Basic semiconductor properties of the Si/6H-SiC interface.
The Si/6H-SiC heterostructure of large lattice mismatch follows domain epitaxy mode, which release most of the lattice-mismatch strain, and the coherent Si epilayers can be grown on 6H-SiC. An interfacial misfit dislocation array is present at the interface that determines the domain’s size. In this chapter, transmission electron microscopy (TEM) and high resolution X-ray diffraction (HRXRD) were employed to reveal in-plane orientation, interface structure and growth mode of the Si/SiC heterostructure. Based on the characterizations, residual lattice mismatch and edge misfit dislocation density at the hetero-interface can be precisely controlled. And these characterization methods are applicable for the heterostructures of large-lattice mismatch, except for the heterostructures with different crystal symmetry on the film and the substrate.
- large lattice mismatch
- domain matching mode
- SiC-based heterostructure
- in-plane orientation
- Interface micro-structure
With advantageous material properties such as a wide bandgap and high thermal conductivity, silicon carbide (SiC) has attracted much attention for its wide applications in the photoelectric devices of high temperature and high power [1, 2, 3, 4, 5]. However, due to the wide bandgap, SiC-based photoelectric devices can be only driven by ultraviolet (UV) light, which essentially limits the application of visible and infrared light detection. Si/SiC heterostructure is suggested to make SiC-based devices to be light-activated by non-UV light, in which Si is used as a non-UV light absorption layer [6, 7]. In our previous work, it was found that the Si films on SiC substrates always have a polycrystalline structure with multiple orientations, while the preferential growth of the Si films with different orientations can be obtained at different growth temperature [8, 9, 10, 11, 12]. The interface-structure of the heterostructure determines some important parameters such as the preferential orientation [13, 14], the interface state density [15, 16, 17] and the carrier mobility, which have significant impact on the heterostructure device performance. By observation of the Si/SiC interface-structure with different growth temperatures, the growth mode of the Si/SiC heterostructure can be revealed, and the accurate control of the growth orientation may be achieved. At present, the studies of the SiC-based Si/SiC heterostructure just focused on the electrical performance of the heterostructures in SiC SBD  and SiC MOSFET [19, 20], the growth mode and interface-structure of the Si/SiC heterostructure is rarely reported.
2. Growth mode, interface micro-structure and in-plane orientation of the Si/SiC heterostructure
2.1. Growth mode of the heterostructures of large lattice-mismatch
The crystal structure of Si and 6H-SiC is face-centered cubic (FCC) and hexagonal close-packed (HCP) with in-plane lattice constants of aSi = 5.430 Å and aSiC = 3.081 Å, respectively. The lattice mismatch of the Si(111)/6H-SiC(0001) is as large as 19.8%, which is given by
Similarly, the residual strain
2.2. Interface micro-structure of the Si/SiC heterostructure
The low magnification cross-sectional transmission electron microscopy (TEM) bright-field image of the Si thin film grown on 6H-SiC(0001) at 900°C is shown in Figure 2(a). In this image, the lower part belongs to the 6H-SiC substrate, while the upper part represents the Si thin film. The Si film with a thickness of about 0.55 μm shows irregular heterogeneous diffraction contrast, which suggests the existence of some structural defects such as stacking faults (SF) and twins in the film, as labeled in Figure 2(a). The selected area electron diffraction (SAED) patterns of the Si/6H-SiC heterostructure corresponding to Si[-110]SiC[-12-10] zone axes are shown in Figure 2(b). It is confirmed that the Si film has epitaxial connection with the 6H-SiC substrate and the orientation relationship of Si/6H-SiC heterostructure is (111)[1-10]Si//(0001)[1-210]6H-SiC. Alignment of the diffraction spots indicates that FFC-on-HCP epitaxial orientation, i.e., (111)Si//(0001)6H-SiC is maintained at a growth temperature of 900°C. It should be pointed out that the extinction diffraction spots of (10-10)SiC and (10-16)SiC can be observed in the SAED patterns because of the multiple diffraction. A superposition of two FCC <110> zone diffraction patterns, which are symmetrical to each other with respect to the (111) mirror plane, indicating that the lamellar structure observed in the film consists of alternate stacks of twins, as shown in Figure 2(b). Furthermore, the faint diffused streaks along the <111> orientation indicate that there exist a large number of SFs. And this agrees with the results of the diffraction contrast study.
