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

$$
\varepsilon\_x = \varepsilon\_y = \left(\mathbf{a}\_{\text{sic}(0001)} - \mathbf{a}\_{\text{Si(111)}}\right) / \mathbf{a}\_{\text{Si(111)}} \tag{1}
$$

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-Al2O3 [23], Sc2O3/GaN [24], GaAs/Si [25]

Figure 1. Schematic illustration of mechanisms for accommodation of lattice mismatch strain in large mismatch systems

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The low magnification cross-sectional transmission electron microscopy (TEM) bright-field image of the Si thin film grown on 6H-SiC(0001) at 900C 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 900C. 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

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

and InxGa1-xN/GaN [26].

with DM mode.

diffraction contrast study.

2.2. Interface micro-structure of the Si/SiC heterostructure

where ɑsic(0001) and ɑSi(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 [22–26]. 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 nɑsic of the SiC substrate does not match perfectly with mɑSi of the Si film and thus a residual domain mismatch strain is present in the film in the x direction, given by

$$
\varepsilon\_x = (m\mathfrak{q}\_{\text{sic x}} - m\mathfrak{q}\_{\text{Si x}}) / n\mathfrak{q}\_{\text{Si x}} \tag{2}
$$

Similarly, the residual strain

$$
\varepsilon\_y = \left( p \mathbf{a}\_{\rm sic} \, y - q \mathbf{a}\_{\rm Si \, y} \right) / q \mathbf{a}\_{\rm Si \, y} \tag{3}
$$

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 Growth Mode and Characterization of Si/SiC Heterostructure of Large Lattice-Mismatch http://dx.doi.org/10.5772/intechopen.74935 69

Figure 1. Schematic illustration of mechanisms for accommodation of lattice mismatch strain in large mismatch systems with DM mode.

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-Al2O3 [23], Sc2O3/GaN [24], GaAs/Si [25] and InxGa1-xN/GaN [26].

#### 2.2. Interface micro-structure of the Si/SiC heterostructure

temperature [8–12]. The interface-structure of the heterostructure determines some important parameters such as the preferential orientation [13, 14], the interface state density [15–17] 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

2. Growth mode, interface micro-structure and in-plane orientation of the

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

where ɑsic(0001) and ɑSi(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 [22–26]. 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 nɑsic of the SiC substrate does not match perfectly with mɑSi of the Si film and thus a residual

=ɑSi 111 ð Þ (1)

εx ¼ ð Þ mɑsic x � nɑSi x =nɑSi x (2)

=qɑSi y (3)

2.1. Growth mode of the heterostructures of large lattice-mismatch

mismatch of the Si(111)/6H-SiC(0001) is as large as 19.8%, which is given by

domain mismatch strain is present in the film in the x direction, given by

εy ¼ pɑsic y � qɑSi 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

εx ¼ εy ¼ ɑsic 0001 ð Þ � ɑSi 111 ð Þ

heterostructure is rarely reported.

Si/SiC heterostructure

68 Heterojunctions and Nanostructures

Similarly, the residual strain

The low magnification cross-sectional transmission electron microscopy (TEM) bright-field image of the Si thin film grown on 6H-SiC(0001) at 900C 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 900C. 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

Figure 2. Cross-sectional low magnification TEM image and the SAED patterns of Si films grown on 6H-SiC (0001) at 900C. (a) TEM image, (b) SAED patterns corresponding to Si[-110]SiC[-12-10] zone axes.

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.

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 εx in the x direction

Figure 3. A HRTEM image of the Si/6H-SiC heterojunction grown at 900C (a), a magnified image (b), and the Fourier-

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XRD data, shown elsewhere [9], indicates that the Si phase with [110] orientation appears when the temperature increases higher than 1000C, 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 1050C. 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)[001]Si//(0001)[10-10]6H-SiC, confirming the epitaxial growth of the Si films with [110] 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

and εy in the y direction are the same as 0.26%.

filtered HRTEM images (c)–(e).

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%, Growth Mode and Characterization of Si/SiC Heterostructure of Large Lattice-Mismatch http://dx.doi.org/10.5772/intechopen.74935 71

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

Figure 2. Cross-sectional low magnification TEM image and the SAED patterns of Si films grown on 6H-SiC (0001) at

900C. (a) TEM image, (b) SAED patterns corresponding to Si[-110]SiC[-12-10] zone axes.

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%,

coincident at the Si/6H-SiC interface.

