Nucleation Of Misfit Dislocations Research Articles

Relaxation process and luminescence of lattice defects in epitaxially grown [formula omitted] layers

In this work, we report on the dependence of strain relaxation in epitaxially grown ZnSe GaAs layers on the growth process. New investigations about luminescence signals are presented, which are caused by lattice defects in these layers. The strain is due to different lattice constants of layer and substrate material and is relaxed by the nucleation of misfit dislocations during growth. This relaxation process depends on layer thickness and growth conditions. We determine the residual strain for different layer thicknesses and growth processes with X-ray diffractometry. Furthermore, the created misfit dislocations give rise to a luminescence at 2.6 eV, the so-called Y line. We found that its behaviour in ZnSe is analogous to that of Y lines in epitaxially grown ZnTe GaAs layers. An excitation of the Y luminescence is only possible with photon energies higher than the recombination energy of free excitons. Its intensity decreases with increasing impurity concentration. It also decreases strongly for a steady state excitation at 2 K. A recovery effect of luminescence intensity is observed, if the sample is heated to room temperature. Furthermore, the strain dependence of the Y luminescence in ZnSe is compared with the maxima of bound exciton luminescence from shallow donors and acceptors.

Role of surface step on misfit dislocation nucleation and critical thickness in semiconductor heterostructures

We discuss the role of surface step formation on misfit dislocation nucleation and critical thickness in thin film semiconductor heterostructures. On the basis of an atomistic model, it is shown that the energy change due to step formation is negative or positive depending upon the sign of the misfit. The step formation energy can even be negative for compressive misfit stress in the heterolayer, while it is definitely positive for tensile misfit stress. This conclusion is in contrast to the classical model where the step energy is always positive and independent of the sign of the misfit. The step formation energy influences the critical thickness and the energy barrier for dislocation nucleation. Using a simple atomistic simulation, we show that the critical thickness depends upon the sign of the misfit; for example, it changes from 4 nm fe films on Si(100) substrates to 6 nm for Si films on Ge(100) substrates having the same misfit.

X-ray study of relaxation process of strained GaAs layers grown on (100) Ge substrates

X-ray topography was used for a comprehensive study of elastic strain and dislocation-induced relaxation of GaAs layers grown on (100) Ge substrates. Measurements of tetragonal distortion and radius of curvature as well as direct transmission imaging indicate that the GaAs layers are highly metastable. Observations also indicate the existence of two sources of misfit dislocations, the threading dislocations and some stacking faults. Threading dislocations are activated first. Their activation involves plastic deformation of both the epilayer and the substrate. Stacking faults are activated later. This unusual misfit dislocation nucleation mechanism is discussed.

High-resolution X-ray diffraction investigations of epitaxially grown ZnSe/GaAs layers

Single-crystalline ZnSe layers were grown by metal-organic vapour phase and molecular beam epitaxy on (001) oriented GaAs substrates. The lattice mismatch between layer and substrate at growth temperature causes a strain in the layer material, which is relaxed by the nucleation of misfit dislocations. The relaxation process starts at the critical thickness, which depends on the growth conditions. The crystalline quality and the residual strain of the epilayers were investigated with a high-resolution X-ray diffractometer. Additionally, the intensity distribution of the scattered X-rays in the directions perpendicular and parallel to the reciprocal lattice vector (004) was observed by a two-reflection analyser crystal. For the system ZnSe/GaAs, this intensity distribution depends on the degree of strain relaxation, which is dependent on the layer thickness. The results are compared with transmission electron microscopy results.

Dislocation Mechanism for Island Diffusion on fcc (111) Surfaces.

We describe a novel mechanism for diffusion of homoepitaxial 2D islands on fcc (111) surfaces by nucleation and motion of misfit dislocations. Embedded atom method molecular dynamics calculations show that this mechanism will compare favorably with edge running and simultaneous gliding for islands up to some tens of atoms in size. We suggest that this mechanism may facilitate diffusion of heteroexpitaxial strained overlayer islands containing dislocations in their lowest energy configurations.

