Grain boundary diffusion and wetting in the analysis of intergranular penetration

Intergranular penetration of liquid bismuth has been analysed in two pure metals, Cu at 500 °C and Ni at 700 °C, used either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves have developed in Cu–Bi, while micrometer-thick films were observed in Ni–Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicate a zone of monolayer Bi segregation followed by a diffusion-type profile over a distance of the order of 100 μm for Cu–Bi and a nanometer-thick film followed by similar diffusion-type profile for Ni–Bi. In both cases the kinetics of intergranular penetration and embrittlement has been shown to be parabolic. It is concluded that no wetting occurs in Cu–Bi system at 500 °C while Bi wets Ni at 700 °C. It is postulated that the mechanism of intergranular penetration operates at a very tip of the penetration front, as opposed to the tip of liquid Bi film observed by scanning electron microscopy, and must be based on diffusion rather than wetting phenomena. Some suggestions are formulated for the future research in the area of intergranular penetration that can be split in two phenomena: grain boundary wetting above the wetting transition temperature and grain boundary diffusion below.     

Equilibrium surface roughness of a strained epitaxial film due to surface diffusion induced by interface misfit dislocations

In recent years, developments in the microelectronics industry have led to extensive studies of the growth and characterization of thin solid films and their implementation in electronic and opto-electronic devices. A goal is to produce thin films with minimal bulk and surface defects. For those systems produced by epitaxial growth of a film on a substrate that has a slightly different lattice parameter, the stress associated with the elastic mismatch strain needed to satisfy the constraint of epitaxy provides a driving force for nucleation and growth of undesirable defects in the film material or on its surface. Among the most common defects are interface misfit dislocations, arranged more or less periodically on the film-substrate interface, which partially relax the elastic mismatch strain in the film. It has been observed that, for some material systems, surface roughness or waviness arises which correlates spatially with the positions of interface misfit dislocations. It is suggested here that the waviness along the surface may be a result of surface diffusion which is driven by a gradient in the chemical potential of the material along the surface. The chemical potential gradient arises from the nonuniform strain field of the interface misfit dislocations, as well as from the unrelaxed elastic mismatch strain. The focus here is on the development of a relatively simple model of this system which leads to an estimate of the magnitude and profile of surface waviness under conditions of thermodynamic equilibrium, i.e., after the material responds to the chemical potential gradient by seeking out a new configuration for which stresses are redistributed and the chemical potential is again uniform. The condition of uniform chemical potential for the final shape leads to an integro-differential equation for the equilibrium surface shape which is solved numerically. For representative values of system parameters, estimates of equilibrium surface roughness are obtained which can vary from less than one percent of film thickness to a significant fraction of film thickness. Although transient aspects of the process are not studied here, the characteristic time for achieving an equilibrium configuration is estimated.     

Ag diffusion in ZnS thin films prepared by spray pyrolysis

ZnS thin films were deposited by spray pyrolysis method on glass substrates. Diffusion of Ag in ZnS thin films was performed in the temperature range 80-400 °C under a nitrogen atmosphere. The diffusion of Ag is determined with XRF, and the obtained concentration profile allows to calculate the diffusion coefficient. The temperature dependence of Ag diffusion coefficient is determined by the equation D = 8 × 10^− 9 exp(− 0.10 eV / kT). It was found that the as-grown undoped high resistive n-type ZnS thin films were converted to the p-type upon Ag doping with a slight increase in resistivity only by rapid thermal annealing at 400 °C in N₂ atmosphere. In addition, the band gap of the p-type film was decreased as compared with the undoped sample annealed under the same conditions. The results were attributed to the migration of Ag atoms in polycrystalline ZnS films by means of both along intergrain surfaces and intragrain accompanied by interaction with native point defect.     

Self diffusion studies on cobalt thin films

The study of lateral diffusion in thin metallic films is important from the application point of view, especially in electromigration reliability studies. Lateral self diffusion in cobalt thin films is studied using a non-destructive tracer scanning method. Neutron irradiation is employed to make a well-defined radioactive (⁶⁰Co) region in the middle of a continuous cobalt thin film stripe of width 3 mm. The experimental data are fitted to the appropriate solution of the diffusion equations by means of a non-linear least square fitting procedure using a computer. The diffusion experiments are conducted in the temperature range 300–600°C in argon atmosphere. This thin film data are compared with the diffusion data available on bulk cobalt. The activation energy for surface diffusion obtained (0·14 eV) is very much smaller than the reported activation energy for grain boundary diffusion in cobalt.     

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.     

