Dislocation-free Islands Research Articles

Simulation of morphological evolution and crystallographic tilt in heteroepitaxial growth using phase-field crystal method

The phase-field crystal (PFC) model is employed to study the morphological evolution and the crystallographic tilt of heteroepitaxial growth on vicinal substrates. The results are as follows: for heterostructures with large misfit (ε > 0.08) the crystallographic tilt of epitaxial layer is approximately proportional to the substrate miscut angle, while the elastic strain energy of the film will lead to the nucleation of dislocation, which contributes to step-flow growth mode. As for the heterostructures with small misfit (ε < 0.04) the elastic strain energy will be released in the form of surface energy, and the surface profile of epitaxial film is dislocation-free island. When exposed to high undercooling, the substrate with large misfit and miscut angle will result in small-angle grain boundary between the substrate and the epitaxial layer. The small-angle grain boundary is composed of arranged dislocations, and it significantly changes the growth orientation of epitaxial layer.

Self-Assembled Islands in the (Ga,Al)As/InAs Heteroepitaxial System Studied by Raman Spectroscopy

Phonon spectra of self-assembled GaAs and AlAs nanometer-sized islands in an InAs matrix as well as InAs quantum dots embedded in AlAs were studied by Raman spectroscopy. Large strain-induced shifts of optical phonon lines of the islands from respective bulk values were observed. The experimental phonon frequencies are characteristic of coherently strained dislocation-free islands. In InAs quantum dots embedded in AlAs the position of optical phonon lines in the Raman spectra was observed to depend on the excitation energy. This behaviour is explained by the presence of smaller-sized dots in which the phonon confinement effect becomes significant. Doublets of folded acoustic phonons similar to those of planar superlattices are present in the low-frequency Raman spectra of multilayer structures with AlAs and GaAs islands embedded in InAs.

Self-organized quantum wires formed by elongated dislocation-free islands in (In,Ga)As/GaAs(100)

Long and fairly uniform quantum wire arrays have been fabricated by the growth of (In,Ga)As/GaAs multilayer structures. The structural properties of the quantum wires are characterized by atomic force microscopy, x-ray diffractometry, and transmission electron microscopy. The lateral carrier confinement in the quantum wires is confirmed by linear polarization dependent photoluminescence (PL) and magneto-PL measurements.

Misfit Dislocation Introduction During The Epitaxial Growth of Inas Islands on Gap

ABSTRACTThe initial growth of InAs on 11% lattice mismatched GaP substrates by molecular beam epitaxy was investigated. High resolution transmission electron microscopy (HREM) images showed that the InAs grew in the form of three-dimensional islands of dissimilar sizes. Mismatch induced strain relief was effected by the direct introduction of (mostly) edge dislocations at the corners of the islands. An examination of HREM images of several islands revealed that the island aspect ratio decreased with the introduction of misfit dislocations. Strain relaxation in the smaller, relatively dislocation-free islands occurred by elastic deformation of InAs lattice planes, which was more effective far from the constrained island-substrate interface. As a result, these islands grew taller and narrower, with a gradient in the elastic strain energy. However, a higher aspect ratio resulted in a higher surface area – to – volume ratio, and increased the surface energy of the InAs islands. Consequently, there was a driving force for the reduction of the aspect ratio if an alternate avenue for strain relaxation existed. The alternate route was plastic deformation by the introduction of misfit dislocations. As the island grew, the strain at the island corners increased, and beyond a critical value, misfit dislocations were added. These dislocations relieved strain at the heterointerface, and promoted the islands to grow laterally, i.e., the aspect ratio decreased. Islands coalesced, and a continuous layer resulted by a nominal thickness of 3 nm. Thus, the morphology of InAs islands grown on GaP was determined by the balance between elastic and plastic deformation.

Inas Nanostructures in a Silicon Matrix: Growth and Properties

AbstractUnder certain growth conditions InAs/Si heteroepitaxial growth proceeds via Stranski-Krastanow or Volmer-Weber growth modes depending on the growth parameters. Critical thickness at which three dimensional InAs islands start to appear at the Si(100) surface is within the range of 0.7 - 4.0 monolayers (substrate temperature range is 350°C - 430 °C). Their size depends critically on the growth conditions and is between 5 nm and 80 nm (uncapped islands). Critical lateral size of the coherent (Si capped) dislocation-free island is equal to 2 - 5 nm depending on the island height. Islands having larger size are dislocated. Optical properties of InAs nanoscale islands capped with Si reveal a luminescence band in the 1.3 μm region.

