4. Terraces at Ohmic contact in SiC electronics

Combining imaging with atomistic simulations, we determine the atomic-scale structures of terraces in between SiC and its contacts and relate the structures to their electronic states and bonding nature, aimed at revealing the impact of the terraces on the contacts of SiC electronics. The terraces were first characterized using the high-resolution TEM (HRTEM) and scanning TEM (STEM), upon which the first-principles calculations were performed. The combined study allows a deeper understanding of the role played by terraces in the ohmic contact formation on a quantum level. The terraces are structurally epitaxial, coherent and atomically ordered, and theoretically predicted to have electronic states at Fermi level (*E* F) regardless of their dimension.

#### 4.1. Atomic structures of terraces

Figure 12 shows a typical cross-sectional HRTEM image of the contact of Ti3SiC2 to SiC which includes terraces of various dimensions. Well arranged (000*l*)-oriented lattice fringes can be observed in both the SiC and Ti3SiC2 layers. The points at which the phase contrast is no longer periodic define the interfacial area, as indicated by a zigzag line. Evidently, the SiC substrate is covered entirely by the Ti3SiC2 layers even in the terrace region (arrows), meaning a direct contact of Ti3SiC2 to SiC at the terraces at atomic scale. The terraces are 0+)%((5z.1,0z\* z+. !.! \_z/\$+3%\*#z\*+z)+.,\$+1/z(5!./\_z/!+\* .5w,\$/!z(5!./\_z+\*¥ taminants, or transition areas. A small number of misfit dislocations are identified at the contact region (not shown here) due to the small lattice mismatch of SiC to Ti3SiC2 (less than 0.5%). Further analyses of selected area diffraction patterns reveal that the Ti3SiC2 layers have epitaxial orientation relationships, (0001)Ti3SiC2//(0001)SiC and [0-110]Ti3SiC2// [0-110]SiC, with the SiC substrate.

Figure 12. A cross-sectional HRTEM image of the contact of the formed Ti3SiC2 to the 4H-SiC substrate viewed from the [11-208<AJ=;LAGF 0@= LOGE9L=JA9DK 9J=<=E9J;9L=<:Q 9RA?R9?DAF= 0=JJ9;=KOAL@N9JQAF?<AE=FKAGFK 9J=G:s

Combining imaging with atomistic simulations, we determine the atomic-scale structures of terraces in between SiC and its contacts and relate the structures to their electronic states and bonding nature, aimed at revealing the impact of the terraces on the contacts of SiC electronics. The terraces were first characterized using the high-resolution TEM (HRTEM) and scanning TEM (STEM), upon which the first-principles calculations were performed. The combined study allows a deeper understanding of the role played by terraces in the ohmic contact formation on a quantum level. The terraces are structurally epitaxial, coherent and atomically ordered, and theoretically predicted to have electronic states at Fermi level

Figure 12 shows a typical cross-sectional HRTEM image of the contact of Ti3SiC2 to SiC which includes terraces of various dimensions. Well arranged (000*l*)-oriented lattice fringes can be observed in both the SiC and Ti3SiC2 layers. The points at which the phase contrast is no longer periodic define the interfacial area, as indicated by a zigzag line. Evidently, the SiC substrate is covered entirely by the Ti3SiC2 layers even in the terrace region (arrows), meaning a direct contact of Ti3SiC2 to SiC at the terraces at atomic scale. The terraces are 0+)%((5z.1,0z\* z+. !.! \_z/\$+3%\*#z\*+z)+.,\$+1/z(5!./\_z/!+\* .5w,\$/!z(5!./\_z+\*¥ taminants, or transition areas. A small number of misfit dislocations are identified at the contact region (not shown here) due to the small lattice mismatch of SiC to Ti3SiC2 (less than 0.5%). Further analyses of selected area diffraction patterns reveal that the Ti3SiC2 layers have epitaxial orientation relationships, (0001)Ti3SiC2//(0001)SiC and [0-110]Ti3SiC2//

served, as indicated by arrows [20].

164 Physics and Technology of Silicon Carbide Devices

(*E* F) regardless of their dimension.

4.1. Atomic structures of terraces

[0-110]SiC, with the SiC substrate.

4. Terraces at Ohmic contact in SiC electronics

Figure 13. a) A typical HAADF-STEM image of a small terrace observed from the [11-208<AJ=;LAGF0@=L=JJ9;=AKAF<As cated by a zigzag line. Bigger dotted circles denote Ti and the smaller ones Si. (b) The same image as in (a) but has been filtered to reduce noise [20].

