5. Discussion and conclusions

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!(!¥

Figure 23. Isosurface plots for (a) small, (b) intermediate, and (c) large terrace along the (11-20) plane in an 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.

tronic role of the Ti3SiC2 in the semiconductor.

172 Physics and Technology of Silicon Carbide Devices

window (*E*F–0.5 eV, *E* F). The charges on Ti3SiC2 are omitted for clarity [20].

4.3. Electron distribution near Fermi energy

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

\$!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 pit-free Ohmic contacts can be fabricated.

\*!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 (/+z2%!3! z/zz+\*0.%10%\*#z"0+.z%\*z.! 1%\*#z0\$!z-^z\$!/!z/00!/z)%#\$0z!z"1.0\$!.z!\*¥ \$\*! z5z 0\$!z,+//%(!z,.!/!\*!z+"z,+%\*0z !"!0/z0z%\*0!."!\_z(0\$+1#\$z 0\$!/!z/0.101.(z !¥ fects have not been detected by the TEM study.

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\$!\*¥ ing interface substantially and reducing the Schottky barrier dramatically. Further, it was reported that the Ohmic contact can be realized by depositing carbon films only onto the SiC substrate, indicative of the determinative role of carbon in the Ohmic contact formation. The important role played by carbon in our study can be traced to the two interfacial Si layers, which provide possible sites for carbon segregation due to the strong Si-C interaction. However, direct imaging of the trapped carbon is still difficult in present study and further characterization requires the high-voltage EM and/or other advanced microscopic techniques.

We then demontrate that atomic-scale TigSiC2-like bilayer can be embeded in the SiC interior, forming an atomically ordered multilayer that exhibits an unexpected electronic state with the point Fermi surface, in stark absence in repestive bulk constituents. The valence charge is tound to be confined largely within the bilayer in a spatially connected way, which serves as a possible conducting channel to enhance the current flow over the semiconductor. Such a heterostructure with unusual properties is mechanically robust, rendering its patterning for technological applications likely. Finally, the atomic structures of terraces at the contacts in SiC devices are investigated and bridged to their electronic properties at an atomic scale. Experimentally, newly formed carbide TigSiC2 is demonstrated to bond directly to silicon carbide in the terrace region in an epitaxial and atomically ordered fashion, regardless of dimension of terraces. Further first-principles calculations reveal gap states in the semiconductor layers and a substantial charge accumulation around terraces in a connected and broadly distributed manner. The presence of gap states at Fermi energy and the likelihood to serve as electron conduction channels to allow current flow over the semiconductor identify the terraces as one of the origins underlying the ohmic contact in silicon carbide electronics. Such a combined experimental and theoretical investigation provides insight into the complex atomic and electronic structures of buried terraces, which should be applicable to addressing contact issues of interest in other electronic devices.

To summarize, we have determined in this chapter atomic-scale structures of Ohmic contacts on SiC and related them to their electronic structures and electric properties, aimed at understanding the formation mechanism of Ohmic contact in TiAl-based system. The combined HAADF-DFT study [31] represents an important advance in relating structures to device properties at an atomic scale and is not limited to the contacts in SiC electronics. Our results show that the main product generated by chemical reaction can be epitaxial and have atomic bonds to the substrate. The contact interface, which could trap an atomic layer of carbon, enables lowered Schottky barrier due to the large interfacial dipole shift associated with the considerable charge transfer. The atomic-scale Ti,SiC-like bilayer is embedded well in SiC bulk interior in an epitaxial, coherent and atomically abrupt manner, which exhibits an unexpected state with a point fermi surface. Moreover, the formed TisSiC2 can even be epitaxial and atomically ordered on SiC substrate near terrace, which inudces pronounced gap states at Ep in the semiconductor layers. Charges are accumulated heavily surrounding terrace in a spatially connected fashion, irrespective of dimension of the terraces, which suggests the possiblity of terraces as likely electron conduction channels to allow current transport across the semiconductor. The inducing of gap states and the capability to enable current flow over the semiconductor identify the terraces as one of the origins underlying the Ohmic nature in the metal/SiC contact system as well. These findings are relevant for technological improvement of contacts in SiC devices, and this chapter presents an important step towards addressing the current contact issues in wide-band-gap electronics.