New method for the growth of highly uniform quantum dots

A new method is realized for the growth of self-formed quantum dots. We identify that dislocation-free islands can be formed by the strain from the strained superlattice taken as a whole. Unlike the Stranski–Krastanow (S–K) growth mode, the islands do not form during the growth of the corresponding strained single layers. Highly uniform quantum dots can be self-formed via this mechanism. The low temperature spectra of self-formed InGaAs/GaAs quantum dot superlattices grown on a (001) GaAs substrate have a full width at half maximum of 26–34 meV, indicating a better uniformity of quantum dot size than those grown in the S–K mode. This method can provide great degrees of freedom in designing possible quantum dot devices.

Alignment of Ge three-dimensional islands on faceted Si(001) surfaces

Arrays of Ge three-dimensional (3D) islands were grown on a Si(001) substrate by molecular beam epitaxy without any lithographic process. Dislocation-free islands 50 nm in diameter were aligned to the [11̄0] direction of a vicinal Si(001) substrate tilted 4° toward [110]. Surface undulations consisting of (001) and (11 x), x=8–10, facets with a 360-nm period were self-organized on the Si buffer layer. Ge 3D islands were then preferentially grown on the upper edge of the (001) facets and were arranged in line accordingly. Nucleation of Ge islands on the surface at atomic-layer steps and deformation of near-surface layers in the Si substrate induced by misfit strain are strongly related to the self-alignment of the islands.

Minimum energy configuration of epitaxial material clusters on a lattice-mismatched substrate

Strained epitaxial material clusters or islands grown on a relatively thick substrate are considered in the context of continuum mechanics. The islands and the substrate are modeled as isotropic elastic solids with similar material properties. The geometry is assumed to be two-dimensional. Under the assumption that the total free energy of the system consists of the energy of the free surface and the strain energy, minimum energy profiles of dislocation-free islands are calculated. It is shown that even though the surface energy contribution to the free energy favors a flat film morphology, an islanded morphology is the preferred growth mode of a strained film with a sufficiently large lattice mismatch. The coalescence of two islands which are relatively close to each other is also considered. It is demonstrated that the free energy of the system is reduced through the coalescence of islands and that coalescence may occur spontaneously. Minimum energy profiles of islands containing interface misfit dislocations are also calculated. Sufficiently large dislocated islands are shown to have a smaller height-to-width aspect ratio and a lower surface chemical potential at equilibrium than dislocation-free equilibrium islands of the same size. This result is used to explain qualitatively how the nucleation of a dislocation can affect the growth characteristics of an island.

In situ RHEED control of direct MBE growth of Ge quantum dots on Si (0 0 1)

Abstract RHEED studies of the morphological transformation of germanium layers during their MBE growth on the Si(0 0 1) surface were carried out. Zero-streak profiles analysis were used to control the formation of coherent and dislocated germanium islands at the film thickness of more than 5 and 8 monolayers (ML), respectively. The dislocation-free islands formed at the first stage (4–8 ML) of the morphological transformation were employed for designing the structures with the self-organized Ge quantum dots. The effects of charge and size quantization of hole transport, such as the Coulomb blockade and the resonance hole tunneling through the discrete energy levels in the quantum dots were observed in these structures.

Growth mechanisms of SiGe on (111) and (100) Si substrates

We present an experimental analysis of strain relaxation in SiGe/Si heterostructures which is strongly dependent on the Si substrate orientation. In particular, different driving forces for the transition from 2D towards 3D growth were evidenced. On (100) Si substrates the primary driving force for SiGe islanding is the ability to elastically relax strain on the top of the islands. At low elastic energy a small corrugation is initiated at the free surface of the SiGe overlayer which is known to lower the energy of the system. The amplitude of this corrugation increases with the accumulated elastic energy up to the formation of dislocation-free islands. The latter kinetically evolve towards plastically relaxed islands. In the same experimental conditions, the strain is at once relaxed by nucleation of dislocations on (111) Si. Dislocation-free islands were not observed in the latter case. Continuous dislocated films are formed up to compressive stresses of about −0.17 GPa (equivalent to a misfit between the SiGe layer and the Si substrate of ≈3%). At higher misfit energy, islands are initiated by the ability of introducing stacking faults on the edges of the islands. The study of defect nucleation processes does not permit one to explain the nucleation of misfit dislocations at lower stresses on (111) Si than on (100) Si. Based on the existing models we suggest two possible mechanisms able to explain the observed discrepancies.

