Investigation of quantum systems with surface plasmons and surface state electrons

We report on investigations of thin films of the quantum systems hydrogen and helium both in thermodynamic equilibrium and in metastable states. Information about the film thickness and surface roughness is obtained from the excitation of surface plasmons and mobility measurements of surface state electrons. The equilibrium studies show triple-point wetting for H ₂ on Au substrates, in agreement with earlier results for similar systems. Unexpectedly a distinct hysteretic behavior of the saturated film thickness is found near the triple-point. Superfluid ⁴ He films on as prepared Ag display complete wetting. Investigations of H ₂ films in metastable states prepared by quench-condensation at 1.5K reveal pronounced changes in the film structure upon annealing. These changes set in far below the desorption temperature and are ascribed to surface diffusion.     

Effects of AlN layer thickness on magnetic anisotropy and transport phenomena of CoPt/AlN layered structure

The magnetic anisotropy was studied as a function of the AlN layer thickness in [AlN(x nm)/CoPt(2 nm)]5/AlN(x nm) layered structure (x is AlN layer thickness, and 5 is the number of multilayer series). The multilayered film was deposited by a sputtering apparatus equipped with two pairs of facing targets. It was found that, in the range of AlN layer thickness below 30 nm, CoPt/AlN multilayers transform from an enhanced in-plane magnetic anisotropy to perpendicular magnetic anisotropy (PMA) through thermal annealing in vacuum, with an optimized AlN thickness of 10 nm for strong PMA. However, beyond this thickness range, the PMA did not occur, and thermal annealing only results in magnetic isotropy in both parallel and perpendicular directions. The related structure analysis revealed that smooth interface and good texture of CoPt (111) make positive contributions to interface anisotropy energy and magnetocrystalline anisotropy energy for producing PMA in CoPt/AlN layered structure. In addition, the transport phenomena were also studied by using a four-probe method.     

Evolution of hole size and shape in {100}, {110} and {111} monocrystalline thin films of gold

Monocrystalline thin films of gold, containing controlled distributions of small holes, were produced by an epitaxial flash deposition process on heated {100}, {110} and {111} monocrystalline substrates of sodium chloride. These films, ranging from 10 to 20 nm in thickness, were then removed from their substrates, annealed for various periods at temperatures ranging from 180 to 290 °C and subsequently examined by transmission electron microscopy in order to record the evolution of hole size and shape as a function of crystallographic orientation and annealing conditions. During annealing, these holes either grow or shrink, depending on the ratio of hole diameter to film thickness, with growing holes developing clearly defined crystallographic facets aligned normal to the film surface. The evolution of hole size is in satisfactory agreement with a kinetic analysis based on atomic surface mobility, whereas the evolution of hole shape is consistent with anisotropy of the surface energy, as computed from a nearest neighbor bond model.     

Morphological evolution driven by strain induced surface diffusion

We have developed a three-dimensional finite element method to simulate the morphological evolution of a strained surface via surface diffusion, with a view to understanding the self-assembly, shape transitions and stability of low-dimensional quantum structures. We model deposition of an elastic film on a large lattice mismatched substrate. The film surface evolves by surface diffusion, driven by a gradient of the surface chemical potential, which includes the elastic strain energy, elastic anisotropy, surface energy, surface energy anisotropy and the interaction between the film and the substrate. Our simulations reveal that surface energy anisotropy and elastic anisotropy have a strong effect on the morphological evolution and shape transitions of the self-assembled islands. Our simulation results show a good qualitative agreement with experimental results.     

激光分子束外延SrTiO3薄膜退火过程中表面扩散的研究

用激光分子束外延研究了SrTiO3同质外延时原位退火中,反射高能电子衍射(RHEED)强度的恢复--驰豫时间,导出了高真空下表面扩散的活化能为0.31 eV,与低真空下的结果相比要小许多,这反映了粒子达到基片时的能量差.对沉积不同厚度的薄膜退火研究,表明当薄膜厚度增加时,表面恢复情况减弱,而导致随后的沉积时RHEED振荡周期的改变.     