70 Heterojunctions and Nanostructures

Figure 3. A HRTEM image of the Si/6H-SiC heterojunction grown at 900C (a), a magnified image (b), and the Fourierfiltered HRTEM images (c)–(e).

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 εx in the x direction and εy 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 1000C, 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 1050C. 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)[001]Si//(0001)[10-10]6H-SiC, confirming the epitaxial growth of the Si films with [110] 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

Figure 4. Schematic diagrams of the of Si(111)/6H-SiC(0001) heterojunction. (a) In-plane orientation, (b) atomic structure at the hetero-interface. The insets show the atomic structures of the Si(111) and 6H-SiC(0001) planes.

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 Si[001]SiC[10-10] 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 0.55%. 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

Figure 6. A HRTEM image of the Si/6H-SiC heterojunction grown at 1050C (a), the Fourier-filtered HRTEM images (b). The insets are the magnified images of region 1 and 2, which show the atomic position of the Si/6H-SiC interface. The atomic positions of Si are slightly different in two regions. It suggests that the Si-to-SiC matching mode at the interface is

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that the Si-to-SiC matching at the interface is merely approximate 1:2.

long-periodic and is merely approximate 1:2.

Figure 5. Low magnification cross-sectional TEM image and the SAED patterns of Si films grown on 6H-SiC(0001) at 1050C. (a) TEM image, (b) SAED patterns of the Si/SiC interface corresponding to Si[-110]SiC[1-210] zone axes.

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Figure 6. A HRTEM image of the Si/6H-SiC heterojunction grown at 1050C (a), the Fourier-filtered HRTEM images (b). The insets are the magnified images of region 1 and 2, which show the atomic position of the Si/6H-SiC interface. The atomic positions of Si are slightly different in two regions. It suggests that the Si-to-SiC matching mode at the interface is long-periodic and is merely approximate 1:2.

Figure 4. Schematic diagrams of the of Si(111)/6H-SiC(0001) heterojunction. (a) In-plane orientation, (b) atomic structure

Figure 5. Low magnification cross-sectional TEM image and the SAED patterns of Si films grown on 6H-SiC(0001) at 1050C. (a) TEM image, (b) SAED patterns of the Si/SiC interface corresponding to Si[-110]SiC[1-210] zone axes.

at the hetero-interface. The insets show the atomic structures of the Si(111) and 6H-SiC(0001) planes.

72 Heterojunctions and Nanostructures

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 Si[001]SiC[10-10] 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 0.55%. 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.

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 [110] 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 1h i � 210 SiC=3 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[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[10-10] and SiC[1-210] respectively, as shown in Figure 8(a). Along Si[001]SiC[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

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Figure 8. Schematic diagrams of the of Si(110)/SiC(0001) heterojunction. In-plane orientation (a), atomic structure at the interface along Si[001]SiC[10-10] (b) and Si[-110]SiC[1-210] (c). The insets show the atomic structures of the Si(110) and

identical with the Si(111)/6H-SiC(0001) heterostructure.

6H-SiC(0001) planes.

Figure 7. HRTEM images and the SAED patterns of Si(110)/6H-SiC(0001) interface. HRTEM image of the Si(110)/6H-SiC(0001) interface (a), SAED patterns (b), the Fourier-filtered HRTEM images of region 1 (c) and region 2 (d). The SAED patterns at the Si/6H-SiC interface corresponding to Si[00-1]SiC[-1010] zone axes.

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[10-10] and SiC[1-210] respectively, as shown in Figure 8(a). Along Si[001]SiC[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.

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 [110] 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 1h i � 210 SiC=3 parallel to the interface,

Figure 7. HRTEM images and the SAED patterns of Si(110)/6H-SiC(0001) interface. HRTEM image of the Si(110)/6H-SiC(0001) interface (a), SAED patterns (b), the Fourier-filtered HRTEM images of region 1 (c) and region 2 (d). The SAED

patterns at the Si/6H-SiC interface corresponding to Si[00-1]SiC[-1010] zone axes.

which are labeled in Figure 7(d).

74 Heterojunctions and Nanostructures

Figure 8. Schematic diagrams of the of Si(110)/SiC(0001) heterojunction. In-plane orientation (a), atomic structure at the interface along Si[001]SiC[10-10] (b) and Si[-110]SiC[1-210] (c). The insets show the atomic structures of the Si(110) and 6H-SiC(0001) planes.

