Dr. Yoshioka of National Institute of Advanced Industrial Science and Technology (AIST), and Matsushita and Iwata of Quemix have jointly discovered an anomalous localization of electronic states at the MOS interface of SiC-MOS devices during device operation (when a gate electric field is applied), and reported in Applied Physics Letters 122, 222104 (2023). （https://doi.org/10.1063/5.0151547）
SiC is currently attracting attention as a power semiconductor material that exhibits excellent performance under severe conditions such as high temperature and high voltage. However, there is a problem of reduced channel conductivity at the SiC/SiO2 interface, the cause of which has been unknown. In this study, the interfacial electronic state under the application of a gate electric field was investigated in detail by using the RSDFT (Real Space Density Functional Theory) code, which is the calculation engine of Quloud.
The calculations revealed an anomalous localization of electron carriers at the SiC (0001)/SiO2 interface when an electric field is applied. Specifically, the wavefunction at the SiC (0001)/SiO2 interface was found to be strongly localized to cubic sites near the interface. Surprisingly, we found that the spread of electron carriers is more strongly localized than expected from the effective mass approximation (typically used in semiconductor science), specifically distributed in the extreme vicinity of the interface (<5 Å). This means that the analysis based on the effective mass approximation fails at the SiC(0001)/SiO2 interface. Furthermore, such a localization phenomenon at the interface is expected to make the electron carriers more sensitive to scattering by the interface defects. On the other hand, we simultaneously found that such anomalous localization does not occur on nonpolar surfaces such as (11-20). This is closely related to the experimentally well-known reason why the channel conductivity of the (0001) plane is particularly low compared to other planes such as (1120). We also investigated the reason for the breakdown of the effective mass approximation at the SiC (0001) plane, and found that it is due to the long structural periodicity along the [0001] direction of the crystal. This finding provides an important insight for basic semiconductor science.
