Real time observation of GeSi/Si(001) island shrinkage due to surface alloying during Si capping

The Si capping of Ge/Si(001) islands was observed by in situ time-resolved transmission electron microscopy. During the initial stages of the Si deposition, islands were observed not only to flatten but also to shrink in volume. This unexpected shrinkage is explained by taking into account the intermixing of the deposited Si with the wetting layer and a consequently induced diffusion of Ge from the islands into the wetting layer. A model of the capping process which takes into account Ge diffusion is presented which is in good agreement with the experimental data.     

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.     

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     

Evolution of thermodynamic potentials in closed and open nanocrystalline systems: Ge-Si:Si(001) islands

An open (closed) system, in which matter is (not) exchanged through surface diffusion, was realized via growth kinetics. Epitaxially grown Si-Ge:Si (001) islands were annealed in different environments affecting the diffusivity of Si adatoms selectively. The evolution of the driving forces for intermixing while approaching the equilibrium was inferred from Synchrotron x-ray measurements of composition and strain. For the open system, intermixing due to the Si inflow from the wetting layer (reservoir) caused a decrease in the Ge content, leading to a lowering of the elastic energy and an increase in the mixing entropy. In contrast, for the closed system, while keeping the average Ge composition constant, atom rearrangement within the islands led to an increase in both elastic and entropic contributions. The Gibbs free energy decreased in both cases, despite the different evolution paths for the composition profiles.     

Strain status of epitaxial Ge film on a Si (001) substrate

An epitaxial Ge film was grown on a Si (001) substrate via a two-step process through the molecular beam epitaxy technique. The strain status of non-annealed and annealed epitaxial Ge films was determined by X-ray diffraction, Raman spectroscopy, and a combination of high-resolution transmission electron microscopy and geometric phase analysis. Results showed that the strain in non-annealed and annealed epitaxial Ge films is nonhomogeneous from the Ge/Si interface to the Ge film surface. The strain parallel to the interface in the non-annealed epitaxial Ge film is compressive; this strain reaches a minimum near the surface and a maximum at the interface. By contrast, the strain parallel to the interface in the annealed epitaxial Ge film is tensile; this strain reaches a minimum at the interface and a maximum near the surface.     

Influence of a predeposited Si1−x Gex layer on the growth of self-assembled SiGe/Si(001) islands

The growth of self-assembled Ge(Si) islands on a strained Si_1?xGe_x layer (0% < x < 20%) is studied. The size and the surface density of islands are found to increase with Ge content in the Si_1?xGe_x layer. The increased surface density is related to augmentation of the surface roughness after deposition of the SiGe layer. The enlargement of islands is accounted for by the decrease of the wetting layer in thickness due to the additional elastic energy accumulated in the SiGe layer and to enhanced Si diffusion from the Si_1?xGe_x layer into the islands. The increase in the fraction of the surface occupied by islands leads to a greater order in the island arrangement.     

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.     

Growth mechanism and morphology of Ge on Pb covered Si(111) surfaces

We study the mechanism and surface morphology in epitaxial growth of Ge on Pb covered Si(111) using a scanning tunneling microscope (STM). We find that Ge adatoms have a very large diffusion length at room temperature. The growth is close to perfect layer-by-layer for the first two bilayers. Surface roughness increases gradually with the film thickness, but no 3D islands are found at room temperature. For growth at 200°C, 3D Ge islands appear after completion of the second bilayer. At room temperature, we believe, the Pb layer enhances surface diffusion and the descending-step motion of Ge adatoms, but the ascending-step motion is hindered and thus 3D island growth is suppressed.     

