Prepyramid-to-pyramid transition of SiGe islands on Si(001)

The morphology of the first three-dimensional islands appearing during strained growth of SiGe alloys on Si(001) was investigated by scanning tunneling microscopy. High resolution images of individual islands and a statistical analysis of island shapes were used to reconstruct the evolution of the island shape as a function of size. As they grow, islands undergo a transition from completely unfacetted rough mounds (prepyramids) to partially {105} facetted islands and then they gradually evolve to {105} facetted pyramids. The results are in good agreement with the predictions of a recently proposed theoretical model

High energy electron diffraction from transverse stacking faults

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High Resolution Z-Contrast Imaging of Semiconductor Interfaces

The structural and compositional integrity of interfaces between semiconductor multilayers can profoundly influence the optical and electronic properties of epitaxially grown heterostructures. Understanding the atomic-scale interfacial structure and chemistry is therefore essential to correctly relate electrical measurements to theoretical models and to correlate such effects with growth conditions. High-resolution electron microscopy (HREM) has played a pivotal role in this process, providing important information on interface commensurability and revealing the presence and nature of defects.More recently, significant advances have been made in applying HREM to the difficult problem of chemical composition mapping in systems where no structural change occurs across the interfaces. The basis of such methods involves using the objective lens as a bandpass filter and tuning in on a specific range of spatial frequencies to form a chemically sensitive interference pattern. By using a suitable low-index zone axis and choosing an optimum range of specimen thickness, the patterns can indeed be extremely sensitive to the strength and periodicities of the projected potential.</jats:p

Atomic Imaging of Crystals using Large-Angle Electron Scattering in STEM

It is well known that conventional atomic resolution electron microscopy is a coherent imaging process best interpreted in reciprocal space using contrast transfer function theory. This is because the equivalent real space interpretation involving a convolution between the exit face wave function and the instrumental response is difficult to visualize. Furthermore, the crystal wave function is not simply related to the projected crystal potential, except under a very restrictive set of experimental conditions, making image simulation an essential part of image interpretation. In this paper we present a different conceptual approach to the atomic imaging of crystals based on incoherent imaging theory. Using a real-space analysis of electron scattering to a high-angle annular detector, it is shown how the STEM imaging process can be partitioned into components parallel and perpendicular to the relevant low index zone-axis.It has become customary to describe STEM imaging using the analytical treatment developed by Cowley. However, the convenient assumption of a phase object (which neglects the curvature of the Ewald sphere) fails rapidly for large scattering angles, even in very thin crystals. Thus, to avoid unpredictive numerical solutions, it would seem more appropriate to apply pseudo-kinematic theory to the treatment of the weak high angle signal. Diffraction to medium order zero-layer reflections is most important compared with thermal diffuse scattering in very thin crystals (&lt;5nm). The electron wave function ψ(R,z) at a depth z and transverse coordinate R due to a phase aberrated surface probe function P(R-RO) located at RO is then well described by the channeling approximation;</jats:p

Mechanisms of strain-induced surface ripple formation and dislocation multiplication in Si{sub x}Ge{sub 1-x} thin films

We discuss the stress driven roughening transition of Si{sub x}Ge{sub 1-x} thin films. In the case of annealed films, nucleation effects dominate the nature of the surface ripple which formed by a cooperative nucleation mechanism. Facetting can however be suppressed at high supersaturations, resulting in a transition with characteristics of the Asaro-Tiller-Grinfeld instability. The relationship between morphological evolution and dislocation nucleation and multiplication is considered

Kinetic Pathways to Strain Relaxation in the Si-Ge System

The strain-induced transition of a planar film to a three-dimensional island morphology is presently a significant issue in the growth of semiconductor thin films. Strain-induced roughening can be problematic in the fabrication of coherently strained device structures where it is important to understand the early stages of the transition to avoid or suppress three-dimensional (3D) growth. On the other hand, the strain-driven transition is beneficial for the self-assembly of quantum dots where it is necessary to control the size distribution and self-organizing behavior of the islands. In both cases, it is clearly important to identify and understand the kinetic pathways to island formation. From a more basic perspective, the strain-induced transition of epitaxial films allows us to study in detail the interplay between elastic stresses and surface energy in a carefully controlled experimental environment. One would therefore hope that the lessons learned from model semiconductor systems will be of relevance to understanding related phenomena in other areas of materials science and physical metallurgy.Ihe strain-induced two-dimensional (2D)-to-3D transition in the Si-Ge system is manifested by a rich variety of observed surface morphologies. In the case of pure Ge on Si(OO1), the 4% misfit strain induces the formation of so-called hut clusters with curious elongated shapes. Such islands form almost immediately after the deposition of a wetting layer. In the case of lower misfit alloys, a more gentle ripple morphology can result that develops far from the interface. A general trend in all of the experiments is the decreasing size of typical morphological features with increasing misfit stress. In this article, largely guided by our experimental results, we adopt a nucleation and growth description of the 2D-to-3D transition. This approach appears particularly well-suited to explaining the wide spectrum of morphological development present in the Si-Ge system.</jats:p

Towards 1-Ångstrom-resolution STEM

Phase contrast TEM has been the leading technique for high resolution imaging of materials for many years, whilst STEM has been the principal method for high-resolution microanalysis. However, it was demonstrated many years ago that low angle dark-field STEM imaging is a priori capable of almost 50% higher point resolution than coherent bright-field imaging (i.e. phase contrast TEM or STEM). This advantage was not exploited until Pennycook developed the high-angle annular dark-field (ADF) technique which can provide an incoherent image showing both high image resolution and atomic number contrast.This paper describes the design and first results of a 300kV field-emission STEM (VG Microscopes HB603U) which has improved ADF STEM image resolution towards the 1 angstrom target. The instrument uses a cold field-emission gun, generating a 300 kV beam of up to 1 μA from an 11-stage accelerator. The beam is focussed on to the specimen by two condensers and a condenser-objective lens with a spherical aberration coefficient of 1.0 mm.</jats:p