Epitaxial graphene perfection vs. SiC substrate quality

Polytype instability of SiC epitaxial films was the main focus of attention in the experiment performed since this factor has a decisive influence on graphene growth, which was the second stage of the experiment. Layers deposited in various initial C/Si ratios were analyzed.     

On a possibility of negative cyclotron resonance in semiconductors

A possibility of negative conductance at the cyclotron resonance frequency of hot carriers in crossed electric and magnetic fields is investigated. It relies on sufficiently strong carrier-optical phonon interaction compared with other scattering processes. The condition is examined numerically but seems to hardly be satisfied in presently available crystals.     

Electronic structure of epitaxial graphene layers on SiC: effect of the substrate

A strong substrate-graphite bond is found in the first all-carbon layer by density functional theory calculations and x-ray diffraction for few graphene layers grown epitaxially on SiC. This first layer is devoid of graphene electronic properties and acts as a buffer layer. The graphene nature of the film is recovered by the second carbon layer grown on both the (0001) and (0001[over]) 4H-SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. Hence the graphene is doped and a gap opens at the Dirac point after three Bernal stacked carbon layers are formed.     

Nucleation of epitaxial graphene on SiC substrate by thermal annealing and chemical vapor deposition

Growth of epitaxial graphene (EG) on silicon carbide (SiC) is regarded as one of the most effective routes to high-quality graphene towards practical applicability. We try to build up a model to illuminate the nucleation process of EG on SiC by thermal decomposition. The model is derived from some experimental results and discloses that surface diffusion plays an important role in the nucleation. For the chemical vapor deposition process used, the organic gas as carbon precursor enables carbon deposition quickly for supporting the growth of high-quality graphene via vapor transformation, so that the nucleated and final graphene becomes almost stress-free and mimics the free-standing graphene. Our findings have a potential in preparing high-quality graphene by controlling the nucleation conditions.     

Intra-Landau-level cyclotron resonance in bilayer graphene

Interaction driven integer quantum-Hall effects are anticipated in graphene bilayers because of the near degeneracy of the eight Landau levels which appear near the neutral system Fermi level. We predict that an intra-Landau-level cyclotron resonance signal will appear at some odd-integer filling factors, accompanied by collective modes which are nearly gapless and have approximate k3/2 dispersion. We speculate on the possibility of unusual localization physics associated with these modes.     

Direct formation of graphene layers on top of SiC during the carburization of Si substrate

We grow graphene film on silicon substrates having various orientations by simple heating in the presence of carbon source gas. We observed that a 3C-SiC (111) film would form upon carburizing silicon with carbon deposited from a carbon source because it is well lattice-matched with Si (110) (less than 2%). Graphene grew on the buffer layer of 3C-SiC (111). The surface consists of hexagonal arrays that can act as a template for graphene growth. This simple and inexpensive method of forming graphene on silicon wafer in situ is compatible with silicon technology.     

Superlattice based on graphene on a strip substrate

A graphene-based superlattice formed due to the periodic modulation of the band gap has been investigated. Such a modulation is possible in graphene deposited on a strip substrate made of silicon oxide and hexagonal boron nitride. The advantages and some possible problems in the superlattice under consideration are discussed. A model describing such a superlattice is proposed and the dispersion relation between the energy and momentum of carriers has been obtained using the transfer matrix method within this model.     

Temperature dependence of the cyclotron resonance in graphene

The dynamic conductivity of graphene in a magnetic field under the conditions of cyclotron resonance has been calculated. The effect of the temperature on cyclotron resonance has been analyzed. So-called multiple resonances have been considered. A dimensionless quantity determining the width and amplitude of the cyclotron peak in the conductivity has been found. Coulomb corrections to the electronic spectrum have been discussed.     

Transition of graphene on a substrate to a semimetallic state

Graphene on a substrate has been shown to exhibit a transition, depending on the substrate material, from a zero-gap semiconductor state to a semimetallic state. The ground-state energy of the electron (hole) gas has been calculated within the random-phase approximation.     

The physics of epitaxial graphene on SiC(0001)

Various physical properties of epitaxial graphene grown on SiC(0001) are studied. First, the electronic transport in epitaxial bilayer graphene on SiC(0001) and quasi-free-standing bilayer graphene on SiC(0001) is investigated. The dependences of the resistance and the polarity of the Hall resistance at zero gate voltage on the top-gate voltage show that the carrier types are electron and hole, respectively. The mobility evaluated at various carrier densities indicates that the quasi-free-standing bilayer graphene shows higher mobility than the epitaxial bilayer graphene when they are compared at the same carrier density. The difference in mobility is thought to come from the domain size of the graphene sheet formed. To clarify a guiding principle for controlling graphene quality, the mechanism of epitaxial graphene growth is also studied theoretically. It is found that a new graphene sheet grows from the interface between the old graphene sheets and the SiC substrate. Further studies on the energetics reveal the importance of the role of the step on the SiC surface. A first-principles calculation unequivocally shows that the C prefers to release from the step edge and to aggregate as graphene nuclei along the step edge rather than be left on the terrace. It is also shown that the edges of the existing graphene more preferentially absorb the isolated C atoms. For some annealing conditions, experiments can also provide graphene islands on SiC(0001) surfaces. The atomic structures are studied theoretically together with their growth mechanism. The proposed embedded island structures actually act as a graphene island electronically, and those with zigzag edges have a magnetoelectric effect. Finally, the thermoelectric properties of graphene are theoretically examined. The results indicate that reducing the carrier scattering suppresses the thermoelectric power and enhances the thermoelectric figure of merit. The fine control of the Fermi energy position is thought to be key for the practical use of graphene as a thermoelectric material, which could be achieved with epitaxial graphene. All of these results reveal that epitaxial graphene is physically interesting.     

