Wetting Layer Thickness Research Articles

Common features of nanostructure formation induced by the surface undulation on the Stranski-Krastanow systems

The nanostructure formation effected by the surface undulation on the Stranski-Krastanow (SK) systems during the annealing process is investigated by simulating the morphological evolution of the systems driven by surface diffusion. The results reveal three common features in the formation process. First, the essence of the nanostructure formation is the transformation of the film material above the wetting layer into faceted islands. Second, the wetting layer thickness is given by the first critical thickness for the SK transition. Third, the width of the nanostructures measured at the average film height is close to a constant during the formation process.

Control of the nematic-isotropic phase transition by an electric field

We use a relatively simple continuum model to investigate the effects of dielectric inhomogeneity within confined liquid-crystal cells. Specifically, we consider, in planar, cylindrical, and spherical geometries, the stability of a nematic-isotropic interface subject to an applied voltage when the nematic liquid crystal has a positive dielectric anisotropy. Depending on the magnitude of this voltage, the temperature, and the geometry of the cell, the nematic region may shrink until the material is completely isotropic within the cell, grow until the nematic phase fills the cell, or, in certain geometries, coexist with the isotropic phase. For planar geometry, no coexistence is found, but we are able to give analytical expressions for the critical voltage for an electric-field-induced phase transition as well as the critical wetting layer thickness for arbitrary applied voltage. In cells with cylindrical and spherical geometries, however, locally stable nematic-isotropic coexistence is predicted, the thickness of the nematic region being controllable by alteration of the applied voltage.

Effect of wetting layer on electron–hole correlation in quantum discs and rings

We propose a simple method for calculating the ground state wavefunction of anelectron–hole pair confined in a heterostructure formed by a flat quantum dotdeposited on a thin wetting layer and imbedded in a matrix made of other material.The calculations of the exciton ground state energy, the electron–hole space paircorrelation function and the density of the charge distribution have been performed forIn0.55Al0.45As–Al0.35Ga0.65As and Ga0.7Al0.3As/GaAs quantum discs and rings. It is shown that the increase of the wetting layer thickness andthe strength of the external magnetic field applied parallel to the axis lead to a lowering ofthe inner barrier height reinforcing the tunnelling of the particles into the central holeregion. We also analyse the charge distribution related to the spatial separation of theparticles due to the difference between masses of the electron and the hole. The variation ofthe quadrupole momentum sign with the increase of the ring inner radius and theappearance of the dipole momentum oriented in the crystal growth direction with theincrease of the wetting layer thickness in the presence of the electron–hole pair arepredicted.

Mechanism of the nanoscale localization of Ge quantum dot nucleation on focused ionbeam templated Si(001) surfaces

We investigate the fundamental mechanism by which self-assembled Ge islands can benucleated at specific sites on Si(001) using ultra-low-dose focused ion beam (FIB)pre-patterning. Island nucleation is controlled by a nanotopography that forms after theimplantation of Ga ions during subsequent thermal annealing of the substrate. Thisnanotopography evolves during the annealing stage, changing from a nanoscale annulardepression associated with each focused ion beam spot to a nanoscale pit, and eventuallydisappearing (planarizing). The correspondence of Ge quantum dot nucleationsites to the focused ion beam features requires a growth surface upon which thenanotopography is preserved. A further key observation is that the Ge wetting layerthickness is reduced in patterned regions, allowing the formation of islands on thetemplated regions without nucleation elsewhere. These results provide routes to thegreatly enhanced design and control of quantum dot distributions and dimensions.

