The nucleation of pentacene thin films

Photoemission Electron Microscopy was used to determine basic factors for nucleation and growth of thin pentacene films. Dependence of both substrate chemistry and deposition rate on the nucleation density was observed. On SiO₂ pentacene shows a high nucleation density and forms small islands consisting of almost vertically oriented molecules. On Si(001) the nucleation density of this thin-film phase is much smaller, but the pentacene film first forms a flat-lying wetting layer. The thin-film phase only forms on top of this wetting layer. Adsorption of a cyclohexene self-assembled monolayer on Si(001) prior to the pentacene growth suppresses the initial pentacene wetting layer but maintains diffusion parameters similar to pentacene on Si(001). The nucleation of pentacene layers on cyclohexene/Si(001) can be described by classical nucleation theory with a critical nucleus size i6. Simple surface modification techniques such as e-beam irradiation of the substrates prior to pentacene adsorption can also have a significant effect on the pentacene nucleation density. PACS 68.37.Nq; 68.43.Fg; 68.47.Fg; 68.55.Ac     

The influence of substrate temperature on growth of para-sexiphenyl thin films on Ir{111} supported graphene studied by LEEM

The growth of para-sexiphenyl (6P) thin films as a function of substrate temperature on Ir{111} supported graphene flakes has been studied in real-time with Low Energy Electron Microscopy (LEEM). Micro Low Energy Electron Diffraction (μLEED) has been used to determine the structure of the different 6P features formed on the surface. We observe the nucleation and growth of a wetting layer consisting of lying molecules in the initial stages of growth. Graphene defects – wrinkles – are found to be preferential sites for the nucleation of the wetting layer and of the 6P needles that grow on top of the wetting layer in the later stages of deposition. The molecular structure of the wetting layer and needles is found to be similar. As a result, only a limited number of growth directions are observed for the needles. In contrast, on the bare Ir{111} surface 6P molecules assume an upright orientation. The formation of ramified islands is observed on the bare Ir{111} surface at 320 K and 352 K, whereas at 405 K the formation of a continuous layer of upright standing molecules growing in a step flow like manner is observed.     

Effects of AlN nucleation layer thickness on crystal quality of AlN grown by plasma-assisted molecular beam epitaxy

In this paper,the effects of thickness of AlN nucleation layer grown at high temperature on AlN epi-layer crystalline quality are investigated.Crack-free AlN samples with various nucleation thicknesses are grown on sapphire substrates by plasma-assisted molecular beam epitaxy.The AlN crystalline quality is analysed by transmission electron microscope and x-ray diffraction (XRD) rocking curves in both (002) and (102) planes.The surface profiles of nucleation layer with different thicknesses after in-situ annealing are also analysed by atomic force microscope.A critical nucleation thickness for realising high quality AlN films is found.When the nucleation thickness is above a certain value,the (102) XRD full width at half maximum (FWHM) of AlN bulk increases with nucleation thickness increasing,whereas the (002) XRD FWHM shows an opposite trend.These phenomena can be attributed to the characteristics of nucleation islands and the evolution of crystal grains during AlN main layer growth.     

Wetting layer thickness and early evolution of epitaxially strained thin films

We propose a physical model which explains the existence of finite thickness wetting layers in epitaxially strained films. The finite wetting layer is shown to be stable due to the variation of the nonlinear elastic free energy with film thickness. We show that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We observe the development of faceted islands in unstable films.     

Fluctuation forces and wetting layers in colloid-polymer mixtures

We present confocal microscopy experiments on the wetting of phase-separated colloid-polymer mixtures. We observe that an unusually thick wetting layer of the colloid-rich phase forms at the walls of the glass container that holds the mixture. Because of the ultralow interfacial tension between the colloid-rich and the polymer-rich phases, the thermally activated roughness of the interfaces becomes very big and measurable. We observe that close to the critical point the roughness of the interface between the wetting layer and the polymer-rich phase decreases with decreasing layer thickness: large excursions of the interface are confined in the wetting layer. The measured relationship between the roughness and the thickness of the wetting layer is in qualitative agreement with the predictions of renormalization group theory for short-range forces and complete wetting.     