Ab initio study of graphene on SiC

Employing density-functional calculations we study single and double graphene layers on Si- and C-terminated 1x1-6H-SiC surfaces. We show that, in contrast with earlier assumptions, the first carbon layer is covalently bonded to the substrate and cannot be responsible for the graphene-type electronic spectrum observed experimentally. The characteristic spectrum of freestanding 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.     

Epitaxial gallium oxide on a SiC/Si substrate

Well-textured gallium oxide β-Ga₂O₃ layers with a thickness of ~1 μm and a close to epitaxial layer structure were grown by the method of chloride vapor phase epitaxy on Si(111) wafers with a nano-SiC buffer layer. In order to improve the growth, a high-quality silicon carbide buffer layer ~100 nm thick was preliminarily synthesized by the substitution of atoms on the silicon surface. The β-Ga₂O₃ films were thoroughly investigated using reflection high-energy electron diffraction, ellipsometry, X-ray diffraction, scanning electron microscopy, and micro-Raman spectroscopy. The investigations revealed that the films are textured with a close to epitaxial structure and consist of a pure β-phase Ga₂O₃ with the (\(\overline 2 01\)) orientation. The dependence of the dielectric constant of epitaxial β-Ga₂O₃ on the photon energy ranging from 0.7 to 6.5 eV in the isotropic approximation was measured.     

About the linewidth of cyclotron resonance in band-gap graphene

The critical amplitude of circularly polarized electromagnetic wave when the hysteresis of cyclotron absorption takes place, was found for band-gap graphene. The dependence of critical amplitude on the gap value and on the relaxation time was investigated. The conditions of applicability of linear theory describing the electromagnetic response of band-gap graphene in a non-zero magnetic field were found. The power of the circularly polarized electromagnetic radiation absorbed by band-gap graphene in the presence of a magnetic field was calculated. The linewidth of cyclotron absorption was shown to be not zero even for pure band-gap graphene.     

The design and construction of a pulsed beam generation system based on high intensity cyclotron

In order to perform the studies on a pulsed beam generation system based on a high intensity cyclotron, a test beam line with a pulsed beam generation for a 10 MeV compact cyclotron (CYCIAE-10) has been designed and constructed at China Institute of Atomic Energy (CIAE). A 70 MHz continuous H-beam can be pulsed to the pulse length of less than 10 ns with a repetition rate of 4.4 MHz. The sine waveform with a frequency of 2.2 MHz is adopted for the chopper and a mesh structure with single drift and dual gaps is used for the 70 MHz buncher. A helical resonator is designed and constructed based on simulations and experiments on the RF matching for the chopper. A helical inductance loop that is exceptionally large of its kind and equipped with water cooling for the resonator has been successfully wound and a 500 W solid RF amplifier has been manufactured. A special measuring device has been designed, which can be used to measure both the DC beam and the pulsed beam. The required pulsed beam was obtained after pulsed beam tuning.

     

The design and construction of a pulsed beam generation system based on high intensity cyclotron

In order to perform the studies on a pulsed beam generation system based on a high intensity cyclotron, a test beam line with a pulsed beam generation for a 10 MeV compact cyclotron (CYCIAE-10) has been designed and constructed at China Institute of Atomic Energy (CIAE). A 70 MHz continuous H-beam can be pulsed to the pulse length of less than 10 ns with a repetition rate of 4.4 MHz. The sine waveform with a frequency of 2.2 MHz is adopted for the chopper and a mesh structure with single drift and dual gaps is used for the 70 MHz buncher. A helical resonator is designed and constructed based on simulations and experiments on the RF matching for the chopper. A helical inductance loop that is exceptionally large of its kind and equipped with water cooling for the resonator has been successfully wound and a 500 W solid RF amplifier has been manufactured. A special measuring device has been designed, which can be used to measure both the DC beam and the pulsed beam. The required pulsed beam was obtained after pulsed beam tuning.     

Raman analysis of epitaxial graphene grown on 4Hben SiC (0001) substrate under low pressure condition

In this paper, we report a feasible route of growing epitaxial graphene on 4H-SiC (0001) substrate in a low pressure of 4 mbar (1 bar=10⁵ Pa) with an argon flux of 2 standard liters per minute at 1200, 1300, 1400, and 1500 ℃ in a commercial chemical vapour deposition SiC reactor. Using Raman spectroscopy and scanning electron microscopy, we confirm that epitaxial graphene evidently forms on SiC surface above 1300 ℃ with a size of several microns. By fitting the 2D band of Raman data with two-Lorentzian function, and comparing with the published reports, we conclude that epitaxial graphene grown at 1300 ℃ is four-layer graphene.     

On the possibility of harmonic operation of cyclotron waveparametric amplifiers

Cyclotron wave amplifiers at the harmonics of the electron cyclotron frequency are investigated. Since the waves on the beam are electrostatic, harmonics are strongly excited in nonrelativistic beams if they are rotating rather than filamentary. These modes at the harmonics can couple to input Cuccia couplers, and pump fields which drive parametric amplification, in very much the same way as they do on filamentary beams at the cyclotron frequency. Harmonic cyclotron wave amplifiers have the possibility of giving rise to a new class of devices at millimeter wave frequencies