(Invited) Impacts of Anisotropic Material Properties on Performance of SiC Power Devices
Authors: Kimoto Tsunenobu , Ishikawa Ryoya , Chi Xilun , Mikami Kyota , Tachiki Keita , Kaneko Mitsuaki 
Published: December 2024

Silicon carbide (SiC) has emerged as a wide bandgap semiconductor suitable for advanced high-voltage and low-loss power devices. Through rapid progress of both SiC material and device technologies in the last two decades, 600–3,300 V SiC power MOSFETs and Schottky barrier diodes are currently in volume production, demonstrating remarkable improvement of energy efficiency. So far, the 4H polytype of SiC (4H-SiC) has been exclusively used for fabrication of SiC electronic devices, owing to it superior physical properties and availability of large (150 mm diameter) and high-quality wafers. However, 4H-SiC exhibits significant anisotropy in physical, chemical, and interface properties, due to its unique hexagonal crystal structure. The authors have recently clarified several important anisotropic material properties of 4H-SiC and, in this paper, impacts of the anisotropic properties on performance of SiC power devices are overviewed.

Since SiC power devices are fabricated as “vertical devices” on SiC(0001) wafers, the current mainly flows along <0001> and high voltage (i.e. high electric field) is applied along the same direction. Thus, carrier mobility and impact ionization coefficients along <0001> are key physical properties which determine the performance of SiC power devices. Regarding the mobility, the authors fabricated Hall-bar structures along <0001> and <1-100> side by side on custom-made SiC(11-20) epitaxial substrates. It turned out that the electron mobility along <0001> is about 15–20% higher than that along <1-100> (or <11-20>), the latter of which was ever measured in almost all the previous studies. The electron mobility along <0001> was determined in wide ranges of the donor density (5x1014–3x1018 cm-3) and temperature (200–600 K), and the highest mobility at room temperature ever reported (1210 cm2/Vs) was obtained along <0001> for a lightly-doped SiC epilayer. On the other hand, the hole mobility showed an opposite anisotropy: The hole mobility along <0001> is about 10–15% lower than that along <1-100>. The authors carefully considered the first Brillouin zone of 4H-SiC to interpret these mobility anisotropies and concluded that these anisotropies can be quantitatively explained by the anisotropy of effective mass, taking account of energy distribution of carriers.

The impact ionization coefficients of electrons and holes along <0001> and <11-20> were also determined by analyzing photo-multiplication current of many different SiC pn diodes fabricated on (0001) and (11-20) substrates, respectively. Although the ionization coefficients of holes along both the directions (<0001> and <11-20>) and the coefficient of electrons along <11-20> are all similar, the coefficient of electrons along <0001> is two- or three-orders-of-magnitude smaller. This unusually small impact ionization coefficient of electrons along <0001> can be explained by considering hot electrons in the unique conduction band (E – k dispersion in the first Brillouin zone) of 4H-SiC. As a result, the critical electric field, which determines the junction breakdown, along <0001> is significantly higher than that expected from the bandgap of SiC. Thus, the anisotropies of both electron mobility and critical electric field (both are higher along <0001>) give 4H-SiC vertical devices fabricated on (0001) wafers much superior performance compared with previous predictions published in literature.

In SiC power MOSFETs with 600–1200 V class blocking voltage, the MOS channel resistance is the major component in the total on-resistance, because the drift resistance of the voltage-blocking layer is extremely low (< 3x10-4 Ω cm2) with SiC. The authors discovered that MOS channels on (11-20) and (1-100), both of which are perpendicular to the (0001) wafer surface, show much higher mobility (about 110–120 cm2/Vs) than those on (0001) (about 40 cm2/Vs) in n-channel MOSFETs with a lightly-doped p-body, as far as appropriate nitridation is performed after gate oxide formation. The high mobilities on (11-20) and (1-100) are attributed to the lower density of interface states especially near the conduction band edge in the SiO2/SiC structure, which leads to less electron trapping and less Coulomb scattering at the interface. These results have promoted development of high-performance SiC trench MOSFETs, where the MOS channels are formed on the trench sidewalls. In recent years, the authors achieved a high mobility of over 130 cm2/Vs in n-channel MOSFETs with a heavily-doped p-body on these planes by using an original oxidation-minimizing process. Furthermore, a reasonably high mobility of over 25 cm2/Vs was obtained in p-channel SiC MOSFETs on (11-20) and (1-100), which is two-fold improvement compared with (0001) MOSFETs. Impacts of these high mobilities on SiC MOSFETs are discussed in detail.