Size distribution and optical properties of self-assembled Ge on Si

The distribution of Ge islands is analysed in order to understand their optical behaviour. The Ge islands described in this paper were deposited by low-pressure chemical vapour deposition at relatively high temperature (700 °C), therefore the diffusion length of adatoms is high (∼100 μm) and thus, not the limiting factor for nucleation. By changing the deposition time and the coverage, square-based pyramids, domes and relaxed domes are nucleated. Mainly domes emit light, the emission being in the wavelength range 1.38–1.55 μm. When pyramids or relaxed domes are present, the photoluminescence broadens and decreases in intensity. The electroluminescence of vertically correlated islands increases with the number of layers, i.e. with the number of islands. The nucleation of islands on patterned (001) Si is changed when the deposition is performed on Si mesas with high index facets. The size distribution becomes narrower when the mesa size is decreased. An intermixing of up to 40% Si in the 2D layer was determined from photoluminescence data. PIN diodes fabricated on patterned wafers show an area-dependent electroluminecence related to a different microstructure of islands on large and small mesas. Finally, the lateral ordering on {hkl} facets is discussed. Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000     

Self-ordering of a Ge island single layer induced by Si overgrowth

We provide a direct experimental proof and the related modeling of the role played by Si overgrowth in promoting the lateral ordering of Ge islands grown by chemical vapor deposition on Si(001). The deposition of silicon induces a shape transformation, from domes to truncated pyramids with a larger base, generating an array of closely spaced interacting islands. By modeling, we show that the resulting gradient in the chemical potential across the island should be the driving force for a selective flow of both Ge and Si atoms at the surface and, in turn, to a real motion of the dots, favoring the lateral order.     

Modified growth of Ge quantum dots using C2H4 mediation by ultra-high vacuum chemical vapor deposition

C₂H₄ mediations were used to modify the Stranski-Krastanow growth mode of Ge dots on Si(0 0 1) at 550 °C by ultra-high vacuum chemical vapor deposition. With appropriate C₂H₄-mediation to modify the Si surface, the elongated Ge hut clusters can be transformed to highly uniform Ge domes with a high Ge composition at the core. These C₂H₄-mediated Ge dots, almost bounded by {1 1 3} facets, have an average diameter and height of 55 and 9 nm, respectively. We propose two major mechanisms to depict the formation of these C₂H₄-mediated Ge dots: (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation, and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. This work provides a useful scheme to tune the topography of Ge dots in an UHV/CVD condition for possible optoelectronic applications.     

An integrated approach to ultraintense laser sciences: The PLASMON-X project

In the present paper, we aim to show the interest of combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence, to obtain structural properties (composition, strain and atomic ordering) of semiconductor heterostructures and nanostructures. As an example we report on preliminary results obtained on a series of Ge/Si(001) nano-island samples: pyramides and domes on nominal and prepatterned surfaces. For free standing domes, it is shown that the Ge content strongly depends on the growth condition with a tendency to increase from the bottom to the top of the nano-islands. There is also some indication of atomic ordering in the upper part of the islands. For small, capped pyramids, we show that the Diffraction Anomalous Fine Structure spectroscopy is the unique non destructive method that allows to recover the actual Ge content, the in-plane and out-of-plane strain and to detect atomic ordering.     

Metal-semiconductor nanocontacts: silicon nanowires

Strain evolution of coherent Ge islands on Si(001) is measured using a newly developed transmission electron microscopy technique based on two-beam dark-field strain imaging. The strain measurements show that a metastable Ge island shape is involved in the shape transition between pyramids and domes; this shape is more readily observed for growth at 550 than 600 degrees C because of the slower rate at which islands cross the kinetic barrier between shapes. The strain relaxation changes discontinuously between pyramids and domes, indicating that the underlying shape transition is first order.     

低温生长的Ge/Si超晶格界面结构的X射线散射研究

低温下用MBE方法生长了Ge/Si超晶格,X射线反射及横向散射研究表明,Ge亚层上下表面的粗糙度呈反对称,下表面大的粗糙度来源于Ge向Si亚层中扩散形成SiGe混合组分结构,这种组分结构可以用一平均成分的SiGe合金层加以拟合,从而使得各亚层均有一个合理的粗糙度,旋转样品进行的X射线散射研究表明,这种SiGe的混合是各向同性的,这与透射电子显微镜的研究结构相一致.     