Figure 14. A typical HAADF image of an intermediate terrace observed from the [11-208<AJ=;LAGF0@=L=JJ9;=AKAF<As cated by a zigzag line. (b) The same image as in (a) but has been filtered to reduce noise [20].

In general, this contact region contains terraces with a wide variety of dimensions that can be affected by numerous factors. However, to develop an understanding of such a complex contact, it is important to first focus on representative terraces. Here, we choose purposely three species of terraces based on the dimension: small, intermediate, and large terrace. The corresponding HAADF images are presented in Figs. 13–15, which confirm the atomically abrupt and ordered terraces. Moreover, heteroepitaxy is retained between the SiC and Ti3SiC2 "+.z!\$z0!..!^z%\*!z0\$!z%\*0!\*/%05z+"z\*z0+)%z+(1)\*z%\*z0\$!zz%)#!z%/z,.+¥ portional to *Z* 1.7 (*Z*: atomic number) [24f\_z.%#\$0!.z/,+0/z%\*z0\$!z%)#!/z.!,.!/!\*0z0+)%z+(¥ umns of Ti while darker ones are Si. In view of the atomic arrangements in the SiC and Ti3SiC2 bulks, we define the terrace by a dashed line and identify the atoms in a few layers (+/!z0+z0\$!z(%\*!z/z%\*z0\$!z0!..!z.!#%+\*z\$!.!"0!.^z+.z0\$!z/)((z0!..!\_z0\$!z%)#!z\*z!z%\*¥ tuitively interpreted as a bonding of Ti (Si) in Ti3SiC2 to Si in SiC (Fig. 13(b)). However, this is not the case for the intermediate terrace showing Si-Si bonding at the hollow site (Fig. 14(b)). Further interpretation of the HAADF mage of large terrace (Fig. 15(b)) reveals a Si-Si bonding as well. Not surprisingly, atomic columns of carbon are not scattered strongly enough to be visualized due to its small *Z*\_z.!\* !.%\*#z0\$!z%)#!z%\*+),(!0!^z1.0\$!.z+),(!¥ menting of these images so as to relate their atomic structures to their electronic properties requires a combination of imaging with the first-principles modeling.

However, these models are constructed upon a careful consideration of the space filling and (+(z\$!)%(z!\*2%.+\*)!\*0\_z\* z%),+.0\*0(5z 0\$!5z!4\$%%0z 0\$!z 05,%(z2.%0%+\*/z+"z 0\$!z+\*¥ tact with terraces, which should be useful as an initial stage to look at the electronic states of terraces. It is also worthy of mentioning that neither the total nor the interface energies could be applicable to justify the models as an interpretation of the HAADF images because c%dz0\$!z0!..!z)+ !(/z\$2!zz %""!.!\*0z\*1)!.z+"z0+)/\_zc%%dz%0z%/z1\*(%'!(5z0+z(1(0!z0\$!z!4¥ act total terrace area, and (iii) there are numerous candidates for each type of terrace. In the optimized model of small terrace (Fig. 16(a)), one can notice a bonding of Ti (Si) atoms in

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Figure 16. 0@=J=D9P=<9LGEA;EG<=DKG>L@=L@J==J=HJ=K=FL9LAN=L=JJ9;=KOAL@<A>>=J=FL<AE=FKAGFK9KE9DD

termediate, and (c) large terrace. The representative atoms surrounding terrace are labeled for the atom-projected

Figure 17 shows PDOS of several representative atoms at the small terrace (labeled in Fig. 17(a)), where a remarkable difference is seen between the atoms near and away from terrace. A key feature of this figure is that strong hybridization takes place between the Ti *d* (T1~T6) and Si *p* (S1~S6) levels below *E* <sup>F</sup>\_z3\$%\$z+\*0%\*1!/z3!((z%\*0+z0\$!z%z.!#%+\*\_z%\* 1%\*#zz,.+¥ nounced gap state at *E* F for the C atoms near the terrace (C1~C4, C6). However, such a gap state at *E* F is vanished completely for the C deeper into the SiC (C5 in Fig. 17). Apart from the C, the Si atoms in SiC near terrace (S1~S3) also exhibit a weak yet visible peak at *E* F that are totally absent in the bulk (S5 in Fig. 17). The presence of gap states can be attributed to the overlap of the Ti *d* with Si *p* levels. A notable hybridization is seen as well between the Si *sp* (S1~S4) and C *sp* (C1~C4, C6) levels, indicating the formation of covalency near terrace. Apart from the SiC, all of the atoms in Ti3SiC2 around terrace (C6, C7, S4, T1~T5) also exhibit a notable peak at *E* F owing to the great degree of overlap between the Ti *d* and C (Si) *p* levels. Moreover, overall feature of PDOS for the Ti atoms around terrace (T1~T4) differs from that in the Ti3SiC2 1('zcId\_z%\* %0%\*#z0\$0z0\$!z0!..!z\*z\$2!z\*z%),0z+\*z!(!0.+\*¥

:AFs

Ti3SiC2 to Si atoms in SiC at the hollow site.

density of states analysis [20].

ic states of Ti as well.