Studies of the surface free energy of sulphur

From adsorption isotherms the water film pressure values on a sulphur surface were determined. An interpretation of the changes with film thickness and the value of the work of wetting processes is proposed. It is concluded that the characteristic film pressure values determined correspond to the work of immersional, adhesional and immersional-cohesional wetting. On the basis of water film pressure values, values of the dispersion component of sulphur surface free energy were determined.     

Micromagnetics of closed-edge laminations

Previous work has shown that lamination of the film core in a recording head eliminates noisy closure domains but introduces potentially noisy curling-edge walls. As a possible means of avoiding the edge walls, attention is given to minimization of the energy of an infinite strip composed of two laminations connected along their edges. Under the conditions that the spacer thickness is ≳0.2×(magnetic-sublayer thickness) and that the energy is less than that of the closure-domain state, it is predicted that: fully stray-field-free flux closure with now walls occurs if the sum of anisotropy and exchange-stiffness energies within magnetic connections is negative; if this energy sum is positive, then part of the flux leaks through the spacer, forming edge walls whose angle is <90° (polarization switching of these walls contributes to hysteresis); and if two would-be edge walls are so thick as to impinge on each other, then the actual ground state is one of saturation along the hard axis. An advantage of closed edges is the need for thicker and therefore fewer sublayers; a disadvantage of thicker sublayers is greater eddy currents     

An experimental investigation of modes of flow under conditions of evaporation of liquid on a vertical heating surface

An experimental investigation is performed of the effect of temperature head on the flow of evaporating film of liquid, defined by the wetting line or by ribs, on a vertical heating surface. The experiments are performed under conditions of evaporation of R11 Freon in a medium of own vapor on a vertical copper plate, including the presence of ribbing. The visualization of flow is performed. Analysis is made of the effect of the evaporation intensity in the neighborhood of liquid-vapor-wall contact line on the conditions of film discontinuity and on the pattern of resultant streamer flow. It is demonstrated that, rather than spreading, the liquid in the case of streamer flow on the heating surface contracts downstream even for a close-to-zero equilibrium wetting angle. This is due to intense evaporation of liquid in the region of liquid-vapor-wall contact line, where the liquid film exhibits a minimal thickness, to the variation of curvature of the interface in this region, and to the emergence of thermal contact angle. The dependence of thermal contact angle on temperature head is determined. Dynamic measurements are performed of the local thickness of flowing films of liquid using a capacitance meter, and spectral analysis is performed of waves which arise because of instability of film flow on the evaporating film surface.     

On Third Sound Amplification by Stimulated Condensation

We describe the process by which condensing ⁴ He vapor atoms are expected to contribute coherently to the macroscopic superfluid motion of a third sound resonance. This phenomenon is similar to the experimentally observed thickening of a film in the fixed velocity state of a persistent current, though with an interesting and subtle difference. As before, the condensing atoms contribute to the kinetic energy of the macroscopic state by the coherent normal-to-superfluid conversion. In the present case, however, the macroscopic state is an oscillatory third sound mode. This allows the energy to remain in the third sound mode even if superfluid is removed by film flow in order to keep the film thickness constant. Thus, a continuous process of third sound amplification by stimulated condensation should accompany a condensing vapor flux under the right conditions.     

Influence of sample surface on thermomagnetic effects in Fe40-Ni60

It is shown that the thickness of the sample, i.e., its surface-to-volume ratio, affects the outcome of thermomagnetic treatments on Fe40-Ni60. The existence of surface effects is proved by the differences of hysteresis loops observed on samples of various thicknesses annealed under identical conditions. Surface influence is further checked by etching-off equivalent surface layers after a given annealing, and comparing the resulting loop changes: these are found to depend markedly on the initial thickness of the sample. The enhancement of thermomagnetic effects on samples with larger surface-to-volume ratios is qualitatively explained by observing thata distribution of directional anisotropy energy probably affects the loop shape more effectively if induced in a surface layer, since it is correlated with domain patterns intrinsically more complex than in the bulk.     