IN SITU STUDIES WITH LOW-ENERGY ELECTRON MICROSCOPY

This paper gives a brief review of low-energy electron microscopy (LEEM) as used for in situ studies of surface dynamical processes. The capabilities of LEEM are illustrated with two examples. One is a kinetic instability observed during growth of the first layer of CaF 2 on Si (111). The second concerns the nucleation of misfit dislocations during the growth of thicker, epitaxial CaF 2 films on Si (111), as the critical thickness is exceeded. Both examples highlight the importance of real time, in situ observations of surface dynamical processes.

Nucleation of misfit dislocations in In0.2Ga0.8As epilayers grown on GaAs substrates

Misfit dislocation arrays in In0.2Ga0.8As epilayers grown on GaAs substrates tilted 2°–10° away from exact (001) toward varied directions have been studied by transmission electron microscopy. A method has been developed to determine the glide plane and the Burgers vector of each misfit dislocation in the tilted InGaAs/GaAs interfaces. Based on experimental observations and theoretical analyses, it is proposed that a stacking fault surrounded by a 30° partial is at first generated by a growth error, followed by thermally activated nucleation of a 90° partial dislocation that removes the stacking fault and forms a 60° dislocation. From the frequency of nucleation events versus the dislocation glide force, the energy barrier for dislocation nucleation of α and β 90° partial dislocations was determined to be equal to 1.5 and 1.4 eV, respectively.

Large Lattice Mismatch Epitaxy

ABSTRACTDuring the early stages of lattice mismatch epitaxy, island growth is observed in several systems. The evolution of this morphology is attributed to a large value of the substrate/layer interface energy. Origins of misfit and threading dislocations are also considered. Nucleation of misfit dislocations could occur either from steps present on surfaces of islands or from substrate/island edges. Arms of glide loops terminating at the surface in the first case form threading segments in the layer, whereas the coalescence of islands in which misfit dislocations are not aligned with respect to each other lead to threading dislocations in the second case. Existing approaches for lowering threading dislocation densities are also evaluated. A novel approach involving controlled, self-assembly of islands is presented to achieve the preceding objective.

Strain Relaxation at Low Misfits: Dislocation Injection vs. Surface Roughening

ABSTRACTTwo competing strain relaxation mechanisms, namely misfit dislocation generation and surface roughening, have been extensively studied using the GexSi1-x/Si (x< 0.5) system as an example. A predictive model has been developed which accurately describes the nature of misfit dislocation nucleation and growth under non-equilibrium conditions. Using optical and electron microscopy, coupled with a refined theoretical description of dislocation nucleation, it is shown that strain relieving dislocations are readily generated at low misfits with a characteristic activation energy barrier regardless of the growth technique employed (i.e. MBE, RTCVD and UHVCVD). Secondly we have studied the alternative elastic strain relaxation mechanism involving surface undulation; x-ray diffraction, electron and atomic force microscopy have been used to characterize GexSi1-x/Si (x<0.5) structures grown by UHVCVD and MBE at relatively higher temperatures. A theoretical model has been used to model the critical thickness for surface wave generation. The conditions governing the interplay between dislocation formation and surface buckling are described in terms of a "morphological instability diagram".

The surface and interface nucleation of misfit dislocations as a possible source of asymmetric strain relaxation in vicinal heterostructures

The surface and interface nucleation of misfit dislocations as a possible source of asymmetric strain relaxation in vicinal heterostructures

Growth ripples upon strained SiGe epitaxial layers on Si and misfit dislocation interactions

Ripple arrays are formed under certain growth conditions upon the surface of epitaxial Si1−xGex on Si, and their nature is examined in detail using transmission electron microscopy, atomic force microscopy, and x-ray diffraction measurements. The driving force for ripple formation on the strained layers is the lowering of the system free energy by partial elastic strain relief due to oscillatory lattice plane distortion. We present the first direct measurement of the lattice distortion and demonstrate that this corresponds to near-complete relaxation in the ripple peaks in a direction normal to the ripple rows. The additional compressive lattice plane distortion present at ripple troughs is also estimated and is likely to promote misfit dislocation nucleation. Surface faceting can reduce the energy of the configuration, allowing troughs to deepen and minimize the dislocation nucleation barrier. The way in which the ripple strain fluctuations couple to already formed underlying misfit dislocations (often pinned bunches) is also demonstrated.

