Heat transfer in YBaCuO thin film/sapphire substrate system

The thermal boundary resistance at the YBaCuO thin film/Al₂O₃ substrate interface was investigated. The transparency for thermal phonons incident on the interface as well as for phonons moving from the substrate was determined. We have measured a transient voltage response of current-biased films to continuously modulated radiation. The observed knee in the modulation frequency dependence of the response reflects the crossover from the diffusion regime to the contact resistance regime of the heat transfer across the interface. The values of transparency were independently deduced both from the phonon escape time and from the time of phonon return to the film which were identified with peculiarities in the frequency dependence. The results are much more consistent with the acoustic mismatch theory than the diffuse mismatch model.We are grateful to A. Elantev for helpful discussion. We acknowledge the financial support of the Russian Scientific Council on the HTS problem (Project No. 90462).     

Charge transfer luminescence in Yb-containing compounds

The characteristics of charge transfer transition of Yb³⁺ in selected compounds and complexes are reviewed. Systematic comparison of the optical and luminescence spectra as well as the decay kinetics of Yb-doped aluminium perovskites and garnets is provided. Effect of intrinsic defects and possible occurrence of Yb²⁺ is discussed and possible application areas are briefly mentioned.     

Some aspects of substrate pretreatment for epitaxial Si nanowire growth

We report on the influence of the surface pretreatment for vapor-liquid-solid growth of epitaxial silicon nanowires with gold catalyst and silane precursor on Si(111) substrates. In this paper we make it obvious that a thin native oxide layer on the Si substrate-as is present under most technological conditions-or a thin layer of oxide formed on top of the catalytic gold particle restrain nucleation and nanowire growth. High resolution transmission electron microscopy, and electron energy loss spectroscopy were utilized to demonstrate Si diffusion from the substrate through the catalytic Au layer and further the formation of a thin oxide layer atop. Based on this observation we present a sample pretreatment practice, making the catalyst insensitive for further oxide formation, thereby preserving epitaxy for nanowire synthesis.     

Effect of thermal treatment on ZnO substrate for epitaxial growth

ZnO is a highly efficient photon emitter, and has optical and piezoelectric properties that are attractive for a variety of applications in sensors and potentially optoelectronic devices such as emitters. Due to its identical stacking order and close lattice match to GaN, it is also being developed as a substrate material for GaN epitaxy. However, the surface finish of the ZnO is such that much of the damage induced by sawing and follow up mechanical polishing remains. We developed a thermal treatment method to eliminate surface damage on the 0 face of ZnO (0 0 0 1) to prepare it for epitaxial growth. Atomic force microscopy images of ZnO (0 0 0 1) annealed at 1050 °C for 3 h etc. show that residual scratches from mechanical polishing are removed and atomically flat, terrace-like surfaces are attained. In addition, low-temperature photoluminescence and high-resolution X-ray diffraction measurements have been employed to investigate the effect of annealing on ZnO substrates.     

The effect of substrate defects on epitaxial magnetic garnet films

The stress patterns associated with various defects in Czochralski-grown gadolinium gallium garnet (GGG) crystals have been observed using a polarizing microscope. The effect of these defects on both the surface topography and the magnetic behaviour of epitaxial magnetic garnet films grown on GGG substrates is reported. In particular, iridium inclusions and defects of a filamentary nature affect the surface topography. Other types of defect influence the magnetic behaviour of the films in a manner attributable to a change in the lattice parameter.     

Transfer-free electrical insulation of epitaxial graphene from its metal substrate

High-quality, large-area epitaxial graphene can be grown on metal surfaces, but its transport properties cannot be exploited because the electrical conduction is dominated by the substrate. Here we insulate epitaxial graphene on Ru(0001) by a stepwise intercalation of silicon and oxygen, and the eventual formation of a SiO(2) layer between the graphene and the metal. We follow the reaction steps by X-ray photoemission spectroscopy and demonstrate the electrical insulation using a nanoscale multipoint probe technique.     

Liquid phase epitaxial growth of gallium arsenide on an etched substrate

A method is described for etching the surface of GaAs substrates with a Ga solution $\text{in}$$\text{situ}$ immediately prior to epitaxial layer growth from the liquid phase. Layers grown by this procedure show excellent surface morphologies and electrical properties ($\text{μ～5100}\text{cm}{}^2\text{v-sec}$ at 300°K for $\text{n～ 7.8×10}{}^{16}\text{cm}{}^3$). Advantages of the method and its applicability to epitaxial growth of other III-V compounds are discussed.     

Charge transfer in two-phase (gas-solid-particle) flows

The contact charging of a single solid particle in contact with a plane wall and of a particle flux bounded by a cylindrical wall is considered.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 34, No. 1, pp. 50–57, January, 1978.     

