Graphene films grown on sapphire substrates via solid source molecular beam epitaxy

A method for growing graphene on a sapphire substrate by depositing an SiC buffer layer and then annealing at high temperature in solid source molecular beam epitaxy (SSMBE) equipment was presented. The structural and electronic properties of the samples were characterized by reflection high energy diffraction (RHEED), X-ray diffraction Φ scans, Raman spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results of the RHEED and Φ scan, as well as the Raman spectra, showed that an epitaxial hexagonal α-SiC layer was grown on the sapphire substrate. The results of the Raman and NEXAFS spectra revealed that the graphene films with the AB Bernal stacking structure were formed on the sapphire substrate after annealing. The layer number of the graphene was between four and five, and the thickness of the unreacted SiC layer was about 1--1.5 nm.     

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.     

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.     

Growth of threaded AIN whiskers by a physical vapor transport method

Threaded aluminum nitride （A1N） whiskers are grown by a physical vapor transport method in a radio-frequency induction heating furnace. The resultant whiskers are characterized by X-ray diffraction, Raman scattering, scanning electron microscopy, transmission electron microscopy and photoluminescence. The analysis shows that the whiskers are single-crystalline, wurtzite AIN. The threaded A1N whiskers are 0.5 μm~100 μm in diameter and several millimeters in length in the fiber direction, and have lots of tiny sawteeth on the surface. The morphology of this threaded A1N whisker is beneficial for bonding when the whisker is used in composite. The growth of the whiskers is dominated by the vapor-solid （VS） mechanism, and the particular morphology might result from an oscillating condition produced in the radio-frequency induction heating furnace.     

Influence of Si doping on the structural and optical properties of InGaN epilayers

Influences of the Si doping on the structural and optical properties of the InGaN epilayers are investigated in detail by means of high-resolution X-ray diffraction （HRXRD）, photolumimescence （PL）, scanning electron microscope （SEM）, and atomic force microscopy （AFM）. It is found that the Si doping may improve the surface morphology and crystal quality of the InGaN film and meanwhile it can also enhance the emission efficiency by increasing the electron concentration in the InGaN and suppressing tile formation of V-defects, which act as nonradiative recombination centers in the InGaN, and it is proposed that the former plays a more important role in enhancing the emission efficiency in the InGaN.     

Effect of Gas Pressure on Nanocrystalline Diamond Films Prepared by Electron-Assisted Chemical Vapour Deposition

With use of electron-assisted chemical vapour deposition （EACVD） technology, nanocrystalline diamond films are successfully deposited on an α-SiC single phase ceramics substrate by means of reduction of the reactive gas pressure. The structure and surface morphology of the deposited films are characterized by Raman spectroscopy, x-ray diffraction （XRD）, field emission scanning electron microscopy （FE-SEM） and atomic force microscopy （AFM）. The results examined by FE-SEM and AFM show that when the gas pressure was reduced to 0.5- 1 kPa, the surface grain size and surface roughness of the diamond film are decreased greatly to 18-32nm and 34-58nm respectively. The grain sizes estimated from full with at half maximum of （111） XRD peak by the Scherrer formula are 6-28 nm. However, too high secondary nucleation rate may result in pores and defects in the deposited films. Only at suitable gas pressure （1 kPa） to deposit films can we obtain densification and better quality nanocrystalline films.     

Preparation and modification of VO_2 thin film on R-sapphire substrate by rapid thermal process

The VO2 thin film with high performance of metal-insulator transition （MIT） is prepared on R-sapphire substrate for the first time by magnetron sputtering with rapid thermal process （RTP）. The electrical characteristic and THz transmittance of MIT in VO2 film are studied by four-point probe method and THz time domain spectrum （THz-TDS）. X-ray diffraction （XRD）, X-ray photoelectron spectroscopy （XPS）, atomic force microscopy （AFM）, and search engine marketing （SEM） are employed to analyze the crystalline structure, valence state, surface morphology of the film. Results indicate that the properties of VO2 film which is oxidized from the metal vanadium film in oxygen atmosphere are improved with a follow- up RTP modification in nitrogen atmosphere. The crystallization and components of VO2 film are improved and the film becomes compact and uniform. A better phase transition performance is shown that the resistance changes nearly 3 orders of magnitude with a 2-~C hysteresis width and the THz transmittances are reduced by 64% and 60% in thermal and optical excitation respectively.     

Direct growth of graphene on gallium nitride by using chemical vapor deposition without extra catalyst

Graphene on gallium nitride(GaN) will be quite useful when the graphene is used as transparent electrodes to improve the performance of gallium nitride devices. In this work, we report the direct synthesis of graphene on GaN without an extra catalyst by chemical vapor deposition. Raman spectra indicate that the graphene films are uniform and about 5–6 layers in thickness. Meanwhile, the effects of growth temperatures on the growth of graphene films are systematically studied, of which 950℃ is found to be the optimum growth temperature. The sheet resistance of the grown graphene is 41.1Ω/square,which is close to the lowest sheet resistance of transferred graphene reported. The mechanism of graphene growth on GaN is proposed and discussed in detail. XRD spectra and photoluminescence spectra indicate that the quality of GaN epi-layers will not be affected after the growth of graphene.     

