One of the most active fields in semiconductor research is the development of electronic devices capable of function at high powder and high frequency levels, high temperatures, and caustic circumstances. This surge of activity is strongly driven by the urgent desire for replacing the current Si- and GaAs-based electronics because they are unable to operate properly under harsh environmental conditions. As a promising substitute, the wide-band-gap semiconductor, silicon carbide (SiC), has captured considerable attention recently due to its excellent intrinsic properties, which involve large breakdown electric field, high electron saturation 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
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 . To form such layers, a wide range of materials have been examined in a trial-and-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, most of research activities have been focused on TiAl-based alloys, the only currently available materials that yield significantly low contact resistance (Ohmic contact) to
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.  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 Schottky barrier in the TiAl-based contact system. However, the role of this interface in realizing the Ohmic nature is still unclear. It is not even clear how the two materials atomically bond together from their experiments, which is very important because it may strongly affect physical properties of the system. Theoretically, we have calculated the atomic structures, adhesive energies, and bonding nature of the SiC/Ti3SiC2 interface . However, this calculation does not actually reveal the formation mechanism of Ohmic contact because it only addresses the interface structure. Furthermore, lacking essential experimental information about the interfacial 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 atomic columns, or missing contrasts of light elements. One possible way out to complement the microscopic data is through atomistic calculation, especially the first-principles calculation. 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
In addition to determining atomic structure of the 4H-SiC/Ti3SiC2 interface, the goal of this work is to clarify the formation mechanism of the TiAl-based Ohmic contacts so as to provide 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
2. Role of coherent SiC/Ti3SiC2 interface
The surface morphology of the TiAl contact layers on 4H-SiC after annealing was observed using a JEOL JSM-6060 scanning electron microscope (SEM). Microstructural analysis and identification of the Ti3SiC2 layers at the contact layers/4H-SiC interfaces after annealing was performed using X-ray diffraction (XRD) and cross-sectional TEM. For XRD analysis, Rigaku RINT-2500 with Cu
Calculations of electronic structure and total energy were carried out using the Vienna
To determine the most stable interface theoretically, one first has to establish feasible models on the basis of the distinct terminations and contact sites and then compare them. However, a direct comparison of total energies of such candidate models is not physically meaningful since interfaces might have a different number of atoms. On the other hand, the adhesion energy (
2.1. Atomic-scale structures of the Ohmic contacts
The electric properties for the TiAl contact systems before and after annealing are meaured first to verify the formation of Ohmic contact. The current almost keeps zero before the annealing, while increasing nearly linear with the rise of applied bias, which unambiguously confirms the formation of the Ohmic contacts after annealing. Further XRD analyses demonstrate that a new reaction product, the ternary Ti3SiC2, is generated after annealing, which shows a strongly (0001)-oriented texture. The SiC retains (0001)-oriented texture after annealing, thereby facilitating development of hetero-epitaxy between reaction products and substrates. The TEM imaging reveals that no any other compounds contact directly the SiC surface, thereby ensuring an exclusive contact of Ti3SiC2 to SiC. Since the carbide itself is metallic in nature, the lowering in Schottky barrier in the TiAl-based contacts is hence attributed qualitatively to the epitaxial and atomically sharp SiC/Ti3SiC2 interface. A careful indexing of the selected area diffraction pattern (SADP) at the contacts/SiC interface reveals that the formed Ti3SiC2 layers have epitaxial orientation relationships, that is, (0001)Ti3SiC2//(0001)SiC and [0¯110]Ti3SiC2//[0¯110]SiC, with the SiC substrate. These orientation relationships are believed to be beneficial for forming a coherent and well matched interface between SiC and Ti3SiC2, since they both belong to the hexagonal space group with lattice constants of
A representative HAADF image of the SiC/Ti3SiC2 interface is shown in Fig. 1, which confirms a clean and atomically sharp contact between the two materials. Since intensity of an atomic column in the Scanning TEM, to good approximation, is directly proportional to the square of atomic number (Z) , brighter spots in image represent atomic columns of Ti, while the comparatively darker ones are Si. Not surprisingly, the columns of C are not scattered strongly enough to be visualized owing to its small Z, thereby making this image incomplete. It should be noted that obtaining a signal of pure interfacial carbon is technically very difficult because the specimen can be easily affected by the environmental carbon, thereby precluding the element-selective imaging of carbon. We therefore rely on the first-principles calculations instead to discuss the possibility in the presence of C at the interface, as will be described later. To see the interface clearer, we magnify the cross-sectional HAADF image in Fig. 2(a) and further filter it to reduce noise, as shown in Fig. 2(b). The Si-terminated Ti3SiC2 is observed intuitively to make a direct contact with the Si-terminated SiC substrate with interfacial Si atoms of Ti3SiC2 sitting above hollow sites of interfacial Si plane of SiC. However, this straightforward interpretation is premature, as will be described later. Since there are no pits, spikes, or dislocations that might act as pathways for current transport, we conclude that this clean and coherent SiC/Ti3SiC2 interface should be critical for the formation of Ohmic contact.
