Theoretical studies of ultrathin film-induced faceting on W(111) surfaces

We have used local volume (or EAM) potentials to study the pyramidal faceting (or reconstruction) of a W(111) surface induced by face center cubic (fcc) metals Pd, Pt, Au, and a body center cubic (bcc) metal Mo. We found that the surface-energy differences of (211) and (111) surfaces of bcc W increases as one or few monolayers of Pd, Pt, Au, and Mo films are deposited. We found that the lateral relaxation which is allowed on the (211) surface further increases the surface energy anisotropy as the thickness of the fcc metal film increases. Our calculated results are consistent with the argument that the surface energy anisotropy is the driving force for the faceting, but do not rule out three-dimensional (3D) island growth as another possible mechanism for the (211) faceting. We also found that there is a possible bilayer growth mode in W(211) surfaces with Pt and Pd films.     

Surface energy anisotropy of iridium

Field-ion microscopy was used to study the faceting behavior and surface energy anisotropy of iridium in vacuum and in hydrogen. In vacuum below approximately 1300 K the order of faceting and the activation energy for growth of {111} facets agreed with previously published results of FIM studies. The unexpected faceting behavior of {210} planes was examined in terms of the geometry of field evaporated specimens. The observed anisotropy at temperatures above 1300 K was in qualitative agreement with Morse and Mie potential calculations and in nearly quantitative agreement with the pairwise bonding model using σ₂ = 0.4, σ₃ = 0.2. The observed maximum anisotropy of 8.8% for iridium at 1360 K fell within the range of extrapolated values for other metals at one-half the absolute melting temperature. Hydrogen appeared to lower the surface energy of each plane by only about 0.1%. An anisotropic effect of hydrogen on the faceting behavior, however, was observed and suggested that surface diffusion rates in {110} and {311} regions were preferentially increased in the presence of hydrogen.     

Oscillations in the surface structure of Sn-doped GaAs during growth by MBE

The presence of predeposited or surface-accumulated Sn during MBE growth modifies the reconstruction of the (001)GaAs surface, as observed by RHEED, at coverages as low as 0.025 monolayer. If growth is initiated on such a surface, oscillation in the intensity modulation of some of the RHEED streaks occurs, with a period equal to the monolayer deposition time of the GaAs. This oscillation decays away at a rate determined by the substrate temperature and Ga flux.     

Nanocrystal formation and faceting instability in Al(110) homoepitaxy: true upward adatom diffusion at step edges and island corners

Using atomic force microscopy and spot-profile analyzing low energy electron diffraction, we have observed the existence of a striking faceting instability in Al(110) homoepitaxy, characterized by the formation of nanocrystals with well-defined facets. These hut-shaped nanocrystals are over tenfold higher than the total film coverage, and coexist in a bimodal growth mode with much shallower and more populous surface mounds. We further use density functional theory calculations to elucidate the microscopic origin of the faceting instability, induced by surprisingly low activation barriers for adatom ascent at step edges and island corners.     

Determination of positron beam parameters by various diagnostic techniques

Various diagnostic techniques have been applied at the neutron-induced positron source Munich NEPOMUC in order to determine the positron beam parameters such as intensity, beam shape and energy distribution. The positron beam intensity is determined by the detection of the annihilation radiation of positrons, which annihilate in a movable target. The use of a micro-channel plate (MCP) detector with a CCD-camera allows a direct measurement of the positron beam shape and the lateral resolved intensity distribution. At NEPOMUC a movable MCP-assembly inside the evacuated beam line enables a quick examination of the beam shape during operation. A retarding grid was mounted inside the homogeneous magnetic guiding field in order to determine the distribution of the longitudinal positron momentum, and hence estimate the energy distribution of the positrons.     

Shocks and curvature dynamics: A test of global kinetic faceting in crystals

We investigate the microscopic mechanisms underlying the dynamical faceting of crystals. Partially faceted crystal shapes of CCl4 are formed from a melt contained in a Bridgman apparatus and pressure is used to control growth which is observed using optical microscopy. In contrast to predictions of models in which the local interfacial motion is greatest where the step density is the highest, the loss of rough orientations is observed to occur via a local decrease in curvature which results in the formation of discontinuities-shocks-in the surface of the growth forms, a feature predicted by a recent theory of kinetic faceting.     

Experimental investigation of the gas discharge dynamics in a solenoidal magnetic field

An electric gas discharge is initiated at the end face of a nonconducting cylinder between a central electrode coaxial with the cylinder and a ring electrode on the lateral surface of the cylinder. The axis of the cylinder and that of a magnetic coil wound on its lateral surface coincide. The dynamics of the gas discharge in a solenoidal magnetic field is studied by observing its position and shape and monitoring the variation of the thermionic center distribution on the surface of the ring electrode with discharge current and magnetic induction. The discharge develops in air at a pressure of 2 kPa.     