Finite element analysis of stress and strain distributions in InAs/GaAs quantum dots

In this paper, we perform systematic calculations of the stress and strain distributions in InAs/GaAs truncated pyramidal quantum dots (QDs) with different wetting layer (WL) thickness, using the finite element method (FEM). The stresses and strains are concentrated at the boundaries of the WL and QDs, are reduced gradually from the boundaries to the interior, and tend to a uniform state for the positions away from the boundaries. The maximal strain energy density occurs at the vicinity of the interface between the WL and the substrate. The stresses, strains and released strain energy are reduced gradually with increasing WL thickness. The above results show that a critical WL thickness may exist, and the stress and strain distributions can make the growth of QDs a growth of strained three-dimensional island when the WL thickness is above the critical value, and FEM can be applied to investigate such nanosystems, QDs, and the relevant results are supported by the experiments.

Signatures of non locality for short-ranged wetting at curved substrates

The binding potential for wetting near planes, spheres, and cylinders in systems with short-ranged forces is shown to have a universal geometrical structure. This arises from the nonlocal nature of the interfacial interactions and is exactly described by a recently proposed binding potential functional, which provides a systematic framework for studying wetting at arbitrarily shaped substrates. The corrections to the equilibrium wetting layer thickness induced by nonlocality are comparable to those arising from a Tolman length and lead to diverging terms in the total mass adsorption.

Growth of InN quantum dots on N-polarity GaN by molecular-beam epitaxy

We investigated the growth behaviors of InN quantum dots (QDs) on N-polarity GaN by molecular-beam epitaxy. The N-polarity growth has been intentionally used to raise the temperature to facilitate formation of high-quality dots. It was found that the InN QDs could be grown up to 550°C the Stranski–Kastanov growth mode with the wetting layer thickness of about 1 monolayer, which was confirmed by the simultaneous in situ observations of reflection high-energy electron diffraction and spectroscopic ellipsometry. The density and the diameter of typical InN QDs grown at 450–550°C were the order of 1011cm−2 and 15–20nm, respectively.

Dark field transmission electron microscope images of III–V quantum dot structures

Multi-layer quantum dot structures are becoming increasingly common in order to improve the efficiency of quantum dot lasers. Each layer of dots may be influenced by the preceding dot layer, and the dot density can vary from layer to layer. Characterization of such structures relies on the reliable determination of the shape, size and density of dots in each layer. Dark field transmission electron microscopy (TEM) images using the 002 diffraction condition are frequently used, viewing the layers edge-on in a cross-section sample. A simple model is used to describe the contrast as a function of dot size and shape, specimen thickness, and the composition of the dot and surrounding materials. Good agreement with experimental results is obtained. It is found that the dot size is not accurately related to the bright region seen in such images. While 002 images can be used to determine the size and shape of dots, a density per unit area cannot be calculated in the cross section geometry without either measuring—or assuming—the specimen thickness. In multilayer structures, plan-view TEM images show the layers as overlapping, losing the information from individual layers. By tilting a cross-section specimen to allow imaging with the dark field 113 diffraction condition, the density in individual layers can be measured. Additional information, such as wetting layer thickness variations and alignment of dots due to surface roughness or substrate offcut, can also be obtained.

Growth and photoluminescence of self-assembled islands obtained during the deposition of Ge on a strained SiGe layer

The growth and photoluminescence of Ge(Si) self-assembled islands on strained Si 1-x Ge x layers (0 < x < 20%) has been investigated. Both island size and density increase when the Ge content in the predeposited Si 1-x Ge x alloy increases. The increased island density is associated with an enhanced surface roughness after SiGe deposition. The increased island size is attributed to enhanced Si intermixing and to a wetting layer thickness reduction. The latter is caused by accumulation of elastic strain energy in the SiGe layer. The increase in both island size and density leads to a strong interaction between neighboring islands. The island-island interaction results in island self-ordering, which is analyzed using a surface autocorrelation function. The lower Ge content in the islands accounts for the observed blueshift of the island related photoluminescence signal. We observe a room temperature photoluminescence signal for Ge islands grown on a predeposited SiGe alloy which is higher than for Ge islands grown on Si(001). This result is explained by an increased island density, which provides an efficient way to capture charge carriers in the islands.