To clarify the mechanism, it is prerequisite to determine the atomic structure of the SiC/Ti3SiC2 interface via complementing the obtained HAADF image (Fig. 1). We have considered a total of 96 candidate interfacial geometries using bulklike slabs, taking into account termination effect, stacking sequence, and full optimization. From the bulk 4H-SiC and Ti3SiC2 structures and the relative stacking order of Ti and Si, the observed image in Fig. 2(a) can be intuitively fitted by a SiSi model shown in Fig. 2(c). In this model, the interfacial Si atoms of Ti3SiC2 sit above the hollow sites of interfacial Si plane of SiC, where the optimal distance between interfacial Si-Si planes (denoted as
To resolve these paradoxes, we notice that a possibility might be ignored, that is, the unseen C might be trapped at the interface, altering local environment there. To test this scenario, we established a new model (named SiCSi) by introducing C into the interfacial layer from the consideration of crystal extension and stacking sequences. The calculated
2.2. Electronic structure and bonding
We then presented in Fig. 3(b) DOS projected on selected atomic layers of the SiCSi interface. A key feature in this figure is that a strong interaction is observed between the sub-interfacial Ti
Figure 4 shows contour plots of charge densities and their differences along (11-20) plane for the optimized SiSi and SiCSi interfaces. We notice in Fig. 4(b) that the bonding interaction between interfacial Si and C for the SiCSi interface is remarkably similar to the Si-C interaction deeper into SiC: the majority of charge is localized on C with humps directed towards their neighboring Si. We thus conclude that the interfacial bonding for the SiCSi is of mixed covalent-ionic nature. The interfacial bonds for the SiSi interface, however, have covalent nature with a small amount of charge accumulated within the interfacial region (Fig. 4(a)). In addition, the amount of charge accumulated on the interfacial Si-Si bonds of SiSi (Fig. 4(c)) is far less significant than that on the interfacial Si-C bonds of SiCSi (Fig. 4(d)). This heaver charge accumulation in the case of SiCSi, together with its mixed covalent-ionic character at interface, accounts for the largest
2.3. Quantum electron transport properties
Although the charge-distribution analysis can reveal valuable information on interfacial bonding, it provides restrained insight into how electrons distribute around
To examine electrical conductivity and gain insight into how the interface influences current transport, we devised a two-probe system , Ti/Ti3SiC2/SiC/Ti3SiC2/Ti, and investigated nonequilibrium quantum transport properties. Figure 6 schematically shows a model of the sandwich transport system, which can be divided into a left semi-infinite electrode, a scattering region, and a right semi-infinite electrode. The atomic and electronic structures of the semi-infinite Ti electrodes are assumed to be the same as those of bulk Ti. On the other hand, the electronic states of scattering region are calculated self-consistently. The scattering region consists of hexagonal SiC and Ti3SiC2 layers and the periodic boundary conditions are imposed along the directions parallel to the interface. The SiC/Ti3SiC2 interface could be either the SiSi or SiCSi, whereas other interfaces are maintained identical for the sandwich systems. In this sense, the difference between the two systems can be mainly attributed to their differing SiC/Ti3SiC2 interfaces. Furthermore, we also calculated the Ti/SiC/Ti system, wherein the SiTi model was taken as the SiC/Ti interface.
