Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Highly-doped p-type few-layer graphene on UID off-axis homoepitaxial 4H–SiC

In this report we investigate structural and electrical properties of epitaxial Chemical Vapor Deposition quasi-free-standing graphene on an unintentionally-doped homoepitaxial layer grown on a conducting 4H–SiC substrate 4° off-axis from the basal [0001] direction towards [11-20]. Due to high density of SiC vicinal surfaces the deposited graphene is densely stepped and gains unique characteristics. Its morphology is studied with atomic force and scanning electron microscopy. Its few-layer character and p-type conductance are deduced from a Raman map and its layers structure determined from a high-resolution X-ray diffraction pattern. Transport properties of the graphene are estimated through Hall effect measurements between 100 and 350 K. The results reveal an unusually low sheet resistance below 100 Ω/sq and high hole concentration of the order of 10¹⁵ cm⁻². We find that the material's electrical properties resemble those of an epitaxially-grown SiC PIN diode, making it an attractive platform for the semiconductor devices technology.     

Effects of carbonization and substrate temperature on the growth of 3C-SiC on Si(1 1 1) by SSMBE

The growth of 3C-SiC on Si(1 1 1) substrate was performed at different carbonization temperatures and substrate temperatures by solid-source molecular beam epitaxy (SSMBE). The properties of SiC film were analyzed with in situ reflection high energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The best carbonization temperature of 810 °C was found to be optimal for the surface carbonization. The quality of SiC film grown on Si at substrate temperature of 1000 °C is best. The worse crystalline quality for the sample grown at higher temperature was attributed to the large mismatch of thermal expansion coefficient between SiC and Si which caused more dislocation when sample was cooled down to room temperature from higher substrate temperature. Furthermore, the larger size of single pit and the total area of the pits make the quality of SiC films grown at higher temperature worse. More Si atoms for the sample grown at lower temperature were responsible for the degradation of crystalline quality for the sample grown at lower temperature.     

Forming mechanism of nitrogen doped graphene prepared by thermal solid-state reaction of graphite oxide and urea

Nitrogen doped graphene was synthesized from graphite oxide and urea by thermal solid-state reaction. The samples were characterized by transmission electron microscopy, atomic force microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectra, element analysis, and electrical conductivity measurement. The results reveal that there is a gradual thermal transformation of nitrogen bonding configurations from amide form nitrogen to pyrrolic, then to pyridinic, and finally to “graphitic” nitrogen in graphene sheets with increasing annealing temperature from 200 to 700 °C. The products prepared at 600 °C and 700 °C show that the quantity of nitrogen incorporated into graphene lattice is ∼10 at.% with simultaneous reduction of graphite oxide. Oxygen-containing functional groups in graphite oxide are responsible for the doping reaction to produce nitrogen doped graphene.     

Effects of sapphire substrate annealing on ZnO epitaxial films grown by MOCVD

The annealing effects of sapphire substrate on the quality of epitaxial ZnO films grown by metalorganic chemical vapor deposition (MOCVD) were studied. The atomic steps formed on (0 0 0 1) sapphire (α-Al₂O₃) substrate surface by annealing at high temperature was analyzed by atomic force microscopy (AFM). The annealing effects of sapphire substrate on the ZnO films were examined by X-ray diffraction (XRD), AFM and photoluminescence (PL) measurements. Experimental results indicate that the film quality is strongly affected by annealing treatment of the sapphire substrate surface. The optimum annealing temperature of sapphire substrates is given.     

Direct growth of graphene film on piezoelectric La3Ga5.5Ta0.5O14 crystal

For the first time, a few layer graphene was grown on the surface of the polar X ‐cut (110) of a piezoelectric La₃Ga_5.5Ta_0.5O₁₄ crystal by the CVD method. This polar X ‐cut is characterized by a good matching of the crystal lattice parameters of La₃Ga_5.5Ta_0.5O₁₄ and two‐dimensional graphene crystal, as well as the presence of piezoelectric fields on the surface of the substrate, which could affect the graphene growth process. Raman spectroscopy investigation has shown the ability for direct growth of graphene on the piezoelectric crystal. The NEXAFS spectroscopy studies of the film grown on the surface of the X ‐cut of an La₃Ga5_.5Ta_0.5O₁₄ crystal also confirmed that the grown film is graphene. Moreover, the NEXAFS spectra enable the conclusion that additional electron states are formed as a result of chemical bonding between the atoms of graphene and the substrate which proceeds through hybridization of the valence electron states of the substrate and graphene atoms. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The annealing effect on structural and optical properties of ZnO thin films produced by r.f. sputtering

In this work, a study of the structure and optical properties of undoped ZnO thin films produced by r.f. magnetron sputtering technique as a function of the growth parameters is reported. Modification under annealing conditions is also analysed. Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and optical transmittance have been used. From the position of the (002) X-ray diffraction peak and the E₂ (high) mode detected in Raman spectra, the residual stress both in the as-grown and in the annealed samples has been estimated.     