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 900C and 1050C, respectively. It is shown that the Si film is [111] oriented when the Si layer is deposited at the lower temperatures of 900C, as the growth temperature increase to 1050C, 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 [11-2] orientation in Si(111) plane, the out-of-plane orientation [110] 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 900C. 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 900C 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 1050C is also characterized, as shown in Figure 11(b). For investigating the [001] orientation in Si(110) plane, the out-of-plane orientation [111] is confirmed firstly. The six-fold symmetry is also observed. It is confirmed that the in-plane orientation relationship is (110)[001]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 [13], and the MD density can be calculated as 4.87 1013 cm2 according to the model

shown in Figure 12(a), which is much smaller than the theoretical value (4.34 1014 cm2

still smaller than the theoretical value (2.57 1014 cm2

(Table 1).

However, the domain size nɑsic of the SiC substrate (n = 5) does not match perfectly with mɑSi 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 [1-210], and the crystal structure model is schematically shown in Figure 12(b). Along 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 identical with the Si(111)/6H-SiC(0001) heterostructure. Along the vertical Si[001]SiC[10-10] orientations, the distinct 1:2 matching is observed and thus MDs are still present at the Si(110)/6H-SiC(0001) interface. The MD density increases to 1.217 1014 cm2 correspondingly, which is

Figure 11. XRD φ scans of the Si/6H-SiC heterostructures, (a) Si(110) (χ = 35.27) reflections of Si(111)/6H-SiC(0001) heterostructure grown at 900C, (b) Si(111) (χ = 35.27) reflections of Si(110)/6H-SiC(0001) heterostructure grown at

1050C. The φ scans from the 6H-SiC(10-11) (χ = 80) reflections is also shown as reference at the bottom.

Figure 10. XRD φ scans schematic diagrams of the Si(111)/6H-SiC(0001) and Si(110)/6H-SiC(0001) heterostructures. For

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

http://dx.doi.org/10.5772/intechopen.74935

77

investigating the in-plane orientations, at least one out-of-plane orientation needs to be confirmed firstly.

mismatch strain ε of 1.84% with in-plane constants 5.43 Å for Si and 5.33 Å for 6H-SiC

).

). The heterostructure has a residual

Figure 9. X-ray θ-2θ scans for Si/SiC(0001) heterostructures with the Si layer grown at (a) 900C and (b) 1050C.

Growth Mode and Characterization of Si/SiC Heterostructure of Large Lattice-Mismatch http://dx.doi.org/10.5772/intechopen.74935 77

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

Figure 9 shows the XRD θ-2θ scans for Si/SiC(0001) heterostructures prepared at 900C and 1050C, respectively. It is shown that the Si film is [111] oriented when the Si layer is deposited at the lower temperatures of 900C, as the growth temperature increase to 1050C, the Si layer

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 [110] 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 900C. 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 900C 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 1050C is also characterized, as shown in Figure 11(b). For investigating the [001] orientation in Si(110) plane, the out-of-plane orientation [111] is confirmed firstly. The six-fold symmetry is also observed. It is confirmed

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 [13], and the MD density can be calculated as 4.87 1013 cm2 according to the model

Figure 9. X-ray θ-2θ scans for Si/SiC(0001) heterostructures with the Si layer grown at (a) 900C and (b) 1050C.

uniform elastic strains, the DM mode is observed.

76 Heterojunctions and Nanostructures

2.3. In-plane orientation of the Si/SiC heterostructure

is mainly [110] oriented, which agrees with the SAED characterizations.

that the in-plane orientation relationship is (110)[001]Si//(0001)[10-10]6H-SiC.

Figure 10. XRD φ scans schematic diagrams of the Si(111)/6H-SiC(0001) and Si(110)/6H-SiC(0001) heterostructures. For investigating the in-plane orientations, at least one out-of-plane orientation needs to be confirmed firstly.

Figure 11. XRD φ scans of the Si/6H-SiC heterostructures, (a) Si(110) (χ = 35.27) reflections of Si(111)/6H-SiC(0001) heterostructure grown at 900C, (b) Si(111) (χ = 35.27) reflections of Si(110)/6H-SiC(0001) heterostructure grown at 1050C. The φ scans from the 6H-SiC(10-11) (χ = 80) reflections is also shown as reference at the bottom.

shown in Figure 12(a), which is much smaller than the theoretical value (4.34 1014 cm2 ). However, the domain size nɑsic of the SiC substrate (n = 5) does not match perfectly with mɑSi 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 [1-210], and the crystal structure model is schematically shown in Figure 12(b). Along 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 identical with the Si(111)/6H-SiC(0001) heterostructure. Along the vertical Si[001]SiC[10-10] orientations, the distinct 1:2 matching is observed and thus MDs are still present at the Si(110)/6H-SiC(0001) interface. The MD density increases to 1.217 1014 cm2 correspondingly, which is still smaller than the theoretical value (2.57 1014 cm2 ). The heterostructure has a residual mismatch strain ε of 1.84% with in-plane constants 5.43 Å for Si and 5.33 Å for 6H-SiC (Table 1).

