1. Introduction

\*!z+"z0\$!z)+/0z0%2!z"%!( /z%\*z/!)%+\* 10+.z.!/!.\$z%/z0\$!z !2!(+,)!\*0z+"z!(!0.+\*%z !¥ vices capable of function at high powder and high frequency levels, high temperatures, and 1/0%z%.1)/0\*!/^z\$%/z/1.#!z+"z 0%2%05z%/z/0.+\*#(5z .%2!\*z5z 0\$!z1.#!\*0z !/%.!z "+.z .!¥ ,(%\*#z0\$!z1..!\*0z%wz\* z/w/! z!(!0.+\*%/z!1/!z0\$!5z.!z1\*(!z0+z+,!.0!z,.+,¥ erly under harsh environmental conditions. As a promising substitute, the wide-band-gap semiconductor, silicon carbide (SiC), has captured considerable attention recently due to its !4!((!\*0z%\*0.%\*/%z,.+,!.0%!/\_z3\$%\$z%\*2+(2!z(.#!z.!' +3\*z!(!0.%z"%!( \_z\$%#\$z!(!0.+\*z/0¥ uration drift velocity, strong hardness, and good thermal conductivity. On the other hand, current significant improvements in the epitaxial and bulk crystal growth of SiC have paved the way for fabricating its electronic devices, which stimulates further interest in developing device processing techniques so as to take full advantage of its superior inherent properties.

One of the most critical issues currently limiting the device processing is the manufacturing of reliable and low-resistance Ohmic contacts especially contacts to *p*-type 4H-SiC [1]. The \$)%z+\*00/z.!z,.%).%(5z%),+.0\*0z%\*z%z !2%!/z!1/!zz\$+00'5z..%!.z+"z\$%#\$z!\*¥ ergy is inclined to form at an interface between metal and wide-band-gap semiconductor, which consequently results in low current driving, slow switching speed, and increased power dissipation. Much of effort expended to date to realize the Ohmic contact has mainly focused on two techniques. One is the high-dose ion implantation approach [2], which can increase carrier density in SiC noticeably while reducing its depletion width significantly so that increasing tunneling current is able to flow across the barrier region. The key problem +"z 0\$%/z)!0\$+ z%/z 0\$!z!/5z "+.)0%+\*z+"z(00%!z !"!0/z+.z)+.,\$%60%+\*z 1.%\*#z 0\$!z%+\*z%)¥ ,(\*00%+\*^z\$!/!z !"!0/z.!z1\*"+.01\*0!(5z2!.5z/0(!z\* z\*!! z0+z!z.!+2!.! z2%z\*\*!(¥

© 2013 Wang; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Wang; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ing at an extremely high temperature of about 2000 K, thereby complicating the mass production of SiC devices.

Recent advances in the high-angle annular-dark-field (HAADF) microscopy [12,13], the highest resolution, have enabled atomic-scale imaging of a buried interface. However, direct interpretation of the observed HAADF images is not always straightforward because there might be abrupt structural discontinuity, mixing of several species of elements on individual 0+)%z +(1)\*/\_z +.z )%//%\*#z +\*0./0/z +"z (%#\$0z !(!)!\*0/^z \*!z ,+//%(!z 35z +10z 0+z +),(!¥ )!\*0z0\$!z)%.+/+,%z 0z%/z0\$.+1#\$z0+)%/0%z(1(0%+\*\_z!/,!%((5z0\$!z"%./0w,.%\*%,(!/z(¥ culation. As well known, the atomistic first-principles simulations have long been confirmed to be able to suggest plausible structures, elucidate the reason behind the observed images, and even provide a *quantitative* insight into how interface governs properties of materials [14,15f^z+\*/!-1!\*0(5\_zz+)%\*0%+\*z+"z0\$!z/00!w+"w0\$!w.0z)%.++,5z\* z1.0!z0+)%/¥ tic modeling [16] is an important advance for determining interface atomic-scale structure \* z.!(0%\*#z%0z 0+z !2%!z,.+,!.0%!/\_z.!2!(%\*#\_z%\*z 0\$%/z35\_z,\$5/%/z+.%#%\*z+"z 0\$!z+\*00z%/¥