Field emission properties of electrophoretic deposition carbon nanotubes film

The field emission properties of electrophoretic deposition(EPD) carbon nanotubes (CNTs) film have been improved by depositing CNTs onto the titanium (Ti)-coated Si substrate, followed by vacuum annealing at 900 °C for 2 h, and the enhanced emission mechanism has been studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. Field emission measurements showed that the threshold electric field was decreased and the emission current stability was improved compared to that of EPD CNTs film on bare Si substrate. XRD and Raman spectroscopy investigations revealed that vacuum annealing treatment not only decreased the structural defects of CNTs but made a titanium carbide interfacial layer formed between CNTs and substrate. The field emission enhancement could be attributed to the improved graphitization of CNTs and the improved contact properties between CNTs and substrate including electrical conductivity and adhesive strength due to the formed conductive titanium carbide.     

Molecular adsorption and growth of naphthalene films on Ag(1 0 0)

The adsorption of naphthalene, vacuum deposited on a Ag(1 0 0) surface, was comprehensively investigated by means of low-energy electron diffraction (LEED), temperature-programmed thermal desorption (TPD) spectroscopy, X-ray photoelectron spectroscopy (XPS), and polarization-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the mono- and multilayer regime. A growth of long-range ordered monolayer at 140 K is observed with LEED. The polarization-dependent C 1s NEXAFS shows that the naphthalene molecules in the monolayer lie almost parallel to the Ag(1 0 0) surface. With increasing film thickness, the molecular orientation turns to upright position. Furthermore, NEXAFS measurements show that in the multilayer regime the molecular orientation depends on the substrate temperature during deposition.     

On the correction of errors in quantum cryptography systems

The Raman and photoluminescence spectra of short-period C/SiC superlattices produced by RF magnetron sputtering are investigated. The Raman data indicate that, in 35-period Sitall/Ni/[C/SiC] superlattices with the C and SiC effective thicknesses of 3.5 and 3 Å, respectively, subjected to postgrowth avalanche annealing, the carbon layers assume the structure of multilayer graphene with 3–5 graphene sheets per superlattice period. A method for the fabrication of graphene-like carbon structures on the basis of short-period superlattices grown by RF sputtering is suggested and implemented.     

Evidence of structural strain in epitaxial graphene layers on 6H-SiC(0001)

The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC (0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows us to resolve submonolayer growth, including individual, localized C=C dimers in a diamondlike carbon matrix for AES C/Si ratio of approximately 3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.     

激光熔敷镍石墨烯立方氮化硼涂层的耐磨性能

在AISI 4140基体上采用预置材料激光熔敷的方法制备了镍石墨烯立方氮化硼(Ni-Graphene-CBN)复合材料涂层。X射线衍射(XRD)和Raman测试证明了石墨烯和CBN存在于所制备的涂层材料中。扫描电镜(SEM)图片给出了所制备的复合材料涂层的表面和断面形貌。进行了复合材料涂层的纳米机械性能和耐磨性的测试。测试结果表明:随着CBN含量的增加,复合涂层的硬度及弹性模量相应提高,分别由4.3 GPa提高到6.2 GPa和101 GPa提高到140 GPa; 同时其耐磨性也有明显改善,6% CBN含量的涂层摩擦系数由基体材料的0.2降低到0.15,最大磨损量降到基体磨损量的一半。     

Studies of Li intercalation of hydrogenated graphene on SiC(0001)

The effects of Li deposition on hydrogenated bilayer graphene on SiC(0001) samples, i.e. on quasi-freestanding bilayer graphene samples, are studied using low energy electron microscopy, micro-low-energy electron diffraction and photoelectron spectroscopy. After deposition, some Li atoms form islands on the surface creating defects that are observed to disappear after annealing. Some other Li atoms are found to penetrate through the bilayer graphene sample and into the interface where H already resides. This is revealed by the existence of shifted components, related to H–SiC and Li–SiC bonding, in recorded core level spectra. The Dirac point is found to exhibit a rigid shift to about 1.25 eV below the Fermi level, indicating strong electron doping of the graphene by the deposited Li. After annealing the sample at 300–400 °C formation of LiH at the interface is suggested from the observed change of the dipole layer at the interface. Annealing at 600 °C or higher removes both Li and H from the sample and a monolayer graphene sample is re-established. The Li thus promotes the removal of H from the interface at a considerably lower temperature than after pure H intercalation.     