GaN epitaxial layers prepared on nano-patterned Si(001) substrate

We report the growth of GaN epitaxial layer on Si(001) substrate with nano-patterns prepared by dry etching facility used in integrated circuit (IC) industry. It was found that the GaN epitaxial layer prepared on nano-patterned Si(001) substrate exhibits both cubic and hexagonal phases. It was also found that threading dislocation observed from GaN prepared on nano-patterned Si(001) substrate was significantly smaller than that prepared on conventional unpatterned Si(111) substrate. Furthermore, it was found that we can reduce the tensile stress in GaN epitaxial layer by about 78% using the nano-patterned Si(001) substrate.     

Studies on silicon-on-insulator-multilayer structures prepared by epitaxial layer transfer

The silicon-on-insulator-multilayer (SOIM) structures were proposed and successfully fabricated by epitaxial layer transfer technology. The properties of the structures were investigated using cross-sectional transmission electron microscopy (XTEM) and spreading resistance profiling. Experimental results show that the buried Si₃N₄ layer is amorphous and the new SOIM sample has good structural and electrical properties. The new SOIM device has been verified in two-dimensional device simulation and indicated that the new structures reduce device self-heating and increase the drain of the SOI MOSFET.     

Electrical control of the superconducting-to-insulating transition in graphene-metal hybrids

Graphene is a sturdy and chemically inert material exhibiting an exposed two-dimensional electron gas of high mobility. These combined properties enable the design of graphene composites, based either on covalent or non-covalent coupling of adsorbates, or on stacked and multilayered heterostructures. These systems have shown tunable electronic properties such as bandgap engineering, reversible metal-insulating transition or supramolecular spintronics. Tunable superconductivity is expected as well, but experimental realization is lacking. Here, we show experiments based on metal-graphene hybrid composites, enabling the tunable proximity coupling of an array of superconducting nanoparticles of tin onto a macroscopic graphene sheet. This material allows full electrical control of the superconductivity down to a strongly insulating state at low temperature. The observed gate control of superconductivity results from the combination of a proximity-induced superconductivity generated by the metallic nanoparticle array with the two-dimensional and tunable metallicity of graphene. The resulting hybrid material behaves, as a whole, like a granular superconductor showing universal transition threshold and localization of Cooper pairs in the insulating phase. This experiment sheds light on the emergence of superconductivity in inhomogeneous superconductors, and more generally, it demonstrates the potential of graphene as a versatile building block for the realization of superconducting materials.     

Group-III-nitride-based light-emitting diode on silicon substrate with epitaxial nanolayer of silicon carbide

A light-emitting diode (LED) structure based on group III nitride has been grown for the first time on Si(111) substrate with SiC buffer nanolayer (50- to 200-nm-thick) obtained by solid-phase epitaxy. This LED structure is characterized by record low (<10⁸ cm⁻²) density of lattice misfit dislocations at a total dislocation density of ∼8 × 10⁸ cm⁻². The photo- and electroluminescence spectra of obtained structures have been measured.     

Charge transfer and retention in directly coupled Au-CdSe nanohybrids

The energy and charge transfer dynamics of directly coupled Au-CdSe hybrid nanocrystals have been studied using time-resolved photoluminescence (PL) techniques. The PL of such nanohybrids was found to be quenched dramatically compared to that of both CdSe quantum dots and mixtures of CdSe quantum dots with Au nanoparticles. Fluorescence decay curves of the Au-CdSe nanohybrids show three distinct decay channels with the fastest one associated with the transfer of electrons from the CdSe portion to the Au portion. The holes on the CdSe portion created by such charge transfer were then quickly taken away by the solution, while the electrons on the Au portion slowly leaked into the solution as well, thus serving as a reductant for redox reactions. Using a model reaction based on the reduction of methylene blue by the leaking electrons, our photocatalytic experiments indicate that the electrons can be temporarily retained in the Au portion (most likely at the Au-capping agent interface) for a dramatically long timescale, up to 100 min. Finally, by merging all of the observations in the time-resolved PL measurements, we were able to figure out a relatively complete picture of charge transfer and retention in the Au-CdSe nanohybrids. This picture is expected to guide researchers in designing modern photocatalysts and solar cells constructed from nanoscale metal-semiconductor hybrids.      

Charge transfer induced polarity switching in carbon nanotube transistors

We probed the charge transfer interaction between the amine-containing molecules hydrazine, polyaniline, and aminobutyl phosphonic acid and carbon nanotube field effect transistors (CNTFETs). We successfully converted p-type CNTFETs to n-type and drastically improved the device performance in both the ON- and OFF-transistor states, utilizing hydrazine as dopant. We effectively switched the transistor polarity between p- and n- type by accessing different oxidation states of polyaniline. We also demonstrated the flexibility of modulating the threshold voltage (Vth) of a CNTFET by engineering various charge-accepting and -donating groups in the same molecule.     

