Catalytic graphitization and Ohmic contact formation on 4H–SiC

Lü, Weijie; Mitchel, W. C.; Landis, G.; Crenshaw, T. R.; Collins, W. E.

doi:10.1063/1.1562737

Cited by 61 publications

(33 citation statements)

References 41 publications

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“…Indeed, Lu et al [63] find a very low resistance of the thermally treated SiC contacts with nickel and cobalt, while other metals, which form carbides and thereby remove the graphitic inclusions, were rectifying. Seyller et al measured the Schottky barrier between n-type 6H-SiC(0001) and graphite by photoelectron spectroscopy and found a low value of 0.3 eV [64].…”

Section: Feature Articlementioning

confidence: 99%

Density functional study of graphene overlayers on SiC

Mattausch

Pankratov

2008

Physica Status Solidi (b)

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Despite the ongoing “graphene boom” of the last three years our understanding of epitaxial graphene grown on SiC substrate is only beginning to emerge. Along with experimental methods such as low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and angle resolved photoemission spectroscopy (ARPES), ab initio calculations help to uncover the geometric and electronic structure of the graphene/SiC interface. In this chapter we describe the density‐functional calculations we performed for single and double graphene layers on Si‐ and C‐terminated 6H‐SiC surfaces. Experimental data reveal a pronounced difference between the two surface terminations. On a Si‐terminated surface the interface adopts a 6√3 × 6√3 unit cell whereas the C‐face supports misoriented (turbostratic) graphene layers. It has been recently realized that, on the Si‐face, the large commensurate cell is subdivided into patches of coherently matching to the substrate carbon atoms. In our calculations we assumed the “coherent match” geometry for the whole interface plane. This reduces the periodic unit to the √3 × √3R 30° cell but requires a substantial stretching of the graphene sheet. Although simplified, the model provides a qualitative picture of the bonding and of the interface electron energy spectrum. We find that the covalent bonding between the carbon layer and the substrate destroys the massless “relativistic” electron energy spectrum, the hallmark of a freestanding graphene. Hence the first carbon layer cannot be responsible for the graphene‐type electron spectrum observed by ARPES and rather plays a role of a buffer between the substrate and the subsequent carbon sheets. The “true” graphene spectrum appears with the second carbon layer which exhibits a weak van der Waals bonding to the underlying structure. For Si‐terminated substrate, we find that the Fermi level is pinned by the interface state at 0.45 eV above the graphene Dirac point, in agreement with experimental data. This renders the interface metallic. On the contrary, for a C‐face the “coherent match” model predicts the Fermi level exactly at the Dirac point. However, this does not necessarily apply to the turbostratic graphene layers that normally grow on the C‐terminated substrate. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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Section: Feature Articlementioning

confidence: 99%

Density functional study of graphene overlayers on SiC

Mattausch

Pankratov

2008

Physica Status Solidi (b)

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“…Indeed, Lu et al [21] find a very low resistance for thermally treated SiC contacts with nickel and cobalt, while other metals, which form carbides and thereby remove the graphitic inclusions, were rectifying. Recently Seyller et al measured the Schottky barrier between n-type 6H-SiC(0001) and graphite by photoelectron spectroscopy and found a low value of 0.3 eV [22].…”

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confidence: 99%

Ab InitioStudy of Graphene on SiC

2007

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Employing density-functional calculations we study single and double graphene layers on Si-and C-terminated 1 × 1 -6H-SiC surfaces. We show that, in contrast to earlier assumptions, the first carbon layer is covalently bonded to the substrate, and cannot be responsible for the graphenetype electronic spectrum observed experimentally. The characteristic spectrum of free-standing graphene appears with the second carbon layer, which exhibits a weak van der Waals bonding to the underlying structure. For Si-terminated substrate, the interface is metallic, whereas on C-face it is semiconducting or semimetallic for single or double graphene coverage, respectively.PACS numbers: 68.35. Ct, 68.47.Fg, The last years have witnessed an explosion of interest in the prospect of graphene-based nanometer-scale electronics [1,2,3,4]. Graphene, a single hexagonally ordered layer of carbon atoms, has a unique electronic band structure with the conic "Dirac points" at two inequivalent corners of the two-dimensional Brillouin zone. The electron mobility may be very high and lateral patterning with standard lithography methods allows device fabrication [1]. Two ways of obtaining graphene samples have been used up to now. In the first "mechanical" method, the carbon monolayers are mechanically split off the bulk graphite crystals and deposited onto a SiO 2 /Si substrate [4]. This way an almost "freestanding" graphene is produced, since the carbon monolayer is practically not coupled to the substrate. The second method uses epitaxial growth of graphite on singlecrystal silicon carbide (SiC). The ultrathin graphite layer is formed by vacuum graphitization due to Si depletion of the SiC surface [5]. This method has apparent technological advantages over the "mechanical" method, however it does not guarantee that an ultrathin graphite (or graphene) layer is electronically isolated from the substrate. Moreover, one expects a covalent coupling between both which may strongly modify the electronic properties of the graphene overlayer. Yet, experiments show that the transport properties of the interface are dominated by a single epitaxial graphene layer [1,2]. Most surprisingly, the electronic spectrum seems not to be affected much by the substrate. As in free-standing graphene one observes the "Dirac points" with the linear dispersion relation around them. The electron dynamics is governed by a Dirac-Weyl Hamiltonian with the Fermi velocity of graphene replacing the speed of light. This leads to an unusual sequence of Landau levels in a magnetic field and hence to peculiar features in the quantum Hall effect [1,4].

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“…The 'free carbon' is a product of the reaction between Ni and SiC to form Ni 2 Si and C during the ohmic contact annealing step, [7,8] while the oxygen is a result of exposure to air between the ohmic contact anneal and the subsequent TaSi x deposition. The AES depth profiles obtained from set A contacts shortly before electrical failure (24 h) and after electrical failure (36 h) show oxidation throughout the TaSi x layer.…”

Section: Table 1 Layer Thicknesses and Doping Concentrations In The mentioning

confidence: 99%

Improved Thermal Stability Observed in Ni-Based Ohmic Contacts to n-Type SiC for High-Temperature Applications

Virshup

Liu

Lukco

et al. 2010

Journal of Elec Materi

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The high-temperature stability of a Pt/TaSi 2 /Ni/SiC ohmic contact metallization scheme was characterized using a combination of current-voltage measurements, Auger electron spectroscopy, and transmission electron microscopy imaging and associated analytical techniques. Increasing the thicknesses of the Pt and TaSi 2 layers promoted electrical stability of the contacts, which remained ohmic at 600C in air for the extent of heat treatment; the specific contact resistance showed only a gradual increase from an initial value of 5.2 x 10 -5 -cm 2 . We observed a continuous silicon-oxide layer in the thinner contact structures, which failed after 36 h of heating. Meanwhile, thicker contacts with enhanced stability contained a much lower oxygen concentration that was distributed across the contact layers, precluding the formation of an electrically insulating contact structure.

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Catalytic graphitization and Ohmic contact formation on 4H–SiC

Cited by 61 publications

References 41 publications

Density functional study of graphene overlayers on SiC

Density functional study of graphene overlayers on SiC

Ab InitioStudy of Graphene on SiC

Improved Thermal Stability Observed in Ni-Based Ohmic Contacts to n-Type SiC for High-Temperature Applications

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