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.     

A Calorimetric Study Assisted with First Principle Calculations of Specific Heat for Si-Ge Alloys within a Broad Temperature Range

Calorimetric measurements are performed to determine the specific heat of Si-xat.% Ge(where x = 0, 10, 30,50, 70, 90 and 100) alloys within a broad temperature range from 123 to 823 K. The measured specific heat increases dramatically at low temperatures, and the composition dependence of specific heat is evaluated from the experimental results. Meanwhile, the specific heat at constant volume, the thermal expansion, and the bulk modulus of Si and Ge are investigated by the first principle calculations combined with the quasiharmonic approximation. The negative thermal expansion is observed for both Si and Ge. Furthermore, the isobaric specific heat of Si and Ge is calculated correspondingly from OK to their melting points, which is verified by the measured results and accounts for the temperature dependence in a still boarder range.     

Is the elongation of Ge huts in the low-temperature regime governed by kinetics?

Contrary to pyramids and domes, elongated huts on a Si(001) substrate are commonly considered as kinetically limited structures. In this work, however, based on detailed scanning tunneling microscopy observations of Ge huts growing on Si(001) at temperatures below 550 degrees C and finite element analysis, the possibility of an equilibrium-driven elongation is raised, where hut-preceding pits are thought to lift the energetic degeneracy of {105} facets and cause elongation along energetically preferred directions.     

溅射沉积自诱导混晶界面与Ge量子点的生长研究

在不同的沉积温度下采用离子束溅射技术,在Si基底上生长得到分布密度高、尺寸单模分布的圆顶形Ge量子点.研究发现:随沉积温度的升高Ge量子点的分布密度增大,尺寸减小,当沉积温度升高到750 ℃时,溅射沉积15个单原子层厚的Ge原子层,生长得到高度和底宽分别为14.5和52.7 nm的Ge量子点,其分布密度高达1.68×10¹⁰ cm^-2;Ge量子点的形貌、尺寸和分布密度随沉积温度的演变规律与热平衡状态下气相凝聚的量子点不同,具有稳定形状特征和尺寸分布的Ge量子点是 关键词： Ge量子点 离子束溅射沉积 表面原子行为 混晶界面     

Sphere-shaped SiGe micro/nanostructures with tunable Ge composition and size formed by laser irradiation

SiGe spheres with different diameters are successfully fabricated on a virtual SiGe template using a laser irradiation method. The results from scanning electron microscopy and micro-Raman spectroscopy reveal that the diameter and Ge composition of the SiGe spheres can be well controlled by adjusting the laser energy density. In addition, the transmission electron microscopy results show that Ge composition inside the Si Ge spheres is almost uniform in a well-defined, nearly spherical outline. As a convenient method to prepare sphere-shaped SiGe micro/nanostructures with tunable Ge composition and size, this technique is expected to be useful for Si Ge-based material growth and micro/optoelectronic device fabrication.     

小尺寸Si/Ge量子点内应变和组分的拉曼光谱表征

本文详细地研究了原始生长和退火处理后的Si/Ge量子点的拉曼光谱。我们观测到了Si/Ge量子点的一系列本征的拉曼振动模以及Ge-Ge模的LO和TO声子峰间4.2cm-1的频率劈裂。通过这些参数,我们自洽地确定了原始生长的平面直径为20nm和高为2nm的Si/Ge量子点内Ge的平均组分为80%,平均应变为-3.4%。分析清楚地表明了这种小尺寸的Si/Ge量子点内的应变仍遵从双轴应变,并且应变的释放主要由量子点和Si隔离层间Si-Ge原子互扩散决定。     

离子束溅射自组装Ge/Si量子点生长的演变

采用离子束溅射技术,通过改变Ge的沉积量,在n型Si(100)衬底上自组装生长了一系列Ge量子点样品. 利用AFM和Raman光谱对样品表面形貌和结构进行表征,系统地研究了Ge量子点形貌、密度、尺寸大小以及Ge的结晶性和量子点中组分等随Ge沉积量的演变规律. 结果表明:Ge层从二维薄层向三维岛过渡过程中,没有观察到传统的由金字塔形向圆顶形量子点过渡,而是直接呈圆顶形生长;且随着Ge沉积量的增加,量子点密度先增大后减小,Ge的结晶性增强同时Ge/Si互混加剧,量子点中Si的组分增加. 关键词： 离子束溅射 量子点 表面形貌 Raman光谱     