射频磁控溅射 Ba0.6Sr0.4TiO3薄膜表层的XPS研究

用射频磁控溅射在Pt／Ti／SiO2／Si基体上沉积Ba0．6Sr0．4TiO3（BST）薄膜，用X射线光电子能谱（XPS）研究BST薄膜表层在常规晶化和快速晶化条件下的结构特征．结果表明，常规晶化时，BST薄膜表层约3-5nm厚度内含有非钙钛矿结构的BST，随着温度的升高该厚度增加；快速晶化时，该厚度减薄至1nm内，随着温度的升高没有明显增加．元素的化学态分析结果表明，非钙钛矿结构的BST并非来自薄膜表面吸附的CO和CO2等污染物，而与表面吸附的其他元素（如吸附氧）对表层结构的影响有关．GXRD和AFM表明，致密的表面结构能有效的阻止表面吸附元素在BST膜体中的扩散，从而减薄含非钙钛矿结构层的厚度．     

Heat and mass transfer through a vapor film in view of the motion of liquid-vapor interface and the rise of temperature at the interface

Unsteady-state processes of heat and mass transfer in a vapor film formed during the interaction of a highly heated body with a cold liquid are investigated using a numerical solution of the kinetic Boltzmann equation. Two options of formulation of the problem are treated, which are more complex compared to those treated in the previous study, namely, (a) the liquid surface moves at a constant velocity, and the liquid temperature is constant; and (b) the liquid surface is stationary, and the liquid is heated (heat is transferred by heat conduction alone). It is demonstrated that, in analyzing various applications (for example, problems on the evolution of vapor cavities under conditions of film boiling), the state of vapor in the film may be taken to be quasi-steady-state; it may be described by steady-state kinetic relations for which the effect of mass flux may be ignored.     

Anisotropy effect on strain-induced instability during growth of heteroepitaxial films

The use of misfit strain to improve the electronic performance of semiconductor films is a common strategy in modern electronic and photonic device fabrication. However, pursuing a favorable higher strain could lead to mechanical instability, on which systematic and quantitative understandings are yet to be achieved. In this paper, we investigate the anisotropy effects on strain-induced thin-film surface roughening by phase field modeling coupled with elasticity. We find that compared with films grown along {111} and {100} surfaces, the instability of {110} film occurs at a much lower strain. Our simulations capture the evolution of interface morphology and stress distribution during the roughening process. Similar characterizations are performed for heteroepitaxial growth from a surface pit. Finally, from 3D simulations, we show that the surface roughening pattern on {110} film exhibits a clear in-plane orientation preference, consistent with experimental observations.     

Magnetic and structural properties of ion beam sputtered Fe-Zr-Nb-B-Cu thin films

Magnetic and structural properties of Fe-Zr-Nb-B-Cu thin films, prepared by ion beam sputtering on silicon substrates by using a target made up of amorphous ribbons of nominal composition Fe₈₄Zr_3.5Nb_3.5B₈Cu₁, are reported. As-deposited thin film samples exhibit an in-plane uniaxial anisotropy, which can be ascribed to the preparation technique and the coupling of quenched-in internal stresses. Structural measurements indicate no significant variation of the grain size with thickness and with the annealing temperature. Increase in surface irregularities with annealing temperature and oxidation results in aggregates that would act as pinning centers, affecting the magnetic properties leading to magnetic hardening of the specimens. The role of the magnetic anisotropy is thoroughly discussed with the help of magnetic and ferromagnetic resonance measurements.     

Low-temperature plasma-enhanced chemical vapor deposition of hard carbon films

Carbon films were prepared by the plasma-enhanced chemical vapor deposition method from methane gas under different regimes of a growing film surface ion bombardment. Ion energy and ion flux was measured for different deposition regimes and were controllably and independently varied for deposition of different film samples. The structure of the films deposited was studied by Raman scattering spectroscopy. It was found that ion bombardment of the growing film surface is a key factor in hard carbon films deposition. A correlation between the sp³ hybridized carbon fraction content with both ion energy and ion flux was found. The maximum sp³ hybridized carbon fraction content was achieved for films deposited under maximum ion energy and ion flux conditions. Additionally, a dependence of the film properties on the substrate material and on the final film thickness was observed.     