Surface effects on kinetics of ordering

We study the effects of surfaces on the kinetics of phase changes in Ising-type systems. If the surface effects can be modelled by a field which couples linearly to the local order parameter, the growth of wetting or drying layers occurs. The numerical solution of the corresponding time-dependent Ginzburg-Landau equation yields a temporally logarithmic growth for the thickness of a wetting (drying) layer growing from an unstable dry (wet) state. On the other hand, if one starts off with a metastable state, the radius of a supercritical plug (wet or dry) grows linearly in time, in accordance with recent experimental results.     

Formation and evolution of multimodal size distributions of InAs/GaAs quantum dots

Self-organized formation and evolution of quantum dot (QD) ensembles with a multimodal size distribution is reported. Such ensembles form after fast deposition near the critical thickness during a growth interruption (GRI) prior to cap layer growth and consist of pure InAs truncated pyramids with heights varying in steps of complete InAs monolayers, thereby creating well-distinguishable sub-ensembles. Ripening during GRI manifests itself by an increase of sub-ensembles of larger QDs at the expense of sub-ensembles of smaller ones, leaving the wetting layer unchanged. The dynamics of the multimodal QD size distribution is theoretically described using a kinetic approach. Starting from a broad distribution of flat QDs, a predominantly vertical growth is found due to strain-induced barriers for nucleation of a next atomic layer on different facets. QDs having initially a shorter base length attain a smaller height, accounting for the experimentally observed sub-ensemble structure. The evolution of the distribution is described by a master equation, which accounts for growth or dissolution of the QDs by mass exchange between the QDs and the adatom sea. The numerical solution is in good agreement with the measured dynamics.     

Hydrodynamic-flow-driven wetting in thin film polymer blends: growth kinetics and morphology

A thin film of deuterated poly(methyl methacrylate) (A) and poly(styrene-ran-acrylonitrile) at the critical composition is annealed in the two phase region to induce simultaneous phase separation and wetting of the A-rich phase at the surface. Using forward recoil spectrometry, the wetting layer thickness is found to grow linearly with time at 185 degrees C and 190 degrees C. After selective etching of A, atomic force microscopy reveals a depletion layer having a bicontinuous, phase separated morphology. The A-rich tubes in this layer provide a pathway for rapid transport of the wetting phase from the bulk to the surface via hydrodynamic flow. Taken together, fast wetting layer growth t(1) and connectivity between the wetting layer and bulk provide unambiguous support for hydrodynamic-flow-driven wetting in thin film polymer blends.     

Features of atomic processes at the formation of a wetting layer and nucleation of three-dimensional Ge islands on Si(111) and Si(100) surfaces

The intermediate stages of the formation of a Ge wetting layer on Si(111) and Si(100) surfaces under quasiequilibrium grow conditions have been studied by means of scanning tunneling microscopy. The redistribution of Ge atoms and relaxation of mismatch stresses through the formation of surface structures of decreased density and faces different from the substrate orientation have been revealed. The sites of the nucleation of new three-dimensional Ge islands after the formation of the wetting layer have been analyzed. Both fundamental differences and common tendencies of atomic processes at the formation of wetting layers on Si(111) and Si(100) surfaces have been demonstrated. The density of three-dimensional nuclei on the Si(111) surface is determined by changed conditions for the surface diffusion of Ge adatoms after change in the surface structure. Transition to three-dimensional growth on the Si(100) surface is determined by the nucleation of single {105} faces on the rough Ge(100) surface.     

What controls the thickness of wetting layers near bulk criticality?

We study the thickness of wetting layers in the binary-liquid mixture cyclohexane methanol. Far from the bulk critical point, the wetting layer thickness is independent of temperature, resulting from the competition between van der Waals and gravitational forces. Upon approaching the bulk critical temperature [t=(T(c)-T)/T(c)-->0], we observe that the wetting layer thickness diverges as t(-beta) with effective critical exponent beta=0.23+/-0.06. This is characteristic of a broad, intermediate scaling regime for the crossover from van der Waals wetting to critical scaling. We predict beta=beta/3 approximately 0.11, with beta the usual bulk-order parameter critical exponent, showing a small but significant difference with experiment.     