Figure 15. A typical HAADF image of a large terrace observed from the [11-20] direction. The terrace is indicated by a zigzag line. (b) The same image as in (a) but has been filtered to reduce noise [20].

#### 4.2. Electronic states and bonding nature of the terraces

+z#%\*z%\*/%#\$0z%\*0+z!(!0.+\*%z,.+,!.0%!/z+"z0\$!z0!..!/z\* z0\$!z.+(!z0\$!5z,(5! z%\*z0\$!z+\$)¥ ic contact formation, we perform first-principles calculations on the three typical terraces. Upon a consideration of bulk structures of SiC and Ti3SiC2, the aforementioned orientation relations, and relative stacking sequence near terraces shown in the HAADF (Figs. 13–15), 0+)%z)+ !(/z+"z0\$!z0\$.!!z.!,.!/!\*00%2!z0!..!/z3!.!z!/0(%/\$! zc%#^zDId\_z0'%\*#z%\*0+z¥ count full structural relaxation. It should be noted that these models may not exactly reflect the real terraces because it is the extreme difficult to interpret directly the HAADF images owing to the intricate atomic arrangements around the terraces and to the invisible C atoms. However, these models are constructed upon a careful consideration of the space filling and (+(z\$!)%(z!\*2%.+\*)!\*0\_z\* z%),+.0\*0(5z 0\$!5z!4\$%%0z 0\$!z 05,%(z2.%0%+\*/z+"z 0\$!z+\*¥ tact with terraces, which should be useful as an initial stage to look at the electronic states of terraces. It is also worthy of mentioning that neither the total nor the interface energies could be applicable to justify the models as an interpretation of the HAADF images because c%dz0\$!z0!..!z)+ !(/z\$2!zz %""!.!\*0z\*1)!.z+"z0+)/\_zc%%dz%0z%/z1\*(%'!(5z0+z(1(0!z0\$!z!4¥ act total terrace area, and (iii) there are numerous candidates for each type of terrace. In the optimized model of small terrace (Fig. 16(a)), one can notice a bonding of Ti (Si) atoms in Ti3SiC2 to Si atoms in SiC at the hollow site.

abrupt and ordered terraces. Moreover, heteroepitaxy is retained between the SiC and Ti3SiC2 "+.z!\$z0!..!^z%\*!z0\$!z%\*0!\*/%05z+"z\*z0+)%z+(1)\*z%\*z0\$!zz%)#!z%/z,.+¥ portional to *Z* 1.7 (*Z*: atomic number) [24f\_z.%#\$0!.z/,+0/z%\*z0\$!z%)#!/z.!,.!/!\*0z0+)%z+(¥ umns of Ti while darker ones are Si. In view of the atomic arrangements in the SiC and Ti3SiC2 bulks, we define the terrace by a dashed line and identify the atoms in a few layers (+/!z0+z0\$!z(%\*!z/z%\*z0\$!z0!..!z.!#%+\*z\$!.!"0!.^z+.z0\$!z/)((z0!..!\_z0\$!z%)#!z\*z!z%\*¥ tuitively interpreted as a bonding of Ti (Si) in Ti3SiC2 to Si in SiC (Fig. 13(b)). However, this is not the case for the intermediate terrace showing Si-Si bonding at the hollow site (Fig. 14(b)). Further interpretation of the HAADF mage of large terrace (Fig. 15(b)) reveals a Si-Si bonding as well. Not surprisingly, atomic columns of carbon are not scattered strongly enough to be visualized due to its small *Z*\_z.!\* !.%\*#z0\$!z%)#!z%\*+),(!0!^z1.0\$!.z+),(!¥ menting of these images so as to relate their atomic structures to their electronic properties

Figure 15. A typical HAADF image of a large terrace observed from the [11-20] direction. The terrace is indicated by a

+z#%\*z%\*/%#\$0z%\*0+z!(!0.+\*%z,.+,!.0%!/z+"z0\$!z0!..!/z\* z0\$!z.+(!z0\$!5z,(5! z%\*z0\$!z+\$)¥ ic contact formation, we perform first-principles calculations on the three typical terraces. Upon a consideration of bulk structures of SiC and Ti3SiC2, the aforementioned orientation relations, and relative stacking sequence near terraces shown in the HAADF (Figs. 13–15), 0+)%z)+ !(/z+"z0\$!z0\$.!!z.!,.!/!\*00%2!z0!..!/z3!.!z!/0(%/\$! zc%#^zDId\_z0'%\*#z%\*0+z¥ count full structural relaxation. It should be noted that these models may not exactly reflect the real terraces because it is the extreme difficult to interpret directly the HAADF images owing to the intricate atomic arrangements around the terraces and to the invisible C atoms.

requires a combination of imaging with the first-principles modeling.

