Surface mobility difference between Si and Ge and its effect on growth of SiGe alloy films and islands

Based on first-principles calculations of surface diffusion barriers, we show that on a compressive Ge(001) surface the diffusivity of Ge is 10(2)-10(3) times higher than that of Si in the temperature range of 300 to 900 K, while on a tensile surface, the two diffusivities are comparable. Consequently, the growth of a compressive SiGe film is rather different from that of a tensile film. The diffusion disparity between Si and Ge is also greatly enhanced on the strained Ge islands compared to that on the Ge wetting layer on Si(001), explaining the experimental observation of Si enrichment in the wetting layer relative to that in the islands.     

Low-energy photoluminescence of structures with GeSi/Si(001) self-assembled nanoislands

The photoluminescence spectra of structures with self-assembled GeSi/Si(001) islands are investigated as functions of the growth temperature. It is shown that the shift of the peak of photoluminescence from islands toward lower energies on decreasing the growth temperature is due to the suppression of Si diffusion into islands and an increase in the fraction of Ge in islands. A photoluminescence signal from the GeSi islands is found in the region of energies down to 0.6 eV, which is considerably smaller than the band-gap width in bulk Ge. The position of the peak of photoluminescence from islands is described well by the model of a real-space indirect optical transition with account of the real composition and elastic strains of the islands. Mono-and multilayer structures are obtained with self-assembled GeSi/Si(001) nanoislands exhibiting a photoluminescence signal in the region 1.3–2 μm at room temperature.     

Mixed PbSi dimer chains on Si(1 0 0): a first-principles study

It has been a common belief that the one-dimensional structures observed by STM at low coverage of Pb on Si(1 0 0) are buckled Pb-Pb dimer chains. However, using first-principles density functional calculations, we found that it is energetically favorable for Pb adatoms to intermix with Si atoms to form mixed dimer chains on Si(1 0 0), instead of Pb-Pb dimer chains as assumed in previous studies. Up to a Pb coverage of 0.125 ML, mixed PbSi dimer chain is 0.19 eV per Pb atom lower in energy than Pb dimer chain.     

Formation of pyramid-like nanostructures in MBE-grown Si films on Si(001)

The growth of Si homoepitaxial layers on Si(001) substrates by molecular beam epitaxy is analyzed for a set of growth conditions in which diverse nanometer-scale features develop. Using Si substrates prepared by exposure to HF vapor and annealing in ultra-high vacuum, a rich variety of surface morphologies is found for different deposited layer thicknesses and substrate temperatures in a reproducible way, showing a critical dependence on both. Arrays of 3D islands (truncated pyramids), percolated ridge networks, and square pit (inverted pyramid) distributions are observed. We analyze the obtained arrangements and find remarkable similarities to other semiconductor though heteroepitaxial systems. The nanoscale entities (islands or pits) display certain self assembly and ordering, concerning size, shape, and spacing. Film growth sequence follows the ‘islands–coalescence–2D growth’ pathway, eventually leading to optimum flat morphologies for high enough thickness and temperature.     

Sb/Si(001)表面的半经验研究

用基于Chadi模型和格林函数方法的一种计算表面应力的半经验方法研究了Sb吸附在Si(001) 衬底上的性质.结果显示,Sb原子在Si(001)表面形成对称的dimer,其键长为0.293nm,表 面以下层的弛豫很小.Sb/Si(001)2×1表面沿着dimer方向的张应力为1.0eV/(1×1cell),而 沿垂直于dimer方向的压应力为-1.1eV/(1×1cell).Sb/Si(001)表面应力的主要贡献来自于 最上面三层表面. 关键词： 表面应力 异质生长 格林函数方法     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

Initial surface roughening in Ge/Si(001) heteroepitaxy driven by step-vacancy line interaction

The initial surface roughening during Ge epitaxy on Si(001) is shown to arise from an effective repulsion between S(A) surface steps and dimer vacancy lines (VLs). This step-VL interaction gradually inactivates a substantial fraction of adatom attachment sites at the growth front, causing a rapid increase in the rate of two-dimensional island nucleation. The mutual repulsion hinders the crossing of S(A) surface steps over VLs in the second layer, thus organizing the developing surface roughness into a periodic array of anisotropic 2D terraces. Isolated (105) facets forming at specific sites on this ordered template mediate the assembly of first 3D Ge islands.     