Homogeneous Nucleation of Misfit Dislocations in Strained Layers

The mechanism of strain relaxation by homogeneous nucleation of misfit dislocations from the interface in strained layers has been investigated. Transmission electron microscopy examination in a coherently strained GaInAsP/GaInP heterostructure demonstrated that the critical thickness of the strained layer for the nucleation of 90° 1/6<112> partial dislocations from a tensile interface is much shallower than that of 60° 1/2<110> perfect dislocations from a compressive interface. A critical thickness model for the interface nucleation of these dislocations is developed as a modification of the classical surface nucleation model.

Effect of stress sign and film thickness on interface nucleation of misfit dislocations in strained multilayers

The strain relaxation mechanism via the homogeneous nucleation of misfit dislocations from the heterointerface in coherently strained layers has been investigated with transmission electron microscopy. It is identified that the dislocations nucleated from the interface are displayed in the form of 90° partial, 30° partial, and 60° perfect dislocations, depending on stress sign and thickness of the strained layer. The tendency is interpreted in terms of the strain relaxation efficiency which is given by the ratio of relieved strain energy to dislocation line energy.

Growth and Processing of Relaxed-Si1-xGex/Strained-Si Structures for Metal-Oxide Semiconductor Applications

Epitaxial growth and processing issues related to strained-Si metal-oxide semiconductor field effect transistor (MOSFET) fabrication are discussed. The material quality of the graded Ge composition, relaxed- Si1-x Gex buffer layers is analyzed. The ion implants used to form complementary metal-oxide semiconductor (CMOS) “wells” prior to epitaxy are found to degrade the quality of these layers by introducing a high density of misfit dislocation nucleation sites. Rough surface morphology, short misfit dislocation line lengths, and high threading defect densities are correlated with the increased nucleation site density. In addition, the oxidation of strained Si is investigated by Rutherford backscattering and Raman spectroscopy. No measurable strain relaxation is found as a result of thermal oxidation at temperatures up to 850° C.

The surface and interface nucleation of misfit dislocations as a possible source of asymmetric strain relaxation in vicinal heterostructures

The surface nucleation of misfit dislocations in vicinal (001) oriented heterostructures with compressive and tensile stresses is discussed. It is shown that beside the asymmetrical stressing of opposite dislocation slip planes due to the vicinal substrate, the surface steps have a similar effect. The effect of the steps has the same-sign asymmetry for a compressive stressed epilayer, but opposite for the tensile case. The effect on dislocation nucleation energy is calculated. For miscut angles used normally, the step energy contribution exceeds that of due to the vicinal substrate. The extension to interface nucleation is treated qualitatively.

Microscopy of stress-induced morphological development and dislocation nucleation during semiconductor epitaxy

Semiconductor films subjected to misfit stresses are unstable to the formation of surface undulations or waves. Atoms located in close proximity to the peaks of the undulations can relax and significantly lower the elastic energy of the film. Provided that the wavelength of the undulation is sufficiently large, this more than compensates for the associated increase in surface energy, leading to a morphological instability of the flat surface.An interesting feature of this instability is that although atomic planes near the peaks of the undulations can relax, stress concentrations develop at the undulation valleys. These stresses can be very large, even for rather shallow surface perturbations. The question, therefore, naturally arises as to the subsequent effect of these stresses on the evolving surface morphology and in particular, whether the magnitude of the stress is large enough to influence the nucleation of misfit dislocations. In order to study these issues experimentally, we have utilized Z-contrast imaging , atomic force microscopy (AFM), and plan-view transmission electron microscopy (TEM) to image the stress-induced surface evolution of SixGe1-x alloys which are compressively strained on Si(100).