高阻GaN的MOCVD外延生长

利用金属有机化合物气相沉积(MOCVD)在蓝宝石衬底上生长了高阻GaN薄膜。对GaN成核层生长的反应室压力、生长时间和载气类型对GaN缓冲层电学特性的影响进行了分析。实验结果表明,延长GaN成核层的生长时间,降低成核层生长时的反应室压力,载气由H₂换为N₂都会得到高阻的GaN缓冲层。样品的方块电阻R_s最高为2.49×10¹¹ Ω/□。以高阻GaN样品为衬底制备了AlGaN/AlN/GaN结构HEMT器件,迁移率最高达1 230 cm²/(V·s)。     

Molecular functionalization of tantalum oxide surface towards development of apatite growth

We have studied the apatite growth dynamics on tantalum oxide surfaces. This nucleation is obtained via an organosilane intermediate layer between the apatite and the substrate surface. Four organosilane layers (differing by their terminal functionality) were investigated. Their characterization with atomic force microscopy and other techniques such as X-ray photoelectron spectroscopy (XPS) and wetting measurements highlighted the influence of the organosilane terminal groups on the apatite growth rates. Results revealed that apatite is indeed growing faster on phosphate terminal groups than on the three other groups studied (vinyl, hydroxyl and carboxyl).     

半导体量子点形成临界尺寸的定量预测

半导体量子点是一类具有显著量子效应的零维量子结构,自组的模型系统,表现为Stranski-Krastanov型生长.其特征为,当超过3—4个Ge单原子层（浸润层）时,则由二维层状生长转变为三维岛状生长.Ge/Si量子点是初期形成的与衬底共格无位错的三维岛,岛表面由{105}晶面组成.文章作者利用第一性原理计算和介观理论模拟相结合的连续式多尺度（sequential multi-scale）方法,第一次对纯Ge和GeSi合金量子点在Si(001)表面的成核临界尺寸进行了定量的理论预测,同时研究了岛边缘的应力不连续对量子点稳定性的影响,实现了对Ge/Si量子点的形成和稳定性定量的理论研究.     

离子束溅射Ge量子点的应变调制生长

采用离子束溅射技术制备了单层和双层Ge量子点, 通过原子力显微镜对比了不同Si隔离层厚度和不同掩埋量子点密度情况下表层量子点的尺寸和形貌差异, 系统研究了掩埋Ge量子点产生的应变对表层量子点的浸润层及形核的影响, 并用埋置应变模型对其进行解释. 实验结果表明, 覆盖Ge量子点的Si隔离层中分布着的应变场, 导致表层量子点浸润层厚度的降低, 从而增大点的体积; 应变强度随隔离层厚度的减小而增加, 造成表层量子点形状和尺寸的变化; 此外, 应变还调控了表层量子点的空间分布. 关键词： Ge量子点 埋层应变 离子束溅射     

脉冲激光原位辐照对InAs/GaAs(001)量子点生长的影响

在InAs/GaAs(001)量子点生长过程中, 当InAs沉积量为0.9 ML时, 利用紫外纳秒脉冲激光辐照浸润层表面, 由于高温下In原子的不稳定性, 激光诱导的原子脱附效应被放大, 样品表面出现了原子层移除和纳米孔. 原子力显微镜测试表明纳米孔呈现以[110]方向为长轴(尺寸: 20-50 nm)、[110]方向为短轴(尺寸: 15-40 nm)的表面椭圆开口形状, 孔的深度为0.5-3 nm. 纳米孔的密度与脉冲激光的能量密度正相关. 脉冲激光的辐照对量子点生长产生了显著的影响: 一方面由于纳米孔的表面自由能低, 沉积的InAs优先迁移到孔内, 纳米孔成为量子点优先成核的位置; 另一方面, 孔外的区域因为In原子的脱附, 量子点的成核被抑制. 由于带有纳米孔的浸润层表面具有类似于传统微纳加工技术制备的图形衬底对量子点选择性生长的功能, 该研究为量子点的可控生长提供了一种新的思路.     

Influence of elastic strains in sublayers on the critical thickness of the Stranski-Krastanow transition for the GeSi/Si(001) system

A novel estimation of the critical thickness of the epitaxial layer in the Stranski-Krastanow (SK) transition is proposed. The transition criterion is based on accumulation of the energy of the effective strain up to a certain critical value. The calculation includes the elastic energy stored in elastically strained layers, takes into account the restriction on the depth of the strained layer still affecting the transition, and a segregation effect described by thermally activated atomic exchange. The equations of growth on the vicinal substrate surface that divide each monolayer into submonolayers parallel to the surface are used. Simulation results are compared with the corresponding experimental data for the heterostructures built of a strained sublayer covered by a spacer layer on which a wetting layer is deposited until the SK transition occurs. The Ge/Si structures of this type are grown by molecular beam epitaxy, and in addition, the experimental results for InAs/GaAs systems, published in the literature, are used. A comparison of experimental and calculated data on the dependence between the critical thickness of the wetting layer and the thickness of the spacer layer shows good agreement for both Ge/Si and InAs/GaAs systems.     