A method to tune the island size and improve the uniformity for the in situ formation of InGaAs quantum dots on GaAs

The use of island formation during the epitaxy of In x Ga 1− x As on GaAs can be applied to the in situ formation of quantum dots. Adjusting the alloy composition toward a larger degree of cation disorder (Ga and In intermixing) can decrease the island size and improve the island uniformity simultaneously. Under the same deposition conditions with a thickness of 15 monolayers, the dot size can be tuned from 125 × 100 to 30 × 28 nm 2 in lateral dimensions as the In composition changes from 1 to 0.5. Among all conditions, In 0.5Ga 0.5As dots show a high density of 5 × 10 10 cm −2 and exhibit the best uniformity. These small-sized and dislocation-free islands are of interest for the formation of high-quality quantum dots.

Direct formation of quantum-sized dots from uniform coherent islands of InGaAs on GaAs surfaces

The 2D–3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum-sized dot structures. Transmission electron micrographs reveal that when the growth of In0.5Ga0.5As is interrupted exactly at the onset of this 2D–3D transition, dislocation-free islands (dots) of the InGaAs result. Size distributions indicate that these dots are ∼300 Å in diameter and remarkably uniform to within 10% of this average size. The areal dot densities can be varied between 109 and 1011 cm−2. The uniformity of the dot sizes is explained by a mechanism based on reduction in adatom attachment probabilities due to strain. We unambiguously demonstrate photoluminescence at ∼1.2 eV from these islands by comparing samples with and without dots. The luminescent intensities of the dots are greater than or equal to those of the underlying reference quantum wells.

Shape transition in growth of strained islands: Spontaneous formation of quantum wires.

Strained epitaxial layers tend initially to grow as dislocation-free islands. Here we show that such islands, as they increase in size, may undergo a shape transition. Below a critical size, islands have a compact, symmetric shape. But at larger sizes, they adopt a long thin shape, which allows better elastic relaxation of the island's stress. We have observed such elongated islands, with aspect ratios greater than 50:1, in low energy electron microscopy studies of growth of Ag on Si (001). These islands represent a novel approach to the fabrication of «quantum wires»

Microstructural evolution during the heteroepitaxy of Ge on vicinal Si(100)

Microstructural evolution during the initial stages of islanding of Ge on vicinal Si(100) has been studied in situ with nanometer resolution in an ultrahigh-vacuum scanning transmission electron microscope. Ge is deposited using molecular-beam-epitaxy (MBE) techniques on vicinal Si(100) misoriented 1° and 5° toward 〈110〉. For MBE-type experiments, there is evidence for metastable growth of the Ge intermediate layer to much greater than the equilibrium critical thickness. The layer may grow up to seven monolayers thick before islanding in the Stranski–Krastanov growth mode. The presence of strong adatom sinks significantly alters the growth and size distribution of the islands when the spacing of these sinks is less than an adatom diffusion distance. Studies of the initial stages of islanding in solid-phase MBE indicate that there is no long-range adatom diffusion. There is an initial fast transformation from a disordered layer growth, followed by a sluggish growth of islands. We have studied the coarsening of these islands at the earliest stages with sensitivity to islands as small as 2 nm in radius. It appears that there is a novel coarsening mechanism influenced by an unstable intermediate layer and dislocation-free islands. In all cases, the dislocation-free islands grow more slowly than those which have relaxed by the introduction of misfit dislocations.

Island Formation in Ge on Si Heteroepitaxy

ABSTRACTWe present a study of island formation (the transition from 2D to 3D growth) during the Stranski-Krastanow (S-K) growth of Ge on Si. The energetic driving force for S-K island formation should be the ability to relax the islands by dislocation introduction. Here, we show that Ge islands formed on Si (100) are initiallydislocation-free, in the presence of a 2D “sea” that is far in excess of the equilibrium 3 monolayers (ML). We call this phase of growth “dislocation-free S-K”. The 2-D sea does not collapse until dislocated islands are produced at an average coverage of = 7 ML. We call the dislocation-free island phase “coherent S-K” growth. The corresponding 2D-3D transition on Si (111) appears to reach equilibrium far faster, and we have not observed dislocation-free island formation: dislocated islands are seen at = 5 ML. As expected, the kinetics allow us to suppress island formation on (100) by reducing the growth temperature. These thick 2D films are analogous to those grown on As-covered surfaces, but have a microstructure dominated by edge dislocations.