Physics Behind the Ohmic Nature in Silicon Carbide Contacts

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

151

In addition to determining atomic structure of the 4H-SiC/Ti3SiC2 interface, the goal of this 3+.'z%/z 0+z(.%"5z 0\$!z "+.)0%+\*z)!\$\*%/)z+"z 0\$!z%(w/! z\$)%z+\*00/z/+z/z 0+z,.+¥ vide suggestions for further improvement of the contacts. 4H-SiC will hereafter be referred to as SiC. In this Chapter, we will first attribute qualitatively the formation of ohmic contacts in the TiAl-deposited SiC system to an epitaxial and atomically abrupt interface between the SiC and Ti3SiC2 generated *via*z\*\*!(%\*#^z\$!z%\*0!."!z,.!/1)(5z/!.2!/z/zz,.%).5z1.¥ .!\*0w0.\*/,+.0z,0\$35z0+z(+3!.z0\$!z\$+00'5z..%!.z"+.)! z0z0\$!z%\*0!."!z!03!!\*z0\$!z !¥ posited metals and SiC. Further quantitative studies reveal that the barrier mitigation arises from trapping of an atomic layer of carbon at the SiC/Ti3SiC2 interface, which assists the electron transport across the SiC [17,18]. The considerations on the role played by interface do not, however, exclude another possibility that the Ti3SiC2 atomic layers can be generated inside the SiC bulk interiors, which presents a behavior that may differ from that of their bulk [19]. Combining the state-of-the-art TEM with atomistic first-principles calculations, we demonstrate the presence of an atomic-scale Ti3SiC2w(%'!z%(5!.z!)! ! z%\*z0\$!z%\_z"+.)¥ ing an atomically ordered multilayer that exhibits an unexpected electronic state with point !.)%z/1."!^z\$!z2(!\*!z\$.#!z%/z+\*"%\*! z0+zz(.#!z!40!\*0z0+z3%0\$%\*z0\$!z%(5!.z%\*zz/,¥ tially connected manner, serving possibly as a conducting channel to enhance the current flow over the semiconductor. Further investigation into the contact regions unveils another new opportunity to allow the electron transport across the semiconductor, namely, *via* the terraces formed at the SiC/Ti3SiC2 interface [20]. Experimentally, the formed carbide Ti3SiC2 is demonstrated to bond directly to the silicon carbide at the terraces in an epitaxial and 0+)%((5z +. !.! z "/\$%+\*\_z .!#. (!//z +"z %)!\*/%+\*z +"z 0\$!z 0!..!/^z \$!.!z ,,!.z ,.+¥ nounced gap states at Fermi level in the semiconductor layers around the terraces, and charges are accumulated heavily around the terraces in a connected and broadly distributed manner. The presence of the metallicity and the likelihood to act as electron conduction \$\*\*!(/z 0+z!\*(!z 0\$!z1..!\*0z "(+3z+2!.z 0\$!z/!)%+\* 10+.z)'!z 0\$!z 0!..!/z0z 0\$!z%\*0!.¥ "!z+\*!z+"z0\$!z+.%#%\*/z1\* !.(5%\*#z0\$!z+\$)%z+\*00z%\*z/%(%+\*z.% !z!(!0.+\*%/^z!z0\$!.!¥ fore demonstrate in this chapter that origin of the long-standing contact issue in SiC devices can be understood and technologically manipulated at the atomic level, and suggest the key

sues in SiC electronics.

physical factors for establishing the ohmic nature.