Sr/Si界面沉积SrTiO3初始生长阶段的扫描遂道显微术研究

本文工作利用脉冲激光沉积术(PLD)和超高真空扫描隧道显微术(UHV-STM),研究了在Sr/Si(001)-(2×1)衬底表面上真空室温沉积几个单层SrTiO₃薄膜的初始生长过程.经660 ℃退火处理后,Sr/Si衬底表面上形成了纳米岛状结构.经分析,这些纳米小岛为C49-TiSi₂和 C54-TiSi₂.实验结果表明,在没有氧气的情况下退火,Sr/Si界面无法有效阻止SrTiO₃薄膜与Si衬底之间的相互作用. 关键词： 脉冲激光沉积术(PLD) 扫描隧道显微镜(STM) 3')" href="#">SrTiO₃ 2')" href="#">C54-TiSi₂     

Perfect alignment of self-organized Ge islands on pre-grown Si stripe mesas

Self-organized Ge islands grown on patterned Si(001) substrates have been investigated. Selective epitaxial growth (SEG) of Si is carried out with gas-source molecular beam epitaxy to form Si stripe mesas followed by subsequent Ge island growth. Self-aligned Ge islands with regular spacing are formed on the <110>-oriented ridges of the Si mesas. The regular spacing is driven by the repulsive interaction between the neighbor islands through the substrates. A mono-modal distribution of the islands has been observed on the ridges of the Si mesas. The spatial confinement as well as the preferential nucleation is believed to be the mechanism of this alignment of the self-organized Ge islands. Received: 16 July 1999 / Accepted: 6 August 1999 / Published online: 24 March 2000     

FI-STM STUDY OF HYDROGEN ADSORPTION ON Si(100) SURFACE

Chemisorption of atomic hydrogen on the Si(100)2×1 surface has been investigated in detail by using a field ion-scanning tunneling microscope (FI-STM). The results showed that the adsorption geometry changed from the 2×1 monohydride phase to the 1×1 dihydride phase with increasing exposure of hydrogen. The data of desorption of the hydrogen-saturated Si surface showed that on annealing at 670K the surface becomes highly disordered: the 1×1 dihydride structure is eliminated and the 2×1 reconstructed monohydride is also hardly to identify. When the temperature rises to as high as 730 K, the surface is domi-nated by the 2×1 structure with missing dimer rows, and some adatom chains occur on the Si substrate ler-races. We attribute the formation of these atomic chains to an epitaxial growth of Si atoms which are formed by the dissociation of SiH_x (x= 1, 2, 3 or 4) compounds on the Si surface.     

Minute SiGe quantum dots on Si(001) by a kinetic 3D island mode

We investigated the initial growth stages of Si(x)Ge(1-x)/Si(001) by real time stress measurements and in situ scanning tunneling microscopy at deposition temperatures, where intermixing effects are still minute (< or =900 K). Whereas Ge/Si(001) is a well known Stranski-Krastanow system, the growth of SiGe alloy films switches to a 3D island mode at Si content above 20%. The obtained islands are small (a few nanometers), are uniform in shape, and exhibit a narrow size distribution, making them promising candidates for future quantum dot devices.     

First-principles molecular dynamics study of acetylene adsorption on the Si(001) surface

We present a first-principles molecular dynamics study of acetylene adsorption on the Si(001) surface. Acetylene molecules are di-σ bonded to the first layer Si dimers with the adsorption energy of 64.8 kcal/mol. It is elucidated that the CC bond is essentially double bond and the Si dimer bonds are not cleaved. The normal mode analyses well reproduce the experimental results, giving a strong support to our results.     

Phase transition from asymmetric to symmetric dimer structure on the Si(001) surface at high temperature

The dimer configurations on the Si(001) surface at high temperatures have been investigated using the rocking curve of reflection high-energy electron diffraction. The Si(001) surface shows a displacive phase transition around 900 K, where a well-known asymmetric (tilted) dimer structure on the Si(001) at room temperature transforms to a symmetric dimer structure around 900 K. The metallic feature of the Si(001) surface above 900 K can be explained by the phase transition.     

Adsorption of Au and Pt dimers on Ge(001) and Si(001): A first-principles study

Based on first-principles total energy calculations, the adsorption of Au and Pt dimers on Ge(001) and Si(001) surfaces are investigated. We find that the Au dimer on both Ge(001) and Si(001) show a similar result with the most stable configuration C, parallel to the substrate dimer row and located in the trough between the dimer rows, and the most unstable configuration A, parallel to and on the top of the substrate dimer row. On the other hand, Pt dimer on Ge(001) prefer the configuration D, perpendicular to the substrate dimer row and located in the trough between the dimer rows, while Pt dimer on Si(001) prefer both A and D configurations. The different structural stabilities of Au and Pt dimers on Ge(001) and Si(001) surfaces are attributed to the different electronic structures of Au and Pt atoms. These results are discussed with the reported data for III, IV and V group elements on Si(001).     