Substrate-step-induced effects on the growth of CaF2 on Si (111)

The growth of CaF₂ on vicinal Si (111) surfaces was studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for the temperature range from 300 to 750 °C. In the submonolayer range a transition from terrace to step nucleation is observed for increasing temperature. The first monolayer grows in the step-flow growth mode since second layer islands are not nucleated before completion of the wetting layer. For the subsequent growth of CaF₂ on the CaF interface layer, substrate-induced steps do not act as preferential nucleation sites, but rather as growth barriers. Thus CaF₂ films grow very inhomogeneously at high temperatures. At lower deposition temperatures, the film homogeneity increases due to the increased (homogeneous) nucleation rate on terraces. The lattice mismatch leads to (lateral) relaxation of thicker CaF₂ film close to substrate steps. In addition, CaF₂ self-decoration effects are caused by the relaxed regions close to the film steps at temperatures above 500 °C. Received: 7 August 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002     

氢化物气相外延生长高质量GaN膜生长参数优化研究

系统研究了低温成核层生长时间、高温生长时的V/Ⅲ 比以及生长温度对氢化物气相外延生长GaN膜晶体质量的影响. 研究发现合适的低温成核层为后续高温生长提供成核中心, 并能有效降低外延膜与衬底间的界面自由能, 促进成核岛的横向生长; 优化的V/Ⅲ比和最佳生长温度有利于降低晶体缺陷密度, 促进横向生长, 增强外延膜的二维生长. 利用扫描电子显微镜、原子力显微镜、高分辨X射线衍射、 低温光致发光谱和室温拉曼光谱对优化条件下生长的GaN外延膜进行了结构和光电特性表征. 测试结果表明, 膜表面平整光滑, 呈现二维生长模式表面形貌; (002)和(102)面摇摆曲线半高宽分别为317和343 arcsec; 低温光致发光谱中近带边发射峰为3.478 eV附近的中性施主束缚激子发射峰, 存在11 meV的蓝移, 半高宽为10 meV, 并且黄带发光强度很弱;常温拉曼光谱中E₂ (high) 峰发生1.1 cm^-1蓝移.结果表明, 优化条件下生长的GaN外延膜具有良好的晶体质量和光电特性, 但GaN 膜中存在压应力. 关键词： 氮化镓 氢化物气相外延 低温成核层     

InAs wetting layer evolution on GaAs(0 0 1)

The evolution of two-dimensional (2D) strained InAs wetting layers on GaAs(0 0 1), grown at different temperatures by molecular beam epitaxy, was studied by in situ high-resolution scanning tunneling microscopy. At low growth temperature (400 °C), the substrate exhibits a well-defined GaAs(0 0 1)-c(4 × 4) structure. For a disorientation of 0.7°, InAs grows in the step-flow mode and forms an unalloyed wetting layer mainly along steps, but also in part on the terrace. The wetting layer displays some local c(4 × 6) reconstruction, for which a model is proposed. 1.2 monolayer (ML) InAs deposition induces the formation of 3D islands. At a higher temperature (460 °C), the wetting layer is obviously alloyed even at low InAs coverage. The critical thickness of the wetting layer for the 2D-to-3D transition is shifted to 1.50 ML in this case presumably since the strain is reduced by alloying.     

Misfit dislocation locking and rotation during gallium nitride growth on SiC/Si substrates

The effect of changing the misfit dislocation propagation direction during GaN layer growth on the AlN/SiC/Si(111) structure surface is detected. The effect is as follows. As the GaN layer growing on AlN/SiC/Si(111) reaches a certain thickness of ~300 nm, misfit dislocations initially along the layer growth axis stop and begin to move in the direction perpendicular to the growth axis. A theoretical model of AlN and GaN nucleation on the (111) SiC/Si face, explaining the effect of changing the misfit dislocation motion direction, is constructed. The effect of changing the nucleation mechanism from the island one for AlN on SiC/Si(111) to the layer one for the GaN layer on AlN/SiC/Si is experimentally detected and theoretically explained.