166 Physics and Technology of Silicon Carbide Devices

zigzag line. (b) The same image as in (a) but has been filtered to reduce noise [20].

4.2. Electronic states and bonding nature of the terraces

Figure 16. 0@=J=D9P=<9LGEA;EG<=DKG>L@=L@J==J=HJ=K=FL9LAN=L=JJ9;=KOAL@<A>>=J=FL<AE=FKAGFK9KE9DD :AFs termediate, and (c) large terrace. The representative atoms surrounding terrace are labeled for the atom-projected density of states analysis [20].

Figure 17 shows PDOS of several representative atoms at the small terrace (labeled in Fig. 17(a)), where a remarkable difference is seen between the atoms near and away from terrace. A key feature of this figure is that strong hybridization takes place between the Ti *d* (T1~T6) and Si *p* (S1~S6) levels below *E* <sup>F</sup>\_z3\$%\$z+\*0%\*1!/z3!((z%\*0+z0\$!z%z.!#%+\*\_z%\* 1%\*#zz,.+¥ nounced gap state at *E* F for the C atoms near the terrace (C1~C4, C6). However, such a gap state at *E* F is vanished completely for the C deeper into the SiC (C5 in Fig. 17). Apart from the C, the Si atoms in SiC near terrace (S1~S3) also exhibit a weak yet visible peak at *E* F that are totally absent in the bulk (S5 in Fig. 17). The presence of gap states can be attributed to the overlap of the Ti *d* with Si *p* levels. A notable hybridization is seen as well between the Si *sp* (S1~S4) and C *sp* (C1~C4, C6) levels, indicating the formation of covalency near terrace. Apart from the SiC, all of the atoms in Ti3SiC2 around terrace (C6, C7, S4, T1~T5) also exhibit a notable peak at *E* F owing to the great degree of overlap between the Ti *d* and C (Si) *p* levels. Moreover, overall feature of PDOS for the Ti atoms around terrace (T1~T4) differs from that in the Ti3SiC2 1('zcId\_z%\* %0%\*#z0\$0z0\$!z0!..!z\*z\$2!z\*z%),0z+\*z!(!0.+\*¥ ic states of Ti as well.

To identify the bonding nature directly, we further show the contour plots of charge density and density difference for the optimized small terrace viewed along the (11-20) plane (Fig. 18). From Fig. 18(a), a remarkable difference is observed in charge distribution on C: charge distribution around C in SiC (away from terrace) has humps directed toward neighboring Si, while that around C in Ti3SiC2 (away from terrace) is of almost spherical symmetry. However, the charge distribution on some C atoms near the terrace (indicated by arrows in Fig. 18(a)) shows a mixed character with a lobe on one side while a spherical outline on the +0\$!.\_z3\$%\$z%/z.!"(!0! z".+)z0\$!%.z %""!.!\*0zzc%#^zDJd^z1.0\$!.)+.!\_z0\$!z\$.#!z %/0.%¥ bution on the Si-C bonds closet to the zigzag line (Fig. 18(a)) shares some features with that +\*z0\$!z%wz+\* /z35z".+)z0\$!z6%#6#z(%\*!zc\*(+#z0+z0\$!z+\* /z%\*z%z1('d`zc%dz0\$!z)&+.%¥ ty of charges are distributed on all the C atoms, and (ii) there are visible distortions in the charge distribution on the C atoms directed toward their adjacent Si atoms. A certain level of covalency is seen on the atomic bonding along the zigzag line (which defines the terrace), which is due to the hybridization of Ti *d* with Si (C) *p* levels (Fig. 17). These imply a mixed covalent-ionic bonding for the small terrace. It is obvious from the density-difference plot (Fig. 18(b)) that the ionic nature arises from the charge transfer of Ti (Si) to C. In the Ti3SiC2 region away from the zigzag line, the Ti-C bonds are found to have more covalent element than the Ti-Si bonds, as more amount of charges are accumulated in between Ti and C,

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which indicates that the Ti-C is a stronger chemical bond than the Ti-Si.

