Direct measurement of the growth mode of graphene on SiC(0001) and SiC(0001¯)

We have determined the growth mode of graphene on SiC(0001) and SiC(0001ˉ) using ultrathin, isotopically labeled Si(13)C "marker layers" grown epitaxially on the Si(12)C surfaces. Few-layer graphene overlayers were formed via thermal decomposition at elevated temperature. For both surface terminations (Si face and C face), we find that the (13)C is located mainly in the outermost graphene layers, indicating that, during decomposition, new graphene layers form underneath existing ones.     

Temperature dependence of the thickness and morphology of epitaxial graphene grown on SiC （0001） wafers

Epitaxial graphene is synthesized by silicon sublimation from the Si-terminated 6H–SiC substrate. The effects of graphitization temperature on the thickness and surface morphology of epitaxial graphene are investigated. X-ray photoelectron spectroscopy spectra and atomic force microscopy images reveal that the epitaxial graphene thickness increases and the epitaxial graphene roughness decreases with the increase in graphitization temperature. This means that the thickness and roughness of epitaxial graphene films can be modulated by varying the graphitization temperature. In addition, the electrical properties of epitaxial graphene film are also investigated by Hall effect measurement.     

Raman Spectrum of Epitaxial Graphene on SiC （0001） by Pulsed Electron Irradiation

We report new Raman features of epitaxial graphene （EG） on Si-face 4H-SiC prepared by pulsed electron irradiation （PEI）. With increasing graphene layers, frequencies of G and 2D peaks show blue-shifts and approach those of bulk highly-oriented pyrolytic graphite. It is indicated that the EG is slightly tension strained and tends to be strain-free. Meanwhile, single Lorentzian line shapes are well fitted to the 2D peaks of EG on SiC（O001） and their full widths at half maximum decrease with the increasing graphene layers, which indicates that the multilayer EG on Si-face can also contain turbostratic stacking by our PEI route instead of only AB Bernal stacking by a traditional thermal annealing method. It is worth noting that the stacking style plays an important role on the charge carrier mobility. Therefore our findings will be a candidate for growing quality graphene with high carrier mobility both on the Si- and C-terminated SiC substrate. Mechanisms behind the features are studied and discussed.     

Comparative study of adsorption characteristics of Cs on the GaN(0001) and GaN(000) surfaces

The adsorption characteristics of Cs on GaN(0001) and GaN(000) surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave ultrasoft pseudopotential method based on first-principles calculations.The results show that the most stable position of the Cs adatom on the GaN(0001) surface is at the N-bridge site for 1/4 monolayer coverage.As the coverage of Cs atoms at the N-bridge site is increased,the adsorption energy reduces.As the Cs atoms achieve saturation,the adsorption is no longer stable when the coverage is 3/4 monolayer.The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer,and then rises with Cs atomic coverage.The most stable position of Cs adatoms on the GaN(000) surface is at H3 site for 1/4 monolayer coverage.As the Cs atomic coverage at H3 site is increased,the adsorption energy reduces,and the adsorption is still stable when the Cs adatom coverage is 1 monolayer.The work function reduces persistently,and does not rise with the increase of Cs coverage.     

The presence of a (1 × 1) oxygen overlayer on ZnO(0001) surfaces and at Schottky interfaces

The atomic surface and interface structures of uncoated and metal-coated epi-polished ZnO(0001) Zn-polar wafers were investigated via surface x-ray diffraction. All uncoated samples showed the presence of a fully occupied (1 × 1) overlayer of oxygen atoms located at the on-top position above the terminating Zn atom, a structure predicted to be unstable by several density functional theory calculations. The same oxygen overlayer was clearly seen at the interface of ZnO with both elemental and oxidized metal Schottky contact layers. No significant atomic relaxations were observed at surfaces and interfaces processed under typical device fabrication conditions.     

Comparative study of adsorption characteristics of Cs on the GaN （0001） and GaN （0001） surfaces

The adsorption characteristics of Cs on GaN （0001） and GaN （0001） surfaces with a coverage from 1/4 to 1 monolayer have been investigated using the density functional theory with a plane-wave uttrasoft pseudopotential method based on first-principles calculations. The results show that the most stable position of the Cs adatom on the GaN （0001） surface is at the N-bridge site for 1/4 monolayer coverage. As the coverage of Cs atoms at the N-bridge site is increased, the adsorption energy reduces. As the Cs atoms achieve saturation, the adsorption is no longer stable when the coverage is 3/4 monolayer. The work function achieves its minimum value when the Cs adatom coverage is 2/4 monolayer, and then rises with Cs atomic coverage. The most stable position of Cs adatoms on the GaN （000i） surface is at H3 site for 1/4 monolayer coverage. As the Cs atomic coverage at H3 site is increased, the adsorption energy reduces, and the adsorption is still stable when the Cs adatom coverage is 1 monolayer. The work function reduces persistently, and does not rise with the increase of Cs coverage.     

