Band structure related wave-function symmetry of amphoteric Si dopants in GaAs

Autocompensated Si-doped GaAs is studied with cross-sectional scanning tunnelling spectroscopy (X-STS). The local electronic contrasts of substitutional Si(Ga) donors and Si(As) acceptors under the (110) cleavage plane are imaged with high resolution. Si(Ga) donor atoms exhibit radially symmetrical contrasts. Si(As) acceptors have anisotropic features. The anisotropic acceptor contrasts are traced back to a tunnel process at the valence band edge. They reflect the probability density distribution of the localized acceptor hole state.     

Self-consistent tight-binding total energy calculations: Application to GaAs/Si and ZnSe/GaAs (100) interfaces

Self-consistent tight-binding total energy calculations are performed for various models of GaAs/Si and ZnSe/GaAs (100) interfaces. A graded GaAs/Si interface with the first monolayer on substrate having 1 As atom per 3 Si atoms followed by a second monolayer with 3 Ga atoms per 1 Si atom and continued with 2 bulk-like As and Ga monolayers is found to be structurally more stable than other interfaces. The instability of the abrupt interface is driven by elastic rather than electrostatic forces. Similar results are obtained for the ZnSe/GaAs (100) interface. The graded interface with Ga atoms exchanged against Zn atoms is found to be energetically most stable. Strong macroscopic electric fields are found in the surface and interface regions for both the GaAs/Si and ZnSe/GaAs interfaces.     

Self-organized in-plane incorporation of Si atoms in GaAs by molecular beam epitaxy

The preferential attachment of Si atoms at misorientation steps on vicinal GaAs(001) surfaces has been studied by RHEED. By analysing the time evolution of the specular beam intensity and the change in surface reconstruction during Si deposition we show that a self-organized Si incorporation along the step edges takes place. The observed (3×2) structure is due to an ordered array of dimerized Si atoms with missing dimer rows. Taking into account the structure of the (3×2) unit mesh and its orientation with respect to the As-terminated or Ga-terminated steps, a characteristic minimum in the RHEED intensity recording corresponds to the number of Ga step-edge sites. Since the preferential path for Ga as well as for Si adatom diffusion is along the [110] direction, the critical terrace width for wirelike Si attachment is much larger for a misorientation toward (111)As than for a misorientation toward (111)Ga. Despite the high local impurity concentration, the Si-modified surface can be overgrown with GaAs without adverse effects on the growth front. This is promising for the fabrication of doping wires.     

Si-W/Si/SiO2/Si(100)的横截面电子显微镜研究

本文用横截面电子显微镜法分析了Si-W/Si/SiO₂/Si(100)在440—1000℃退火后的晶化过程,以及各个界面的变化情况.发现Si-W合金膜中,WSi₂并未优先在表面、界面处形成晶核.当退火温度不高于700℃时,反应在合金膜内发生,表面、界面起伏和缓.退火温度高达800—1000℃时,界面、表面出现原子扩散,造成剧烈的界面起伏;表面则出现小的热沟槽,Si/SiO₂界面也出现高分辨电子显微镜才能观察到的起伏.表面、界面的原子迁移的动力来源于晶界与表面、界面张力.由于SiO₂中Si—O键很稳定,不易发生Si和O在界面处的互扩散,所以Si/SiO₂界面起伏很小. 关键词：     

Structural properties of GaAs nanostructures formed by a supply of intense As4 flux in droplet epitaxy

We investigated detailed structural properties of GaAs nanostructures formed by a supply of intense As₄ flux to Ga droplets. Scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM) revealed that whisker-like nanostructures had formed on the truncated cone-shaped bases after crystallization. Moreover, electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) revealed that elemental Ga atoms remained inside the nanostructures while outside, some had crystallized into GaAs. These findings suggest that crystallization started at the edges of the droplets and the GaAs grew upward along the periphery of the droplets until the droplets were completely covered with crystallized GaAs.     

Mechanisms of dislocation generation in Si structures with strained layers: Intrinsic point defects in dislocation nucleation

The peculiarities of dislocation production in silicon compositions with elastically strained layers fabricated by the molecular-beam epitaxy technique (SiGe/Si heterostructures) and by direct bonding of Si(110)/Si(100) wafers are studied with transmission electron microscopy. The role of intrinsic point defects during the process of nucleation of misfit dislocations is explained. The surplus concentration of these defects in heterostructures was produced via low-temperature epitaxial growth of buffer layers or with ion implantation of elastically strained heterostructures. The model of “optimal” and “inverse” intrinsic point defects providing an explanation for the relaxation of misfit strains in heterostructures is proposed.     

Schottky barrier at the Al/Si(111) doped and double-doped interfaces: a local-density cluster study

Using the local-density approximation method we have investigated a cluster model of the Al/Si(111) interface. P atoms were placed in the fifth silicon double-atomic layer (DAL) counting from the interface. The second DAL was doped with either Ga or As. The Schottky barrier height was found to be 0.75 eV at the Al/Si(intrinsic) system and 0.73 eV at the Al/Si(P-doped) case. The additional doping of the near-interface layer by Ga increased the barrier to 0.90 eV while the As doping decreased it to 0.5 eV.     

