Role of the Potential Barrier in the Electrical Performance of the Graphene/SiC Interface
In spite of the great expectations for epitaxial graphene (EG) on silicon carbide (SiC) to be used as a next-generation high-performance component in high-power nano-and micro-electronics, there are still many technological challenges and fundamental problems that hinder the full potential of EG/SiC structures and that must be overcome. Among the existing problems, the quality of the graphene/SiC interface is one of the most critical factors that determines the electroactive behavior of this
... behavior of this heterostructure. This paper reviews the relevant studies on the carrier transport through the graphene/SiC, discusses qualitatively the possibility of controllable tuning the potential barrier height at the heterointerface and analyses how the buffer layer formation affects the electronic properties of the combined EG/SiC system. The correlation between the sp 2 /sp 3 hybridization ratio at the interface and the barrier height is discussed. We expect that the barrier height modulation will allow realizing a monolithic electronic platform comprising different graphene interfaces including ohmic contact, Schottky contact, gate dielectric, the electrically-active counterpart in p-n junctions and quantum wells. Crystals 2017, 7, 162 2 of 18 temperature sublimation). In this context, many attempts at growing epitaxial graphene were dedicated to breaking up the covalent bonds between SiC and zero layer graphene and to saturating as-formed dangling bonds by guest species, so-called intercalants           . Experimental studies clearly reveal a strong effect of the buffer layer on the electronic properties of graphene; specifically, it was documented that due to the charge transfer through interfacial dangling bonds, the buffer layer is found to pin the Fermi level to ≈0.49 eV in the conduction band, making the material n-type  . As a consequence of the charge transfer from the interface states to graphene and spontaneous appearance of the interface dipole moment, the Fermi level and work function of graphene can be modulated, thereby determining its electroactive behavior  . Furthermore, the existence of a giant inelastic tunneling (50% of total tunneling current) caused by localized states at the interface layer of graphene/SiC was confirmed by atomically-resolved scanning tunneling microscopy and spectroscopy  . Another surprising fact related to the buffer layer effect was the observation of a small gap (~0.26-0.5 eV) in epitaxial graphene on SiC induced by breaking the sublattice symmetry, but the fundamental nature of this band gap opening is controversial [18, 19] . New insights into the origin of the band-gap opening induced by the structural periodicity in the epitaxial graphene buffer layer have been recently reported by Nair et al  . Taking the aforementioned aspects into account, it is reasonable to assume that a control of the sp 2 /sp 3 hybridization ratio in epitaxial graphene is a good strategy towards the atomistic-level engineering of the graphene/SiC heterointerface to tailor the electronic properties of graphene. Indeed, it was recently reported that epitaxial graphene, depending on the material quality, can play different roles when being interfaced with SiC, such as the ohmic contact  , the Schottky contact  , the gate electrode  , the heterojunction counterpart  and/or even the quantum well component  . Although a great deal of attention has been paid to buffer layer effects, the physical reasons why the same material exhibits such diverse electronic features are not fully understood. There is still a point to be discussed: the correlation between buffer layer "physics" and mechanisms/possible scenarios underlying the electroactive behavior of epitaxial graphene. Undoubtedly, for a deep understanding and correct interpretation of experimental data on the electronic properties of epitaxial graphene, one should pay proper attention to the role of the buffer layer. On the other hand, the presence of interfacial states at the heterointerface may be responsible for other physical processes and phenomena underlying the heat transport and ferromagnetism. In particular, perturbation of the ballistic heat transport caused by strong phonon scattering at the graphene/SiC interface was discussed in  . Thermal transport through graphene/SiC depending on the kind of SiC polytype, face polarity and atomic bond has been intensively investigated in    . These results show that the heat transfer is highly sensitive to the kind of interface between the graphene and SiC. Thus, solving the so-called thermal management problem can be achieved via controlling the geometry and the chemical nature of the interface region, i.e., the buffer layer. As another example of the crucial role of the interfacial layer on the physical properties of graphene, Giesbers et al.  have reported on strong room temperature ferromagnetism (with magnetic moment of 0.9 µB per carbon hexagon projected area) and suggested that such ferromagnetic behavior may be attributed to an exchange interaction between the Coulomb-induced localized silicon dangling bonds (belonging to the buffer layer) and the localized mid-gap state. Zhou et al.  have also proposed some ideas towards using graphene/SiC interface-induced magnetism for spintronic applications. The aim of this paper is to review the current status of the main experimental and theoretical studies of graphene on SiC towards understanding the physical nature of the interfacial layer formation and how this layer manifests itself in the carrier transport. In the next section, key features of the buffer layer structure will be described. We will discuss possible scenarios regarding the buffer layer-assisted interaction between graphene and SiC: we will show that the electroactive behavior of epitaxial graphene is strongly dependent on the interface chemistry and quality of the buffer layer. Finally, we make some concluding remarks regarding the relation between the quality of the graphene-SiC heteroboundary and the expected behavior of epitaxial graphene on SiC. Crystals 2017, 7, 162 3 of 18 Electrical Properties of the Graphene/SiC Interface Being combined in a single system, graphene and silicon carbide exhibit unique behavior under the influence of an external electric field, which differs from the behavior of the contact between the metal and the semiconductor under classical considerations. In the first place, the difference in the properties is caused by a possibility to control the work function of graphene and the polarizability of its π orbitals [31, 32] . Therefore, the energy properties of the heterojunction may be purposefully altered by changing the interfacial chemistry between the materials. Since the presence of the buffer layer significantly influences the electrical properties of the graphene/SiC structure, first of all, it is important to understand how current flows through a buffer-free graphene/SiC structure. From the theoretical point of view, there are, at least, three cases when we can avoid the formation of the buffer layer: Figure 2. Band line-up at the interfaces between epitaxial graphene and (left panel) the Si-terminated 6H-SiC (0001) and (right panel) the C-terminated 6H-SiC . E C and E V represent the energies of the conduction and valence band edge, respectively. E F is the Fermi energy. The Fermi levels of the two materials are aligned. E G is the band gap energy of the 6H-SiC. Reprinted from Chen et al. .