Epitaxial Growth of Graphene on 6H-SiC （0001） by Thermal Annealing

An epitaxial graphene （EG） layer is successfully grown on a Si-terminated 6H-SiC （（9001） substrate by the method of thermal annealing in an ultrahigh vacuum molecular beam epitaxy chamber. The structure and morphology of the EG sample are characterized by reflection high energy diffraction （RHEED）, Raman spectroscopy and atomic force microscopy （AFM）. Graphene diffraction streaks can are clearly observed in the Raman spectrum. The AFM about 4-10 layers. be seen in RHEED. The G and 2D peaks of graphene results show that the graphene nominal thickness is     

Influence of defects in SiC （0001） on epitaxial graphene

Defects in silicon carbide(SiC) substrate are crucial to the properties of the epitaxial graphene(EG) grown on it. Here we report the effect of defects in SiC on the crystalline quality of EGs through comparative studies of the characteristics of the EGs grown on SiC(0001) substrates with different defect densities. It is found that EGs on high quality SiC possess regular steps on the surface of the SiC and there is no discernible D peak in its Raman spectrum. Conversely, the EG on the SiC with a high density of defects has a strong D peak, irregular stepped morphology and poor uniformity in graphene layer numbers. It is the defects in the SiC that are responsible for the irregular stepped morphology and lead to the small domain size in the EG.     

MD simulation of nanoindentation on (001) and (111) surfaces of Ag–Ni multilayers

We perform MD simulations of the nanoindentation on (001) and (111) surfaces of Ag–Ni multilayers with different modulation periods, and find that both the hardness and maximum force increase with the increase of modulation period, in agreement with the inverse Hall–Petch relation. A prismatic partial dislocation loop is observed in the Ni(111)/Ag(111) sample when the modulation period is relatively large. We also find that misfit dislocation network shows a square shape for the Ni(111)/Ag(111) interface, while a triangle shape for the Ni(001)/Ag(001) interface. The pyramidal defect zones are also observed in Ni(001)/Ag(001) sample, while the intersecting stacking faults are observed in Ni(111)/Ag(111) sample after dislocation traversing interface. The results offer insights into the nanoindentation behaviors in metallic multilayers, which should be important for clarifying strengthening mechanism in many other multilayers.     

Raman Spectrum of Epitaxial Graphene Grown on Ion Beam Illuminated 6H-SiC （0001）

Patterning SiC substrates with focused ion beam for growth of confined graphene nanostructures is interesting for fabrication of graphene devices. However, by imposing an ion beam, the morphology of illuminated SiC substrate surface is inevitably damaged, which imposes significant effects on the subsequent growth of graphene. By using confocal Raman spectroscopy, we investigate the effects of ion beam illumination on the quality of graphene layers that are grown on 6H-SiC （0001） substrates with two different growth methods. With the first method, the 6H-SiC （0001） substrate is flash annealed in ultra-high vacuum. Prominent defects in graphene grown on illuminated areas are revealed by the emergence of Raman D peak. Significant changes in D peak intensity are observed with Ga＋ ion fluence as low as 10^5 μm^-2. To eliminate the damage from the ion beam illumination, hydrogen etching is employed in the second growth method, with which prominent improvement in the quality of crystalline graphene is revealed by its Raman features. The defect density is significantly reduced as inferred from the disappearance of D peak. The Raman shift of G peak and 2D peak indicates strain-released graphene layers as grown in such a method. Such results provide essential information for patterning graphene nano-devices.     

Graphene on SiC as a Q-switcher for a 2 μm laser

Double-layer graphene epitaxially grown on silicon carbide was used to Q-switch a Tm:YAG laser. Stable Q-switched laser pulses at the central wavelength of 2.01 μm were obtained. The maximum average output power, pulse repetition rate, and single pulse energy were 38 mW, 27.9 kHz, and 1.74 μJ, respectively. Our results illustrate that graphene can be used as a saturable absorber at the 2 μm region.     

Spectroscopic and scanning probe analysis on large-area epitaxial graphene grown under pressure of 4 mbar on 4H-SiC （0001） substrates

We produced epitaxial graphene under a moderate pressure of 4 mbar(about 400 Pa) at temperature 1600℃. Raman spectroscopy and optical microscopy were used to confirm that epitaxial graphene has taken shape continually with slight thickness variations and regularly with a centimeter order of magnitude on 4H-SiC(0001) substrates. Then using X-ray photoelectron spectroscopy and Auger electron spectroscopy, we analyzed the chemical compositions and estimated the layer number of epitaxial graphene. Finally, an atomic force microscope and a scanning force microscope were used to characterize the morphological structure. Our results showed that under 4-mbar pressure, epitaxial graphene could be produced on a SiC substrate with a large area, uniform thickness but a limited morphological property. We hope our work will be of benefit to understanding the formation process of epitaxial graphene on SiC substrate in detail.     