Figure 7(a) shows transmission spectra for the relaxed SiSi, SiCSi, and SiTi systems, where one can see that the spectra differ from one another suggesting variations in electronic structures with interface geometries. The most interesting feature is the presence of transmission peaks at
3. Atomic-scale Ti3SiC2 bilayers embedded in SiC
3.1. Atomic structure of the embedded system
Figure 8(a) shows a HAADF image of the annealed TiAl/SiC system, where the SiC substrate is covered entirely by a layered compound, Ti3SiC2, as reported previously. In addition to the formation of this epitaxial and coherent SiC/Ti3SiC2 interface, another interesting feature is that an atomic-scale bilayer is generated in the SiC interior (marked by a square in Fig. 8(a)), which is located approximately 9.5 nm away from the interface. An enlarged image of the region surrounding the bilayer shows that it has a Ti3SiC2-like structure, as shown in Fig. 8(b), where brighter spots represent atomic columns of Ti (smaller circles), while dark ones those of Si (larger circles), since intensity of an atomic column in STEM is, to a good approximation, directly proportional to Z1.7 (Z: atomic number) .
Evidently, the bilayer is embedded in the SiC in an atomically coherent and ordered fashion with no transitional or intermixing layers (see Fig. 8(c)). In view of bulk structures of 4H-SiC and Ti3SiC2 and the relative stacking sequence of Ti and Si, the image in Fig. 8(c) can be qualitatively fitted by an energetically stable model shown in Fig. 8(d). In this model, the optimal distances between layers around the bilayer (denoted as
3.2. Formation of point Fermi surface
To gain insight into how the embedded layer influences SiC electronically, we present in Fig. 9 band structure and density of states (DOS) of the multilayer system calculated using the optimal atomic geometry (Fig. 8(d)). Unexpectedly, several bands with a quadratic dispersion cross the Fermi level (
In addition, extensive calculations using the LDA and PBE functional corroborate once again the peculiar crossing of the bands (band structure and DOS spectra are almost identical to those calculated using the PW91), which therefore indicates that the crossing at
3.3. Electronic states
Further investigation of DOS projected on selected atomic layers provides evidence that the bands close to
To shed further light on bonding nature and charge distribution in the multilayer, we present contour plots of charge density (Fig. 11(a)) and its difference (Fig. 11(b)) along the (11-20) plane. From the figures, we notice that (i) majority of charge is localized on C atoms with humps distorted toward their neighboring atoms, suggesting that bonds in both SiC and bilayer are of a mixed covalent-ionic nature, (ii) charge distribution on C in SiC exhibits more pronounced lobes than that on C in the bilayer (Fig. 11(a)), indicative of more covalent element for bonds in SiC, and (iii) ionicity originates from the large charge gain on C at an expense of charge loss on its neighboring cations (Fig. 11(b)). Further examination on electron distribution around
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 (
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
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 for each terrace. Since the intensity of an atomic column in the HAADF image is proportional to
4.2. Electronic states and bonding nature of the terraces
To gain insight into electronic properties of the terraces and the role they played in the ohmic 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), atomic models of the three representative terraces were established (Fig. 16), taking into account 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 local chemical environment, and importantly they exhibit the typical variations of the contact 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 (i) the terrace models have a different number of atoms, (ii) it is unlikely to calculate the exact 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.