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     

Strain-driven morphology of Si1-xGex islands grown on Si(100)

A study has been carried out on the morphology and structure of three-dimensional (3D) SiGe islands grown by molecular beam epitaxy (MBE) on Si(100) substrates. Samples of Si1-xGex alloys have been prepared to investigate the effects either of the alloy composition or of the growth temperature. Atomic force microscopy (AFM) evidenced the growth of 3D islands and transmission electron microscopy (TEM) demonstrated wetting layer growth on Si(100), independently on the deposition conditions. Energy dispersive spectroscopy (EDS) micro-analyses carried out on cross-sections of large Si1-xGex islands with defects allowed a measurement of the Ge distribution in the islands. To the best of our knowledge, these have been the first experimental evidences of a composition change inside SiGe islands. The interpretation of the experimental results has been done in terms of strain-enhanced diffusion mechanisms both of the growing species (Si and Ge) and of small islands.     

Si/Gen/Si(001)异质结薄膜的掠入射荧光X射线吸收精细结构研究

利用掠入射荧光X射线吸收精细结构(XAFS)方法研究了在400℃的温度下分子束外延生长的Si/Ge_n/Si(001)异质结薄膜(n=1，2，4和8个原子层)中Ge原子的局域环境结构.结果表明，在1至2个Ge原子层(ML)生长厚度的异质结薄膜中，Ge原子的第一近邻配位主要是Si原子.随着Ge原子层厚度增加到4ML，Ge原子的最近邻配位壳层中的Ge-Ge配位的平均配位数增加到1.3.当Ge原子层厚度增加到8ML时，第一配位壳层中的Ge-Ge配位占的比例只有55%.这表明在400℃的生长条件下，Ge原子有很强的迁移到Si覆盖层的能力.随着Ge层厚度从1 增加到2，4和8ML，Ge原子迁移到Si覆盖层的量由0.5ML分别增加到1.5，2.0和3.0ML.认为在覆盖Si过程中Ge原子的迁移主要是通过产生Ge原子表面偏析来降低表面能和Ge层的应变能. 关键词： XAFS n/Si(001)异质膜')" href="#">Si/Ge_n/Si(001)异质膜 迁移效应     

Initial stages of Ge epitaxy on Si(111) under quasi-equilibrium growth conditions

The results of the structural and morphological studies of Ge growth on a Si(111) surface at the initial stages of epitaxy by means of scanning tunneling microscopy and high-resolution transmission electron microscopy are presented. Epitaxy of Ge has been performed in the temperature range of 300 to 550°C under the quasi-equilibrium growth conditions and low deposition rates of 0.001–0.01 bilayers per minute. The stages of the formation and decay of the nanoclusters as a result of the redistribution of the Ge atoms into two-dimensional pseudomorphic Ge islands before the formation of the continuous wetting layer have been experimentally detected. The positions of the preferable nucleation of three-dimensional Ge islands on the wetting layer formed after the coalescence of the two-dimensional islands have been analyzed. The c2 × 8 → 7 × 7 → c2 × 8 phase transitions due to the lateral growth of the islands and the plastic relaxation of the misfit strains occur on the surface of the three-dimensional Ge islands when their strain state changes. The misfit dislocations gather at the interface and two types of steps lower than one bilayer are formed on the surface of the three-dimensional islands during the relaxation process.     

纳米尺度下Si/Ge界面应力释放机制的分子动力学研究

采用分子动力学方法研究了纳米尺度下硅(Si)基锗(Ge)结构的Si/Ge界面应力分布特征,以及点缺陷层在应力释放过程中的作用机制.结果表明:在纳米尺度下, Si/Ge界面应力分布曲线与Ge尺寸密切相关,界面应力下降速度与Ge尺寸存在近似的线性递减关系;同时,在Si/Ge界面处增加一个富含空位缺陷的缓冲层,可显著改变Si/Ge界面应力分布,在此基础上对比分析了点缺陷在纯Ge结构内部引起应力变化与缺陷密度的关系,缺陷层的引入和缺陷密度的增加可加速界面应力的释放.参考对Si/Ge界面结构的研究结果,可在Si基纯Ge薄膜生长过程中引入缺陷层,并对其结构进行设计,降低界面应力水平,进而降低界面处产生位错缺陷的概率,提高Si基Ge薄膜质量,这一思想在研究报道的Si基Ge膜低温缓冲层生长方法中初步得到了证实.     

