Characterization of Interface State Density of SiO2/SiC (000-1) Based on Oxygen Concentration at the Interface during Thermal Oxidation
Authors: Hasunuma Ryu , Hanasato Kohei , Yamabe Kikuo 
Published: February 2020

Inferior interfacial electrical properties of thermally grown SiO2 films on SiC, compared to those on Si, is probably brought by carbon atoms in the substrates. Although few direct evidence f has been reported, it is quite natural to consider that some carbon atoms exist as impurities at the interface as long as we assume a finite diffusion coefficient for carbon atoms. It is well known that we cannot avoid formation of high density of interface states in dry oxide interface. Various processes for interface state reduction have been reported, such as FGA for instance. The mechanism is not clear, however, it is apparently different from termination of Si dangling bonds, which is the case for SiO2/Si, since FGA for SiO2/SiC requires much higher temperature than Si case.
For reduction of interface states, it is important to quickly remove carbon atoms out of the film to the ambient. In this paper, we focus on the role of oxygen atoms, in addition to being oxidants, as a carbon removers by generating CO molecules. We report how the interface states density is related to oxygen concentration at the interface.
The substrate in our experiments was 4H-SiC(000-1) epi-wafers with the miscut angle of 4°. After modified RCA cleaning, thermal oxidation was carried out at 900-1200°C in O2 ambient. The oxide thickness was characterized by spectroscopic ellipsometry. Then the Al gate electrodes were fabricated on top of the oxide films, and we characterized interface state density by C-V measurement. From the oxidation date, that is thickness vs. oxidation time, we have extracted the linear rate and the parabolic rate constants by fitting with the Deal-Grove model. Using these constants, we have calculated the oxygen concentration at the interface, which should be determined with oxidation rate at the interface and the flux of the oxygen diffusion from the ambient to the interface.
Figure 1 is the oxide thickness vs. oxidation time with variation of oxidation temperature. The dotted lines are the fitting curves. The experimental data are well fitted by the Deal-Grove model, indicating that thermal oxidation of SiC can be basically explained by diffusion of oxidant species through oxide film and oxidation reaction at the interface, as same as for Si oxidation. Figure 2 is the calculated oxygen density at the interface as a function of oxide thickness. It is natural that the oxygen density decreases as the oxide becomes thicker due to decrease in oxygen diffusion to the interface. The decrease in the oxygen density for higher oxidation temperature is due to decrease in the oxygen solubility. Figure 3 shows the summary of the relation between interface state density and the calculated oxygen density at the interface. In this figure the interface state density was evaluated at 1.0 eV below the bottom of SiC conduction band, which is usually observed for C-face substrate and strongly affect the reliability properties, such as flatband voltage shift. It is clear that the interface state density decreases as the oxygen density increases. It could be explained by that oxygen atoms act as the carbon remover, if we assume that the interface states are carbon related. Decrease in the interface sate density for higher oxidation temperature can be explained by fact that temperature dependence of the rate of CO formation is larger than that of SiO2 formation, that is supplied oxygen atoms are more used for carbon removal at higher temperature.

Figure 1