Adsorption and desorption of fullerene on graphene/SiC(0001)

Adsorption and desorption of fullerene on a single layer of graphene grown on SiC(0001) were investigated by photoemission spectroscopy (PES). No significant change in the band structure of graphene was observed after fullerene deposition on the graphene layer under vacuum conditions, and subsequent exposure to the air. After annealing the fullerene layer at 275 °C in a vacuum, complete desorption of fullerene was observed without any resulting damage to the graphene structure. The desorption temperature of fullerene was significantly higher than that of pentacene, indicating that fullerene layers show higher stability than pentacene as protection layers of graphene-based devices.     

Thermal annealing effects on a compositionally graded SiGe layer fabricated by oxidizing a strained SiGe layer

Thermal annealing effects on a thin compositionally graded SiGe buffer layer on silicon substrate fabricated by oxidizing a strained SiGe layer are investigated with X-ray diffraction, ultraviolet Raman spectra and atomic force microscopy. Interestingly, we found that the surface roughness and the threading dislocation densities are kept low during the whole annealing processes, while the Ge concentration at the oxidizing interface decreases exponentially with annealing time and the strain in the layer is only relaxed about 66% even at 1000 °C for 180 min. We realized that the strain relaxation of such a compositionally graded SiGe buffer layer is dominated by Si-Ge intermixing, rather than generation and propagation of misfit dislocations or surface undulation.     

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.     

Crystalline quality of 3C-SiC formed by high-fluence C-implanted Si

Carbon ions at 40 keV were implanted into (1 0 0) high-purity p-type silicon wafers at 400 °C to a fluence of 6.5 × 10¹⁷ ions/cm². Subsequent thermal annealing of the implanted samples was performed in a diffusion furnace at atmospheric pressure with inert nitrogen ambient at 1100 °C. Time-of-flight energy elastic recoil detection analysis (ToF-E ERDA) was used to investigate depth distributions of the implanted ions. Infrared transmittance (IR) and Raman scattering measurements were used to characterize the formation of SiC in the implanted Si substrate. X-ray diffraction analysis (XRD) was used to characterize the crystalline quality in the surface layer of the sample. The formation of 3C-SiC and its crystalline structure obtained from the above mentioned techniques was finally confirmed by transmission electron microscopy (TEM). The results show that 3C-SiC is directly formed during implantation, and that the subsequent high-temperature annealing enhances the quality of the poly-crystalline SiC.     

Ta/Ni/Ta multilayered ohmic contacts on n-type SiC

The interface formation, electrical properties and the surface morphology of multilayered Ta/Ni/Ta/SiC contacts were reported in this study. It was found that the conducting behavior of the contacts so fabricated is much dependent on the metal layer thickness and the subsequent annealing temperature. Auger electron spectroscopy (AES) and X-ray diffraction analyses revealed that Ni₂Si and TaC formed as a result of the annealing. The Ni atoms diffused downward to metal/SiC interface and converted into Ni₂Si layer in adjacent to the SiC substrate. The released carbon atoms reacted with Ta atoms to form TaC layer. Ohmic contacts with specific contact resistivity as low as 3 × 10⁻⁴ Ω cm² have been achieved after thermal annealing. The formation of carbon vacancies at the Ni₂Si/SiC interface, probably created by dissociation of SiC and formation of TaC during thermal annealing, should be responsible for the ohmic formation of the annealed Ta/Ni/Ta contacts. The addition of Ta into the Ni metallization scheme to n-SiC restricted the accumulation of carbon atoms left behind during Ni₂Si formation, improving the electrical and microstructure properties.     

Analysis of Y1Ba2Cu3O7−y thin films on sapphire substrates made by electron beam evaporation

We have analyzed the structure and properties of Y₁Ba₂Cu₃O_7–y films on (012) sapphire substrates. The films were obtained by evaporation of the metals in a UHV system with an oxygen beam directed at the substrate. Analysis was performed by X-ray diffraction and electron probe microanalysis [EPMA], after annealing in addition by scanning electron microscopy [SEM] and Auger electron spectroscopy. Without annealing no superconductivity was obtained although sufficient oxygen could be incorporated during growth. The copper does not oxidize. After annealing in oxygen superconductivity is found with an onset temperature of 80–90 K and zero resistance at 30–40 K. A grain-like pattern of 3–5 m typical size is seen. Chemical reactions between layer and substrate are observed.