A blueprint for producing scalable digital graphene electronics has remained elusive. Current methods to produce semiconducting-metallic graphene networks all suffer from either stringent lithographic demands that prevent reproducibility, process-induced disorder in the graphene, or scalability issues. Using angle resolved photoemission, we have discovered a unique one-dimensional metallic-semiconducting-metallic junction made entirely from graphene, and produced without chemical functionalization or finite size patterning. The junction is produced by taking advantage of the inherent, atomically ordered, substrate-graphene interaction when it is grown on SiC, in this case when graphene is forced to grow over patterned SiC steps. This scalable bottomup approach allows us to produce a semiconducting graphene strip whose width is precisely defined within a few graphene lattice constants, a level of precision entirely outside modern lithographic limits. The architecture demonstrated in this work is so robust that variations in the average electronic band structure of thousands of these patterned ribbons have little variation over length scales tens of microns long. The semiconducting graphene has a topologically defined few nanometer wide region with an energy gap greater than 0.5 eV in an otherwise continuous metallic graphene sheet. This work demonstrates how the graphene-substrate interaction can be used as a powerful tool to scalably modify graphene's electronic structure and opens a new direction in graphene electronics research.Patterning a flat graphene sheet to alter its electronic structure was envisaged to be the foundation of graphene electronics. 1 The early focus was to open a finite-size gap in lithographically patterned nanoribbons, a necessary step for digital electronics. 1-5 While early transport measurements supported this possibility, 6 it soon became apparent that these "transport gaps" originated from a series of mismatched-level quantum dots caused by the inability of current lithographically to produce sufficiently narrow, well ordered, and crystallography define graphene edges. 7-10 A working solution to the graphene "gap problem" has yet to be formulated, let alone demonstrated. We show that in fact such a solution exists, not by patterning graphene, but instead by controlling the graphene-substrate geometry.We have been able to construct a unique, reproducible, and scalable semiconducting graphene ribbon with a gap larger than 0.5 eV. Using pre-patterned SiC trenches to force graphene to bend between a high symmetry (0001) face to a low symmetry (112n) facet, we produce a narrow curved graphene bend with localized strain. This "topologically-defined" ribbon is a wide-gap graphene semiconductor strip a few lattice constants wide that extends hundreds of microns long. The strip is connected seamlessly to metallic graphene sheets on both of its sides. One metallic sheet is n-doped and the other pdoped. From this simple morphology, we have not only produced a gap suitable for room temperature...

The maximum oscillation frequency (fmax) quantifies the practical upper bound for useful circuit operation. We report here an fmax of 70 GHz in transistors using epitaxial graphene grown on the C-face of SiC. This is a significant improvement over Si-face epitaxial graphene used in the prior high-frequency transistor studies, exemplifying the superior electronics potential of C-face epitaxial graphene. Careful transistor design using a high κ dielectric T-gate and self-aligned contacts further contributed to the record-breaking fmax.

We report the observation of the fractional quantum Hall effect in the lowest Landau level of a two-dimensional electron system (2DES), residing in the diluted magnetic semiconductor Cd 1−x Mn x Te. The presence of magnetic impurities results in a giant Zeeman splitting leading to an unusual ordering of composite fermion Landau levels. In experiment, this results in an unconventional opening and closing of fractional gaps around the filling factor ν = 3/2 as a function of an in-plane magnetic field, i.e., of the Zeeman energy. By including the s-d exchange energy into the composite Landau level spectrum the opening and closing of the gap at filling factor 5/3 can be modeled quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES provides a means to manipulate fractional states. The fractional quantum Hall effect (FQHE) is a collective high-magnetic field phenomenon, originating from Coulomb repulsion of electrons confined in two dimensions. At certain fractional fillings, ν = p/q, of the Landau levels (LLs) (ν = filling factor, p,q = integers), quantized plateaus in the Hall resistance ρ xy and the vanishing longitudinal resistance ρ xx herald the presence of peculiar electron correlations [1,2]. Here, the electrons condense into a liquidlike ground state that is separated by a gap from the excited states. Most experiments to date have been carried out on GaAsbased systems, being still the cleanest material system with the highest electron mobilities [3]. When the direction of the magnetic field B is tilted, the orbital LL splitting is given by the field component B ⊥ normal to the two-dimensional electron system (2DES) while the total field strength B determines the Zeeman splitting E Z . Early experiments on GaAs revealed that the ν = 4/3, 5/3, and 8/5 states behaved differently upon tilting the sample [4,5]: While the ν = 4/3 and 8/5 states were undergoing a transition from a spinunpolarized state to a polarized one, the ν = 5/3 state was always fully spin polarized.Although the FQHE has been reported in quite a number of different materials [6][7][8][9][10][11][12], the FQHE has never been observed in a diluted magnetic semiconductor in which atoms with magnetic moment (e.g., Mn 2+ ) are placed in a 2DES. Then, the localized spins in the magnetic impurities' d orbitals interact with the correlated electron system via the quantum mechanical s-d exchange interaction, causing giant Zeeman splitting [13] which is tunable in magnitude, sign, and field dependence [14]. The constant αN 0 specifies the s-d exchange strength and is the largest energy scale in the system. It hence has remained unclear whether FQHE states survive in the presence of magnetic impurities. Below we demonstrate that (i) the FQHE indeed exists in magnetic 2DESs and (ii) the opening and closing of gaps in an in-plane field can be described within a modified composite fermion (CF) picture, in which the s-d exchange is taken into account.Let us first recall the CF model which maps the FQHE onto the integer quantum Hall effe...

