1. Introduction

Silicon carbide (SiC) is a representative wide-bandgap semiconductor with a wide bandgap, high breakdown electric field, excellent thermal conductivity, and high electron saturation velocity. Due to these characteristics, it has attracted attention as a key material for high-voltage and high-power semiconductor devices in electric vehicles, renewable energy conversion devices, and industrial power conversion systems ^[1].

Specifically, power semiconductors, such as SiC MOSFET (metal–oxide–semiconductor field-effect transistor), Schottky barrier diodes, and positive-intrinsic-negative diodes are highly dependent on the quality of the epitaxial layer ^{[2, 3]}.

Homo-epitaxy, which grows the same SiC material on a SiC wafer, can minimize problems arising from differences in lattice constants and thermal expansion coefficients. However, minute residual stresses, crystal defects, step bunching, and changes in surface roughness that occur during the growth process can affect device characteristics ^{[4– 6]}. Therefore, as a key study determining the reliability and performance of high-voltage power semiconductors, the structural analysis of the 4H-SiC epitaxial layer grown on a SiC substrate was conducted using various analytical techniques, such as atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and high-resolution transmission electron microscopy (HR-TEM).

2. Experimental Details

2.1 SiC epitaxial growth

This study used 4H-SiC single-crystal wafers as substrates. To grow high-quality SiC epitaxial layers, a chloride-based chemical vapor deposition process was employed to grow 4H-SiC epitaxial layers with a thickness of 12 μm ^{[7, 8]}. The growth temperature was maintained in the range of 1600 ℃, and dichlorosilane (DCS, H₂SiCl₂) and ethylene (C₂H₄) were used as precursors to supply Si and C, respectively. Hydrogen was used as the carrier gas to form high-quality SiC single-crystal epitaxial layers.

SiC includes cubic, hexagonal, and rhombohedral forms that are classified based on the stacking structure of tetrahedra along the [0001] direction, which corresponds to the C-axis. Crystallographically, the cubic phase with a zinc-blende structure is referred to as the β phase, while all others (hexagonal and rhombohedral phases) with a wurtzite structure are referred to as the α phase ^{[9, 10]}. Furthermore, SiC polytypes can be distinguished by the stacking sequence even within the same crystal structure. Over 200 SiC polytypes exist thermodynamically, and according to Ramsdell’s notation, the stacking period of the SiC unit layer is indicated by a number in the front, followed by C for cubic, H for hexagonal, and R for rhombohedral, depending on the structure. For example, in Figure 1, if the stacking sequence is ‘ABCABC…’ with a cubic structure, it is denoted as 3C-SiC, as shown in Figure 1(a). Stacking periods such as ‘ABCBABCBA…’ and ‘ABCACBABCACB…’ in hexagonal structures are labeled as 4H-SiC and 6H-SiC, respectively, as shown in Figures 1(b) and 1(c) ^[9]. The SiC polytype is determined by the order of the A, B, C positions in which the SiC bilayer repeats along the C-axis [0001] direction. This stacking sequence is most clearly observed in a cross-section of the (11-20) plane ^{[11- 13]}.

Fig. 1 Crystallographic structure of SiC polytypes. SiC crystal structures viewed from the zone axis [11-20]: (a) 3C-SiC, (b) 4H-SiC, and (c) 6H-SiC.

In particular, compared to other polytypes, 4H-SiC has a higher band gap energy. Its band gap energy of 3.23 eV is about three times that of Si (the most widely used semiconductor material). Furthermore, its saturated electron drift velocity of about 2.2 × 10⁷ cm/s is about twice that of Si, its thermal conductivity of 4.9 W/cm·K is about three times that of Si, and its breakdown field of 3 × 10⁶ V/cm is over ten times higher ^[11]. These are highly advantageous characteristics for high-temperature device operation, high breakdown voltage, high current, and low loss. This study analyzed the behavioral characteristics of epitaxial layers with a 4H-SiC structure.

3. Results and Discussion

The cross-section of a six-inch SiC wafer with a 4H-SiC epitaxial layer is shown in Figure 2. Typically, when an N-type dopant is introduced into a 4H-SiC substrate, it appears light amber or yellow, and as the doping concentration increases, the color turns closer to dark brown. Figure 2(a) depicts a cross-sectional image observed by SEM of the 4H-SiC epitaxial layer grown on the SiC wafer. Since SiC is the second hardest (hardness 9.5) and brittle material after diamond, the cross-sectional samples were prepared by laser sawing to maximize quality without physical contact ^{[14, 15]}. As shown in Figure 2(a), the interface between the SiC wafer and the SiC epitaxial layer could be distinguished by the contrast, and the interface is indicated with arrows. Furthermore, elemental analysis of Si and C was conducted via scanning transmission electron microscopy (STEM)-EDS, as shown in Figure 2(b). SiC was clearly confirmed.

Fig. 2 SiC epitaxial wafer, SEM cross-section, TEM cross-section, and EDS analysis images: (a) SEM cross-sectional image of the interface between the grown epitaxial layer and the 4H-SiC wafer and (b) TEM cross-sectional image and EDS elemental analysis image of the interface between the epitaxial layer and the SiC wafer.