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
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 other, which is reflected from their different PDOS (Fig. 17). Furthermore, the charge distribution on the Si-C bonds closet to the zigzag line (Fig. 18(a)) shares some features with that on the Si-C bonds away from the zigzag line (analog to the bonds in SiC bulk): (i) the majority 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
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
Figure 20 shows contour plots of charge density and density difference for the relaxed intermediate 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 (
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
4.3. Electron distribution near Fermi energy
Although the PDOS and charge analyses can reveal valuable information on the bonding nature near terrace, they provide limited insights into matters regarding electron distribution around
5. Discussion and conclusions
The current understanding of formation origin of Ohmic contact, which is based mainly on experimental studies of property measurement and structure characterization, can be summarized 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
The findings presented first demonstrate that no Al is clearly segregated around the interfacial region, in particular at the top few layers of SiC, which rules out the possibility of additional 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 layer directly contacted to the SiC, thus playing a negligible role in Ohmic contact formation. The majority of deposited Al is evaporated during annealing because of its low melting point and high equilibrium vapor pressure. The dominant role played by Al in the TiAl system is to assist the formation of liquid alloy so as to facilitate chemical reaction. Furthermore, 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.
One remaining theory is the alloy-assisted Ohmic contact formation. This alloy is determined 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
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 DFT and the TiNi Ohmic contacts on 4H-SiC experimentally by Auger electron spectroscopy. It was proposed that the carbon could be segregated to the interfacial area, strengthening 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 Ti3SiC2-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 found 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 Ti3SiC2 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  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 Ti3SiC2-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 Ti3SiC2 can even be epitaxial and atomically ordered on SiC substrate near terrace, which inudces pronounced gap states at EF 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.
The author acknowledges M. Saito, S. Tsukimoto, at the WPI Research Center, Advanced Institute for Materials Research, Tohoku University and Y. Ikuhara at the The University of Tokyo for their collaborations. The author thanks S. Watanabe at The University of Tokyo for allowing our use of computational resources. The present study was supported in part by a Grant-in-Aid for Scientific Research on Priority Area, “Atomic Scale Modification (474)” from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. Z. W acknowledges financial supports from the Grant-in-Aid for Young Scientists (A) (Grant No. 24686069) and the Challenging Exploratory Research (Grant No. 24656376). The calculations were carried out on a parallel SR11000 supercomputer at the Institute for Solid State Physics, Univ. of Tokyo.
Perez-Wurfl I. Krutsinger R. Torvik J. T. Van Zeghbroeck B. 2003 4H-SiC Bipolar Junction Transistor with High Current and Power Density