Morphology and photoluminescence of ultrasmall size of Ge quantum dots directly grown on Si(0 0 1) substrate

High density and ultrasmall size of Ge quantum dots (QDs) have been achieved directly on Si(0 0 1) (2 × 1) reconstruction surface. Their detailed morphology was observed by atomic force microscope (AFM) and shows that small pyramids, small domes, huts, and multi-headed large domes coexist in the film grown at 400 °C, while small domes and multi-headed large domes formed at 450 °C. Their low temperature photoluminescence (PL) showed that a very strong non-phonon (NP) peak with a large blue shift of 0.19 eV at 14 K, which can be attributed to their very high areal density, 5.2 × 10¹¹ cm⁻², and sub-10-nm mean size, 7.6 ± 2.3 nm.     

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.     

Dangling bond modulating the electronic and magnetic properties of zigzag SiGe nanoribbon

First-principles nonmagnetic calculations reveal a metallic character in zigzag SiGe nanoribbons (ZSiGeNRs) regardless of their width. The partial DOS projected onto the Si and Ge atoms of ZSiGeNR shows that a sharp peak at the Fermi level is derived from the edge Si and Ge atoms. The charge density contours show the Si–Ge bond is covalent bond, while for the Si–H bond and Ge–H bond, the valence charges are strongly accumulated around H atoms due to their stronger 1 s potential and the higher electronegativity of 2.20 than that of 1.90 for Si atom and 2.01 for Ge atom, so that a significant charge transformation from Si or Ge atoms to H atoms and thus an ionic binding feature. Spin–polarization calculations show that the band structures of ZSiGeNR are modified by the dangling bonds. Compared with perfect ZSiGeNR which is a ferrimagnetic semiconductor, the bands of the ZSiGeNRs with bare Si edge, bare Ge edge, and bare Si and Ge edges shift up and nearly flat extra bands appear at the Fermi level. The ZSiGeNR with bare Si edge or bare Ge edge is a ferrimagnetic metal, while ZSiGeNR with bare Si and Ge edges is a nonmagnetic metal.     

Stoichiometry issues in pulsed-laser deposition of alloys grown from multicomponent targets

We have examined the degree of congruent transfer in pulsed-laser deposition (PLD) of alloy thin films in phases that are stable over a wide range of compositions. SiGe films were deposited by PLD onto high-purity glassy carbon substrates. We analyzed the average composition of these films using Rutherford backscattering spectrometry (RBS), and results show that the deposited films have a higher relative concentration of Ge than the initial targets. We infer that the noncongruent transfer is due to differential scattering in the plume itself. Additionally, the local composition of the particulates was measured by the use of microprobe analysis, and observations of a sintered target of Si and Ge powders and a solidified target of melted Si and Ge were compared. We found that the sintered target produces particulates with a wide range of compositions, whereas the solidified target produces five times fewer particulates with a tighter distribution of compositions. In contrast with the average composition of the films, the average composition of the particulates is the same as that of the targets. These results are discussed in terms of the microstructure of the targets and the melting process at the surface. The implications of these observations for composition determination by laser ablation are discussed.     

硅(001)图形衬底上锗硅纳米线的定位生长

纳米线的定位生长是实现纳米线量子器件寻址和集成的前提.结合自上而下的纳米加工和自下而上的自组装技术,通过分子束外延生长方法,在具有周期性凹槽结构的硅(001)图形衬底上首先低温生长硅锗薄膜然后升温退火,实现了有序锗硅纳米线在凹槽中的定位生长,锗硅纳米线的表面晶面为(105)晶面.详细研究了退火温度、硅锗的比例及图形周期对纳米线形成与否,以及纳米线尺寸的影响.     

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.     

Ge/Si混合物纳米薄膜中Ge钠米颗粒氧化态的XPS研究(英文)

pacc:7300,3320R TheoxidationstatesofGenanoparticlesin thesurfacelayerofGe/SifilmwerestudiedbyX-rayphotoelectronspectroscopy.Newfeatures appearedatthehighbindingenergysideofthe XPSGe3dpeakwhensampleswereannealedin atmosphere,whichwerecausedbythelargein ter…