Figure 3(a) shows a high-resolution TEM image of the Si/6H-SiC interface, exhibiting interfaces of Si(111)/6H-SiC(0001) without any indication of interfacial reactions, however, it is not crystallographic sharp, which reflects the roughness of the Si layer’s surface and the poor crystallographic match between interatomic distances in close-packed layers of between Si(111) and SiC (0001) planes (in-plane constants 3.84 Å for Si and 3.08 Å for 6H-SiC). Moreover, typical structural defects such as SFs and twins are clearly observed in the Si film, which are labeled in Figure 3(a). Figure 3(b) is the magnified image of the region b in Figure 3(a), which further confirms the epitaxial relation of the Si/6H-SiC heterostructure. Fourier-filtering technique is applied to remove the non-periodic information such as background signal and the structural defects in the Si film. The Fourier-filtered high-resolution TEM images of the Si thin film, the 6H-SiC substrate and the Si/6H-SiC interface are shown in Figure 3(c)–3(e), respectively. It is clearly observed that the SiC substrate with the HCP stacking sequence as ABCACB has a crystal plane spacing of 2.57 Å, while the Si film with the FCC stacking as ABC has a crystal plane spacing of 3.21 Å. Calculated from the crystal plane spacing, lattice mismatch of the Si(111)/6H-SiC(0001) heterostructure is about 19.8%, which is in accordance with the calculation results based on the fast-Fourier transform (FFT) pattern. Nevertheless, the (0001) lattice planes of SiC and (111) lattice planes of Si are well aligned, and the Si film grows epitaxially but with MDs (indicated by the arrows) at the interface between the Si film and the 6H-SiC substrate, which can be easily identified by extra lattice fringes in the 6H-SiC. The Si epitaxial growth follows the DM mode, every five 6H-SiC(1-210) planes match with four Si(1-10) planes along the interface, as shown in Figure 3(e). Moreover, the invariant crystal plane spacings of the Si film and the 6H-SiC substrate at the Si/6H-SiC interface demonstrate that the interfacial MD array accommodates most of the lattice mismatch strain and makes the lattice coincident at the Si/6H-SiC interface.
Based on the results shown above, the in-plane orientation of the (111)Si//(0001)6H-SiC heterostructure is schematically shown in Figure 4(a). Both of the 6H-SiC(0001) and Si(111) lattice planes have the same triangular lattice in two-dimensions (2D). And the Si(111) layers epitaxially grow on 6H-SiC(0001) without rotation of the 2D triangular lattice. However, the in-plane lattice constant of the Si(111) (3.84 Å) is larger than that of the 6H-SiC(0001) (3.08 Å), as shown in Figure 4(b). The FCC-on-HCP epitaxial relationship with a four-to-five mode of Si-to-SiC is clearly observed. The residual mismatch calculated by the DM mode is only 0.26%, which is much smaller than the mismatch of 19.8% calculated by the conventional lattice matching (LM) mode. Because the 2D triangular lattice of the Si(111) film has no rotation during epitaxial growth on 6H-SiC(0001), the domain mismatch strain
XRD data, shown elsewhere , indicates that the Si phase with  orientation appears when the temperature increases higher than 1000°C, which is confirmed by the TEM characterizations. Figure 5(a) is a low magnification cross-sectional TEM image of the Si/6H-SiC(0001) heterostructure grown at 1050°C. The Si/SiC heterostructure haves a sharp interface and consist of columnar grains. SAED patterns at the Si/6H-SiC interface corresponding to Si[-110]SiC[1-210] zone axes in Figure 5(b) clearly show the FCC-on-HCP orientation relationship of (110)Si//(0001)[10-10]6H-SiC, confirming the epitaxial growth of the Si films with  orientation. The high-resolution TEM image of the Si(110)/SiC(0001) heterostructure is shown in Figure 6(a). The Si/SiC interface is crystallographic sharp without any indication of the interfacial reaction products. Figure 6(b) is the Fourier-filtered HRTEM image, which confirms the epitaxial connection of the Si/6H-SiC heterostructure. Calculated from the crystal plane spacings and the FFT patterns, lattice mismatch of the Si(110)/6H-SiC(0001) heterostructure is 1.84%. The interfacial MD array can also be observed by extra lattice fringes in the 6H-SiC. Every two 6H-SiC(10-10) planes match with one Si(001) planes along the interface, as shown in Figure 6(b). Compared with the Si(111)/6H-SiC(0001) heterostructure with a residual mismatch of 0.26%, the Si(110)/6H-SiC(0001) heterostructure has higher residual mismatch of 1.84% along SiSiC[10-10] orientation. If the Si-to-SiC matching mode is not 1:2 but a long-period structure of 53:54, the lattice mismatch can decrease to −0.55%. Of course the long-period matching is very difficult to be confirmed by experimental observations; however, the trend of this large-period matching can be observed in Figure 6(b). The atomic position of Si in region 1 is slightly different from that of Si in region 2, as shown in the insets. It is suggested that the Si-to-SiC matching at the interface is merely approximate 1:2.