We have measured optical absorption in mid-infrared spectral range on hydrogen intercalated single layer epitaxial graphene and buffer layer grown on silicon face of SiC. We have used attenuated total reflection geometry to enhance absorption related to the surface and SiC/graphene interface. The Raman spectroscopy is used to show presence of buffer layer and single layer graphene prior to intercalation. We also present Raman spectra of quasi free standing monolayer and bilayer graphene after hydrogen intercalation at temperatures between 790 and 1510°C. We have found that although the Si-H bonds form at as low temperatures as 790°C, the well developed bond order has been reached only for samples intercalated at temperatures exceeding 1000°C. We also study temporal stability of hydrogen intercalated samples stored in ambient air. The optical spectroscopy shows on a formation of silyl and silylene groups on the SiC/graphene interface due to the residual atomic hydrogen left from the intercalation process.

The fractional quantum Hall ͑FQH͒ effect is reported in a high mobility CdTe quantum well at millikelvin temperatures. Fully developed FQH states are observed at filling factor 4/3 and 5/3 and are found to be both spin-polarized ground state for which the lowest energy excitation is not a spin flip. This can be accounted for by the relatively high intrinsic Zeeman energy in this single valley two-dimensional electron gas. FQH minima are also observed in the first excited ͑N =1͒ Landau level at filling factor 7/3 and 8/3 for intermediate temperatures. In contrast, the 5/2 FQH state remains absent down to T ϳ 10 mK. Interacting carriers in certain fractional quantum Hall ͑FQH͒ ground states can have opposite spins provided the Zeeman energy is sufficiently small. This is typically observed in GaAs-based two-dimensional electron gases ͑2DEGs͒, where an increase in the Zeeman energy induces a change in the spin polarization of the ground state from unpolarized to fully spin polarized. This transition has been reported for the FQH states at filling factor =4/ 3, =8/ 5, =2/ 3, or =2/ 5, 1-4 as well as in a GaAs 2D hole gas. 5 Subsequently, this behavior was elegantly interpreted within the composite fermions ͑CFs͒ model 6 for the FQH effect by invoking Zeeman energy-induced crossings between spinsplit composite fermion Landau levels ͑LLs͒, leading to possible changes in the spin configuration of the ground state. 7 More recently, the =4/ 3 FQH state was investigated in a strained Si quantum well, 8 where the associated resistance minimum was found to maintain its strength with increasing Zeeman energy, which was interpreted as the consequence of a spin-polarized ground state. The latter work addresses the interesting question of how the FQH effect manifests itself in a 2D system with an intrinsically larger Zeeman energy than in GaAs. However, the influence of the valley degeneracy inherent in Si is another degree of freedom that may also interfere with the FQH physics.In the present work, we study the evolution of FQH states under relatively high intrinsic Zeeman energy in a single valley electron system. This is made possible by investigating the FQH effect in a high-quality 2D electron gas in CdTe, a single valley, direct gap, semiconductor in which the bare electronic g factor is about four times larger than in GaAs. A fundamental asset of this system is the possibility to incorporate magnetic ions to form a so-called diluted magnetic semiconductor, which offers possible applications in the fields of spintronics and quantum computing. The transport measurements performed at millikelvin temperature reveal fully developed FQH states ͑i.e., zero longitudinal resistance and exact quantization of the Hall resistance͒ in the upper spin branch of the lowest ͑N =0͒ LL, which constitutes to our knowledge the first observation of the FQH effect in a II-VI semiconductor. Tilted magnetic fields experiments up to 28 T show no significant changes in the FQH gap both at filling factor 4/3 and 5/3, a behavior typical of spin-pola...

The development of graphene electronics [1,2] requires the integration of graphene devices withSi-CMOS technology. Most strategies involve the transfer of graphene sheets onto silicon, with the inherent difficulties of clean transfer [3][4][5] and subsequent graphene nano-patterning that degrades considerably the electronic mobility of nanopatterned graphene [6,7] . Epitaxial graphene (EG) by contrast is grown on an essentially perfect crystalline (semi-insulating)surface, and graphene nanostructures with exceptional properties [8][9][10][11] have been realized by a selective growth process on tailored SiC surface that requires no graphene patterning [9,12,13] .However, the temperatures required in this structured growth process are too high for silicontechnology. Here we demonstrate a new graphene to Si integration strategy, with a bonded and interconnected compact double-wafer structure. Using silicon-on-insulator technology (SOI) [14][15][16] a thin monocrystalline silicon layer ready for CMOS processing is applied on top of epitaxial graphene on SiC. The parallel Si and graphene platforms are interconnected by metal vias. This method inspired by the industrial development of 3d hyper-integration stacking thin-film electronic devices [17,18] preserves the advantages of epitaxial graphene and enables the full spectrum of CMOS processing.

A method is proposed to extract pure Raman spectrum of epitaxial graphene on SiC by using a Non-negative Matrix Factorization. It overcomes problems of negative spectral intensity and poorly resolved spectra resulting from a simple subtraction of a SiC background from the experimental data. We also show that the method is similar to deconvolution, for spectra composed of multiple sub- micrometer areas, with the advantage that no prior information on the impulse response functions is needed. We have used this property to characterize the Raman laser beam. The method capability in efficient data smoothing is also demonstrated.Comment: 3 figures, regular pape

Magnetotransport measurements of Shubnikov-de Haas (SdH) oscillations have been performed on twodimensional electron gases (2DEGs) confined in CdTe and CdMnTe quantum wells. The quantum oscillations in CdMnTe, where the 2DEG interacts with magnetic Mn ions, can be described by incorporating the electron-Mn exchange interaction into the traditional Lifshitz-Kosevich formalism. The modified spin splitting leads to characteristic beating pattern in the SdH oscillations, the study of which indicates the formation of Mn clusters resulting in direct anti-ferromagnetic Mn-Mn interaction. The Landau-level broadening in this system shows a peculiar decrease with increasing temperature, which could be related to statistical fluctuations of the Mn concentration.