Real-time analysis of wetting-layer evolution and island nucleation using spectroscopic ellipsometry with Tauc–Lorentz parametrization

Real-time spectroscopic ellipsometry was used to monitor the nucleation behavior of Ge grown on H-terminated Si(100) at 500 °C and H-terminated Si(111) with <0.3° and ∼4° miscut angles at 570 °C. To model the dielectric functions of layers with space-separated 3D islands the Tauc–Lorentz parametrization (TLP) was used. This parametrization was found to be much better than the generally used Brüggemann effective media approximation. It is shown that the wetting layer (WL) in the system of Ge on flat Si(111) is of nearly constant thickness during island nucleation, whereas the nucleation of Ge islands on stepped Si(111) and Si(100) is accompanied by a strong decrease of the wetting-layer thickness. It is concluded that in the first case 3D islands grow at the expense of atoms adsorbed on the wetting layer surface, whereas in the latter cases 3D islands grow at the expense of atoms of the wetting layer itself, which corresponds to stress-driven nucleation.

Kinetics of the initial stage of coherent island formation in heteroepitaxial systems

The stress-driven formation of coherent islands in heteroepitaxial systems is studied within the frame of the kinetic theory of nucleation under dynamical conditions. The kinetic model for a description of the time evolution of the island size distribution and wetting layer thickness in Stranski-Krastanow growth mode is developed. The time scale hierarchy of the nucleation stage, the size relaxation stage, and the stage of critical wetting layer formation enable us to construct analytical solutions to the model kinetic equations. The time dependence of the lateral size distribution, wetting layer thickness, and other characteristics of the island formation process are calculated. Analytical expressions for the substrate temperature and growth rate dependence of the mean lateral size and surface density of islands are obtained and analyzed.

Quantum dots in InAs layers of subcritical thickness on GaAs(100)

Ensembles of InAs quantum dots formed at the GaAs(100) surface have been studied by the methods of reflection high-energy electron diffraction and photoluminescence. The amount of deposited InAs corresponds to a wetting layer thickness smaller than the critical value necessary for the transition from two-to three-dimensional growth. It is experimentally shown that, at a deposited film thickness of 1.5 and 1.6 monolayers, islands are formed after keeping the sample in a flow of As4. The influence of the substrate temperature on the kinetic characteristics of the formation of InAs/GaAs islands has been studied.

Theoretical and experimental study of the effect of InAs growth rate on the properties of QD arrays in InAs/GaAs system

The dependence of properties of quantum dot (QD) arrays in an InAs/GaAs system on the InAs growth rate has been investigated theoretically and experimentally. The derived kinetic model of the formation of coherent nanoislands allows the calculation of the average size, surface density of islands, and wetting layer thickness as functions of the growth time and conditions. Optical properties of InAs/GaAs QDs have been studied for the case of two monolayers (ML) of the material deposited at different growth rates. Predictions of the theoretical model are compared with the experimental data. It is shown that with two ML of the deposited material the characteristic lateral size of QDs decreases and the thickness of the residual wetting layer increases with rising growth rate.

Photoluminescence of self‐assembled Ge islands grown on Si by MBE at low temperatures

The optical properties (photoluminescence and the Raman spectrum) as well as structural characterization (by atomic force microscopy and high-resolution transmission electron microscopy) of self-assembled Ge islands grown at low temperatures (Tg ≤ 350 °C) are systematically studied in connection with Ge-layer thickness. With growth-temperature reduction, the sizes of the islands decrease with increase of their density. The wetting layer thickness was shown to be reduced up to 2–3 monolayers and is the same at the growth temperatures Tg = 250 °C and 300 °C.