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 900C 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 1050C, 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 1014 cm2

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

http://dx.doi.org/10.5772/intechopen.74935

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

We declare that we have no financial and personal relationships with other people or organi-

2 Department of Electronic Engineering, Xi'an University of Technology, Xi'an, China

[1] Seely JF, Kjornrattanawanich B, Holland GE, Korde R. Response of a SiC photodiode to extreme ultraviolet through visible radiation. Optics Letters. 2005;30:3120-3122. DOI:

respectively, which is much smaller than the theoretical value.

Science Foundation (Grant No. 2013M532072).

zations that can inappropriately influence our work.

\*Address all correspondence to<EMAIL_ADDRESS>
1 School of Science, Xi'an Polytechnic University, Xi'an, China

Acknowledgements

Conflict of interest

Author details

Lianbi Li1,2\*

References

10.1364/OL.30.003120

,

79

Figure 12. Schematic diagrams of the Si/6H-SiC(0001) heterojunction. Si/6H-SiC heterostructure with the [111] preferential orientation (a), Si/6H-SiC heterostructure with the [110] preferential orientation (b). As the preferential orientations are [111] and [110], the in-plane orientations are Si[01-1]//SiC[11-20] and Si[001]//SiC[10-10], respectively.

The lattice-mismatch of the Si/6H-SiC heterostructure calculated with the domain matching model.

Table 1. Basic semiconductor properties of the Si/6H-SiC interface.


**7. Conclusions**

In this research, the acetylene hydrogenation over Pd-Ag supported α-Al2O3 was investigated in a differential reactor. The full factorial design method, based on the cubic pattern was used to determine the experiments, considering hydrogen to acetylene ratio, temperature, gas hourly space velocity, and pressure as dependent variables. The fresh and spent catalysts were characterized by SEM, TEM, DTG-TGA, XRD, and BET tests. It is concluded from SEM and BET tests, that coke build-up on the external surface of the catalyst increases surface area and decrease pore mean diameter. Then, a detailed reaction network was proposed based on the Langmuir-Hinshelwood-Hougen-Watson approach, considering ethane, 1-butene, and 1,3-butadine as side products. The coefficients of the proposed kinetic model were calculated, based on experimental data. In addition, the industrial Tail-End hydrogenation reactors were modelled, and a decay model was proposed to predict catalyst activity. The results showed that applying hydrogen rich stream increases 1-butene concentration in the reactor. Based on the simulation results the acetylene molar flow rate in the outlet stream from Guard bed increases during the process run-time and approaches from 0.19 to 0.21 mol s−1 .

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2227-9717/7/3/136/s1.

**Author Contributions:** O.D.: Conceived and designed the analysis, Collected the data, Contributed data or analysis tools, Performed the analysis, Wrote the paper; M.R.R.: Conceived and designed the analysis, Contributed data or analysis tools, Performed the analysis, Wrote the paper; A.S.: Conceived and designed the analysis, Performed the analysis, Wrote the paper.

**Conflicts of Interest:** The authors confirm that there are no known conflict of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.


**Chapter 8**

*Sarcoidosis and Granulomatosis - Diagnosis and Management*

[31] Vaidya VR, Abudan AA,

[32] Yazaki Y, Isobe M, Hiroe M, Morimoto S, Hiramitsu S, Nakano T, et al. Prognostic determinants of longterm survival in Japanese patients with cardiac sarcoidosis treated with prednisone. The American Journal of Cardiology. 2001;**88**(9):1006-1010

[33] Tokuda M, Tedrow UB, Kojodjojo P, Inada K, Koplan BA, Michaud GF, et al.

