which explains their range of ionization energies and the thermally activated capture mechanism. No distinct interface traps are observed on 6H-SiC samples. In general, the electrical characteristics of the 6H-SiC/SiO2 interface can be satisfactorily explained in terms of conventional “fast” interface states.