The other alternative is to generate an intermediate semiconductor layer with narrower band gap or higher carrier density at the contacts/SiC interface via depositing and annealing technique [3]. To form such layers, a wide range of materials have been examined in a trialand-error designing fashion, including metals, silicides, carbides, nitrides, and graphite. Of all these materials, the metallic alloys have been investigated extensively, largely because their fabrication process is simple, standard, and requires no exotic materials. In particular, )+/0z+"z.!/!.\$z0%2%0%!/z\$2!z!!\*z"+1/! z+\*z%(w/! z((+5/\_z0\$!z+\*(5z1..!\*0(5z2%(¥ ble materials that yield significantly low contact resistance (Ohmic contact) to *p*-SiC [4]. Moreover, they demonstrate high thermal stability. Although a lot of intriguing results have been obtained regarding the TiAl-based contact systems, the mechanism whereby the Schottky becomes Ohmic after annealing has not been well clarified yet. In other words, the key factor to understanding the formation origin of Ohmic contact remains controversial. Mohney et al. [5] proposed that a high density of surface pits and spikes underneath the contacts contributes to the formation of Ohmic behavior based on their observations using scanning electron microscopy and atomic force microscopy. Nakatsuka et al. [6], however, concluded that the Al concentration in the TiAl alloy is important for the contact formation. Using the liquid etch and ion milling techniques, John and Capano [7fz.1(! z+10z0\$!/!z,+//%¥ bilities and claimed that what matters in realizing the Ohmic character is the generation of carbides, Ti3SiC2 and Al4C3, between the metals and semiconductor. This, however, differs, to some extent, from the X-ray diffraction (XRD) findings of Chang et al. [8] showing that the compounds formed at the metal/SiC interface are silicides, TiSi2, TiSi, and Ti3SiC2^z \*z ¥ dition, Ohyanagi et al. [9] argued that carbon exists at the contacts/SiC interface and might play a crucial role in lowering Schottky barrier. These are just a few representative examples illustrating the obvious discrepancies in clarifying the formation mechanism of the Ohmic contact. Taking the amount of speculations on the mechanism and the increasing needs for !00!.z !2%!z !/%#\*z\* z,!."+.)\*!z+\*0.+(\_z1\* !./0\* %\*#z0\$!z1\* !.(5%\*#z"+.)0%+\*z+.%¥ gin is timely and relevant.

To develop an understanding of the origin in such a complex system, it is important to focus first on microstructure characterization. Transmission electron microscopy (TEM) studies by Tsukimoto et al. [10] have provided useful information in this aspect. They have found that the majority of compounds generated on the surface of 4H-SiC substrate after annealing consist of Ti3SiC2 and hence proposed that the SiC/Ti3SiC2 interface is responsible for the lowering of \$+00'5z..%!.z%\*z0\$!z%(w/! z+\*00z/5/0!)^z+3!2!.\_z0\$!z.+(!z+"z0\$%/z%\*0!."!z%\*z.!(%6¥ ing the Ohmic nature is still unclear. It is not even clear how the two materials atomically bond 0+#!0\$!.z".+)z0\$!%.z!4,!.%)!\*0/\_z3\$%\$z%/z2!.5z%),+.0\*0z!1/!z%0z)5z/0.+\*#(5z""!0z,\$5/%¥ cal properties of the system. Theoretically, we have calculated the atomic structures, adhesive energies, and bonding nature of the SiC/Ti3SiC2 interface [11]. However, this calculation does \*+0z01((5z.!2!(z0\$!z"+.)0%+\*z)!\$\*%/)z+"z\$)%z+\*00z!1/!z%0z+\*(5z .!//!/z0\$!z%\*¥ 0!."!z/0.101.!^z1.0\$!.)+.!\_z('%\*#z!//!\*0%(z!4,!.%)!\*0(z%\*"+.)0%+\*z+10z0\$!z%\*0!."¥ cial atomic-scale structure, such calculations have been incomplete.