Figure 19. PDOS (states/eV atom) of several selected atoms at or far from the intermediate terrace. Refer to Fig. 16(b)

for the sites of the selected atoms [20].

Figure 17. PDOS (states/eV atom) of several selected atoms near or far from the small terrace. All of atoms in the slab are fully relaxed. Refer to Fig. 16(a) for the sites of the selected atoms. The *E* F is set to zero and indicated by a vertical dashed line [20].

Figure 18. Contour plots of (a) charge density and (b) charge-density difference for the small terrace viewed along the (11-20) plane. Difference in charge density shows redistribution of charge near terrace relative to its isolated system. The position of terrace is indicated by a zigzag dashed line and the atoms intersecting the contour plane are labeled [20].

To identify the bonding nature directly, we further show the contour plots of charge density and density difference for the optimized small terrace viewed along the (11-20) plane (Fig. 18). From Fig. 18(a), a remarkable difference is observed in charge distribution on C: charge distribution around C in SiC (away from terrace) has humps directed toward neighboring Si, while that around C in Ti3SiC2 (away from terrace) is of almost spherical symmetry. However, the charge distribution on some C atoms near the terrace (indicated by arrows in Fig. 18(a)) shows a mixed character with a lobe on one side while a spherical outline on the +0\$!.\_z3\$%\$z%/z.!"(!0! z".+)z0\$!%.z %""!.!\*0zzc%#^zDJd^z1.0\$!.)+.!\_z0\$!z\$.#!z %/0.%¥ bution on the Si-C bonds closet to the zigzag line (Fig. 18(a)) shares some features with that +\*z0\$!z%wz+\* /z35z".+)z0\$!z6%#6#z(%\*!zc\*(+#z0+z0\$!z+\* /z%\*z%z1('d`zc%dz0\$!z)&+.%¥ ty of charges are distributed on all the C atoms, and (ii) there are visible distortions in the charge distribution on the C atoms directed toward their adjacent Si atoms. A certain level of covalency is seen on the atomic bonding along the zigzag line (which defines the terrace), which is due to the hybridization of Ti *d* with Si (C) *p* levels (Fig. 17). These imply a mixed covalent-ionic bonding for the small terrace. It is obvious from the density-difference plot (Fig. 18(b)) that the ionic nature arises from the charge transfer of Ti (Si) to C. In the Ti3SiC2 region away from the zigzag line, the Ti-C bonds are found to have more covalent element than the Ti-Si bonds, as more amount of charges are accumulated in between Ti and C, which indicates that the Ti-C is a stronger chemical bond than the Ti-Si.

Figure 17. PDOS (states/eV atom) of several selected atoms near or far from the small terrace. All of atoms in the slab are fully relaxed. Refer to Fig. 16(a) for the sites of the selected atoms. The *E* F is set to zero and indicated by a vertical

Figure 18. Contour plots of (a) charge density and (b) charge-density difference for the small terrace viewed along the (11-20) plane. Difference in charge density shows redistribution of charge near terrace relative to its isolated system. The position of terrace is indicated by a zigzag dashed line and the atoms intersecting the contour plane are labeled [20].

dashed line [20].

168 Physics and Technology of Silicon Carbide Devices

Figure 19. PDOS (states/eV atom) of several selected atoms at or far from the intermediate terrace. Refer to Fig. 16(b) for the sites of the selected atoms [20].

one can note that (i) gap states at *E* F appear for the C (C2~C4) and Si (S1,S2) atoms in SiC near terrace, (ii) a hybridization is observed between the Ti *d* (Si *p*) and O *p* levels just below *E* <sup>F</sup>\_z\*z%\* %0%+\*z+"z 0\$!z "+.)0%+\*z+"z+2(!\*5\_z\* z c%%%dz 0\$!z%),0z+"z 0!..!z+\*z 0\$!z!(!¥ tronic states of both SiC and Ti3SiC2 is confined, as the PDOS returns to its bulk value for the atoms away from terrace (*e.g.*, C5, C8, S4, S6, T6). Interestingly, overall feature of the PDOS "+.z!%0\$!.z 0\$!zz cDWGdz+.z %z cDWFdz 0+)/z .+1\* z 0!..!z %""!./z ".+)z+\*!z \*+0\$!.\_z /1#¥

