Growth Mode and Characterization of Si/SiC Heterostructure of Large Lattice-Mismatch Growth Mode and Characterization of Si/SiC Heterostructure of Large Lattice-Mismatch

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 orienta- tion, 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. 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 (cid:3) 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 (cid:3) C, the preferential orientation of the Si film transitions to [110]. SAED patterns show that the in-plane orientation relationship is (110)[001] Si //(0001)[10-10] 6H-SiC . Along Si[-110] orientation, the Si-to-SiC matching is still 4:5; along the vertical orientation Si[001], 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 (cid:4) 10 14 cm (cid:2) 2 , respectively, which is much smaller than the theoretical value.


Introduction
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 [18] 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 a Si = 5.430 Å and a SiC = 3.081 Å, respectively. The lattice mismatch of the Si(111)/6H-SiC(0001) is as large as 19.8%, which is given by where ɑ sic(0001) and ɑ Si(111) are the lattice constants of the SiC(0001) and Si(111) crystalline planes, respectively. If the lattice mismatch of the heterostructure is sufficiently low, the mismatch strain can be released by interfacial atomic relaxation of the heterostructure, and the strained-layer heterostructure with no interfacial misfit dislocations (MD) will be attained. However, the Si/SiC heterostructure has a large lattice mismatch, the epitaxial growth is still followed except that domain matching (DM) mode [21] in order to reduce the mismatch, and therefore an interfacial MD array is present at the interface that determines the domain's size [22][23][24][25][26]. A schematic illustration of mechanisms for accommodation of lattice mismatch strain in large-mismatch systems with domain epitaxial growth is shown in Figure 1. And this matching mode is applicable for the heterostructures with similar crystal symmetry on the film and the substrate. In the Si/6H-SiC system, domains consisting of m lattice constants of the Si film match with n of the SiC substrate. During domain matching system, the domain size nɑ sic of the SiC substrate does not match perfectly with mɑ Si of the Si film and thus a residual domain mismatch strain is present in the film in the x direction, given by Similarly, the residual strain ε y ¼ pɑ sic y À qɑ Si y =qɑ Si y where p and q are integers, is present in the y direction. The lattice-mismatch of the Si/6H-SiC heterostructure calculated with the DM mode are very small, the coherent Si epilayer can be grown on 6H-SiC. This type of edge misfit dislocation is also observed in other heterostructure of large lattice-mismatch, such as TiN/Si [23], ZnO/α-Al 2 O 3 [23], Sc 2 O 3 /GaN [24], GaAs/Si [25] and In x Ga 1-x N/GaN [26].

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.  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 ε x in the x direction and ε y in the y direction are the same as 0.26%.
XRD data, shown elsewhere [9], indicates that the Si phase with [110] orientation appears when the temperature increases higher than 1000 C, which is confirmed by the TEM characterizations. Figure 5 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   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 Si[001]SiC  orientation. If the Si-to-SiC matching mode is not 1:2 but a longperiod structure of 53:54, the lattice mismatch can decrease to À0.55%. Of course the longperiod 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.   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[001] 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  and SiC  respectively, as shown in Figure 8(a). Along Si[001]SiC  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  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 , 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 [111] 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 [110] 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  orientation in Si(111) plane, the out-of-plane orientation [110] must be confirmed, as demonstrated in Figure 10. Figure 11 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 //SiC , 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 [13], and the MD density can be calculated as 4.87 Â 10 13 cm À2 according to the model shown in Figure 12(a), which is much smaller than the theoretical value (4.34 Â 10 14 cm À2 ). However, the domain size nɑ sic of the SiC substrate (n = 5) does not match perfectly with mɑ Si of the Si film (m = 4), and thus a residual domain mismatch strain ε, given by Eq. (2) is present in the film. The residual mismatch strain ε of the Si(111)/6H-SiC(0001) heterostructure calculated with the DM mode is 0.26%, which is much smaller than conventional LM mode (19.8%). The Si(110) layers epitaxial grow on 6H-SiC(0001) with an in-plane orientation of Si[-110]//SiC  , and the crystal structure model is schematically shown in Figure 12