Sawtooth faceting in silicon nanowires

We observe in situ the vapor-liquid-solid (VLS) growth of Si nanowires, in UHV-CVD using Au catalyst. The nanowire sidewalls exhibit periodic sawtooth faceting, reflecting an oscillatory growth process. We interpret the facet alternation as resulting from the interplay of the geometry and surface energies of the wire and liquid droplet. Such faceting may be present in any VLS growth system in which there are no stable orientations parallel to the growth direction. The sawtooth structure has important implications for electronic mobility and scattering in nanowire devices.     

Three-dimensional characterization of multiply twinned nanoparticles by high-angle tilt series of lattice images and tomography

A new electron tomography methodology is presented which allows the reconstruction of external particle shape from lattice resolved high-resolution electron microscopy images. The technique is based on the shape-from-silhouette binary backprojection algorithm after filtering of the lattice contrast. The simultaneous availability of particle shape and crystallographic lattice plane orientations allows the correlation of faceting with crystallographic orientations. The method is demonstrated using a multiply twinned decahedral gold nanoparticle. Eligibility limits in terms of degree of convexity are derived.     

地面对激光雷达信号散射的统计研究

从理论角度研究了利用高斯光束照射远场目标时,激光散斑的统计特性.导出了散射光场的自相关函数和光强度的二阶矩的解析表达式,计算了激光散斑的面积.研究表明:在接收面上,光强的分布与目标表面高度的相关长度和均方根高度密切相关,激光散斑面积和散斑光强的相关系数只与激光束腰的尺度有关,与目标的均方根高度和相关长度无关. 关键词： 激光散斑 自相关函数 高斯分布 协方差     

Optical near field scattered by surface defects: Two-dimensional numerical analysis

Recently developed scanning near-field optical microscopy has drawn attention to the problem of describing the electromagnetic field in the close vicinity of a surface. In this work, we present a numerical simulation that solves rigorously the field equations for a dielectric-air-dielectric layer system with arbitrary one-dimensional structure at its interfaces. Our theory is applied to calculate the intensity of the near field transmitted at the center of the tip of a probe as it is moved at a constant height above a surface with two identical topographic defects. The effects on the optical image due to the separation of the objects, and the shape difference between the ridges and grooves are discussed. The resolution limit and the conditions for near field interaction are determined. This paper was originally presented at the first Asia-Pacific Workshop on Near Field Optics, which was held on August 17 and 18, 1996 at Seoul Education and Culture Center, Seoul, Korea, organized by Condensed Matter Research Institute, Seoul National University.     

Faceting of sapphire crystals grown from a melt by the Stepanov method

The appearance of faces on cylindrical sapphire single crystals grown from a melt by the Stepanov method is studied by photogoniography and optical microscopy. The crystallographic indices of the detected faces are established, and the microstructure of the growth layers is investigated. A relationship between the faceting and the growth conditions is found. The experimental results are compared with data on the faceting of the sapphire crystals grown from a solution-melt and with the calculated specific free surface energies of the faces.     

反射光偏振特性分析与物体形状的测量

基于反射光偏振特性,提出了利用图像处理技术测量透明物体三维形状的理论和方法.分析了物体表面反射光的偏振特性,表明自然光在经透明物体表面反射后,反射率随光的振动方向不同而不同,即反射光表现出部分偏振光的特性.研究了强度反射率与入射角以及光强与偏振片方向之间的函数关系,得到了光强大小与入射面方向的关系;根据偏振度概念并结合菲涅耳公式和折射定律,建立了偏振度和入射角之间的表达式,可求得物体表面法线方向,进而得到透明物体的形状.研制了光学实验平台,获得了物体反射光的偏振图像,经过图像处理,获得了被测物体的三维形状.实验结果表明,这种方法对透明物体形状测量是有效和实用的.     

Sculpting semiconductor heteroepitaxial islands: from dots to rods

In the Ge on Si model heteroepitaxial system, metal patterns on the silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shape including nanorods and truncated pyramids is set by the metal species and substrate orientation. Analysis of island faceting elucidates the prominent role of the metal in promoting growth of preferred facet orientations while investigations of island composition and structure reveal the importance of Si-Ge intermixing in island evolution. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays in heteroepitaxial systems.     

