High Electron Mobility along the c-axis in SiC: Evidence for the Advantage of SiC(0001) Vertical Devices

Silicon carbide (SiC) exhibits unique material properties, making it a promising semiconductor for high-power and high-temperature electronic devices. The advancement in SiC technology has led to production of MOSFETs and Schottky barrier diodes for commercial applications in power devices. However, accurate simulation of the device current as well as the design of optimal devices has remained a big challenge. This is mainly attributed to a poor understanding of the physical and material properties of SiC. An example of such properties is mobility which influences the on-resistance of SiC devices. As well, the mobility exhibits anisotropy that produces the mobility differences along the c-axis (〈0001〉) direction and in the direction perpendicular to the c-axis (e.g. 〈1100〉). Since current flows along the c-axis in typical SiC power devices, the mobility along this direction is of great significance in determining the performance of SiC power devices.

Despite the extensive study on electron and hole mobilities measurement in SiC, most of the available studies concentrate on the mobility perpendicular to the c-axis determined by Hall effect measurement. To date, there are limited efforts to study the mobility along the c-axis. Accurate mobility characterization along the c-axis requires thorough knowledge of the device structure and substrate orientation. Based on the Hall effect measurement principle, substrates such as (1100) and (1120) can be used because they comprise the c-axis in the in-plane direction. Likewise, the van der Pauw structure is considered suitable only when the in-plane mobility is isotropic because current does not flow in one direction. Thus, fabricating Hall bar structures which enables us to determine the mobility along a particular direction by restricting current flow in one direction will be an important step towards measuring the mobility along the c-axis. The mobility depends on the doping concentration and the temperature, whose effects are poorly understood.

Herein, Mr. Ryoya Ishikawa, Mr. Masahiro Hara, Dr. Hajime Tanaka, Dr. Mitsuaki Kaneko, and Professor Tsunenobu Kimoto investigated the Hall electron mobility in 4H-SiC along and perpendicular to the c-axis over a wide range of temperature (298 – 600 K) and donor concentration (2.1 × 10¹⁵ – 3 .2 × 10¹⁸ cm^-3). They used Hall bar structures produced on n-type 4H-SiC(1120) epitaxial layers. The study also investigated the drift mobility anisotropy from that of the resistivity. Lastly, the contribution of the anisotropy of the electron effective mass to that of the drift mobility was discussed based on the donor concentration and temperature dependencies. Their work is currently published in the journal, Applied Physics Express.

The researchers reported electron mobility of 1160 cm² V^­-1s^-1 along the c-axis for a donor concentration of 2.1 × 10¹⁵ cm^-3. So far, this is the highest value ever to be reported for SiC at room temperature. Besides, the anisotropy of the drift mobility reduced as the temperature and donor concentration increased. This phenomenon was attributed to the reduction in the anisotropy of the electron effective mass with the increase of the electron energy in the conduction band.

In summary, the research team investigated the electron mobility along and perpendicular to the c-axis with various temperatures and donor concentrations using SiC(1120) Hall bar structures. The measured electron mobility for SiC is the highest to ever been reported. Based on the first-principles calculations, it was revealed that the relationship between the anisotropy of the electron mobility and that of the electron effective mass provided a reliable explanation. In a statement to Advances in Engineering, the authors said the study findings provide a thorough understanding of electron mobility and would contribute to the design of high-performance SiC devices.