Study of the effect of initial conditions on the scaling properties of CaF2 thin films

The growth of thermally deposited CaF₂ films was studied using three different substrates for deposition: glass, gold and silicon. Each substrate was chosen because of its different topography and used to determine the effect of substrate roughness on the growth of CaF₂ films. After thermally depositing a range of CaF₂ film thicknesses on the substrates, the CaF₂ surfaces were imaged using atomic force microscopy. The images were then used to determine the characteristic exponents which described the surface. In each case the Hurst exponent, H was found to rapidly increase from the initial substrate condition to a constant value (H ≈ 0.85) with increasing CaF₂ film thickness. This rapid crossover is quite remarkable and occurs in films with nominal thicknesses less than ≈20 nm. These data indicate that the roughness of the substrate, or in other words the initial conditions, have little effect on the growth properties of CaF₂ films beyond the crossover at very small values of the film thickness. The scaling of the dynamic exponent, β, is also presented as are measurements of the CaF₂ film porosity.     

Influence of the surface microroughness of a Si(001) substrate on the morphology of CaF2 epitaxial layers under high-temperature growth conditions

The influence of the surface microroughness of Si(001) substrates on the morphology of CaF₂ epitaxial layers is investigated in the high-temperature growth mode. The atomic-force microscopy method is used to demonstrate that the silicon surface relief substantially affects the nucleation, growth, and formation of the morphology of CaF₂ layers.     

Homogeneous Si films on CaF2/Si(1 1 1) due to boron enhanced solid phase epitaxy

The structure and morphology of Si/CaF₂/Si(1 1 1) structures have been investigated by X-ray diffraction (XRD, GIXRD) and X-ray photoelectron spectroscopy (XPS). While CaF₂ films were grown via molecular beam epitaxy (MBE), Si films on CaF₂/Si(1 1 1) are fabricated by surfactant enhanced solid phase epitaxy (SE-SPE). Here Boron was used as a surfactant to obtain semiconductor films of homogeneous thickness. The Si films are entirely relaxed while the CaF₂ films have both pseudomorphic and relaxed crystallites. After exposure to ambient conditions, the Si films have a very thin native oxide film. The homogeneous Si film partially prevents the incorporation of impurities at the interface between the Si substrate and CaF₂ via migration along residual defects of the CaF₂ film.     

Observation of plasmon excitations in X-ray production following heavy ion bombardment

Four equidistant peaks are observed below the Kα_1,2 line in the X-ray spectra of Na, Al, Si, SiO₂, Ca, Sc and Ti, produced by oxygen bombardment. With much weaker intensity, these lines are also seen in proton excitation. The lines are interpreted to be due to the excitation of one or more volume plasmons in the target material by the Kα_1,2 X-ray decay.     

Features of the surface morphology of CaF<Subscript>2</Subscript>/Si(100) films obtained by solid-phase epitaxy

The dielectric parameters of calcium fluoride films grown on the Si(100) surface by solid-phase epitaxy (group 2) and without it (group 1) are analyzed. It is established experimentally that the deposition mode of CaF₂ films immediately after growth of the Si buffer layer at a substrate temperature of 530°C is not suitable for producing high-quality dielectric layers. The use of solid-phase epitaxy at the initial stage of the nucleation of CaF₂ layers enables the production of single crystal uniformly thick films with high dielectric properties.     

Accurate Design of Finline Tapers for Millimeter Wave Applications

The design of nonuniform bilateral finlines on anisotropic substrates for millimeter wave applications is presented. The taper consists of a planar circuit with smooth variation of the slot width profile along the structure. The spectral domain method and Galerkin procedure are combined with the transmission line theory for studying the behavior of tapered bilateral finlines and evaluation of their design parameters. The influence of the substrate anisotropy on the propagation characteristics of these structures is also examined. This technique is general and can be applied to investigate a broad class of planar transmission line tapers.     

Fabrication of tapers with Gaussian refractive-index profile in azo polymers

We have fabricated a refractive-index-tapered waveguide by exposing an azo polymer film to a 532-nm wavelength laser beam focused to about 60-μm spot size at the film with no surface deformation. The converging ability of the taper (ability to increase intensity along the taper) was measured to be 5 db/cm while the loss was 0.39 db/cm. According to the data obtained, the power density increases along the taper while loss is nearly constant along the tapered and non-tapered segments.     

