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.     

Comparison of the formation process and properties of epitaxial graphenes on Si- and C-face 6Hben SiC substrates

In this paper, the epitaxial graphene layers grown on Si- and C-face 6H-SiC substrates are investigated under a low pressure of 400 Pa at 1600 ℃. By using atomic force microscopy and Raman spectroscopy, we find that there are distinct differences in the formation and the properties between the epitaxial graphene layers grown on the Si-face and the C-face substrates, including the hydrogen etching process, the stacking type, and the number of layers. Hopefully, our results will be useful for improving the quality of the epitaxial graphene on SiC substrate.     

Freestanding graphene writing on a silicon carbide wafer

Freestanding graphene on a trench has been fabricated extensively using a transfer process of chemical vapor deposition grown graphene. Here, we demonstrate that freestanding graphene can be grown directly on a trench without a transfer process. A shallow trench was made on a 6H–SiC(0001) wafer using a focused ion beam lithography. The shallow trench was heated to a high temperature under Ar atmosphere. The heat treatment made the shallow trench become deeper and wider. Subsequently, epitaxial graphene was floating on the trench, resulting in freestanding graphene, where underlying bulk SiC was self-etched after the growth of epitaxial graphene. The freestanding graphene on a trench was characterized using Raman spectroscopy and atomic force microscopy. Such freestanding graphene writing can be applied to semiconductor fabrication process of freestanding graphene devices without a transfer process.     

Magnetotransport in a graphene monolayer with two tunable magnetic barriers

The magnetotransport property for a monolayer graphene with two turnable magnetic barriers has been investigated by the transfer-matrix method. We show that the parameters of barrier height, width, and interval between two barriers affect the electron wave decaying length, which determine the conductance with parallel or antiparallel magnetization configuration, and consequently the tunneling magnetoresistance (TMR) for the system. Interestingly, a graphene attached by two barriers with different heights can produce a resonant TMR peak at low energy region one order of magnitude larger than that for the system with two same height barriers because that the asymmetry of magnetic barriers block the electron transmission in the case of antiparallel magnetization configuration. The results obtained here may be useful in understanding of electron tunneling in graphene and in designing of graphene-based nanodevices.     

Modulating doping and interface magnetism of epitaxial graphene on SiC(0001)

On the basis of first principles calculations, we report that the type and density of charge carriers of epitaxial graphene on Si C(0001) can be deliberately controlled by decorating the buffer layer with specific atoms(i.e., F, Cl, O, or N). More importantly, a fine tuning of the doping behavior from intrinsic n-type to charge neutrality to p-type and interface magnetism is achieved via increasing the doping concentration of F atoms on the buffer layer. Our results suggest an interesting avenue to the application of epitaxial graphene in nanoscale electronic and spintronic devices.     

Automated detection and characterization of graphene and few‐layer graphite via Raman spectroscopy

Several processes have to be automated in order to use graphene in future industrial applications. One of these is the detection and characterization of graphene and few‐layer graphite (FLG) flakes on a substrate. Raman spectroscopy is an ideal tool for this purpose, as it allows not only the identification of these graphitic materials on arbitrary substrates but also monitoring the quality of flakes within the sample. In this paper, we report how graphene and FLG crystallites can be automatically detected and characterized by monitoring the evolution of Raman bands. We present an algorithm that achieves this purpose and thus has special potential in industrial applications of graphene. Copyright © 2010 John Wiley & Sons, Ltd.     

Strain status of epitaxial Ge film on a Si (001) substrate

An epitaxial Ge film was grown on a Si (001) substrate via a two-step process through the molecular beam epitaxy technique. The strain status of non-annealed and annealed epitaxial Ge films was determined by X-ray diffraction, Raman spectroscopy, and a combination of high-resolution transmission electron microscopy and geometric phase analysis. Results showed that the strain in non-annealed and annealed epitaxial Ge films is nonhomogeneous from the Ge/Si interface to the Ge film surface. The strain parallel to the interface in the non-annealed epitaxial Ge film is compressive; this strain reaches a minimum near the surface and a maximum at the interface. By contrast, the strain parallel to the interface in the annealed epitaxial Ge film is tensile; this strain reaches a minimum at the interface and a maximum near the surface.     