纳米薄膜界面滑移现象的分子动力学模拟进展

随着先进制造技术的发展,许多精密系统中的薄膜厚度已经接近纳米量级。由于纳米尺度的特殊性,宏观尺度下的界面性质将不再适用,因此探索纳米尺度下的界面性质对于微机电系统(MEMS)甚至纳机电系统(NEMS)中涉及到的润滑薄膜研究具有非常重要的价值。Couette流动作为剪切驱动流动的原形,在研究薄膜界面滑移现象上具有较高的代表性。在总结国内外该方向研究成果的基础上,详细介绍和讨论了纳米尺度薄膜Couette流动的分子动力学方法,总结了现有的滑移现象和规律,并对今后的发展进行了展望。     

Multilayer films by spraying on spinning surface — Best of both worlds

The layer-by-layer buildup of poly(styrene sulfonate), poly(diallyldimethyl ammonium) and/or TiO₂-nanoparticles was carried out using an automated system which allowed spraying the polyion solutions on a spinning substrate. The different spraying and spinning arrangements were studied. The films were prepared by varying ionic strength, electrolyte species (NaBr and NaF) and spraying time. The results were compared to the films prepared by using the traditional dipping method. Depending on the electrolyte and the ionic strength the thickness of the films prepared with spraying on spinning substrate was thinner than or equal to the dipped films. It was found that addition of salt on spraying solutions is necessary in order to prepare thin films with a good quality. Spraying on spinning substrate generated evenly occupied smooth surfaces. The benefits of the automated method are attributed to the noticeably lower solution consumption and to the fact that the film deposition process could be accelerated by a factor of 100 comparing to the traditional dipping method.     

On mechanics of deformation and crushing processes

The mechanics of crushing and breaking of particles is one of the most intractable problems in materials science. The stressed states of processed materials are significantly inhomogeneous, and thus the deformation and disintegration mechanisms vary greatly. Two techniques have been developed for realizing these processes as a quasi-homogeneous transition. The device and method developed by Enikolopian transform a solid polymer spontaneously into powder. The same loading system is now used for obtaining fine-grained metals, similarly as when using the ECAP device developed by Valiev. Both techniques are now used for obtaining nanostructured materials. The common feature of both types of methods is the formation of new physical surfaces. These are particle-free oversurfaces or grain boundaries. The method requires a supply of energy in the form of mechanical work, and this is mostly done by simultaneous action of pressure and shear stress. The formation of free oversurfaces in stressed solid bodies is the subject of fracture mechanics. The Griffith equation is employed to describe the problem. __________ Translated from Problemy Prochnosti, No. 1, pp. 64–68, January–February, 2008.     

Effects of sapphire substrates surface treatment on epitaxial ZnO thin films grown by MOCVD

The surface treatment effects of sapphire substrate on the quality of epitaxial ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) were studied. The sapphire substrates have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best-quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements. Results show that the intensity of (002) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching is strongest. FWHM of (002) diffraction peak is narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property.     

真空还原制备的VO2热致相变薄膜Raman光谱和红外光谱研究

报道了利用真空还原制备的VO2薄膜的红外透射光谱和Raman光谱,并进行370－900nm波段的光透射测试以及900nm波长的热滞回线特性测试,表明所制备VO2薄膜具有优良的热致相变光学特性,结晶状态不同的薄膜其Raman谱位置有明显改变,室温时的红外光谱表现出较好的红外振动特性。讨论了薄膜结晶状态对Raman位移的影响以及VO2薄膜的红外光谱。     

AFM surface analysis of ZnO layers prepared by pulsed laser deposition at different oxygen pressures

Polycrystalline thin films of zinc oxide were deposited by pulsed laser deposition onto silicon substrates at different oxygen partial pressures in the range of 1-35 Pa. For ablation of the sintered zinc oxide target a pulsed Nd:YAG laser was used. Other processing parameters such as laser pulse energy, pulse repetition rate, substrate temperature and deposition pressure were identical. The effect of oxygen pressure on the structural properties of the films was systematically studied by using atomic force microscopy. The surface morphology, average roughness S_a, root mean square S_q, and mean size of grains on selected places with 2 × 2 μm² area of prepared samples were evaluated. Detailed structural analysis confirmed that partial oxygen pressure leads to the modification of surface morphology. Mean grain size in height and lateral direction decreases with raising oxygen pressure from 1 to 5 Pa while the further increase of oxygen pressure from 5 to 35 Pa results in grain size enlargement. The zinc oxide film formed at oxygen partial pressure 5 Pa shows smallest values of evaluated parameters (S_a = 0.6 nm, S_q = 0.7 nm and mean size of grains 50 nm).     

Surface-tension-driven flow on a moving curved surface

The leading-order equations governing the flow of a thin viscous film over a moving curved substrate are derived using lubrication theory. Three possible distinguished limits are identified. In the first, the substrate is nearly flat and its curvature enters the lubrication equation for the film thickness as a body force. In the second, the substrate curvature is constant but an order of magnitude larger; this introduces an extra destabilising term to the equation. In the final regime, the radius of curvature of the substrate is comparable to the lengthscale of the film. The leading-order evolution equation for the thin film is then hyperbolic, and hence can be solved using the method of characteristics. The solution can develop finite-time singularities, which are regularised by surface tension over a short lengthscale. General inner solutions are found for the neighbourhoods of such singularities and matched with the solution of the outer hyperbolic problem. The theory is applied to two special cases: flow over a torus, which is the prototype for flow over a general curved tube, and flow on the inside of a flexible axisymmetric tube, a regime of interest in modelling pulmonary airways.     