Substrate effects on the oxygen gas sensing properties of SnO2/TiO2 thin films

In this study, SnO₂/TiO₂ thin films are fabricated on SiO₂/Si and Corning glass 1737 substrates using a R.F. magnetron sputtering process. The gas sensing properties of these films under an oxygen atmosphere with and without UV irradiation are carefully examined. The surface structure, morphology, optical transmission characteristics, and chemical compositions of the films are analyzed by atomic force microscopy, scanning electron microscopy and PL spectrometry. It is found that the oxygen sensitivity of the films deposited on Corning glass 1737 substrates is significantly lower than that of the films grown on SiO₂/Si substrates. Therefore, the results suggest that SiO₂/Si is an appropriate substrate material for oxygen gas sensors fabricated using thin SnO₂/TiO₂ films.     

Asymmetrical plasmon reflections in tapered graphene ribbons with wrinkle edges

Asymmetrical graphene plasmon reflection patterns are found in infrared near-field images of tapered graphene ribbons epitaxially grown on silicon carbon substrates. Comparing experimental data with numerical simulations, the asymmetry of these patterns is attributed to reflection of plasmons by wrinkled edges naturally grown in the graphene. These graphene wrinkles are additional plasmon reflectors with varying optical conductivity, which act as nanometer scale plasmonic modulators and thus have potential applications in photoelectric information detectors, transmitters, and modulators.     

金属衬底上石墨烯的红外近场光学

对石墨烯/铜体系开展了系统性的近场光学实验研究,成功观测到了区别于铜衬底的、来自石墨烯的近场光学响应信号,发现在表面台阶几何参数相同的铜衬底上的不同石墨烯样品表现出了截然不同的近场光学响应.     

Analysis of the oscillation intensity of RHEED specular reflection during the MBE growth of CaF<Subscript>2</Subscript>/Si/CaF<Subscript>2</Subscript> structures

The results of analysis of the oscillation intensity of RHEED specular reflection during the MBE growth of CaF₂/Si(111) structures in a wide temperature range from 100 to 600°С are presented. It is shown that the preliminary formation of a 2D Si buffer layer provides the two-dimensional growth of CaF₂ layers. Possible reasons which for the disruption of 2D growth at high substrate temperatures are discussed.     

Effect of high-energy electron beam irradiation on the surface morphology of CaF2/Si(100) heterostructures

The effect of high-energy electron beam irradiation on the surface morphology of CaF₂ films in the course of molecular-beam epitaxy on Si(100) substrates is studied by atomic-force microscopy. It is shown that not only do morphological defects significantly change their shape in the electron beam spot, but also their average height more than doubles.     

Influence of Si substrate preparation on surface chemistry and morphology of L-CVD SnO2 thin films studied by XPS and AFM

Results of experimental studies of the influence of substrate preparation on the surface chemistry and surface morphology of the laser-assisted chemical vapour deposition (L-CVD) SnO₂ thin films are presented in this paper. The native Si(1 0 0) substrate cleaned by UHV thermal annealing (TA) as well as thermally oxidized Si(1 0 0) substrate cleaned by ion bombardment (IBA) have been used as the substrates. X-ray photoemission spectroscopy (XPS) has been used for the control of surface chemistry of the substrates as well as of deposited films. Atomic force microscopy (AFM) has been used to control the surface morphology of the L-CVD SnO₂ thin films deposited on differently prepared substrates. Our XPS shows that the L-CVD SnO₂ thin films deposited on thermally oxidized Si(1 0 0) substrate after cleaning with ion bombardment exhibit the same stoichiometry, i.e. ratio [O]/[Sn] = 1.30 as that of the layers deposited on Si(1 0 0) substrate previously cleaned by UHV prolonged heating. AFM shows that L-CVD SnO₂ thin films deposited on thermally oxidized Si(1 0 0) substrate after cleaning with ion bombardment exhibit evidently increasing rough surface topography with respect to roughness, grain size range and maximum grain height as the L-CVD SnO₂ thin films deposited on atomically clean Si substrate at the same surface chemistry (nonstoichiometry) reflect the higher substrate roughness after cleaning with ion bombardment.     