Signatures of epitaxial graphene grown on Si-terminated 6H-SiC (0 0 0 1)

Epitaxial graphene layers thermally grown on Si-terminated 6H-SiC (0 0 0 1) have been probed using Auger electron spectroscopy, Raman microspectroscopy, and scanning tunneling microscopy (STM). The average multilayer graphene thickness is determined by attenuation of the Si (L₂₃VV) and C (KVV) Auger electron signals. Systematic changes in the Raman spectra are observed as the film thickness increases from one to three layers. The most striking observation is a large increase in the intensity of the Raman 2D-band (overtone of the D-band and also known as the G′-band) for samples with a mean thickness of more than ∼1.5 graphene layers. Correlating this information with STM images, we show that the first graphene layer imaged by STM produces very little 2D intensity, but the second imaged layer shows a single-Lorentzian 2D peak near 2750 cm⁻¹, similar to spectra acquired from single-layer micromechanically cleaved graphene (CG). The 4-10 cm⁻¹ higher frequency shift of the G peak relative to CG can be associated with charge exchange with the underlying SiC substrate and the formation of finite size domains of graphene. The much greater (41-50 cm⁻¹) blue shift observed for the 2D-band may be correlated with these domains and compressive strain.     

Comparison of the formation epitaxial graphenes on Si- and process and properties of C-face 6H-SiC substrates

In this paper,the epitaxial graphene layers grown on Si-and C-face 6H-SiC substrates are investigated under a low pressure of 400 Pa at 1600 C.By using atomic force microscopy and Raman spectroscopy,we find that there are distinct differences in the formation and the properties between the epitaxial graphene layers grown on the Si-face and the C-face substrates,including the hydrogen etching process,the stacking type,and the number of layers.Hopefully,our results will be useful for improving the quality of the epitaxial graphene on SiC substrate.     

Si面4H-SiC衬底上外延石墨烯近平衡态制备

SiC热解法是制备大面积、高质量石墨烯的理想选择之一.外延石墨烯的晶体质量仍是制约其应用的关键因素之一.本文通过SiC热解法在4H-SiC(0001)衬底上制备单层外延石墨烯.通过引入氩气惰性气氛和硅蒸气,使SiC衬底表面的Si原子升华与返回概率接近平衡,外延石墨烯生长速率大大减慢,单层石墨烯的生长时间从15 min延长至75 min.测试分析表明,生长速率减慢,外延石墨烯中缺陷减少,晶体质量提高,使得外延石墨烯的电性能都得到改善,单层外延石墨烯的最高载流子迁移率达到1200 cm2/V·s,方阻604?/.以上结果表明,控制生长气氛,减慢生长速率是实现高质量外延石墨烯的可行途径之一.     

Fabrication and characterization of magnetron sputtered arsenic doped p-type ZnO epitaxial thin films

Arsenic doped p-type ZnO thin films were grown on sapphire substrate by magnetron sputtering. As grown films reveal p-type conduction confirmed by Hall-effect and photoluminescence measurements. The p-type film with a hole concentration of 2.16× 10¹⁷ cm⁻³, mobility of 1.30 cm²/V.s and resistivity of 22.29 Ω-m were obtained at substrate temperature of 700 °C. ZnO homojunction synthesized by in-situ deposition of As doped p-ZnO layer on Al doped n-ZnO layer showed p-n diode like characteristics. X-ray pole figure and Transmission Electron Microscope studies confirm epitaxial nature of the films. Photoluminescence results exhibit the peaks associated with donor acceptor pair emission.     

TiN/SiC纳米多层膜的生长结构与力学性能

研究了TiN/SiC纳米多层膜中立方SiC（B1cubic SiC）的形成及其对TiN/SiC多层膜力学性能的影响.结果表明：在TiN/SiC多层膜中，非晶态的SiC层在厚度小于0.6nm时形成立方结构并与TiN形成共格外延生长的超晶格柱状晶，使多层膜产生硬度和弹性模量显著升高的超硬效应，最高硬度超过60GPa.SiC随着层厚的增加转变为非晶相，从而阻止了多层膜的共格外延生长，使薄膜呈现TiN纳米晶和SiC非晶组成的层状结构特征，同时多层膜的硬度和弹性模量下降.TiN/SiC纳米多层膜产生的超硬效应与立方 关键词： 立方碳化硅 TiN/SiC纳米多层膜 外延生长 超硬效应     

Study of a 4H-SiC epitaxial n-channel MOSFET

Epitaxial channel metal-oxide semiconductor field-effect transistors (MOSFETs) have been proposed as one possible way to avoid the problem of low inversion layers in traditional MOSFETs. This paper presents an equation of maximum depletion width modified which is more accurate than the original equation. A 4H--SiC epitaxial n-channel MOSFET using two-dimensional simulator ISE is simulated. Optimized structure would be realized based on the simulated results for increasing channel mobility.     