ZnO薄膜气相法制备

ZnO薄膜具有压电、光电、压敏、气敏、发光等多种特性,应用十分广泛。介绍了ZnO薄膜气相法制备原理中的各类主要方法,包括脉冲激光沉积、磁控溅射、分子束外延、金属有机化合物化学气相沉积、单源化学气相沉积和等离子体增强化学气相沉积等技术;分析了这些方法的优缺点;展望了ZnO薄膜今后的研究方向。     

YBaCuO超导薄膜的显微结构

在白宝石,ZrO_2,SrTiO_3单晶基片上,用射频磁控反应溅射工艺淀积 YBaCuO 超导薄膜。用SEM,TEM 方法研究了超导薄膜的形貌和显微结构。讨论了它们与超导性质的关系。指出了几种不同类型的晶界。     

On diffusion creep of polycrystals

We present a variant of diffusion creep in polycrystalline bodies, which is based on solving a diffusion-viscous flow model problem. A polycrystal is treated as a composite material and the relevant boundary-value problem is solved through approximation of the self-consistent method. Simple expressions have been derived for viscosity coefficients of the polycrystal and grain boundaries. The creep equation is analyzed for a uniaxial compression case. The conclusions inferred from this equation are qualitatively consistent with the available experimental findings. Translated from Problemy Prochnosti, No. 3, pp. 14–24, May–June, 2009.     

Tunnel diode oscillator circuit for surface impedance measurement of thin films near 20 MHz

We describe and analyse a simple method for the measurement of thin film surface impedance in the frequency range 10–20 MHz. This method is based on the analysis of both the frequency shift and the amplitude variation of the signal in a tunnel diode oscillator circuit inductively coupled to the sample. The sensitivity of the method to the conducting properties of the thin films is a function which is greatly dependent on geometrical parameters and which can be easily optimized and included in the response calculation. The analysis is illustrated by examples with superconducting and normal metal films. The resolution of the measurements is also discussed.     

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).     

脉冲激光沉积ZnO及其掺杂薄膜的研究

本文用脉冲激光沉积（PLD）法在SiO2基片上制备了ZnO薄膜和Zn1-xMnxO薄膜。X射线衍射、原子力显微镜、紫外-可见分光光度计对ZnO薄膜的测试结果表明：薄膜具有（103）面的择优取向，表面比较平坦；SiO2基片上制备的薄膜在387nm附近存在明显的吸收边，且薄膜的吸收对基片温度变化不明显。通过对Zn1-xMnxO薄膜的吸收光谱分析得出：Mn离子的掺杂改变了ZnO薄膜的禁带宽度，随Mn离子的掺杂量的增加，薄膜禁带宽度增加；薄膜的光吸收也从直接跃迁过渡为间接跃迁过程。     

The effective surface Debye temperature of Yb:GaN

The effective Debye temperature of ytterbium and gallium in Yb:GaN thin films has been obtained using X-ray photoemission spectroscopy. The vibrational motion normal to the surface results in a dimunition of photoemission intensities from which we have estimated the effective Debye temperatures of 221 ± 30 K and 308 ± 30 K for Yb and Ga, respectively. The difference between the measured values for Yb and Ga suggests that the Debye temperatures are influenced by the local environment. The smaller effective surface Debye temperature for Yb correlates to a soft, strained surface, possibly due to an increased Yb―N bond length as compared to the Ga―N bond length.     

Ellipsometric studies of microscopic surface roughness of CdS thin films

Analysis of changes in surface roughness of CdS thin films with preparation temperature was carried out using variable angle spectroscopic ellipsometry (VASE). The films studied were prepared by spray pyrolysis technique, in the substrate temperature range 200–360°C. The VASE measurements were carried out in the visible region below the band gap (E _g=2·4eV) of CdS so as to reduce absorption by the film. The thickness of the films was in the range 500–600 nm. Bruggeman’s effective medium theory was used for analysis of the surface roughness of the film. The roughness of the film had a high value (∼ 65 nm) for films prepared at low temperature (200°C) and decreased with increase in substrate temperature. This reached minimum value (∼ 27 nm) in the temperature range 280–300°C. Thereafter roughness increased slowly with temperature. The growth rate of the films was calculated for different temperature ranges. It was found that the deposition rate decreases with the increase in substrate temperature and have an optimum value at 300°C. Above this temperature deposition rate decreased sharply. The scanning electron micrograph (SEM) of the film also showed that the film prepared at 280–300°C had very smooth surface texture.