MoO_3/Si界面区钼掺杂非晶氧化硅层形成的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,通过模拟MoO_3/Si界面反应,研究了MoO_x薄膜沉积中原子、分子的吸附、扩散和成核过程,从原子尺度阐明了缓冲层钼掺杂非晶氧化硅(a-SiO_x(Mo))物质的形成和机理.结果表明,在1500 K温度下, MoO_3/Si界面区由Mo, O, Si三种原子混合,可形成新的稳定的物相.热蒸发沉积初始时, MoO_3中的两个O原子和Si成键更加稳定,同时伴随着电子从Si到O的转移,钝化了硅表面的悬挂键. MoO_3中氧空位的形成能小于SiO_2中氧空位的形成能,使得O原子容易从MoO_3中迁移至Si衬底一侧,从而形成氧化硅层;替位缺陷中, Si替位MoO_3中的Mo的形成能远远大于Mo替位SiO_2中的Si的形成能,使得Mo容易掺杂进入氧化硅中.因此,在晶硅(100)面上沉积MoO_3薄膜时, MoO_3中的O原子先与Si成键,形成氧化硅层,随后部分Mo原子替位氧化硅中的Si原子,最终形成含有钼掺杂的非晶氧化硅层.     

Si,GaAs(111)表面弛豫效应

本文将赝原子轨道的线性组合方法应用于计算半导体表面电子结构。除了赝势的形状因子以外,不引入任何可调参量。用这方法计算了Si和GaAs(111)理想表面和弛豫表面的电子态和波函数。Si的结果与Appelbaum和Hamann的自洽计算结果在表面能级位置和表面电荷分布两方面都符合得比较好。计算结果表明,当表面Si或Ga原子向内位移时,表面能级向上移动;表面As原子向外位移时,表面能级向下移动。同时,表面态波函数的性质往往也发生较大的变化。 关键词：     

Transformation mechanism of n-butyl terminated Si nanoparticles embedded into Si1−xCx nanocomposites mixed with Si nanoparticles and C atoms

Bright-field transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and fast-Fourier transformed electron-diffraction patterns showed that n-butyl terminated Si nanoparticles were aggregated. The formation of Si_1−xC_x nanocomposites was mixed with Si nanoparticles and C atoms embedded in a SiO₂ layer due to the diffusion of C atoms from n-butyl termination shells into aggregated Si nanoparticles. Atomic force microscopy (AFM) images showed that the Si_1−xC_x nanocomposites mixed with Si nanoparticles and C atoms existed in almost all regions of the SiO₂ layer. The formation mechanism of Si nanoparticles and the transformation mechanism of n-butyl terminated Si nanoparticles embedded into Si_1−xC_x nanocomposites mixed with Si nanoparticles and C atoms are described on the basis of the TEM, HRTEM, and AFM results. These results can help to improve the understanding of the formation mechanism of Si nanoparticles.     

Chemical interactions in noncontact AFM on semiconductor surfaces: Si(111), Si(100) and GaAs(110)

Total-energy pseudopotential calculations are used to study the imaging process in noncontact atomic force microscopy (AFM) on Si(111), Si(100) and GaAs(110) surfaces. The chemical bonding interaction between a localised dangling bond on the atom at the apex of the tip and the dangling bonds on the adatoms in the surface is shown to dominate the forces and the force gradients and, hence, to provide atomic resolution. The lateral resolution capabilities are tested in both the Si(100) and the GaAs(110) surfaces. In the first case, the two atoms in a dimer can be resolved due to the dimer flip induced by the interaction with the tip during the scan, while in the GaAs(110), we identify the anion sublattice as the one observed in the experimental images.     

Selective growth of carbon nantoubes on SiO2/Si substrate

Three-step raising temperature process was employed to fabricate carbon nanotubes by pyrolysis of ferrocene/melamine mixtures on silica and single crystalline silicon wafers respectively. Then the morphologies, structures and compositions of obtained carbon nanotubes are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscope (EDX) and electron energy-loss spectroscopy (EELS). TEM and SEM observation shows that on silica substrate, high-oriented carbon nanotube can grow compactly to form continuous film on both frontal and cross-section surfaces, but on silicon substrate, only can form on cross-section surface. These carbon nanotubes have much irregular cup-like structure, and with outer diameter varying from 25 nm to 35 nm. At the top end of carbon nanotube there is a catalyst particle. EDX analysis reveals that the particle are iron cluster, and EELS spectrum indicates that the nanotube is composed of pure carbon. Finally, the effect of substrate surface roughness on the growth behavior of carbon nanotubes has been discussed.     

