Interfacial Transitional Layer in SiO2 Film Thermally Grown on SiC(000-1)
Authors: Nagai Ryu , Iitsuka Nozomu , Ozawa Kodai , Hasunuma Ryu , Yamabe Kikuo 
Published: February 2020

SiC is one of the materials which attracts much attention for its application to power devices because of its excellent electrical, thermal, and mechanical properties. It is an advantage that SiC is a wide gap semiconductor, and then it is expected to be used in high power, high temperature, and high frequency devices. In addition, it is also an advantage that insulating SiO2 films can be easily formed directly on SiC substrates by thermal oxidation. However, SiO2/SiC interface properties, which determines the electrical characteristics of them, has not been well controlled. One of the problems is higher density of interface traps which might be related to C atoms at the interface. To control the interface properties, oxidation mechanism needs to be understood in detail. We have reported that two dimensional fluctuation of thermal oxide film thickness on SiC brings about dispersion of leakage current for MOS capacitors. However, there was no relation between the film thickness fluctuation and lifetime uniformity characterized by TDDB (Time-Dependent-Dielectric-Breakdown) measurement. It is considered that oxide film quality of SiC has an effect on the lifetime uniformity. In this research, we focused on thermal oxide film quality in the vicinity of the interface and investigated non-uniformity of the film with etching characteristics in diluted HF solution.
The substrate in our experiments was 4H-SiC substrate with the miscut angle of 4°. After modified RCA cleaning, thermal oxidation was carried out at 1200°C in O2 ambient. The grown oxide thickness was measured by spectroscopic ellipsometry. The morphology of the oxide surface was characterized by Atomic Force Microscopy (AFM). Then, step-etching, at 3 ~ 5 nm intervals, using 0.5 % diluted HF solution was done for the oxide film. The morphology of the surface after each step- etching was characterized by AFM.
Figure is surface roughness growth with step-etching of the oxide films on SiC(000-1) C-face. The oxide thickness is 30 nm. The surface roughness obtained in the case of Si substrate is also plotted for comparison. In the case of Si, the surface roughness increases with etching in the initial stage, which means that the film is not two-dimensionally uniform giving rise to fluctuation of the etching rate. As being close to the SiO2/Si interface, the surface roughness RMS maintains at a constant value, which means that the film is uniform at this region. The drastic decrease in the roughness at the interface is because it is the roughness of the substrate surface obtained after complete removal of the SiO2 film. For SiC case, on the other hand, the surface roughness still increases even at near-interface region. This means that the film quality at near interface is not two-dimensionally uniform. However, the tendency of the roughness growth at near-surface and middle regions are comparable to the Si case, indicating the middle region is uniform for SiC substrate, too. It should be also emphasized that the specific characteristics of SiO2 films at near-interface region was also seen for other oxide thicknesses 10 and 50 nm. These results apparently show that SiO2film, after its formation at the interface, keeps non-uniformity for a while, then gradually turns to be two-dimensionally uniform during subsequent oxidation time. The non-uniform region at near interface, which we call interfacial-transitional layer, was found to be ~10 nm-thick in our present experiments.
The non-uniformity might come from existence of C-related impurity and it non-uniform distribution. Homogenization and uniformization might proceed by out-diffusion of the impurities.

Figure 1