Center wavelength shift dependence on substrate coefficient of thermal expansion for optical thin-film interference filters deposited by ion-beam sputtering

Single-layer films of Ta2O5 and multilayer thin-film filters of Ta2O5 and SiO2 were deposited by ion-beam-sputter deposition. Postdeposition annealing of the structures resulted in increased optical thickness of the films, resulting in an upward shift in the wavelength of the transmission-reflection spectra. Modeling of the single-layer films by means of the effective media approximation indicates an increase in the void fraction of the film after annealing. This increase is consistent with an observed decrease in refractive index and an increase in physical film thickness. The multilayer structures, deposited on substrates of varying coefficient of thermal expansion (CTE), were annealed at various temperatures, and the change in the center wavelength was measured. The measured change is dependent on the annealing temperature and the substrate CTE, indicating that the increase in the void fraction is caused in part by thermally induced stress during the annealing process. A simple model is proposed that relates the void fraction present in the films after annealing with the substrate CTE and the annealing temperature.     

Epoxy resin-based nanocomposite films with highly oriented BN nanosheets prepared using a nanosecond-pulse electric field

Facile orientation of boron nitride (BN) with high anisotropy in epoxy resin-based nanocomposite films was performed in a polyepoxide matrix using a nanosecond-pulse electric field to generate a high electric flux. Control of the BN anisotropy was achieved in the polymer without damaging the composite films or requiring surface modification of the BN. The degree of BN orientation perpendicular to the nanocomposite film plane, which was parallel to the electric flux, could be controlled by applying the nanosecond pulse for different lengths of time before cross-linking. The resulting composite films with oriented BN nanosheets manifested improved thermal diffusivity compared to a composite prepared without orientation.     

离子束溅射铜钨薄膜的结构

为了进一步探讨离子束溅射铜钨薄膜的结构,在铁片上离子束溅射铜钨薄膜,研究了轰击离子束能量及低能辅助轰击方式对薄膜相结构和厚度的影响.结果表明:随轰击铜靶离子束能量增加,钨由近似非晶亚稳态转变成晶态;由于溅射粒子落到基片前的反射效应,薄膜中间比边缘薄,且随轰击铜靶离子束能量增加,薄膜变薄到一定程度时开始增厚;当使用低能辅...     

Angular dependence of magnetic annealing effects in permalloy base films

Attempts have been made to interpret magnetic annealing effects by assuming that the uniaxial anisotropy is caused by a directional order of local atomic configurations. By studying both evaporated and electroplated films, it is found that during an anneal the number of such elementary anisotropy sources is conserved. The angular dependence of magnetic annealing effects can be described by assuming a reorder of elementary anisotropy sources parallel to the magnetization at a rate independent of the magnetization direction. In the absence of a physical model describing the kinetics of reorder, it is shown how experimental values of the anisotropy field measured during hard-axis anneals can be used to predict for any annealing direction, the changes in anisotropy field, skew, and coercivity. Changes in angular dispersion were also studied but could not be explained satisfactorily. The variations of all main film parameters are found to be reversible during subsequent easy-axis anneals, and the recovery of the anisotropy field during such an anneal can be used to predict the recovery of all other parameters.     

The Conditions for the Existence of the Optimal Thickness of a Cooled Anisotropic Wall Subjected to Local Heat Exposure

The problem of the determination of sufficient conditions for the existence of the optimal thickness of an anisotropic wall, one of whose surfaces of is exposed to axisymmetric stationary heat flux with the intensity of the Gaussian type, while the other is cooled by the external medium with a constant temperature, was formulated and solved using the two-dimensional exponential integral Fourier transform. The requirement for minimization of the temperature of the most heated point of object of study was used as an optimality criterion. The sufficient condition that was obtained is an inequality that establishes the link between the thermophysical characteristics of the anisotropic material of a wall, the intensity of heat transfer on its cooled surface, and the factor of the concentration of the outer heat flux. These results confirm the well-known effect of the “drift” of the temperature field in an anisotropic material with the common type of anisotropy of its properties.