Catheter ablation of ventricular tachycardia in nonischemic heart disease. Circulation. Arrhythmia and Electrophysiology. 2012;**5**(5):992-1000

Vasudevan K, Shantha G, Cooper LT, Kapa S, et al. The efficacy and safety of electroanatomic mapping-guided endomyocardial biopsy: A systematic review. Journal of Interventional Cardiac Electrophysiology. 2018

**122**


**2. Urinary tract obstruction**

The peak incidence of urinary tract obstruction in males is in the eighth and ninth decades secondary to benign prostatic hyperplasia and prostatic carcinoma, whereas the peak incidence in females is in the fourth to six decades secondary to pregnancy and carcinoma of the cervix or uterus. (Gulmi et al., 1998)


*5.1. Volatility Measurement*

We first calculated the GARCH-based volatility for the exchange rate in terms of the monthly data and then converted it to annual data. The results and diagnostic tests for estimated the GARCH model are presented in Table A3 in the Appendix. Figure 3 depicts, on average, the yearly volatility of the bilateral exchange rate between Vietnam and 26 of its export partners in three different regions. The volatility fluctuates within a range of around 8%, the only exception being Italy. The magnitude of the fluctuation tends to be higher during the global crisis.

**Figure 3.** Exchange rate volatility over the 2000–2015 period.

### *5.2. Effects of Exchange Rate Volatility on Exports*

This section presents the estimation results pertaining to the effects of exchange rate volatility on manufacturing exports for the whole sample and in three regions—Asia, Europe, and America. We first show the result for the manufacturing sector, followed by its 10 subsectors.

Regarding the manufacturing sector, Tables 3 and 4 show the results of three kinds of panel unit root tests—IPS (Im et al. 2003), Maddala and Wu (1999), and Choi (2001)—for the whole sample and in three regions, respectively. At the manufacturing sector level, strong evidence of unit roots is found for the foreign GDP and real bilateral exchange rate, while, for the difference, the hypothesis that unit roots are present is strongly rejected. The variable for GARCH-based volatility is stationary for the whole sample and for Asia and America, but it contains unit roots in Europe. A similar pattern is found for manufacturing exports. Thus, the variables of interest are a mixed integration of I(0) and I(1).

Numbers indicate the *p*-values. A maximum of 2 lags were included.

### **Table 4.** The unit root test for three regions.

Numbers indicate the *p*-values. A maximum of 2 lags were included.

The cointegration tests introduced by Pedroni (1999, 2001) were performed to determine the long-run relationship between manufacturing exports and the other variables of interest. Table 5 displays the results for the whole sample and the three regions. Three statistics—group augmented Dickey-Fuller (ADF) test, panel ADF test, and group rho test—strongly support the hypothesis of cointegration. It is worth noting that the group small *t*-test and the group ADF test have a more powerful feature compared with other types of panel statistics, while the panel variance test and group rho test seem to perform poorly. Thus, based on this feature, we conclude that there is a long-run association among the given variables. An exception is the case for countries in the American region, as none of the calculated statistics are significant.

**Table 5.** The cointegration test for the full sample and three regions.

\*\*, \*\*\* indicate the hypothesis of no cointegration is rejected at significance levels of 5%, and 1%.

Next, we performed the long-run estimation among the relevant variables using the panel DOLS estimation in light of the confirmation by cointegration tests. Table 6 presents the estimation result for the whole sample and in three regions—Asia, Europe, and America. Overall, the foreign income is found to be positively related to Vietnam's exports for the whole sample and in Asia and Europe, as the estimated coefficients are significant at a level of at least 10%. The positive sign illustrates that an increase in income among Vietnam' trading partners enhances the exporting performance of manufactured goods for the country. This is consistent with the trade theory that a higher income in foreign nations will lead to an increase in domestic good demands. Also, Hooy et al. (2015) asserted that Vietnam has been deeply engaged in supply chain production due to the rise of China, enhancing both economic growth and exports. According to the results of the present study, the depreciation of the Vietnam Dong is not expected to cause an adverse impact on manufacturing exports in the long run. The effect is found to be negative, although insignificant, for the three regions. Exchange rate volatility has strong reverse effects on manufacturing exports, not only for the whole sample but also in America.

**Table 6.** The panel DOLS estimation for the full sample and three regions.

"Ln" represents variables defined in terms of logarithm. Standard errors are numbers in the parentheses. \*, \*\*, and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.