Electrodeposition of epitaxial nickel films on GaAs

The structural and magnetic properties of Ni films grown by electrodeposition from simple sulfate solutions directly onto the (0 0 1) and (0 1 1) surfaces of n-GaAs have been studied. In-plane X-ray diffraction has been used to show that Ni grows on (0 0 1) GaAs with two different preferred epitaxial relationships: (1) perpendicular to plane (0 0 1)_Ni(0 0 1)_GaAs and preferred orientation in-plane [1 0 0]_Ni[1 1 0]_GaAs and (2) perpendicular to plane (0 1 1)_Ni(0 0 1)_GaAs and preferred orientation in-plane [1 1 1]_Ni[1 1 0]_GaAs. Nickel films grown on (0 1 1) n-GaAs show only a single preferred growth relationship: perpendicular to plane (1 1 1)_Ni(0 1 1)_GaAs and in-plane [1 1 0]_Ni[1 1 0]_GaAs. The magnetic properties were strongly dependent on the substrate orientation. The films grown on GaAs (0 0 1) showed a small but definite four-fold magnetic anisotropy in plane with the highest remanence being found along the GaAs [1 0 0] direction. In contrast, the Ni films grown on the (0 1 1) GaAs showed a pronounced uniaxial anisotropy with an anisotropy field of approximately 500 Oe.     

Engineering and metrology of epitaxial graphene

Here we review the concepts and technologies, in particular photochemical gating, which contributed to the recent progress in quantum Hall resistance metrology based on large scale epitaxial graphene on silicon carbide.     

Raman scattering investigations on Co-doped ZnO epitaxial films: Local vibration modes and defect associated ferromagnetism

Raman scattering spectroscopy has been performed on high quality Co-doped ZnO epitaxial films, which were grown on Al₂O₃ (0001) by oxygen-plasma assisted molecular beam epitaxy. Raman measurements revealed two local vibration modes (LVMs) at 723 and 699 cm⁻¹ due to the substitution of Co²⁺ in wurtzite ZnO lattice. The LVM at 723 cm⁻¹ is found to be an elemental sensitive vibration mode for Co substitution. The LVM at 699 cm⁻¹ can be attributed to enrichment of Co²⁺ bound with oxygen vacancy, the cobalt–oxygen vacancy–cobalt complexes, in Zn_1−xCo_xO films associated with ferromagnetism. The intensity of LVM at 699 cm⁻¹, as well as saturated magnetization, enhanced after the vacuum annealing and depressed after oxygen annealing.     

Buffer layer free large area bi-layer graphene on SiC(0 0 0 1)

The influence of hydrogen exposures on monolayer graphene grown on the silicon terminated SiC(0 0 0 1) surface is investigated using photoelectron spectroscopy (PES), low-energy electron microscopy (LEEM) and micro low-energy electron diffraction (μ-LEED). Exposures to ionized hydrogen are shown to have a pronounced effect on the carbon buffer (interface) layer. Exposures to atomic hydrogen are shown to actually convert/transform the monolayer graphene plus carbon buffer layer to bi-layer graphene, i.e. to produce carbon buffer layer free bi-layer graphene on SiC(0 0 0 1). This process is shown to be reversible, so the initial monolayer graphene plus carbon buffer layer situation is recreated after heating to a temperature of about 950 °C. A tentative model of hydrogen intercalation is suggested to explain this single to bi-layer graphene transformation mechanism. Our findings are of relevance and importance for various potential applications based on graphene-SiC structures and hydrogen storage.     

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.     

Manifestation of LO-LA phonons in Raman scattering in graphene

We analyze the spectral density of Raman scattering in graphene accompanied by the emission of a pair of LO or LA phonons from the corner of the Brillouin zone. Using a minimal tight-binding model approach, we find that the lineshape of the corresponding Raman signal consists of two peaks with a strongly non-Lorentzian (almost triangular) form with their width and the splitting between the peaks being strongly dependent on the energy of the incoming photon. The asymmetric lineshape is determined by the kinematics of the fully-resonant two-phonon process, and it reflects a strong anisotropy of LO-LA phonons’ dispersion around the Brillouin zone corners.     

The thermodynamics of electrons and the thermoelectric transport in epitaxial graphene on the size-quantized films

We investigated the thermodynamic parameters (chemical potential, heat capacity and thermodynamic potential) and a thermoelectric transport in an epitaxial graphene on the size-quantized metal and semiconductor films within the framework of simple analytical model. We considered limiting cases of high and low temperatures. We showed that the chemical potential of epitaxial graphene is smaller than the chemical potential of isolated graphene at the same carrier concentration. Conversely, the heat capacity of the epitaxial graphene is greater than the heat capacity of the isolated graphene. We investigated a conductivity and thermopower of the epitaxial graphene. We showed that in such system there are the kinks of conductivity and peaks of thermoelectric power. These peaks are several times greater than those of isolated graphene. We compared our system with cases of 2D and 3D substrates.