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Figure 20 /\$+3/z+\*0+1.z,(+0/z+"z\$.#!z !\*/%05z\* z !\*/%05z %""!.!\*!z"+.z0\$!z.!(4! z%\*0!.¥ mediate terrace intersected along the same plane as in Fig. 18. Like what was seen in the small terrace, the majority of charges remain concentrated on C in two different ways: the charge distribution on the C in Ti3SiC2 (*e.g.*, C8) is of spherical symmetry, while that on the C in SiC (*e.g.*, C5) has notable lobes pointed toward their adjacent Si. However, some C atoms near terrace (indicated by arrows in Fig. 12(a)) have lobe and sphere outline simultaneously. Moreover, charges are distributed along the bonds near terrace (on two sides of zigzag line), which together with the charge localization on C atoms infers that the intermediate terrace \$/z z)%4! z +2(!\*0z \* z%+\*%z +\* %\*#z /z3!((^z2% !\*0(5\_z 0\$!z \$.#!z #%\*z +\*zz%/z 0z!4¥

Figure 22. Contour plots of (a) charge density and (b) charge-density difference for the large terrace viewed along the HD9F=0@=K;9D=AKL@=K9E=9KAF"A?0@=KAL=G>L=JJ9;=AKAF<A;9L=<:Q9RA?R9?DAF=9F<L@=9LGEKAFL=Js

Figure 16(c) illustrates optimized atomic geometry of the large terrace, where a Si-Si bonding is revealed. Figure 21 shows the PDOS of the large terrace, where one can notice that electronic structure is influenced remarkably by terrace. The key point is that there emerge notable peaks at *E* F for both the C (C1~C5) and Si (S1~S3, S5, S6) surrounding terrace. However, such gap states are screened rapidly, since the atoms in SiC away from terrace (S4) show no peak at *E* F at

gesting a strong effect of terrace on electronic states of both SiC and Ti3SiC2.

pense of charge loss on their neighboring cations (Fig. 20(b)).

secting the contour plane are labeled [20].

Figure 20. Contour plots of (a) charge density and (b) charge-density difference for the intermediate terrace viewed along the (11-20) plane. The scale is the same as in Fig. 18. The location of terrace is indicated by a zigzag line and the atoms intersecting the contour plane are labeled [20].

Figure 21. PDOS (states/eV atom) of several selected atoms at or far from the large terrace. Refer to Fig. 16(c) for the sites of the selected atoms [20].

The fully relaxed structure of the intermediate terrace is shown in Fig. 16(b), where one can see a Si-Si bonding at the hollow site (on two sides of the zigzag line). Figure 19 shows PDOS plot of several representative atoms on or near terrace (labeled in Fig. 16(b)), where one can note that (i) gap states at *E* F appear for the C (C2~C4) and Si (S1,S2) atoms in SiC near terrace, (ii) a hybridization is observed between the Ti *d* (Si *p*) and O *p* levels just below *E* <sup>F</sup>\_z\*z%\* %0%+\*z+"z 0\$!z "+.)0%+\*z+"z+2(!\*5\_z\* z c%%%dz 0\$!z%),0z+"z 0!..!z+\*z 0\$!z!(!¥ tronic states of both SiC and Ti3SiC2 is confined, as the PDOS returns to its bulk value for the atoms away from terrace (*e.g.*, C5, C8, S4, S6, T6). Interestingly, overall feature of the PDOS "+.z!%0\$!.z 0\$!zz cDWGdz+.z %z cDWFdz 0+)/z .+1\* z 0!..!z %""!./z ".+)z+\*!z \*+0\$!.\_z /1#¥ gesting a strong effect of terrace on electronic states of both SiC and Ti3SiC2.

Figure 20 /\$+3/z+\*0+1.z,(+0/z+"z\$.#!z !\*/%05z\* z !\*/%05z %""!.!\*!z"+.z0\$!z.!(4! z%\*0!.¥ mediate terrace intersected along the same plane as in Fig. 18. Like what was seen in the small terrace, the majority of charges remain concentrated on C in two different ways: the charge distribution on the C in Ti3SiC2 (*e.g.*, C8) is of spherical symmetry, while that on the C in SiC (*e.g.*, C5) has notable lobes pointed toward their adjacent Si. However, some C atoms near terrace (indicated by arrows in Fig. 12(a)) have lobe and sphere outline simultaneously. Moreover, charges are distributed along the bonds near terrace (on two sides of zigzag line), which together with the charge localization on C atoms infers that the intermediate terrace \$/z z)%4! z +2(!\*0z \* z%+\*%z +\* %\*#z /z3!((^z2% !\*0(5\_z 0\$!z \$.#!z #%\*z +\*zz%/z 0z!4¥ pense of charge loss on their neighboring cations (Fig. 20(b)).