We also examined the short-run relationship on the basis of the ECM model using Equation (3). The results are shown in Table 7, which paint a completely different picture of the effect of exchange rate volatility on manufacturing exports in Vietnam. The coefficients of the lagged error correction terms are negative and significant, supporting the long-run cointegration tests above. The foreign GDP and the bilateral real exchange rate have a positive association in the estimation for the whole sample and Asia. Europe has a positive significant coefficient for foreign income. The volatility of the exchange rate, on average, has no impact on exports in general in Asia, Europe, and America. The dummy variable representing the participation in the WTO is significant only for the case of Asia, implying that Vietnam gained a significant benefit in exporting goods to Asian countries. The global financial crisis is expected to be harmful, to some extent, to exports. The evidence of negative effects of volatility is weak, suggesting that it can be mostly insured against at low cost. Meanwhile, the price mechanism works via the real exchange rate to ensure that export supply equals demand. These findings imply that the manufacturing exports in Vietnam rely heavily on the partner's income and largely benefit from the depreciation of the Vietnam Dong.

**Table 7.** The panel OLS estimation for the full sample and three regions.

Δ represents variables defined in terms of difference, indicating growth rate. "Ln" represents variables defined in terms of logarithm. Standard errors are numbers in the parentheses. \*, \*\*, and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.

When it comes to manufacturing exports at disaggregated levels, we applied the same econometrics procedures to all 10 subsectors for the whole sample and for the three regions. Before the panel DOLS and the panel OLS estimations were run, unit root tests for stationarity and panel cointegration tests were conducted.2

The long-run effects of exchange rate volatility on each subsector for the whole sample and in the three regions are depicted in Tables 8 and 9, respectively. Based on all of the data in Table 8, 8 out of 10 manufacturing subsectors suffered adverse effects due to exchange rate volatility. The coefficients of

2 For minimizing space, the estimated results will be provided upon request. The findings for the 10 subsectors are similar to those for the manufacturing sector. The long-run relationship is confirmed for all 10 subsectors.

two subsectors, namely, *textiles, wearing apparel, leather, and related* products and *chemicals, rubber, plastics, and fuel products*, are statistically insignificant although negative. The effect of the bilateral real exchange rate is also found to be negative in five subsectors in the long run. Thus, a depreciation policy in Vietnam would lead to a decline in export value in the long term as it generates volatility in the exchange rate.

When the geographical factor is taken into consideration, we observe a completely different picture of the relationship between exchange rate devaluation, exchange rate volatility, and export performance at the subsector level. As can be seen from Table 9, exchange rate volatility has almost no effect on exports in Asia and Europe, but America has five subsectors that are negatively related to the exchange rate volatility.3 It is only the subsector of *non-metallic mineral* products in the American region that enjoys a favorable gain from exchange rate depreciation without being influenced by the exchange rate fluctuation.

Using Equation (3), the short-run effects of exchange rate volatility on export performance at disaggregated levels were regressed and are provided in the next four tables with the application of panel estimation. Table 10 presents the result for the whole sample. The bilateral real exchange rate has a favorable impact on exports in such subsectors as (i) *textiles, wearing apparel, leather, and related products* and (ii) *furniture and other manufacturing products*. This implies that depreciation boosts exports in the short run rather than in the long run in some subsectors. Similarly, exchange rate volatility is found to be positively associated with exports for the subsector of *transport equipment.* Exporters in this subsector may pursue a strategy of exporting more in order to maintain its trading value, as hypothesized by De Grauwe (1988). The subsector of *chemicals, rubber, plastics, and fuel products* is found to be quite sensitive to exchange rate volatility in the short run, given that its estimated coefficient is positive in the current period but becomes negative in the first lag. We find no statistically significant effect of exchange rate depreciation and volatility on export values for the eight remaining subsectors.

A significantly different pattern is seen in Asia, Europe, and America, as indicated in Tables 11–13, respectively. The short-run effects of exchange rate volatility vary considerably across the subsectors, as well as in the given regions. Asia has three subsectors that are positively associated with exchange rate volatility, together with one that reacts negatively. In Europe, the number of subsectors experiencing favorable and harmful effects is three and two, respectively. In America, 3 out of 10 subsectors are found to be positively related to exchange rate fluctuations. Interesting to note is that the export performance in the subsector of *Transport equipment* is observed to benefit from the exchange rate fluctuations, as its estimated coefficients are statistically positive in all three regions. There is an increase in the export performance of *textiles, wearing apparel, leather, and related products* to countries in Europe and America when the exchange rate is volatile. The *Paper and printing* subsector is negatively related to the exchange rate fluctuations in Asia but positively associated in Europe. Other subsectors, such as (i) *wood and products of wood and cork,* (ii) *Chemicals, rubber, plastics, and fuel products*, (iii) *Non-metallic mineral products*, and (iv) *Furniture and other manufacturing products* are influenced by exchange rate volatility, either positively or negatively.