STM and LEED studies of CaF2 submonolayer coverage on Si(001)

In the present work we report on the initial stage of CaF₂ growth on Si(001) at 750 °C. Samples with submonolayer coverage are grown and in situ studied by low energy electron diffraction and scanning tunneling microscopy. The unusual diffraction patterns with asymmetrically split fractional reflections and streaks are ascribed to the antiphase relations at the surface characteristic for the studied CaF₂ coverage. A statistical model built to describe the partially ordered array of 3× cells shows good agreement with the observed diffraction patterns. Similarities between CaF₂/Si(001) and metallic Ca/Si(001) interfaces are discussed.     

Rapid Preparation of Large-Area Densely Packed Plasmonic Hot-Spots for Reliable Sers Sensing

A facile, efficient and time-saving strategy is proposed to obtain large-area and reliable surface-enhanced Raman scattering (SERS) substrates via artificial heat-treatment of Au nanoparticles or ultrathin Au films sputtered on the silica substrates. Excellent Raman enhancements with the detection limitation down to 10^–9 mol/L are obtained due to the highly-dense plasmonic hot-spots and strong plasmons near-field coupling. Decreased intensity of Raman peaks with the increased sputtering time of Au nanoparticles or ultrathin films mainly originates from the excited and hybridized coupling of multiple surface plasmons. The simple fabrication strategy and superior performance make these substrates promising candidates for the development of inexpensive and reliable SERS substrates.

     

Study of the features of BaF<Subscript>2</Subscript> heteroepitaxy on CaF<Subscript>2</Subscript>/Si(100) layers obtained in the high-temperature growth mode

The surface morphology of BaF₂ epitaxial films grown by MBE (molecular beam epitaxy) in various modes on the surface of CaF₂/Si(100) is investigated by AFM. The CaF₂ layers on Si(100) are obtained in the high-temperature growth mode (Т _S = 750°C). It is shown that the epitaxy of BaF2 at a temperature of 600°C at the initial stage of growth leads to the formation of defects such as perforations in the epitaxial film, while epitaxy at a temperature of 750°C provides a defect-free film with a surface morphology suitable for the subsequent growth of semiconductors of IV–VI type and solid solutions based on them.     

Raman spectroscopic analysis of the CaF2Si heterostructure interface

The lattice constants of CaF₂ and Si differ by only 0.6%. In addition, these materials have very similar cubic structures. These similarities suggest that CaF₂Si should form a low stress heterostructure interface. We have used Raman scattering to probe Si at the epitaxial CaF₂Si interface. Measurements of these MBE grown films indicate that there is very little interfacial stress (<0.5 kbars). Comparison is made with Si films grown on sapphire and spinel.     

Ion tracks in ultrathin polymer films: The role of the substrate

Surface damage produced by single MeV-GeV heavy ions impacting ultrathin polymer films has been shown to be weaker than those observed under bulk (thick film) conditions. The decrease in damage efficiency has been attributed to the suppression of long-range effects arising from excited atoms lying deeply in the solid. This raises the possibility that the substrate of the films itself is relevant to the radiation effects seen at the top surface. Here, the role of the substrate on cratering induced by individual 1.1 GeV Au ions in ultrathin poly(methyl methacrylate) (PMMA) layers is investigated. Materials of different thermal and electrical properties (Si, SiO₂, and Au) are used as substrates to deposit PMMA thin films of various thicknesses from ∼1 to ∼300 nm. We show that in films thinner than ∼40 nm craters are modulated by the underlying substrate to a degree that depends on the transport properties of the medium. Crater size in ultrathin films deposited on the insulating SiO₂ is larger than in similar films deposited on the conducting Au layer. This is consistent with an inefficient coupling of the electronic excitation energy to the atomic cores in metals. On the other hand, the damage on films deposited on SiO₂ is not very different from the Si substrate with a native oxide layer, suggesting, in addition, poor energy transmission across the film/substrate interface. The experimental observations are also compared to calculations from an analytical model based on energy addition and transport from the excited ion track, which describe only partially the results.     

Effect of an electron beam on CaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> epitaxial layers on Si

Atomically smooth CaF₂ and BaF₂ layers have been sequentially grown on Si(111) substrates by molecular beam epitaxy. Pore macrodefects have been revealed at the points of the action of an electron beam from a diffractometer when analyzing the crystal structure of the surface during the growth with the subsequent observation using atomic force microscopy. The formation of these macrodefects is associated with the decomposition of fluorides by high-energy electrons, which is accompanied by the desorption of fluorine and the drift current of positive ions from the electron charge drains.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.