Self-organized patterning of an insulator-on-metal system by surface faceting and selective growth: NaCl/Cu(211)

We report experimental results on an insulator-on-metal system which is inherently unstable against lateral pattern formation on the nanometer scale. NaCl deposition on Cu(211) at substrate temperatures >300 K leads to faceting into (311) and (111) facets and selective NaCl growth on (311) facets only, thereby creating alternating stripes of bare Cu and NaCl-covered areas. The mesoscopic restructuring process is brought about by (1) the tendency to form (100)-terminated NaCl layers, (2) epitaxial matching between NaCl(100) and Cu(311), and (3) sufficient mobility of the Cu substrate surface.     

Theory of surface dynamic variation during the high-temperature field evaporation

We consider the heated surface of a metallic tip to which a strong electric field is applied. At temperatures activating surface self-diffusion, crystalline outgrowths and microprotrusions arise on the surface. The latter generate ion fluxes, i.e., act as sources of high-temperature field evaporation, when a positive potential is applied to the emitter. The existence conditions for the microprotrusions on the emitter surface are discussed. It is shown that their stability is provided by the balance between three atomic fluxes: diffusion from the top of the tip, diffusion toward the top, and field evaporation from the top. Different ways of providing such a balance are discussed. In a desorption-type field ion microscope, the microprotrusions and evaporating ions are visualized as bright spots. These spots execute random motion and, at the same time, exhibit ordered cyclic displacements: the microprotrusions first form dotted rings along the developed faces of the crystalline emitter, and then these rings quickly collapse toward the center of the face. A quantitative theory of these cyclic processes is developed for the first time. It explains why the rings “calm down” before collapse and why subsequent collapse develops in an avalanchelike manner. The electric field distribution over the surface in the presence of an outgrowth is calculated, and diffusion fluxes at different stages of its growth and dissolution are analyzed. The calculation shows that the outgrowth heights are relatively small and their slopes are rather smooth.     

Roughening (faceting) transitions on the helium crystal-superfluid interface

The concept of roughening transitions (RT) was first introduced into theory by Burton and Cabrera (1949). RT, which manifests itself in disappearance of smooth facets in equilibrium shape of the crystal (faceting transition), has been under intensive theoretical study in the last few years (Andreev 1981; Rottman and Wortis 1984). Current interest in this field is connected mainly with the problems of “quantum roughening’ and critical behaviour of the surface stiffness. Experimental studies of RT in usual classical crystals meet with serious difficulties due to very long crystal shape relaxation times. In this respect, helium crystals seem to be the best candidates because of their extremely fast growth kinetics which ensures sufficiently short shape relaxation times (Keshishevet al 1982). We have recently investigated the equilibrium shape of large⁴He crystals in the vicinities of the two different RT (T _R=1.2 K and 0.9 K) by a simple optical technique which provides the temperature and angular dependences of the surface stiffness (Babkinet al 1958, 1984, 1985). The data obtained by this method show, in accord with theory, that both studied RT being continuous phase transitions. However, the measured critical behaviour of the surface stiffness cannot be explained satisfactorily by current theories, either by the phenomenological “mean field” theory (Andreev 1981) or by the lattice model calculation (Rottman and Wortis 1984).     

Reverse roughening transition in carbon dioxide

We report the observation of a roughening transition in carbon dioxide along the melting line of phase I, which we call reverse as faceting appears with increasing temperature. The characteristics of the transition are discussed in light of modern theories of roughening and the causes of its reverse behavior investigated. We propose that high temperature faceting is related to a pressure-induced increase of the surface stiffness.     

Low-energy electron diffraction study of the reconstruction and orientational stability of Si(331)

Using low-energy electron diffraction, we have studied the reconstruction of Si(331). We find a previously unreported (12 × 1) reconstruction is characteristic of the clean surface; we present evident that a previously observed (13 × 1) reconstruction is not the equilibrium structure. The (12 × 1) reconstruction disorders via a strongly first-order phase transition at approximately 810°C. Heating carbon contaminated surfaces to about 950°C causes faceting to (111) and {17151} orientations.     

Temperature-dependent facet development in the deposition of GaInP on non-planar surfaces

The low-pressure (20 mbar) organometallic vapour-phase epitaxy (LP-OMVPE) of GaInP on non-planar {001} GaAs substrates has been examined. The encountered and {1 0} faceting features develop along the bottom corner and the top edge configurations of the inverted and dovetail grooves, respectively. At higher temperatures (T ≥ 720°C) these features are no longer present. The results have been compared to computer simulations of surface concentration profiles, whereby the inversely proportional relation between temperature and supersaturation, along with varying growth rate on adjacent surfaces of different crystallographic orientations, is found to be the driving force behind the occurrence of these features. The stability of the observed facets is related to the decrease in dangling-bond densities upon surface reconstruction.