Formation Of 3D Islands Research Articles

Kinetic Monte Carlo simulations of quantum dot self-assembly

In the Stranski–Krastanov growth mode for heteroepitaxial systems, layer-by-layer growth is followed by the formation and growth of three-dimensional (3D) islands. In this paper, we use a kinetic Monte Carlo method to simulate this growth mode behavior. We present a detailed and systematic investigation into the effects of key model parameters including strain, growth temperature, and deposition rate on this phenomenon. We show that increasing the strain lowers the apparent critical thickness that is defined by the onset of 3D island formation. Similarly, increasing the growth temperature lowers the apparent critical thickness, until intermixing, and the resulting relevance of entropic contributions, become more significant. We also report the impact on Stranski–Krastanov growth of more model-specific parameters, such as bond strengths between constituent atoms of the system, and surface energy anisotropies.

Oxidation of a Platinum–Tin Alloy Surface during Catalytic CO Oxidation

We have investigated the surface composition of a well-ordered Pt3Sn(111) surface during CO oxidation with ambient pressure X-ray photoemission spectroscopy. Oxidation of tin in the surface coincides with the onset of catalytic conversion, and we observe significant differences in the oxidation state and morphology of the oxide formed depending on the gas composition, with an oxygen-rich mixture leading to formation of 2D wetting layers and a CO-rich mixture leading to formation of 3D oxide islands. Spontaneous oscillations in conversion are observed at 300 °C in the oxygen-rich gas mixture and attributed to the combined effects of site blocking by tin oxides and by CO. The results highlight the importance of gas–surface interactions in determining the nature of oxides formed and thus the type and number of interfacial sites under reaction conditions.

In situ reflection electron microscopy for investigation of surface processes on Bi2Se3(0001)

The sublimation and van der Waals (vdW) epitaxy on Bi2Se3(0001) surface have been first visualized using in situ reflection electron microscopy. When Bi2Se3(0001) surface was exposed to a Se molecular beam (up to 0.1 nm/s) and heated to ∼400°C, we observed ascending motion of atomic steps corresponding to congruent Bi2Se3 sublimation. During the sublimation, grooves made by probe lithography act as sources of atomic steps: groove depth increases and generates atomic steps that move in the ascending direction away from the source. We used this phenomenon to create self-organized regularly-spaced zigzag atomic steps having 1 nm height on the Bi2Se3(0001) surface. The deposition of Bi (up to ∼0.01 nm/s) onto the Bi2Se3(0001) surface at constant Se flux (up to ∼0.1 nm/s) reversed the direction of the step flow, and vdW epitaxy was observed. The deposition of In and Se onto the Bi2Se3(0001) surface at ∼400°C led to the epitaxial growth of layered In2Se3. This vdW heteroepitaxy started with 2D island nucleation and, after 3–5 nm growth, continued with a screw-dislocation-driven formation of 3D islands. Ex situ Raman scattering measurements have shown that the grown 20-nm-thick In2Se3 film exhibits vibrational modes that originate from the β-In2Se3 crystal phase.

Metal Microsubstrates on Si(111): Crystallography, Morphology, and Interactions with a Molecular Adsorbate

We describe the growth and characterization of 10 nm to 10 micron-scale islands created by depositing Ag onto Si(111) at elevated temperatures. By varying the surface coverage of Ag, we are able to produce and control the distribution of Ag(111) and/or Ag(100) single-crystal islands. These were examined using scanning tunneling microscopy (STM), X-ray diffraction (XRD), helium ion microscopy (HIM), and atomic force microscopy. The formation of 3D islands with orientations of either (111) or (100) was confirmed by STM and XRD analysis. As these islands manifest as tiny single crystals of elemental Ag, they can be ideally used as low-cost or disposable substrates to host molecular architectures; we demonstrate this by comparing absorption and thermal behavior of 1,3,5-benzenetricarboxylic acid molecules on the islands and noting identical behavior and reactivity to experiments performed on Ag single-crystal counterparts. These islands provide an economical and practical method for facilitating molecular adsorption on two distinct microsubstrates at the same time while under identical experimental conditions.

Improvement of GaN crystalline quality by SiNx layer grown by MOVPE

In this work the mechanism which helps to reduce the dislocation density by deposition of a SiNx interlayer is discussed. It is shown that the dislocation reduction by SiNx interlayer deposition is influenced by dislocation density in the underlying GaN layers. The SiNx interlayer is very effective when the original dislocation density is high, while in the case of lower dislocation density the deposition of SiNx is not effective for crystal quality improvement. Although it is widely accepted that SiNx serves as a barrier for dislocation propagation, similarly to the enhanced lateral overgrowth method, it is shown that after masking the SiNx deposition cannot be the dominant dislocation reduction mechanism. The most probable mechanism is the annihilation of bended neighbouring dislocations during the coalescence of 3D islands. The SiNx layer cannot serve as a barrier for dislocations, since it is probably dissolved during the following GaN growth and dissolved Si atoms are incorporated into the above-grown GaN layer which stimulates the 3D island formation. Then the use of the SiNx interlayer for dislocation reduction is recommended only for the improvement of layers with a high dislocation density. On the other hand, the PL signal was strongly enhanced for both low and high dislocation density structures with the SiNx interlayer, suggesting that the interlayer might help to suppress the nonradiative recombination in subsequent GaN that is not related to the dislocation density, which remained the same. But its origin has to be studied further.

