S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Materials Science and Engineering (재료공학부) Theses (Ph.D. / Sc.D._재료공학부)
4H-SiC Homoepitaxial Growth on Various SiC Substrates for Power Device Application
- Hunhee Lee
- 공과대학 재료공학부
- Issue Date
- 서울대학교 대학원
- 4H-SiC; homoepitaxial growth; BTMSM; Si- and C-face substrate; off-axis angle; on-axis substrate
- 학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2015. 8. 김형준.
- Silicon carbide (SiC) has been considered as a promising wide bandgap material for high power, high frequency, and high temperature devices owing to its high breakdown field (~3×106 V/cm), high thermal conductivity, high saturated electron drift velocity (~2×107 cm/s), and chemical stability. The commercialization of its large single-crystal wafer and excellent epitaxial growth technique provide a better candidate for application in high-power and high-frequency electronic devices compared with other wide-bandgap materials such as GaN, ZnO, and diamond. Among the many SiC polytypes, most of the recent work has focused on 4H-SiC, owing to its high saturated electron drift velocity and commercial availability. For growth of high quality SiC epitaxial layers, many techniques were tried such methods as molecular-beam epitaxy (MBE), liquid-phase epitaxy (LPE), and vapor-phase epitaxy (VPE), there are some remaining problems. To solve this problem, the author focuses on the homoepitaxial growth of a 4H-SiC epilayer by MOCVD using bis-trimethylsilylmethane (BTMSM, C7H20Si2). BTMSM is an organo-silicon source which has a non-toxic and non-flammable nature, thus giving it certain advantages in comparison with the normal process using silane (SiH4). Moreover, because BTMSM has a Si-C bonding structure, low temperature epitaxial growth is also possible.
Homoepitaxial growth of SiC epitaxial layers on various substrates including 4H-SiC 4°, 8° off-axis, on-axis Si-face substrates, and C-faces substrates was carried out at temperature ranging from 1240 to 1550 °C and carrier gas flow rates of 5-20 sccm for the BTMSM source.
In the case of homoepitaxial growth on various off-angle substrates, it was found that the structure perfection of SiC epilayers is improved with higher temperature and lower flow rate of BTMSM. This growth behavior can be explained by the step-controlled epitaxy model. At the nominally on-axis surface spiral growth occurs via micro-steps provided by screw dislocations intersecting the surface. When growth temperature became 1480 °C on the nominally on-axis, we conducted island growth, which possesses a number of blocks, but hexagonal shapes were sparsely appeared. In addition, epilayer began to have smooth surface in macroscopic view as well as a shape of continuous chain of hexagonal-shape, when the temperature reached 1550 °C. When epilayer grows at 1480 °C, 4H-SiC was grown to a hexagonal shape, and nearby blocks were composed of 3C-SiC. This means the application of BTMSM source enabled growth of 4H-SiC at a relatively lower temperature of 1480 °C, which is the same as the polytype of substrate. Moreover, we could identify that 4H-SiC could grow at 1550 °C, satisfying stability of 4H-SiC polytype for 100%. The whole area was grown to 4H-SiC epilayer, revealing two types of surface regions which are hexagonal chain-shape and smooth surface. The smooth surface could be obtained due to high dislocation density of the area when compared to the hexagonal chain region. Increasing the growth temperature helps to enhance the mobility of the adatoms on the surface, lowering the probability for a two dimensional nucleation of 3C.
Many researchers have attempted epitaxial growth using 4H-SiC Si-face substrates, but there are few studies that have investigated the epitaxial growth on the C-face. Therefore, it is essential to determine the dependence of the quality of epitaxial layers on substrate polarity. This study has its significance in that an organosilicon source material of BTMSM was used and the epitaxial growth on C-face 4H-SiC substrates was analyzed in detail, which has not been previously attempted by other groups. In situ H2 etching and homoepitaxial growth of 4H-SiC have been carried out on 4° off-axis Si-face and C-face 4H-SiC substrates by low-pressure CVD. H2 etching characteristics and epitaxial growth behaviors on two different polarities using a BTMSM were systematically analyzed and discussed. When the temperature of in situ H2 etching was 1500 °C, the Si-face and C-face showed macro step-bunching and some clusters, respectively, whereas both faces showed fairly good quality when treated at 1450 °C for 10 min. High-quality 4H-SiC epitaxial layers with less crystallographic defects and free of step-bunching were demonstrated on both Si-face and C-face substrates. The optimal growth temperature on the Si-face substrate was 1320-1440 °C with a BTMSM source flow rate of 5-10 sccm, while the growth temperature should be increased to 1500 °C on the C-face substrate with a lower source flow rate of 5 sccm. A mechanism for the observed generation of step-bunching and surface morphological defect on both substrates depending on the growth temperature and source flow rate was also proposed.