Figure 20. Contour plots of (a) charge density and (b) charge-density difference for the intermediate terrace viewed along the (11-20) plane. The scale is the same as in Fig. 18. The location of terrace is indicated by a zigzag line and the

Figure 21. PDOS (states/eV atom) of several selected atoms at or far from the large terrace. Refer to Fig. 16(c) for the

The fully relaxed structure of the intermediate terrace is shown in Fig. 16(b), where one can see a Si-Si bonding at the hollow site (on two sides of the zigzag line). Figure 19 shows PDOS plot of several representative atoms on or near terrace (labeled in Fig. 16(b)), where

atoms intersecting the contour plane are labeled [20].

170 Physics and Technology of Silicon Carbide Devices

sites of the selected atoms [20].

Figure 22. Contour plots of (a) charge density and (b) charge-density difference for the large terrace viewed along the HD9F=0@=K;9D=AKL@=K9E=9KAF"A?0@=KAL=G>L=JJ9;=AKAF<A;9L=<:Q9RA?R9?DAF=9F<L@=9LGEKAFL=Js secting the contour plane are labeled [20].

Figure 16(c) illustrates optimized atomic geometry of the large terrace, where a Si-Si bonding is revealed. Figure 21 shows the PDOS of the large terrace, where one can notice that electronic structure is influenced remarkably by terrace. The key point is that there emerge notable peaks at *E* F for both the C (C1~C5) and Si (S1~S3, S5, S6) surrounding terrace. However, such gap states are screened rapidly, since the atoms in SiC away from terrace (S4) show no peak at *E* F at all, which implies that the effect of terrace is confined to within a couple of layers. Moreover, a substantial hybridization is seen between the Ti *d* and Si (C) *p* levels below *E* F, which indicates the formation of covalent bonding. The covalency is also reflected in the charge-density plot (Fig. 22(a)) showing lobes for the C in SiC and charge accumulation along the bonds in Ti3SiC2. These, along with the charge distribution on C at expense of Ti and Si (Fig. 22(b)), indicate that +\* %\*#z\*!.z0!..!z%/z#%\*z,.%).%(5z%+\*%\_z5!0z)%\*0%\*/zz!.0%\*z !#.!!z+"z+2(!\*5^z%¥ nally, both the electron density and density difference for some atoms near terrace (indicated 5z..+3/dz !2%0!z/!2!.!(5z".+)z0\$+/!z"+.z0\$!%.z1('z+1\*0!.,.0/\_z%),(5%\*#z\*z!//!\*0%(z!(!¥ tronic role of the Ti3SiC2 in the semiconductor.

5. Discussion and conclusions

pit-free Ohmic contacts can be fabricated.

(/+z2%!3! z/zz+\*0.%10%\*#z"0+.z%\*z.! 1%\*#z0\$!z-

fects have not been detected by the TEM study.

The current understanding of formation origin of Ohmic contact, which is based mainly on !4,!.%)!\*0(z/01 %!/z+"z,.+,!.05z)!/1.!)!\*0z\* z/0.101.!z\$.0!.%60%+\*\_z\*z!z/1)¥ marized in three main points [27-30]: (1) the deposited Al (80 at%) might diffuse in part into the SiC and dope heavily the semiconductor because Al is well-known to act as a *p*-type +,\*0z"+.z%\_zcEdz0\$!z\$%#\$z !\*/%05z+"z,%0/\_z/,%'!/\_z+.z %/(+0%+\*/z)5z!z#!\*!.0! z1\* !.¥ \*!0\$z0\$!z+\*00/z"0!.z\*\*!(%\*#z/+z0\$0z1..!\*0z\*z0.\*/,+.0z,.%).%(5z0\$.+1#\$z0\$!/!z !¥ fects due to the possible enhancement of electric field at these features and semiconductor +,%\*#z0z0\$!/!z(+0%+\*/\_zcFdz"+.)0%+\*z+"z%\*0!.)! %0!z/!)%+\* 10+.z(5!.z!03!!\*z0\$!z !¥ posited metals and semiconductor, which consists of silicides or carbides, could divide the