Figure 7(a) shows a HRTEM image of the Si(110)/6H-SiC(0001) interface. Because the observation orientation is SiC[-1010], the 6H stacking sequence as ABCACB of 6H-SiC is not observed. The SAED patterns at the Si/6H-SiC interface corresponding to Si[00-1]SiC[-1010] zone axes are shown in Figure 7(b). SAED patterns at the Si/6H-SiC interface clearly show the FCC-on-HCP orientation relationship of (110)[-110]Si//(0001)[1-210]6H-SiC, confirming the epitaxial growth of the Si films with  growth orientation. Figure 7(c) shows the Fourier-filtered image of region 1, which further confirms the epitaxial connection of the Si(110)/6H-SiC(0001) heterostructure. The crystal plane spacing at the Si/6H-SiC interface also has no significant change. The Si epitaxial growth follows the DM mode, every five 6H-SiC(1-210) planes match with four Si(-110) planes along the interface, as shown in Figure 7(d). According to the extra SiC lattice planes at the hetero-interface, Burgers vectors of the MDs can be determined uniquely. The MDs are of the pure edge type with a Burgers vector of parallel to the interface, which are labeled in Figure 7(d).
Base on the HRTEM observations and SAED analysis, the lattice-structure model of the Si(110)/6H-SiC(0001) heterostructure is constructed and schematically shown in Figure 8. It is known that the Si(110) plane has a rectangular 2D lattice with different in-plane constants of 5.43 Å and 3.84 Å along the vertical orientations Si and Si[-110], which is different from the 2D triangular lattice of the 6H-SiC(0001). However, the triangular lattice of 6H-SiC(0001) can be transformed to rectangular 2D lattice by missing partial Si-C atoms, which has in-plane constants of 5.33 Å and 3.08 Å along SiC[10-10] and SiC[1-210] respectively, as shown in Figure 8(a). Along SiSiC[10-10] orientations, the heterostructure has a lattice mismatch of 1.84% with in-plane constants 5.43 Å for Si and 5.33 Å for 6H-SiC. The residual lattice mismatch strain can be released by interfacial atomic relaxation of the Si/6H-SiC heterostructure and the strained-layer with no MDs will be attained. However, the 2D rectangular lattice of 6H-SiC(0001) is converted from the triangular lattice by missing every other Si-C atoms along [10-10] orientation. Therefore, MDs are still present at the Si(110)/6H-SiC(0001) interface and 1:2 mode of Si-to-SiC is observed. Along the vertical orientations Si[-110]SiC[1-210], the Si(110)/6H-SiC(0001) heterostructure has in-plane constants 3.84 Å for Si and 3.08 Å for 6H-SiC, and the interface with a four-to-five mode of Si-to-SiC is shown in Figure 8(c), which is identical with the Si(111)/6H-SiC(0001) heterostructure.
Because of the large lattice mismatch strain, the conventional LM epitaxy is not expected. The lattice mismatch between 6H-SiC and Si is totally accommodated by MDs rather than by uniform elastic strains, the DM mode is observed.
2.3. In-plane orientation of the Si/SiC heterostructure
Figure 9 shows the XRD θ-2θ scans for Si/SiC(0001) heterostructures prepared at 900°C and 1050°C, respectively. It is shown that the Si film is  oriented when the Si layer is deposited at the lower temperatures of 900°C, as the growth temperature increase to 1050°C, the Si layer is mainly  oriented, which agrees with the SAED characterizations.