In the short run, the impact of the bilateral exchange rate on the export performance at disaggregated levels is considerably different across the regions. When the real bilateral exchange devaluates, it raises Vietnam's export value in three subsectors—*textiles, wearing apparel, leather, and related products*; *wood and products of wood and cork*; and *paper and printing*—to countries in Europe and America, and it has no effect on the remaining subsectors. In Europe, the estimated results indicate that two subsectors are positively influenced by exchange rate fluctuations, another two react negatively, and results for the rest of the subsectors are inconclusive. All results prove that the impact of the exchange rate on exports at the manufacturing disaggregated level depends on two factors: (i) the type of export and (ii) the exporting destination.

3 Table A4 presented in indicates the percent change in exports for the manufacturing subsectors resulting from a 1% increase in exchange rate volatility.

"Ln" represents variables defined in terms of logarithm. ex1—Food products, beverages and tobacco; ex2—Textiles, wearing apparel, leather and related products; ex3—Wood and products of wood and cork; ex4—Paper and printing; ex5—Chemicals, rubber, plastics and fuel products; ex6—Non-metallic mineral products; ex7—Basic metals and fabricated metal products; ex8—Machinery and equipment; ex9—Transport equipment; ex10—Furniture and other manufacturing. Standard errors are numbers in the parentheses. \*, \*\*, and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.

products of wood and cork; ex4—Paper and printing; ex5—Chemicals, rubber, plastics and fuel products; ex6—Non-metallic mineral products; ex7—Basic metals and fabricated metal products; ex8—Machinery and equipment; ex9—Transport equipment; ex10—Furniture and other manufacturing. Standard errors are numbers in the parentheses. \*, \*\*, and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.

fabricated metal products; ex8—Machinery and equipment; ex9—Transport equipment; ex10—Furniture and other manufacturing. Standard errors are numbers in the parentheses.

\*, \*\*, and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.

*J. Risk Financial Manag.* **2019**, *12*, 12

fabricated metal products; ex8—Machinery and equipment; ex9—Transport equipment; ex10—Furniture and other manufacturing. Standard errors are numbers in the parentheses.

\*, \*\*, and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.

fabricated metal products; ex8—Machinery and equipment; ex9—Transport equipment; ex10—Furniture and other manufacturing. Standard errors are numbers in the parentheses. \*, \*\*,

and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.

fabricated metal products; ex8—Machinery and equipment; ex9—Transport equipment; ex10—Furniture and other manufacturing. Standard errors are numbers in the parentheses. \*, \*\*,

and \*\*\* indicate the 10%, 5% and 1% significance level, respectively.


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31, pp. 3300-3309

66-67


**14. Nerve lesion**

252 Urinary Incontinence

after one year showing recurrence of a stone in the same site of the previous surgery; surgery was performed in the office since the patient refused major surgery. Siegel (2006) reported one case of urethral necrosis and a urethra-vaginal fistula in a 64 year old woman, who needed 3 surgeries; initially TVT mesh fragments were removed and adjacent tissue needed debridement. A urethroplasty was performed as a second surgical procedure and in the third

E Kobashi's (2009) reviewed the different materials and reported the rate of extrusion of

Silicone Monofilament Macro 40-71 TVT Monofilament Macro 2-3 Prolapse repair Monofilament Macro Up to 26 Obtape Monofilament Micro 5.4-16.6 Sparc/Monarc Monofilament Macro 1.7-2.4

Table 5. Synthetic mesh types, Characteristics and Associated Vaginal Extrusion rates.

the TVT is safe and technically possible and solves patient's symptoms.

objective rate than TOT, but the subjective cure is similar for both.

Miraliakbari and Tse (2011) reported the first case of ureteral erosion in a 78 year old woman, the erosion was located in the distal third of the ureter, and the patient was treated

Rouprêt et al (2010) reported resection of the mesh via laparoscopy in 38 women with bladder erosion, vaginal extrusion, bladder obstruction and groin pain. The resection was complete with an operating time of 110 minutes (50 to 240 minute range) all patients reported a decrease in symptoms in a follow up period of 37.9 months (2-80 months range). However, the incontinence recurrence rate is 65.7% (25 patients). Laparoscopic resection of

Novara et al (2010) did a systematic review of pubovaginal sling; retropubic tape (RT) and transobturator tape (TOT) the subjective cure rate is similar among those procedures. Patients who have a TOT inserted have less risk of bladder or vaginal perforation (OR: 2.5 IC: 1.75-3.57; p<0.00001); less risk of hematoma (OR: 2.62; CI: 1.35-5.08; p=0.005) and less risk of urinary tract injury (OR: 1.35; CI: 1.05-1.72; p=0.02). This meta-analysis showed similar results between TVT-O and Monarc. The use of a retropubic tape had a higher

Mendoca et al (2011) report two cases with late urethral erosion with transobturator suburethral mesh (Obtape) the first one diagnosed 1 year after the surgery and the second

With an intestinal lesion, patients refer abdominal pain, peritoneal irritation and sometimes fecal matter leak through the incisions of the kit used (TVT) as was reported by Meschia et al

one, a very late complication, occurring 4 years after the placement of the sling.