Physics Behind the Ohmic Nature in Silicon Carbide Contacts

http://dx.doi.org/10.5772/50767

173

\$!z"%\* %\*#/z,.!/!\*0! z"%./0z !)+\*/0.0!z0\$0z\*+z(z%/z(!.(5z/!#.!#0! z.+1\* z0\$!z%\*0!."¥ %(z.!#%+\*\_z%\*z,.0%1(.z0z0\$!z0+,z"!3z(5!./z+"z%\_z3\$%\$z.1(!/z+10z0\$!z,+//%%(%05z+"z %¥ tional Al doping. Though a small amount of residual Al is found to be present, mostly in a form of Al4C3 compound, it may locate on the surface of annealed contacts rather than in the (5!.z %.!0(5z +\*00! z 0+z 0\$!z %\_z 0\$1/z,(5%\*#z z \*!#(%#%(!z .+(!z%\*z\$)%z +\*00z "+.)¥ tion. The majority of deposited Al is evaporated during annealing because of its low melting ,+%\*0z\* z\$%#\$z!-1%(%.%1)z2,+.z,.!//1.!^z\$!z +)%\*\*0z.+(!z,(5! z5z(z%\*z0\$!z%(z/5/¥ 0!)z%/z 0+z //%/0z 0\$!z "+.)0%+\*z +"z(%-1% z ((+5z /+z /z 0+z "%(%00!z \$!)%(z .!0%+\*^z 1.0\$!.¥ more, careful characterization of the interfacial region reveals that the substrate and the generated compound are epitaxially oriented and well matched at interface with no clear evidence of high density of defects. This suggests that the morphology might not be the key to understanding the contact formation. In support of this speculation, it has been observed previously that Ti Ohmic contacts can be possibly generated without any pitting and that

\*!z .!)%\*%\*#z 0\$!+.5z %/z 0\$!z ((+5w//%/0! z \$)%z +\*00z "+.)0%+\*^z \$%/z ((+5z %/z !0!.¥ mined to be ternary Ti3SiC2, which has also been corroborated by other expriments. Since the bulk Ti3SiC2 has already been found to be of metallic nature both in experiment and theory, the contact between Ti3SiC2 and its covered metals should show Ohmic character and thus the SiC/Ti3SiC2 interface should play a significant role in Ohmic contact formation. This idea is supported by the fact that the determined interface has a lowered SBH due to the large dipole shift at interface induced by the partial ionicity and the considerable charge transfer. In addition, the interfacial states, as indicated by the electron distribution at *E <sup>F</sup>*, are

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Interestingly, our calculations predict that an atomic layer of carbon emerges as the first monolayer of Ohmic contacts, which eventually affects interface electronic structure. Such trapped carbon was previously studied in both other interfacial systems theoretically by z\* z0\$!z%%z\$)%z+\*00/z+\*zGw%z!4,!.%)!\*0((5z5z1#!.z!(!0.+\*z/,!0.+/+¥ ,5^z 0z3/z,.+,+/! z0\$0z0\$!z.+\*z+1( z!z/!#.!#0! z0+z0\$!z%\*0!."%(z.!\_z/0.!\*#0\$!\*¥

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high barrier height into lower ones, thus reducing the effective barrier height.

Figure 23. Isosurface plots for (a) small, (b) intermediate, and (c) large terrace along the (11-20) plane in an energy window (*E*F–0.5 eV, *E* F). The charges on Ti3SiC2 are omitted for clarity [20].

#### 4.3. Electron distribution near Fermi energy

(0\$+1#\$z0\$!zz\* z\$.#!z\*(5/!/z\*z.!2!(z2(1(!z%\*"+.)0%+\*z+\*z0\$!z+\* %\*#z\*¥ ture near terrace, they provide limited insights into matters regarding electron distribution around *E* F, which is strongly related to the electronic conduction over terrace. Figure 23 shows electron-density isosurface near *E* F along the (11-20) plane for the three terraces. The !(!0.+\*/z.!z1)1(0! z/1/0\*0%((5z.+1\* z((z0\$!z0!..!/z%\*zz/,0%((5z+\*\*!0! z"/\$¥ ion, which are extended as far as several atomic layers into SiC, regardless of dimension of 0\$!z 0!..!/^z1\$zz.+ z!(!0.+\*z %/0.%10%+\*z,.+2% !/zz(%'!(5z!(!0.+\*z+\* 10%+\*z\$\*¥ nel to allow current transport across a few layers of the semiconductor, which indicates that terraces could also be one of the origins underlying the observed ohmic nature in the metal/SiC contact system. One can also note that the three terraces are comparable in the amount of accumulated charge at *E* <sup>F</sup>\_z%\*"!..%\*#z0\$0z0\$!z %)!\*/%+\*z+"z0!..!z,(5/z\*z%\*/%#¥ nificant role in affecting electron transport across the contact. It is worthy of mentioning that the electron density at *E* F is extremely high in Ti3SiC2zc/!z+"z!(!0.+\*/\_z\*+0z/\$+3\*z"+.z(.%¥ ty), yet turns almost nil for the SiC atoms away from terrace, which can be understood on the fact that the Ti3SiC2 is intrinsically metallic, whereas the SiC is semiconducting.