Positioning of self-assembled Ge islands on stripe-patterned Si(001) substrates

Self-assembled Ge islands were grown on stripe-patterned Si(001) substrates by solid source molecular beam epitaxy. The surface morphology obtained by atomic force microscopy and cross-sectional transmission electron microscopy images shows that the Ge islands are preferentially grown at the sidewalls of pure Si stripes along the [−110] direction at 650 °C or along the trenches, whereas most of the Ge islands are formed on the top terrace when the patterned stripes are covered by a strained GeSi buffer layer. Reducing the growth temperature to 600 °C results in a nucleation of Ge islands both on the top terrace and at the sidewall of pure Si stripes. A qualitative analysis, based on the growth kinetics, demonstrates that the step structure of the stripes, the external strain field, and the local critical wetting layer thickness for the islands formation contribute to the preferential positioning of Ge islands on the stripes.

Modified droplet epitaxy GaAs/AlGaAs quantum dots grown on a variable thickness wetting layer

We show that the use of modified droplet epitaxy allows to tune the wetting layer thickness in GaAs quantum dot structures. Morphological observations demonstrate that the wetting layer at the base of the dots can be controlled or even removed by changing the surface stoichiometry of substrates before droplet formations. Spectroscopical measurements show that the variation of the wetting layer thickness strongly influences the optical properties of the dots. The experimental transition energies of the dots well agree with a theoretical model based on effective mass approximation.

Elastic Fields of Quantum Dots in Multilayered Semiconductors: A Novel Green’s Function Approach

We present an efficient and accurate continuum-mechanics approach to predict the elastic fields in multilayered semiconductors due to buried quantum dots (QDs). Our approach is based on a novel Green’s function solution in anisotropic and linearly elastic multilayers, derived within the framework of generalized Stroh formalism and Fourier transforms, in conjunction with the Betti’s reciprocal theorem. By using this approach, the induced elastic fields due to QDs with general misfit strains are expressed as a volume integral over the QDs domains. For QDs with uniform misfit strains, the volume integral involved is reduced to a surface integral over the QDs boundaries. Further, for QDs that can be modeled as point sources, the induced elastic fields are then derived as a sum of the point-force Green’s functions. In the last case, the solution of the QD-induced elastic field is analytical, involving no numerical integration, except for the evaluation of the Green’s functions. As numerical examples, we have studied a multilayered semiconductor system of QDs made of alternating GaAs-spacer and InAs-wetting layers on a GaAs substrate, plus a freshly deposited InAs-wetting layer on the top. The effects of vertical and horizontal arrays of QDs and of thickness of the top wetting layer on the QD-induced elastic fields are examined and some new features are observed that may be of interest to the designers of semiconductor QD superlattices.

Optical phonon localization in self-assembled Ge islands

We present a Raman scattering study for self-organized Ge islands on Si substrate carried out at 10 K . By means of difference Raman spectroscopy technique, we have separated the Raman signals from the Ge islands and Si substrate. The wetting layer thickness and strain were estimated from the line width and frequency peak. The estimated wetting layer thickness values are comparative with the Ge dot height obtained from microscopy measurements. The strain is decreased with an increase of the thickness.

Raman spectroscopy of self-assembled Ge islands on Si

We present a Raman scattering study for self-organized Ge dots on Si substrate. By means of difference Raman spectroscopy technique, we have separated the Raman signals from the Ge islands and Si substrate. The wetting layer thickness and strain were estimated from the line width and peak frequency. The estimated wetting layer thickness values are comparative with the Ge dot height obtained from microscopy measurements. The strain is decreased with an increase of the thickness.

Surface-directed phase separation with off-critical composition: analytical and numerical results.

We study the interplay of wetting and phase separation in an unstable binary mixture (AB) with off-critical composition, placed in contact with a surface which prefers the component A. We consider surface potentials V(z) approximately z(-n), where z is the distance from the surface, and present analytical arguments and detailed numerical results to elucidate wetting-layer kinetics for arbitrary mixture compositions. If the preferred component is the minority phase, the wetting-layer thickness exhibits a potential-specific behavior at early times tau, R1 approximately tau(1/(n+2)), before crossing over to the universal growth law, R1 approximately tau(1/3). On the other hand, if the preferred component is the majority phase, there is a crossover from potential-specific growth (as before) to a slower growth regime.