Deformation behavior during nanoindentation of epitaxial ZnO thin films on sapphire substrate

Nanoindentation studies were carried out on epitaxial ZnO thin films on (0001) sapphire substrates grown by radio frequency magnetron sputtering. A single discontinuity (‘pop-in’) in the load–displacement curve was observed at a specific depth (13−16 nm) irrespective of the film thickness. The physical mechanism responsible for the ‘pop-in’ event in these epitaxial films may be due to the nucleation, propagation and interaction behavior of the glissile threading dislocations during mechanical deformation. Indentation well below the critical depth was found to be plastic deformation behavior (residual impression of 4 nm).     

Simulation of epitaxial growth on convex substrate using phase field crystal method

Phase field crystal （PFC） model is employed to simulate the process of growth of epitaxial layer on plane-convex substrate with a lattice mismatch and a small inclination angle. The variation of the systematic free energy, the total atomic number of the epitaxial layer, and the effect of the curvature and the angle of the substrate are analyzed. The results show that when the surface of the substrate is plane, the free energy increases with the increase of the substrate inclination angle, and also the total atomic number of the epitaxial layer increases; while the surface of the substrate is convex, the free energy decreases with the increase of substrate angle and so also the total atomic number of the epitaxial layer decrease. This is the reason that the frontier of surface of epitaxial layer changes from the step bunching to the hill-and-valley facet structure with the increasing of the inclination angle of convex substrate. These results are in good agreement with the other method results.     

Influence of substrate on the in-plane ferromagnetic spectrum of epitaxial YIG films

We find a direct correspondence between the lattice mismatch and the in-plane angular dependence of the FMR field for epitaxial YIG films on non-magnetic garnet substrates.     

Charge transfer reactions across oxide covered Titanium Electrodes

The kinetics of the charge transfer process of the ferro-/ferricyanide Redox system on anodically formed titanium oxide layers is studied. The oxides were formed potentiodynamically in borate buffer solutions of pH = 8. Anodic and cathodic Tafel lines were measured at a scan rate of 1.0 mV · S^?1. Oxide thickness was measured using capacitance technique at a frequency of 1.0 KHz. The log of the exchange cd, i₀ for the Redox system was found to decrease linearly with the oxide thickness in agreement with the theoretical treatment of Heusler. Tunneling of the electron represents the main process for the charge transfer for oxides formed at potentials below O₂-evolution. Temporary breakdown of the oxide around the O₂-evolution potential, causes an increase of i_O with thickness. The reaction order was 0.8–0.85 with respect to the Redox system. The low values of the anodic transfer coefficient supports earlier results on similar n-type semiconductor electrodes.     

Charge transfer and ionisation by intermediate-energy heavy ions

The use of heavy ion beams for microbeam studies of mammalian cell response leads to a need to better understand interaction cross sections for collisions of heavy ions with tissue constituents. For ion energies of a few MeV u(-1) or less, ions capture electrons from the media in which they travel and undergo subsequent interactions as partially 'dressed' ions. For example, 16 MeV fluorine ions have an equilibrium charge of 7(+), 32 MeV sulphur ions have an equilibrium charge of approximately 11(+), and as the ion energies decrease the equilibrium charge decreases dramatically. Data for interactions of partially dressed ions are extremely rare, making it difficult to estimate microscopic patterns of energy deposition leading to damage to cellular components. Such estimates, normally obtained by Monte Carlo track structure simulations, require a comprehensive database of differential and total ionisation cross sections as well as charge transfer cross sections. To provide information for track simulation, measurement of total ionisation cross sections have been initiated at East Carolina University using the recoil ion time-of-flight method that also yields cross sections for multiple ionisation processes and charge transfer cross sections; multiple ionisation is prevalent for heavy ion interactions. In addition, measurements of differential ionisation cross sections needed for Monte Carlo simulation of detailed event-by-event particle tracks are under way. Differential, total and multiple ionisation cross sections and electron capture and loss cross sections measured for C(+) ions with energies of 100 and 200 keV u(-1) are described.     

Charge transfer in single-walled carbon nanotubes filled with cadmium halogenides

In this work the internal channels of the single-walled carbon nanotubes (SWCNTs) were filled with cadmium chloride, cadmium bromide, and cadmium iodide by a capillary method using melts of these salts. The influence of incorporated chemical compounds on the electronic properties of the carbon nanotubes was investigated by optical absorption spectroscopy, Raman spectroscopy, near edge X-ray absorption fine structure spectroscopy, and X-ray photoelectron spectroscopy. It was found that there is the chemical bonding between carbon atoms of nanotube walls and metal atoms of encapsulated CdX₂ nanocrystals. The obtained data testify acceptor doping effect of cadmium halogenides incorporated into the SWCNT channels, which is accompanied by the charge transfer from nanotube walls to introduced substances.