Theoretical study of ZnO adsorption and bonding on Al_2O_3(0001) surface

Sapphire (α-Al2O3) and silicon (Si) are widely applied as the substrates of the highquality ZnO thin films prepared by pulse laser deposition (PLD) and molecule beamepitaxy (MBE) technology. The adhesion, diffusivity, and bonding of the particles on thesubstrates play a significant role in the forming and initial growing of nucleation for filmgrowth, and directly influence the quality of the entire thin films[1]. No sufficient studiesand experiments are available on the surface atomic str…     

(0001)SiC衬底MgB_2超薄膜的制备和性质研究

利用混合物理化学气相沉积法(Hybrid physical-chemical vapor deposition简称为(HPCVD)在(0001)SiC衬底上制备了干净的MgB2超导超薄膜.在背景气体、压强、载气氢气流量以及B2H6的流量一定的情况下,改变沉积时间,制得一系列MgB2超薄膜样品。通过观察样品的表面形貌变化探究了MgB2超薄膜的生长过程.该系列超薄膜的生长遵循Volmer-Weber岛状生长模式且沿c轴外延生长.以20nm超薄膜作为例子,可知其表面连接性良好,超导转变温度Tc(0)≈38.5K,临界电流密度Jc≈0.82×107 A/cm2,表明了利用HPCVD在(0001)SiC衬底上制备的MgB2超薄膜有很好的性能.这预示其在超导电子器件上具有广阔的应用前景.     

Decay Dynamics of Hydrogen Clusters on Surfaces of Graphene and Stone–Wales Graphene

Physics of the Solid State - The thermal stability of hydrogen clusters disposed on surfaces of graphene and the Stone–Wales graphene that is a graphene allotrope discovered recently is...     

Molecular dynamics study of temperature-dependent ripples in monolayer and bilayer graphene on 6H—SiC surfaces

Using classical molecular dynamics and a simulated annealing technique,we show that microscopic corrugations occur in monolayer and bilayer graphene on 6H-SiC substrates.From an analysis of the atomic configurations,two types of microscopic corrugations are identified,namely periodic ripples at room temperature and random ripples at high temperature.Two different kinds of ripple morphologies,each with a periodic structure,occur in the monolayer graphene due to the existence of a coincidence lattice between graphene and the SiC terminated surface(Si-or C-terminated surface).The effect of temperature on microscopic ripple morphology is shown through analysing the roughness of the graphene.A temperature-dependent multiple bonding conjugation is also shown by the broad distribution of the carbon-carbon bond length and the bond angle in the rippled graphene on the SiC surface.These results provide atomic-level information about the rippled graphene layers on the two polar faces of the 6H-SiC substrate,which is useful not only for a better understanding of the stability and structural properties of graphene,but also for the study of the electronic properties of graphene-based devices.     

Nanostructured Graphene on β-SiC/Si(001): Atomic and Electronic Structures,Magnetic and Transport Properties (Brief Review)

JETP Letters - The studies of the properties of graphene synthesized on the surface of epitaxial films of cubic single-crystal silicon carbide preliminarily grown on Si(001) wafers have been...     

Growth and characterization of Au,Ni and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh

The nucleation and thermal stability of Au, Ni, and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh as well as the interaction between Au–Ni bimetallic clusters and reactive gases have been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Both Au and Ni atoms grow as three-dimensional (3D) clusters. Annealing the Au/carbon nanomesh surface up to 1150 °C leads to complete desorption of the Au clusters, while interfacial reaction occurs between Ni clusters and the substrate surface when the Ni clusters are subjected to the same annealing process. The nucleation of Au–Ni clusters depends critically on the deposition sequence. Au atoms preferentially nucleate on the existing Ni clusters, leading to the formation of bimetallic clusters with Au enriched on the surface. If the deposition sequence is reversed, a part of Ni atoms nucleate between the Au clusters. The thermal stability of the Au–Ni clusters resembles that of the Ni/carbon nanomesh surface, irrespective of the deposition sequence. XPS characterization reveals that Ni atoms in Au–Ni bimetallic clusters are oxidized upon exposure to 5.0 × 10^? 7 mbar O₂ for 5 min at room temperature while negligible structure change can be detected when the bimetallic clusters are exposed to CO gas under the similar conditions.