Ultrasmall Ge islands with low diameter-to-height aspect ratio on Si(1 0 0)-(2 × 1) surfaces

Scanning tunneling microscopy (STM) and high resolution cross-sectional transmission electron microscopy (XTEM) studies have been used to investigate the formation of Ge nanocrystals grown on Si(1 0 0)-(2 × 1) surfaces by molecular beam epitaxy (MBE). We observe relatively high density of Ge islands where small ‘pyramids’, small ‘domes’ and facetted ‘domes’ of various sizes co-exist in the film. As revealed from XTEM images, a large fraction of islands, especially dome-shaped Ge islands have been found to have an aspect ratio of ∼1 (diameter):1 (height). Observation of truncated-sphere-shaped Ge islands with a narrow neck contact with the wetting layer is reported.     

Alloying mechanisms for epitaxial nanocrystals

The different mechanisms involved in the alloying of epitaxial nanocrystals are reported in this Letter. Intermixing during growth, surface diffusion, and intraisland diffusion were investigated by varying the growth conditions and annealing environments during chemical vapor deposition. The relative importance of each mechanism was evaluated in determining a particular composition profile for dome-shaped Ge:Si (001) islands. For samples grown at a faster rate, intermixing during growth was reduced. Si surface diffusion dominates during H2 annealing, whereas Ge surface diffusion and intraisland diffusion prevail during annealing in a PH3 environment.     

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.     

Photoluminescence of Ge nanostructures grown by gas source molecular beam epitaxy on silicon (1 1 8) surface

In this paper, we present a photoluminescence (PL) study of Si/Ge/SiGe/Si structures grown by gas source molecular beam epitaxy on an (1 1 8) undulated surface with various Ge coverage. Nucleation and growth of Ge films is obtained by the Stranski–Krastanov mechanism. The influence of the substrate orientation on the changeover 2D–3D growth mode is investigated. Furthermore, we show the use of growing an SiGe wetting layer to control the uniformity of the Ge island size. The PL signal related to the Ge islands is found to be highly dependent of the power excitation and is observed up to room temperature.     

Ge(5 × 5) as a wetting layer on Si(1 1 1)

Ab initio total energy methods are used to investigate the effects on a Ge(1 1 1)-5 × 5 surface of the lateral compressive stress that would be due to a Si substrate, and the effects of intermixing at the interface with the substrate. The effects of stress due to the lattice mismatch between Si and Ge are studied on a Ge slab by changing the lattice constant in the surface plane from that of experimental bulk diamond Ge to that of Si. When this is done the height difference of the Ge adatoms in the faulted half-cell from those in the unfaulted half is accentuated. Effects on the Ge surface due to the presence of the Si-Ge interface were studied using a thin Ge layer on a Si substrate. The presence of the substrate leads to corrugations with significant height differences appearing among the faulted adatoms. The energetics of intermixing were investigated for Si-Ge single atom interchanges. Additional corrugations resulted from the shortened bondlengths due to the Si impurity in the wetting layer.     

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.     

Ge self-diffusion in epitaxial Si(1)-(x)Ge(x) layers

Diffusion coefficients and activation energies have been determined for Ge diffusion in strain-relaxed Si(1)-(x)Ge(x) with x = 0.00, 0.10, 0.20, 0.30, 0.40, and 0.50. The activation energy drops from 4.7 eV in Si and Si(0.90)Ge(0.10) to 3.2 eV at x = 0.50. This value compares with the literature value for Ge self-diffusion in Ge, suggesting Ge-like diffusion already at x approximately equal to 0.5. The effect of strain on the diffusion was also studied showing a decrease in diffusion coefficient and an increase in activation energy upon going from compressive over relaxed to tensile strain.