Effect of self-assembled Ge nanostructures on Si surface electronic properties

Incorporating self-assembled Ge islands on Si surfaces into electronic devices has been suggested as a means of forming small features without fine-scale litho- graphy. For use in electronic devices, the electrical properties of the deposited Ge and their relation to the underlying Si substrate must be known. This report presents the results of a surface photovoltage investigation of the surface energy barrier as increasing amounts of Ge are added to a Si surface by chemical vapor deposition. The results are interpreted in terms of band discontinuities and surface states. The surface barrier increases as a wetting layer is deposited and continues to increase as defect-free islands form. It saturates as the islands grow. As the amount of Ge continues increasing, defects form, and the surface barrier decreases because of the resulting allowed states at the Ge/Si interface. Qualitatively similar behavior is found for Si(001) and Si(111). Covering the Ge with Si reduces the surface-state density and possibly modifies the wetting layer, decreasing the barrier to one more characteristic of Si. Initial hydrogen termination of the surface decreases the active surface-state density. As the H desorbs, the surface barrier increases until it stabilizes as the surface oxidizes. The behavior is briefly correlated with scanning-tunneling spectroscopy data. Received: 13 November 2000 / Accepted: 14 November 2000 / Published online: 23 May 2001     

Ion-beam-assisted MBE growth of semi-spherical SiGe/Si nano-structures

Semi-spherical SiGe/Si nano-structures of a new type are presented. Epitaxial islands of 30–40 nm in base diameter and 11 nm in height and having a density of about 6×10¹⁰ cm^-2 were produced on (001) Si by molecular beam epitaxial growth of Si/Si_0.5Ge_0.5 layers with in situ implantation of 1-keV As⁺ ions. It was found by cross-section transmission electron microscopy that the islands have a complicated inner structure and consist of a micro-twin nucleus and semi-spherical nano-layers of various SiGe compositions. The nature of the surface patterning is interpreted by stress relaxation through implantation-induced defects. Received: 12 July 2001 / Accepted: 4 September 2001 / Published online: 2 October 2001     

Monte carlo simulation of transitions related to growth on (001) faces of Si and Ge

Based on Monte Carlo simulations for molecular beam epitaxy, three types of growth related transitions on the Si(001) and Ge(001) surfaces have been studied. In the thermal roughening simulations on a Ge(001) surface, a different type of transition from the Kosterlitz and Thouless type is obtained. The simulated result is consistent with the experimental x-ray diffraction data, at least qualitatively. In the growth simulations, a transition in the shape of growing islands is shown at the very initial stage of the homoepitaxial growth on a Si(001)-2x1 flat surface. During the transition, the step density variations as a function time show different behaviors at various temperatures. In the homoepitaxial growth on Si(001)-2x1 vicinal surfaces, the growth mode transition from two-dimensional island formation to the step-flow mode is reproduced by increasing the system temperature, which agrees qualitatively with the observed results. At the intermediate temperature, a transient growth mode is obtained, in which the two-dimensional island formation and the step flow growth modes coexist on two types of terraces on the surface.     

Enhanced stability of 1D molecular lines on the H-terminated Si(001) surface

We present a facile method for the self-directed growth of 1D molecular lines on the H-terminated Si(001) surface. Instead of a previously employed single dangling bond, we here employ a single H-free Si dimer as a reaction site, resulting in an enhanced stability of the radical intermediate for the O-phthalaldehyde (OP) molecule containing two carbonyl groups. This radical intermediate easily abstracts two H atoms from a neighboring Si dimer, thereby allowing the chain reaction for a 1D molecular line. Such a fabricated OP line will be stable at higher temperatures compared to previously reported alkene lines because of its enhanced stability.     

One-dimensional Ge nanostructures on Si(001) and Si(1 1 10): Dominant role of surface energy

Ge/Si(001) is a prototypical system for investigating three-dimensional island self-assembly owed to the Stranski–Krastanow growth mode. More than twenty years of research have produced an impressive amount of results, together with various theoretical interpretations. It is commonly believed that lattice-mismatch strain relief is the major driving force leading to the formation of these islands. However, a set of recent results on Si(001) and vicinals point out that, under suitable conditions, this is not the case. Indeed, we here review experimental and theoretical results dealing with nanostructures mainly determined by surface-energy minimization. Results are intriguing, as they reveal the existence of magic sizes, show the presence of very peculiar morphologies, such as micron-long wires, and distinguish among attempts to facet the wetting-layer and true SK islands.     

Surface stress anisotropy of Ge(001)

By analyzing the equilibrium shape of vacancy islands on the Ge(001) surface we have determined the surface stress anisotropy, i.e., the difference between the compressive stress component along the substrate dimer rows and the tensile stress component perpendicular to the substrate dimer rows. In order to extract the surface stress anisotropy we have used a model recently put forward by Li et al. [Phys. Rev. Lett. 85, 1922 (2000)]. The surface stress anisotropy of the clean Ge(001) surface is found to be 80+/-30 meV/A(2). This value is comparable to the surface stress anisotropy of the closely related Si(001) surface.