Misfit dislocation nucleation in doped and undoped ZnSe/GaAs

We have observed that undoped ZnSe films grown on GaAs substrates by molecular beam epitaxy show an irregular array of interfacial 60° misfit dislocations. However, N and Cl doping of the ZnSe thin films changes the interfacial dislocation structure. p-type ZnSe with N concentrations of ∼1018/cm3 shows a regular array of interfacial 60° misfit dislocations and a lower (∼1×106/cm2) density of threading dislocations compared to undoped films. However, samples with doping levels higher than 1019/cm3 show a density of threading dislocations of ∼108/cm2. These differences are explained in terms of Frank partial dislocations observed only in doped ZnSe. The Frank partial dislocations act as nucleation sites for the misfit dislocations. Thus, different mechanisms for the formation of misfit dislocations in doped and undoped films occur in this system.

Atomistic simulation of formation of misfit dislocations if f.c.c. heterostructures

Atomistic simulations were performed to study formation and characteristics of misfit dislocations in heterostructures. The f.c.c. Au/Ni (15.9% mismatch) system was used for analysis. When the strain energy of the system, computed using Lennard-Jones potentials [ Physica status solidi (a) 30, 619 (1975)], (or potentials based on embedded atom method [ Phys. Rev. B 41, 9717 (1990)]) was minimized, misfit dislocations were generated. The dislocation type depended upon the orientation of the substrate, being 90° type for a (001) interface and 60° type for a (111) interface. The latter orientation also resulted in greater relaxation of the strain energy. Multiple dislocations were generated at appropriate spacings in large computational cells relaxed using rigid boundaries. When infinitely large computational cells were simulated using periodic boundaries, the size of the periodic unit affected the dislocation spacing and the energy relaxation. Based on the structure of the intermediate defect configurations, it is hypothesized that the nucleation of misfit dislocations starts with formation of vacancies at the film surface. As the vacancies migrated towards the substrate, a dislocation loop moved to the interface.

Spontaneous nucleation of misfit dislocations in strained epitaxial layers

Although existing theoretical calculations predict relatively large activation energy barriers (>10 eV) for misfit dislocation nucleation in relatively low misfit heteroepitaxial systems, it is shown that the nucleation of misfit dislocations can occur spontaneously demonstrating a vanishingly small activation energy barrier. A wide range of GexSi1−x/Si (x < 0.5) heterostructures grown by MBE and CVD techniques are studied using optical and electron microscopical techniques in order to elucidate a novel misfit dislocation nucleation mechanism which is found to be rate-controlled solely by glide-activated processes with activation energies of ≈ 2 eV. Obwohl vorhandene theoretische Berechnungen eine relativ hohe Aktivierungsenergiebarriere (> 10 eV) fur die Erzeugung von Misfit-Versetzungen in heteroepitaktischen Systemen mit relativ geringem Misfit voraussagen, wird gezeigt, das die Bildung von Misfit-Versetzungen spontan auftritt, was auf eine verschwindend geringe Aktivierungsenergiebarriere schliesen last. Eine grose Anzahl mit MBE- und CVD-Technik hergestellte GexSi1−x/Si (x < 0.5)-Heterostrukturen werden mit Licht- und Elektro-nenmikroskopie untersucht, um einen neuartigen Erzeugungsprozes fur Misfit-Versetzungen naher zu untersuchen. Es wird festgestellt, das dieser Erzeugungsprozes ausschlieslich durch einen Gleitprozes im Wachstum kontrolliert wird. Die gesamte Aktivierungsenergie betragt ≈ 2eV.

The role of surface steps in the asymmetric surface dislocation nucleation in vicinal heterostructures with compressive and tensile stresses

The surface nucleation of misfit dislocations in vicinal (001)-oriented heterostructures is discussed. It is shown that beside the asymmetrical stressing of opposite dislocation slip planes due to the vicinal substrate, the surface steps have a similar effect. The effect of the steps has the same-sign asymmetry for a compressive stressed epilayer, but is opposite in the tensile case. The effect on dislocation nucleation energy is calculated. For miscut angles used normally, the step energy contribution exceeds that due to the vicinal substrate. The effect on epilayer tilting is also discussed.