HPHT Synthesis of Micron Grade Boron-Doped Diamond Single Crystal in Fe-Ni-C-B Systems

Micron grade boron-doped diamond crystals with octahedral morphology are successfully synthesized in a Fe-Ni- C-B system under high pressure and high temperature （HPHT）. The effects of the additive boron on synthesis conditions, nucleation and growth, crystal morphology of diamond are studied. The synthesized micron grade diamond crystals were characterized by optical microscope （OM）, scanning electron microscope （SEM）, x-ray diffraction （XRD） and Raman spectroscopy. The research results show that the V-shaped section of synthetic diamond moves downwards to the utmost extent due to 0.3a wt% （a is a constant.） boron added in the synthesis system. The crystal colour is black, and the average crystal size is about 25μm. The crystal faces of synthetic diamond are mainly （111） face. The synthesis of this kind of diamond is few reported, and it will have important and widely applications.     

Dynamic surface wettability of three-dimensional graphene foam

In this work, three-dimensional graphene foams （GFs） are synthesized and characterized by scanning electron micro- scope （SEM） and Raman spectroscopy. The SEM images indicate that after the growth of graphene, the graphene covers the surface of nickel （Ni） foam uniformly. Raman spectra show that the percentages of monolayer, bilayer, trilayer, and multilayer graphenes are - 58%, - 32%, - 8%, and ,.o 2%, respectively. The contact angle （CA） （-- 12°） of water droplet （3 p-L） on GF is found to be larger than that on Ni foam （,- 107°）, indicating that graphenes have changed the surface wettability of the Ni foam. Meanwhile, the dynamic characteristics of CA of water droplet on GF are different from those on Ni foam. The mechanisms for different behaviors are discussed, which are attributed to volatilization and seepage of water droplets.     

Stress Analysis of ZnO Film with a GaN Buffer Layer on Sapphire Substrate

A 5.35-μm-thick ZnO film is grown by chemical vapour deposition technique on a sapphire （0001） substrate with a GaN buffer layer. The surface of the ZnO film is smooth and shows many hexagonal features. The full width at half maximum of ZnO （0002） u-rocking curve is 161 arcsec, corresponding to a high crystal quality of the ZnO film. From the result of x-ray diffraction 0 - 20 scanning, the stress status in ZnO film is tensile, which is supported by Raman scattering measurement. The reason of the tensile stress in the ZnO film is analysed in detail. The lattice mismatch and thermal mismatch are excluded and the reason is attributed to the coalescence of grains or islands during the growth of the ZnO film.     

MOCVD Growth and Characterization of Epitaxial AlxGa1-xN Films

We study the growth of AlxGa1-x N epilayers on （0001） sapphire by low-pressure MOCVD, using a lowtemperature AIN buffer. By varying the input flow rates of trimethylgallium （TMGa）, we obtain crack-free AlGaN films in the whole range of composition. A linear relationship between gas and solid Al content is observed. The structural properties of the layers （x =0- 1） are investigated by x-ray diffraction, atomic force microscopy （AFM） and scanning electron microscopy （SEM）. It is found that a two-direction growth appears along the c-axis and the （1011） directions for x ≥ 0.45. From the results of Raman spectroscopy, we suggest that the compressive stain and the lack of mobility orAl adatoms can induce the formation of （1011） grains.     

Structure Characterization of Modified Polyimide Films Irradiated by 2MeV Si Ions

Structures of polyimide （6051） films modified by irradiation of 2.0 MeV Si ions with different fluences are studied in detail. Variations of the functional groups in polyimide are investigated by attenuated total reflection Fourier transform infrared spectroscopy （ATR-FTIR） and Raman spectroscopy. The results indicate that the functional groups can be destroyed gradually with the increasing ion fluenee. The variations of structure and element contents are characterized by x-ray diffraction （XRD）, Rutherford backscattering spectrometry （RBS） and x- ray photoelectron spectroscopy （XPS）. The results indicate that the contents of N and O decrease significantly compared with the original samples, some graphite-like and carbon-rich phases are formed in the process of irradiation.     

Structural and Optical Properties of Nonpolar m-Plane GaN and GaN-Based LEDs on γ-LiAlO_2

We report the structural and optical properties of nonpolar m-plane GaN and GaN-based LEDs grown by MOCVD on a 7-LiAlO2 （100） substrate. The TMGa, TMIn and NH3 are used as sources of Ga, In and N, respectively. The structural and surface properties of the epilayers are characterized by x-ray diffraction, polarized Raman scattering and atomic force microscopy （AFM）. The films have a very smooth surface with rms roughness as low as 2nm for an area of 10×10μm^2 by AFM scan area. The XRD spectra show that the materials grown on 7-LiAl02 （100） have （1 - 100） m-plane orientation. The EL spectra of the m-plane InGaN/GaN multiple quantum wells LEDs are shown. This demonstrates that our nonpolar LED structure grown on the 7-LiAlO2 substrate is indeed free of internal electric field. The current voltage characteristics of these LEDs show the rectifying behaviour with a turn on voltage of 1-3 V.     