Recent advances in the high-angle annular-dark-field (HAADF) microscopy [12,13], the highest resolution, have enabled atomic-scale imaging of a buried interface. However, direct interpretation of the observed HAADF images is not always straightforward because there might be abrupt structural discontinuity, mixing of several species of elements on individual 0+)%z +(1)\*/\_z +.z )%//%\*#z +\*0./0/z +"z (%#\$0z !(!)!\*0/^z \*!z ,+//%(!z 35z +10z 0+z +),(!¥ )!\*0z0\$!z)%.+/+,%z 0z%/z0\$.+1#\$z0+)%/0%z(1(0%+\*\_z!/,!%((5z0\$!z"%./0w,.%\*%,(!/z(¥ culation. As well known, the atomistic first-principles simulations have long been confirmed to be able to suggest plausible structures, elucidate the reason behind the observed images, and even provide a *quantitative* insight into how interface governs properties of materials [14,15f^z+\*/!-1!\*0(5\_zz+)%\*0%+\*z+"z0\$!z/00!w+"w0\$!w.0z)%.++,5z\* z1.0!z0+)%/¥ tic modeling [16] is an important advance for determining interface atomic-scale structure \* z.!(0%\*#z%0z 0+z !2%!z,.+,!.0%!/\_z.!2!(%\*#\_z%\*z 0\$%/z35\_z,\$5/%/z+.%#%\*z+"z 0\$!z+\*00z%/¥ sues in SiC electronics.

ing at an extremely high temperature of about 2000 K, thereby complicating the mass

The other alternative is to generate an intermediate semiconductor layer with narrower band gap or higher carrier density at the contacts/SiC interface via depositing and annealing technique [3]. To form such layers, a wide range of materials have been examined in a trialand-error designing fashion, including metals, silicides, carbides, nitrides, and graphite. Of all these materials, the metallic alloys have been investigated extensively, largely because their fabrication process is simple, standard, and requires no exotic materials. In particular, )+/0z+"z.!/!.\$z0%2%0%!/z\$2!z!!\*z"+1/! z+\*z%(w/! z((+5/\_z0\$!z+\*(5z1..!\*0(5z2%(¥ ble materials that yield significantly low contact resistance (Ohmic contact) to *p*-SiC [4]. Moreover, they demonstrate high thermal stability. Although a lot of intriguing results have been obtained regarding the TiAl-based contact systems, the mechanism whereby the Schottky becomes Ohmic after annealing has not been well clarified yet. In other words, the key factor to understanding the formation origin of Ohmic contact remains controversial. Mohney et al. [5] proposed that a high density of surface pits and spikes underneath the contacts contributes to the formation of Ohmic behavior based on their observations using scanning electron microscopy and atomic force microscopy. Nakatsuka et al. [6], however, concluded that the Al concentration in the TiAl alloy is important for the contact formation. Using the liquid etch and ion milling techniques, John and Capano [7fz.1(! z+10z0\$!/!z,+//%¥ bilities and claimed that what matters in realizing the Ohmic character is the generation of carbides, Ti3SiC2 and Al4C3, between the metals and semiconductor. This, however, differs, to some extent, from the X-ray diffraction (XRD) findings of Chang et al. [8] showing that the compounds formed at the metal/SiC interface are silicides, TiSi2, TiSi, and Ti3SiC2^z \*z ¥ dition, Ohyanagi et al. [9] argued that carbon exists at the contacts/SiC interface and might play a crucial role in lowering Schottky barrier. These are just a few representative examples illustrating the obvious discrepancies in clarifying the formation mechanism of the Ohmic contact. Taking the amount of speculations on the mechanism and the increasing needs for !00!.z !2%!z !/%#\*z\* z,!."+.)\*!z+\*0.+(\_z1\* !./0\* %\*#z0\$!z1\* !.(5%\*#z"+.)0%+\*z+.%¥

To develop an understanding of the origin in such a complex system, it is important to focus first on microstructure characterization. Transmission electron microscopy (TEM) studies by Tsukimoto et al. [10] have provided useful information in this aspect. They have found that the majority of compounds generated on the surface of 4H-SiC substrate after annealing consist of Ti3SiC2 and hence proposed that the SiC/Ti3SiC2 interface is responsible for the lowering of \$+00'5z..%!.z%\*z0\$!z%(w/! z+\*00z/5/0!)^z+3!2!.\_z0\$!z.+(!z+"z0\$%/z%\*0!."!z%\*z.!(%6¥ ing the Ohmic nature is still unclear. It is not even clear how the two materials atomically bond 0+#!0\$!.z".+)z0\$!%.z!4,!.%)!\*0/\_z3\$%\$z%/z2!.5z%),+.0\*0z!1/!z%0z)5z/0.+\*#(5z""!0z,\$5/%¥ cal properties of the system. Theoretically, we have calculated the atomic structures, adhesive energies, and bonding nature of the SiC/Ti3SiC2 interface [11]. However, this calculation does \*+0z01((5z.!2!(z0\$!z"+.)0%+\*z)!\$\*%/)z+"z\$)%z+\*00z!1/!z%0z+\*(5z .!//!/z0\$!z%\*¥ 0!."!z/0.101.!^z1.0\$!.)+.!\_z('%\*#z!//!\*0%(z!4,!.%)!\*0(z%\*"+.)0%+\*z+10z0\$!z%\*0!."¥

