Investigation of Residual Strain in 4H-SiC/Si Heterostructures Fabricated By Surface Activated Bonding
Authors: Liang Jianbo , Zhou Yan , Yamajor Shoji , Arai Manabu , Kuball Martin , Shigekawa Naoteru 
Published: February 2020

Silicon carbide (SiC) is one of the most potential materials for the next generation power devices because of its advantageous material properties and the maturity of processing technology. SiC/Si heterojunction is a potentially useful system for realizing high-performance bipolar transistors with wide band gap emitters, switching devices, electroluminescence devices, and sensors. On the other hand, it is difficult to grow SiC on Si due to the large mismatch in the lattice constant and the thermal expansion coefficients between Si and SiC. We previously fabricated 4H-SiC/Si junctions using surface activated bonding (SAB) method and investigated their electrical characteristics. It was found that the electrical properties of 4H-SiC/Si junctions improved with increasing annealing temperature and the amorphous layer formed at the interface without annealing vanished after the interface annealing at 1000 °C. It is possible to fabricate 4H-SiC/Si junctions at room temperature due to the amorphous layer absorbed the residual stresses caused by the mismatch in the lattice constant. However, the stress state of the bonded interface with annealing at 1000 °C is not clear after the recrystallization of the amorphous layer. The knowledge of the stress state near the bonded interface is very important for better understanding the vibrational properties and improving the electrical properties of the devices fabricated by SAB.

In this work, the residual stress near the 4H-SiC/Si bonded interface was investigated using confocal micro-Raman scattering technique. The effects of thermal annealing process on the residual stress of the bonded interface were also investigated. The magnitude of the residual stress near the bonded interface was calculated from the measured frequency shift with respect to the stress-free Raman peak position. The structural properties of the bonded interfaces were examined by transmission electron microscopy (TEM) observation.

B–doped (100) p+-Si substrate and n-4H-SiC epitaxial substrates (6 um, 5 × 10E15 cm-3 epitaxial layer / 0.5 um, 1 × 10E18 cm-3 buffer layer / substrate 3 × 10E18 cm-3) were used for the bonding experiment. Si and 4H-SiC substrates were bonded to each other by SAB. And then, the bonded samples were annealed separately at 400, 700, and 1000 °C for 60 s in N2 gas ambient. The residual stress in 4H-SiC/Si heterostructures was systematically investigated by micro-Raman spectroscopy. Micro-Raman mapping measurement was performed for an area of 40 × 40 μm2 with a step of 2 μm. A Renishaw InVia confocal micro-Raman spectroscopy measured under backscattering geometry, an Ar+ laser of 488 nm, a diffraction grating of 2400 lines/mm, a 50× long working distance objective lens, and a laser sport size of about 1.5 μm was used. 4H-SiC and Si peak frequencies were determined using the curve-fitting option integrated in the operation software by performing a Lorentz fitting to the spectral. The accuracy of the peak frequency is within ±0.02 cm-1. The interfacial structures of the bonded samples were investigated using TEM (JEM-2200FS)

The Raman peak shift mappings of 4H-SiC E2 (TO) and Si phonon mode in 4H-SiC/Si heterostructures without annealing are shown in Fig. 1(a) and 1(b), respectively. The averaged Raman peak shift position for SiC and Si is observed to be 776.57 and 519.86 cm-1, respectively. A uniform distribution of Raman peak position is obtained for both SiC and Si phonon modes. The stress state of SiC and Si can be considered nearly stress free in 4H-SiC/Si heterostructures with respect to their bulk counterpart values of 4H-SiC and Si substrate at 776.54 and 519.93 cm-1, respectively. This result indicates that stress free 4H-SiC/Si heterostructures was achieved by SAB method.

Figure 1