The surface image of the epitaxial layer grown on the SiC wafer is shown in Figure 3. The surface images were analyzed by AFM, with scan sizes of 15 μm, 5 μm, and 2 μm, as shown in Figures 3(a), (b), and (c), respectively. In addition, the surface roughness is denoted by root mean square (RMS). The RMS values were 178.899 pm for Figure 3(a) at a scan size of 15 μm, 188.563 pm for Figure 3(b) at a scan size of 5 μm, and 197.106 pm for Figure 3(c) at a scan size of 2 μm, confirming that the roughness was at the picometer (pm) scale and thus extremely flat. The terrace-step structure of the 4H-SiC epitaxial layer is formed by the step-flow growth mechanism, and local step bunching and variations in terrace width influence the RMS roughness measured by AFM. Therefore, as the scan size decreased, the influence of individual step edges and local surface undulations was relatively more pronounced, which may have caused a slight increase in RMS; this finding is consistent with the previously reported surface characteristics of SiC epitaxial layers ^[16].

Fig. 3 AFM image showing the typical surface structure of the SiC epitaxial wafer. The scan sizes were (a) 15 × 15 μm, (b) 5 × 5 μm, and (c) 2 × 2 μm.

As shown in Figure 4, the HR-TEM results revealed homo-epitaxial characteristics, with matched crystal orientation and lattice spacing between the SiC wafer and the epitaxial layer. Figure 4 presents a high-angle annular dark-field (HAADF)–STEM image of the SiC wafer and epitaxial layer fabricated using FIB. Notably, as in Figure 1(a), the 4H-SiC stacking structure of both the SiC wafer and the grown epitaxial layer is indicated with red dots, which can be clearly observed in Figure 4(a) ^{[17, 18]}. In this study, to confirm both the 4H-SiC homo-epitaxial structure and the behavior of local strain accommodation at the interface between the SiC wafer and the SiC epitaxial layer, HAADF-STEM analysis was conducted with the zone axis rotated to [11-20] and [01-12], respectively, for detailed examination. Figure 4(a) displays the 4H-SiC structure described in Figure 1(b). The fast Fourier transform (FFT) pattern and crystal plane direction as viewed along the [11-20] zone axis are shown in Figure 4(a). The actual analysis image shows that growth occurred in a homo-epitaxial manner along the crystal orientation of the 4H-SiC wafer at the interface between the SiC wafer and the epitaxial layer. Consequently, it was confirmed that the epitaxial layer and wafer exhibited a homo-epitaxial relationship with the matched crystal structure and spacing ^[6].

Figure 4(b) shows an HAADF-STEM image obtained with the zone axis rotated to the [01-12] direction; unlike Figure 4(a), the stacking periodicity could not be seen, but the local strain accommodation of atoms at the interface could be observed. The FFT pattern and crystal plane directions, as viewed from the [01-12] zone axis, are shown in Figure 4(b). As denoted by the red squares and arrows, local shifts of the Si atomic columns by approximately 0.01–0.05 nm (±3 pm) at the interface indicate the distribution of micro-strain. This displacement was derived by measuring the HAADF-STEM images along the zone axis [01-12] and analyzing the atomic column positions. Prior to measurement, the initial normal 4H-SiC crystal structure and atomic arrangement were corrected based on a reference lattice.

Fig. 4 SiC HAADF-STEM cross-section and FFT pattern images. (a), (b) HAADF-STEM image of a 4H-SiC wafer with an epitaxial layer grown on it. Structural analysis and FFT pattern image of the interface between the epitaxial layer and the SiC wafer according to the zone axis [11-20], [01-12].

Such local stress relaxation mechanisms are also consistent with previous studies that have reported that interfacial stress and lattice misfit can be relaxed during the growth process in 4H-SiC homoepitaxy. However, since this study did not directly compare samples of different thicknesses, the thickness dependence of the strain will be further verified in future research ^{[19, 20]}. In other words, the slight lattice mismatch and residual stress at the interface were diffused and absorbed through very small displacements of atomic positions, thereby maintaining the structural stability of the entire crystal. Such strain relaxation served as a key mechanism that suppressed the generation of new defects and enabled the epitaxial layer to form a stable homo-epitaxial structure with the substrate.

In conclusion, the high-quality SiC homo-epitaxial structure of the epitaxial layer confirmed in this study is expected to provide several positive effects for power semiconductor devices. This study’s significance lies not in reconfirming the previously reported homo-epitaxial relationship through HAADF-STEM and FFT pattern analysis but in directly observing and analyzing the minute displacement of Si atomic columns and local strain accommodation behavior occurring at the homo-epitaxial interface. These results are may contribute to improved reliability and stability of power semiconductor devices during their long-term operation by alleviating local stress.

4. Conclusion

This study confirmed that a 4H-SiC epitaxial layer with a thickness of 12 μm grown on a SiC wafer exhibited a well-developed homo-epitaxial structure; furthermore, its excellent crystalline quality, the same crystal structure as the substrate, and the local stress generated during growth could be explained by strain accommodation. Strain accommodation refers to the phenomenon in which stress concentrated at the interface is gradually dispersed to adjacent atomic rows rather than accumulating at a specific location. Therefore, the effective relaxation of stress through only minor shifts in local atomic columns implies that a high-quality homo-epitaxial SiC epitaxial layer was maintained without any increase in crystal defects. In particular, the HAADF-STEM images and FFT pattern results demonstrated that local strain arising during the epitaxial growth process could be relaxed while maintaining the continuity of the crystal structure. Notably, these results yield crucial microstructural evidence for understanding the structural stability of high-quality SiC epitaxial layers.