SrO/Si(100)表面去氧过程的研究

借助高温扫描隧道显微镜和光电子能谱技术,深入研究了SrO/Si(100)表面向Sr/Si(100)再构表面的动态转化过程.Sr/Si(100)再构表面在硅基氧化物外延生长中起重要作用.在该动态转化过程中,样品在500 ℃的退火温度下,表面出现SrO晶化的现象;在550—590 ℃的退火温度下,SrO/Si(100)开始向Sr/Si(100)转化,界面和表面上的氧以气态的SiO溢出,使得表面出现大量凹槽状缺陷.并且在此动态转化过程中表面的电子态表现出金属特性,这是由于表层硅原子发生断键重排,从而在表面出现悬 关键词： SrO/Si表面 Sr/Si表面 扫描隧道显微镜 去氧过程     

Atomic configurations during Si incorporation on GaAs(001) in As atmosphere evidenced by reflectance difference spectroscopy

The structural ordering of surface atoms during Si deposition on singular and vicinal GaAs(001) surfaces has been studied by reflectance difference (RD) spectroscopy using the difference function between the Si-covered and the bare surface. In dependence on the Si coverage the difference spectra correspond to RD spectra of the bare Si(001)-(1×2) or of the As-terminated Si(001):As(2×1) surface. This finding and the behaviour of RD transients recorded at 3.8 eV photon energy allows to define a (3×2)α phase with Si dimers in the top layer and Ga dimers in the third layer, and a (3×2)β phase with As-dimer rows on top of Si in the second layer.     

Growth Kinetics of Silicon Carbide Film Prepared by HeatingPolystyrene/Si(111)

在不同的温度下热处理用sol-gel法制的PS/Si(111)叠层凝胶膜制备出了SiC薄膜．用XRD、SEM、XPS、FTIR等分析方法研究了SiC薄膜的结构、组成和表面形貌等．根据FTIR光谱计算了不同温度下得到的SiC薄膜的厚度,并研究了PS/Si(111)热解法生长SiC薄膜的生长动力学．结果表明,随着生长温度的增加,SiC薄膜生长速率变化趋势为：1200～1250 oC生长速率增加缓慢是2D生长机制,1250～1270 oC生长速率快速增加是3D生长机制,1270～1300 oC生长速率为负增长是由于SiC薄膜生长与Si和C原子的挥发共同作用所致．由速率变化求得各段表观生长激活能分别是122.5、522.5、-127.5 J/mol．     

Influence of the Si cap layer on the SiGe islands morphology

Transmission electron microscopy (TEM), atomic force microscopy (AFM), and EDX methods were used to study morphology and chemical composition of SiGe/Si(001) islands grown at 700 degrees C and covered at 550 degrees C and 700 degrees C by Si layers of different thickness. The samples were grown in ultra high vacuum chemical vapor deposition process (UHVCVD) controlled with in situ reflection of high-energy electron diffraction (RHEED). The islands transformed from initial pyramid and dome shapes to lens shape for 1.4 nm and 3.7 nm cap layer thickness at 550 degrees C and 700 degrees C, respectively. An increase of lateral to vertical ratio was observed during the transformation. For the higher depositing temperature the ratio was bigger and was increasing continuously with cap layer thickness. Also, with increasing Si cap layer thickness, the Ge concentration was decreasing, which was more observable for higher capping temperature.     

Preparation and structural properties of MgO films grown on GaAs substrate

Epitaxial MgO thin films have been grown on semiinsulating GaAs (0 0 1) substrates using electron beam (e-beam) evaporation. X-ray diffraction indicates c-axis oriented MgO with (0 0 2) reflection only and rocking curve widths ∼2.2-3°. Transmission electron microscopy (TEM) analyses confirm an epitaxial growth of the MgO films. We study the microstructure and the defects at the interface between the MgO film and the GaAs substrate. Auger electron Spectroscopy (AES) concentration depth profiles reveal no contamination of the MgO films by As and Ga at different temperatures of the deposition process.     

Role of surface defects in room-temperature growth of metals on Si(100)2 × 1

Nucleation and formation of silver islands on the Si(100)2 × 1 surface was for the first time studied by scanning tunneling microscopy directly during deposition. Crucial role of C-type defects on growth has been observed. Rapidly diffusing Ag atoms nucleate on these defects and during island evolution the defects represent stable terminations of Ag atomic chains. Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005.     

Intrinsic vacancy-induced nanoscale wire structure in heteroepitaxial Ga2Se3/Si(001)

A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga2Se3) on the lattice matched substrate, arsenic-terminated Si(001). Scanning tunneling microscopy of Ga2Se3 films reveals nanoscale, wirelike structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect the bulk, defected zinc-blende structure of beta-Ga2Se3, which contains ordered 110 arrays of Ga vacancies. These ordered vacancy lines are proposed to be responsible for the observed growth anisotropy in heteroepitaxial Ga2Se3.     

High temperature reaction and defect chemistry at the Si/SiO2 interface

The generation of structural and electrical defects in Si/SiO₂ structures upon high temperature annealing by the oxide decomposition reaction Si+SiO₂→2SiO ↑ has been studied using scanning electron microscopy (SEM) and ramped current-voltage measurements. The SiO decomposition is nucleated at crystalline defects in the substrate and results in the formation of voids in the oxide. The voids grow laterally with annealing time, independent of the nature of the defect. Prior to the formation of physical voids in the oxide, defects become electrically active, leading to low field dielectric breakdown. The breakdown degradation is prevented when the O₂ pressure in the annealing ambient is sufficient to reverse the decomposition reaction by reoxidizing the SiO product at the interface.