Lee S. K. Zetterling C. M. Danielsson E. Östling M. Palmquist J. P. Högberg H. Jansson U. 2000 Electrical Characterization of TiC Ohmic Contacts to Alumimum Ion Implanted 4H-Silicon Carbide
Pécz B. 2001 Contact Formation in SiC Devices
Crofton J. Barnes P. A. Williams J. R. 1993 Contact Resistance Measurements on p-type 6H-SiC
Mohney S. E. Hull B. A. Lin J. Y. Crofton J. 2002 Morphological Study of the Al-Ti Ohmic Contact to p-type SiC
Nakatsuka O. Takei T. Koide Y. Murakami M. 2002 Low Resistance TiAl Ohmic Contacts with Multi-Layered Structure for p-Type 4H-SiC
Johnson B. J. Capano M. A. 2004 Mechanism of Ohmic Behavior of Al/Ti Contacts to p-type 4H-SiC After Annealing
Chang S. C. Wang S. J. Uang K. M. Liou B. W. 2005 Investigation of Au/Ti/Al Ohmic Contact to N-type 4H-SiC
Ohyanagi T. Onose Y. Watanabe A. 2008 Ti/Ni Bilayer Ohmic Contact on 4H-SiC
Tsukimoto S. Nitta K. Sakai T. Moriyama M. Murakami M. 2004 Correlation Between the Electrical Properties and the Interfacial Microstructurs of TiAl-Based Ohmic Contacts to p-type 4H-SiC
Wang Z. C. Tsukimoto S. Saito M. Ikuhara Y. 2009 SiC/Ti3SiC2 Interface: Atomic Structure, Energetics, and Bonding
Nellist P. D. Chisholm M. F. Dellby N. Krivanek O. L. Murfitt M. F. Szilagyi Z. S. Lupini A. R. Borisevich A. Sides Jr W. H. Pennycook S. J. 2004 Direct Sub-Angstronm Imaging of a Crystal Lattice
Wang Z. C. Saito M. Mc Kenna K. P. Gu L. Tsukimoto S. Shluger A. L. Ikuhara Y. 2011 Atom-Resolved Imaging of Ordered Defect Superstructures at Individual Grain Boundaries
Wang Z. C. Tsukimoto S. Saito M. Ikuhara Y. 2009 Atomic and Electronic Structure of the YBa2Cu3O7/SrTiO3 Interface from First Principles
Wang Z. C. Tsukimoto S. Saito M. Ikuhara Y. 2009 Individual Charge-Trapping Dislocations in an Ionic Insulator
Wang Z. C. Zeng W. Gu L. Saito M. Tsukimoto S. Ikuhara Y. 2010 Atomic-Scale Structure and Electronic Property of the LaAlO3/TiO2 Interface
Wang Z. C. Saito M. Tsukimoto S. Ikuhara Y. 2009 Interface Atomic-Scale Structure and its Impact on Quantum Electron Transport.
Wang Z. C. Tsukimoto S. Saito M. Ito K. Murakami M. Ikuhara Y. 2009 Ohmic Contacts on Silicon Carbide: The First Monolayer and its Electronic Effect
Wang Z. C. Tsukimoto S. Sun R. Saito M. Ikuhara Y. 2011 Atomic-Scale Ti3SiC2 Bilayers Embedded in SiC: Formation of Point Fermi Surface
Wang Z. C. Saito M. Tsukimoto S. Ikuhara Y. 2012 Terraces at Ohmic Contact in SiC Electronics: Structure and Electronic States
Kresse G. Hafner J. 1993 Ab initio Molecular Dynamics for Liquid Metals
Wang X. G. Smith J. R. Evans A. 2002 Fundamental Inflenence of C on Adhesion of the Al2O3/Al Interface
Wang Z. C. Saito M. Tsukimoto S. Ikuhara Y. 2011 Heterointerfaces: Atomic Structures, Electronic States, and Related Properties
Pennycook S. J. Boatner L. A. 1988 Chemically Sensitive Structure-Imaging with a Scanning Transmission Electron Microscope
Ching W. Y. Xu Y. N. Rulis P. Ouyang L. Z. 2006 The Electronic Structure and Spectroscopic Properties of 3C, 2H, 4H, 6H, 15R and 21R Polymorphs of SiC
Wang Z. C. Kadohira T. Tada T. Watanabe S. 2007 Nonequilibrium Quantum Transport Properties of a Silver Atomic Switch
Gao M. Tsukimoto S. Goss S. H. Tumakha S. P. Onishi T. Murakami M. Brillson L. J. 2007 Role of Interface Layers and Localized States in TiAl-Based Ohmic Contacts to p-Type 4H-SiC
Tanimoto S. Kiritani N. Hoshi M. Okushi H. 2002 Ohmic Contact Structure and Fabrication Process Applicable to Practical SiC Devices
Morkoc H. Strite S. Gao G. B. Lin M. E. Sverdlov B. Burns M. 1994 Large-band-gap SiC, III-V Nitride, and II-VI ZnSe-based Semiconductor Device Technologies
Lu W. Mitchel W. C. Thornton CA Collins. W. E. Landis G. R. Smith S. R. 2003 Ohmic Contact Behavior of Carbon Films on SiC
Wang Z. C. Okude M. Saito M. Tsukimoto S. Ohtomo A. Tsukada M. Kawasaki M. Ikuhara Y. 2010 Dimensionality-Driven Insulator-Metal Transition in A-site Excess Non-stoichiometric Perovskites