cial atomic-scale structure, such calculations have been incomplete.

production of SiC devices.

150 Physics and Technology of Silicon Carbide Devices

gin is timely and relevant.

In addition to determining atomic structure of the 4H-SiC/Ti3SiC2 interface, the goal of this 3+.'z%/z 0+z(.%"5z 0\$!z "+.)0%+\*z)!\$\*%/)z+"z 0\$!z%(w/! z\$)%z+\*00/z/+z/z 0+z,.+¥ vide suggestions for further improvement of the contacts. 4H-SiC will hereafter be referred to as SiC. In this Chapter, we will first attribute qualitatively the formation of ohmic contacts in the TiAl-deposited SiC system to an epitaxial and atomically abrupt interface between the SiC and Ti3SiC2 generated *via*z\*\*!(%\*#^z\$!z%\*0!."!z,.!/1)(5z/!.2!/z/zz,.%).5z1.¥ .!\*0w0.\*/,+.0z,0\$35z0+z(+3!.z0\$!z\$+00'5z..%!.z"+.)! z0z0\$!z%\*0!."!z!03!!\*z0\$!z !¥ posited metals and SiC. Further quantitative studies reveal that the barrier mitigation arises from trapping of an atomic layer of carbon at the SiC/Ti3SiC2 interface, which assists the electron transport across the SiC [17,18]. The considerations on the role played by interface do not, however, exclude another possibility that the Ti3SiC2 atomic layers can be generated inside the SiC bulk interiors, which presents a behavior that may differ from that of their bulk [19]. Combining the state-of-the-art TEM with atomistic first-principles calculations, we demonstrate the presence of an atomic-scale Ti3SiC2w(%'!z%(5!.z!)! ! z%\*z0\$!z%\_z"+.)¥ ing an atomically ordered multilayer that exhibits an unexpected electronic state with point !.)%z/1."!^z\$!z2(!\*!z\$.#!z%/z+\*"%\*! z0+zz(.#!z!40!\*0z0+z3%0\$%\*z0\$!z%(5!.z%\*zz/,¥ tially connected manner, serving possibly as a conducting channel to enhance the current flow over the semiconductor. Further investigation into the contact regions unveils another new opportunity to allow the electron transport across the semiconductor, namely, *via* the terraces formed at the SiC/Ti3SiC2 interface [20]. Experimentally, the formed carbide Ti3SiC2 is demonstrated to bond directly to the silicon carbide at the terraces in an epitaxial and 0+)%((5z +. !.! z "/\$%+\*\_z .!#. (!//z +"z %)!\*/%+\*z +"z 0\$!z 0!..!/^z \$!.!z ,,!.z ,.+¥ nounced gap states at Fermi level in the semiconductor layers around the terraces, and charges are accumulated heavily around the terraces in a connected and broadly distributed manner. The presence of the metallicity and the likelihood to act as electron conduction \$\*\*!(/z 0+z!\*(!z 0\$!z1..!\*0z "(+3z+2!.z 0\$!z/!)%+\* 10+.z)'!z 0\$!z 0!..!/z0z 0\$!z%\*0!.¥ "!z+\*!z+"z0\$!z+.%#%\*/z1\* !.(5%\*#z0\$!z+\$)%z+\*00z%\*z/%(%+\*z.% !z!(!0.+\*%/^z!z0\$!.!¥ fore demonstrate in this chapter that origin of the long-standing contact issue in SiC devices can be understood and technologically manipulated at the atomic level, and suggest the key physical factors for establishing the ohmic nature.