Ahnormal Crystallization of Silicon Thin Films Deposited by ICP-CVD

Silicon thin films are deposited by inductively coupled plasma chemical vapour deposition （ICP-CVD） at a low temperature of 350℃ using a mixture of SiH4 and H2. The structures of the films are characterized by x-ray diffraction and Raman spectra. Under the optimum experimental conditions, we observe that the crystallinity of Si films becomes more excellent and the preferred orientation changes from （111） to （220） with the decreasing dilution of SiH4 in H2. Such an abnormal crystallization is tentatively interpreted in term of the high density, low electron temperature and spatial confinement of the plasma in the process of ICP-CVD.     

Reactive Mechanical Alloying Synthesis of Nanocrystalline Cubic Zirconium Nitride

Zirconium nitride powders with rock salt structure （γ-ZrNx） are prepared by mechanical milling of a mixture of Zirconium and hexagonal boron nitride （h-BN） powders. The products are analysed by x-ray diffraction （XRD）, scanning electron microscopy （SEM）, and Raman spectroscopy （ITS）. The formation mechanism of γ-ZrNx by ball milling technique is investigated in detail. N atoms diffuse from amorphous BN （a-BN） into Zr to form Zr（N） solid solution alloy, then the Zr（N） solid solution alloy decomposes into γf-ZrNx. No ZrB2 is observed in the as-milled samples or the samples annealed at 1050° C for 2h.     

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.     

Fluctuations of optical phase of diffracted light for Raman–Nath diffraction in acousto–optic effect

The Raman–Nath diffraction in acousto–optic effect was studied theoretically and experimentally in the paper.Up to now,each order of diffracted light in Raman–Nath diffraction was still considered simply to be just frequency-shifted and to be a plane wave.However,we find that the phase and frequency shifts occur simultaneously and individually in Raman–Nath diffraction.The findings demonstrate that,in addition to the frequency shift,the optical phase of each order of diffracted light is also shifted by the sound wave and fluctuates with the sound wave and is related to the location in the acoustic field from which the diffracted light originates.As a result,the wavefront of each order of diffracted light is modulated to fluctuate spatially and temporally with the sound wave.Obviously,these findings are significant for applications of Raman–Nath diffraction in acousto–optic effect because the optical phase plays an important role in optical coherence technology.     

Raman Scattering Detection of Stacking Faults in Free-Standing Cubic-SiC Epilayer

We report on stacking fault （SF） detection in free-standing cubic-SiC epilayer by the Raman measurements. The epilayer with enhanced SFs is heteroepitaxially grown by low pressure chemical vapour deposition on a Si（100） substrate and is released in KOH solution by micromechanical manufacture, on which the Raman measurements are performed in a back scattering geometry. The TO line of the Raman spectra is considerably broadened and distorted. We discuss the influence of SFs on the intensity profiles of TO mode by comparing our experimental data with the simulated results based on the Raman bond polarizability （BP） model in the framework of linearchain concept. Cood agreement with respect to the linewidth and disorder-induced peak shift is found by assuming the mean distance of the SFs to be 11 A in the BP model.     

Optical behavior of self-assembled high-density Ge nanoislands embedded in SiO2

The radio frequency magnetron sputtering method is used to prepare well-dispersed pyramidal-shaped Ge nanoislands embedded in amorphous SiO2 sublayers of various thicknesses. The estimated size and number density of Ge nanoislands in SiO2 sublayer thicknesses beyond 30 nm are approximately 15 nm and 1011 cm-2, respectively. Atomic force microscopy （AFM） reveals root mean square （RMS） roughness sensitivity as the SiO2 sublayer thickness varies from 30 to 40 nm. The formation of nanoislands with high aspect ratios is attributed to the higher rate of surface reactions between Ge adatoms and nucleated Ge islands than reactions associated with SiO2 and Ge. The Ge nanoisland polyorientation on SiO2 （50-nm thickness） is revealed by X-ray diffraction （XRD） patterns. Photoluminescence （PL） peaks of 2.9 and 1.65 eV observed at room temperature （RT） are attributed to the radiative recombination of electrons and holes from the Ge nanoislands/SiO2 and Si02/Si interfaces, respectively. The mean island sizes are determined by fitting the experimental Raman profile to two models, namely, the phonon confinement model and the size distribution combined with phonon confinement model. The latter model yields the best fit to the experimental data. We confirm that SiO2 matrix thickness variations play a significant role in the formation of Ge nanoislands mediated via the minimization of interfacial and strain energies. OCIS codes： 250.5230, 170.5660.