Material Fiber type Pore Size Extrusion Rate (%)

surgery a coaptation with an occlusive sling was made to repair the continence.

these. Table 5.

successfully.

**13. Intestinal lesion**

Geis and Dietl (2002) reported an ilioinguinal nerve lesion after insertion of a TVT, this due to the closeness of the nerve to the sites where the suprapubic incision was made. Rigaud et al (2010) said that pelvic or perineal pain may be a consequence of the obturator nerve or pudendal nerve lesion, a clinical sign that is underestimated.


*3.3. Contribution of the Botanical Origin to the Phenolic Content and Antioxidant Activity of Pollen*

The relationships between the botanical origin of the bee pollens and the polyphenol and flavonoid contents and the antioxidant capacity have been evaluated using a PCA. This multivariate technique reduced the dataset and revealed a six-component model with 81.47% of the variance of the data (Table 3). The first three components explained 53.40% of the data variability. The variables with higher weight in the first component were *Taraxacum officinale* type, *Plantago*, DPPH, TPC and *Castanea* (with coefficients above 0.30). In the second component, the higher coefficients corresponded to *Erica*, TFC and *Campanula* type (above 0.35), while the third component was related with the higher coefficients of TFC, DPPH, *Echium*, *Trifolium repens* type and *Rubus* (coefficients above 0.26).

N = number of pollen samples containing it. \* Significant differences according to Student's *t*-test (*p* < 0.05).

\* Significant differences according to Student's *t*-test (*p* < 0.05).

The projection of the relationships among the palynological and chemical variables on the three first components is shown in Figure 3. The graphic representation shows the close relationship of the pollen variables *Erica*, *Castanea*, *Trifolium repens* type and *Echium* with TPC and DPPH, whereas *Campanula* type and *Lythrum* are closely related to ABTS and TFC. On the contrary, *Genista* type, *Plantago* and *Taraxacum officinale* type had an inverse relationship. Therefore, the bee pollen samples with higher presence of *Erica*, *Castanea*, *Echium* and *Trifolium repens* type had higher TPC and antioxidant activity. Bee pollen with high presence of *Campanula* pollen type and *Lythrum* were characterized by higher concentration of TFC and antioxidant content.

**Table 3.** Number of components extracted and component weights for each variable included in PCA.

**Figure 3.** Plot of the first three principal components with the palynological and chemical variables obtained by PCA.

The cluster analysis was carried out with the palynological variables of greater representation in the pollen spectra, TPC, TFC and antioxidant activity (DPPH and ABTS). The results of this multifactorial analysis classified the pollen samples into three groups (Figure 4).

**Figure 4.** Dendrogram of cluster analysis with the distribution of bee pollen samples (1: group 1; 2: group 2; 3: group 3).

The first group (1) included six pollen samples with significantly higher percentages in *Erica* (20.1%), *Campanula* type (14.5%), *Lythrum* (11.5%), significantly higher in TFC (454.1 mg/100 g) in comparison to the other two groups (two and three), and DPPH (69.3%) respect to group three (21.0%) (Table 4). In group two, there is the largest number of pollen samples (22) and it was characterized by significantly higher mean percentage in the *Genista* type (12.1%), *Castanea* (28.4%) and *Rubus* (37.7%) pollen types, TPC (1741.4 mg/100 g), TFC (219.2 mg/100 g) and antioxidant activity (70.8%) compared to group three (21.0%). Finally, group three (with three bee pollen samples) included the samples with the significantly lower mean values in TPC, TFC and DPPH, but significantly higher proportions of the pollen types *Taraxacum officinale* type (50.2%) and *Plantago* (24.3%).

**Table 4.** Groups of bee pollen samples of each class obtained by the cluster analysis.

N: number of samples. Same letter shows the significant differences between groups by Bonferroni test (*p* < 0.05).