The in-plane orientation at the hetero-interface was carefully examined using X-ray phi(φ) scan. For investigating the [11-2] orientation in Si(111) plane, the out-of-plane orientation  must be confirmed, as demonstrated in Figure 10. Figure 11(a) shows XRD 360° φ scans of the Si(110) (χ = 35.27°) reflections of Si(111)/6H-SiC(0001) heterostructure grown at 900°C. Moreover, for investigating [10-10] orientation in 6H-SiC(0001) plane, the φ scans from the 6H-SiC(10-11) (χ = 80°) reflections are also characterized. Narrow and intense peaks with six-fold symmetry are observed. On the basis of the Si(110) and 6H-SiC(10-11) reflections shown in Figure 11(a), it can be concluded that a FCC-on-HCP parallel epitaxy is achieved at 900°C and the in-plane orientation relationship is (111)[1-10]Si//(0001)[1-210]6H-SiC. The in-plane orientation of the Si(110)/6H-SiC(0001) heterostructure grown at 1050°C is also characterized, as shown in Figure 11(b). For investigating the  orientation in Si(110) plane, the out-of-plane orientation  is confirmed firstly. The six-fold symmetry is also observed. It is confirmed that the in-plane orientation relationship is (110)Si//(0001)[10-10]6H-SiC.
By means of the in-plane orientation characterizations, the 3D Si/SiC(0001) hetero-interface structures with different orientations are confirmed and schematically shown in Figure 12. The Si(111) layers grow epitaxially on 6H-SiC(0001) with an in-plane orientation relationship of Si[11-2]//SiC[10-10], as shown in Figure 12(a). As mentioned above, the Si(111)/SiC(0001) heterostructure follows DM mode, the epitaxial growth is described by 4 (111) interatomic distances of Si matching with 5 (0001) interatomic distances of 6H-SiC, which releases most of the lattice-mismatch strain. The 4:5 matching generates edge-MD array at the Si/6H-SiC interface , and the MD density can be calculated as 4.87 × 1013 cm−2 according to the model shown in Figure 12(a), which is much smaller than the theoretical value (4.34 × 1014 cm−2). However, the domain size
|Growth orientation||Si-to-SiC DM mode||Residual mismatch||MD density theoretical value||MD density DM mode|
|4.34 × 1014 cm−2||0.487 × 1014 cm−2|
|Si[-110] SiC[1-210]||Si[-110] SiC[1-210]||2.57 × 1014 cm−2||1.217 × 1014 cm−2|
In this chapter, Si/SiC heterostructures with different orientations were prepared on 6H-SiC(0001) by LPCVD. The heterostructure of large lattice-mismatch grows by DM mode, which releases most of the lattice-mismatch strain, and the coherent Si epilayers can be grown on 6H-SiC. Si(111)/6H-SiC(0001) heterostructure obtained at 900°C has an in-plane orientation relationship of (111)[1-10]Si//(0001)[1-210]6H-SiC. The Si(111)/6H-SiC(0001) interface has the 4:5 Si-to-SiC matching mode with a residual lattice-mismatch of 0.26% along both the Si[11-2] and Si[1-10] orientations. As the growth temperature increases to 1050°C, the preferential orientation of the Si film transitions to . SAED patterns show that the in-plane orientation relationship is (110)Si//(0001)[10-10]6H-SiC. Along Si[-110] orientation, the Si-to-SiC matching is still 4:5; along the vertical orientation Si, the matching mode is approximate 1:2 and the residual mismatch is 1.84% correspondingly. The atom quantity in one DM period decreases with increasing residual mismatch and vice versa. The Si film epitaxially grows but with MDs at the Si/6H-SiC interface. The MD density of the Si(111)/6H-SiC(0001) and Si(110)/6H-SiC(0001) obtained by experimental observations is as low as 0.487 and 1.217 × 1014 cm−2, respectively, which is much smaller than the theoretical value.
This work was supported financially by the National Natural Science Foundation of China (Grant Nos. 51402230, 51177143), the Project Supported by Natural Science Basic Research Plan in Shaanxi Province of China (Grant Nos. 2015JM6282) and the China Postdoctoral Science Foundation (Grant No. 2013M532072).
Conflict of interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work.