Bismuth-surfactant-induced growth and structure of InAs/GaAs(110) quantum dots

We explore the Bi-surfactant-directed self-assembly and structure of InAs quantum dots (QDs) grown on GaAs(110) by molecular beam epitaxy. The addition of a Bi flux during InAs deposition changes the InAs growth mode from two-dimensional (2D) Frank–van der Merwe to Stranski–Krastanov (SK), resulting in the formation of three-dimensional (3D) InAs islands on the surface. Furthermore, exposing static InAs 2D layers to Bi induces a rearrangement of the strained layer into 3D islands. We explore the effect of varying the InAs thickness and Bi flux for these two growth approaches, observing a critical thickness for 3D island formation in both cases. Characterization of (110) InAs QDs with high-resolution transmission electron microscopy reveals that larger islands grown by the SK mode are plastically relaxed, while small islands grown by the on-demand approach are coherent. Strain relaxation along the [] direction is achieved by 90° pure-edge dislocations with dislocation lines running along [001]. In contrast, strain relief along [001] is by 60° misfit dislocations. This behavior is consistent with observations of planar (In, Ga)As/GaAs(110) layers. These results illustrate how surfactant Bi can provoke and control QD formation where it normally does not occur.

Formation and morphological evolution of self-similar 3D nanostructures on weakly interacting substrates

Vapor condensation on weakly interacting substrates leads to the formation of three-dimensional (3D) nanoscale islands (i.e., nanostructures). While it is widely accepted that this process is driven by minimization of the total film/substrate surface and interface energy, current film-growth theory cannot fully explain the atomic-scale mechanisms and pathways by which 3D island formation and morphological evolution occurs. Here, we use kinetic Monte Carlo simulations to describe the dynamic evolution of single-island shapes during deposition of Ag on weakly interacting substrates. The results show that 3D island shapes evolve in a self-similar manner, exhibiting a constant height-to-radius aspect ratio, which is a function of the growth temperature. Furthermore, our results reveal the following chain of atomic-scale events that lead to compact 3D island shapes: 3D nuclei are first formed due to facile adatom ascent at single-layer island steps, followed by the development of sidewall facets bounding the islands, which in turn facilitates upward diffusion from the base to the top of the islands. The limiting atomic process which determines the island height, for a given number of deposited atoms, is the temperature-dependent rate at which adatoms cross from sidewall facets to the island top. The overall findings of this study provide insights into the directed growth of metal nanostructures with controlled shapes on weakly interacting substrates, including two-dimensional crystals, for use in catalytic and nanoelectronic applications.

Quantum Dot Self-Assembly Driven by a Surfactant-Induced Morphological Instability.

In strained heteroepitaxy, two-dimensional layers can exhibit a critical thickness at which three-dimensional islands self-assemble, relieving misfit strain at the cost of an increased surface area. Here we show that such a morphological phase transition can be induced on demand using surfactants. We explore Bi as a surfactant in the growth of InAs on GaAs(110), and find that the presence of surface Bi induces Stranski-Krastanov growth of 3D islands, while growth without Bi always favors 2D layer formation. Exposing a static two monolayer thick InAs layer to Bi rapidly transforms the layer into 3D islands. Density functional theory calculations reveal that Bi as well as Sb reduce the energetic cost of 3D island formation by modifying surface energies. These 3D nanostructures behave as optically active quantum dots. This work illustrates how surfactants can enable quantum dot self-assembly where it otherwise would not occur.

Self-Assembly of InAs Nanostructures on the Sidewalls of GaAs Nanowires Directed by a Bi Surfactant.

Surface energies play a dominant role in the self-assembly of three-dimensional (3D) nanostructures. In this Letter, we show that using surfactants to modify surface energies can provide a means to externally control nanostructure self-assembly, enabling the synthesis of novel hierarchical nanostructures. We explore Bi as a surfactant in the growth of InAs on the {11̅0} sidewall facets of GaAs nanowires. The presence of surface Bi induces the formation of InAs 3D islands by a process resembling the Stranski-Krastanov mechanism, which does not occur in the absence of Bi on these surfaces. The InAs 3D islands nucleate at the corners of the {11̅0} facets above a critical shell thickness and then elongate along ⟨110⟩ directions in the plane of the nanowire sidewalls. Exploiting this growth mechanism, we realize a series of novel hierarchical nanostructures, ranging from InAs quantum dots on single {11̅0} nanowire facets to zigzag-shaped nanorings completely encircling nanowire cores. Photoluminescence spectroscopy and cathodoluminescence spectral line scans reveal that small surfactant-induced InAs 3D islands behave as optically active quantum dots. This work illustrates how surfactants can provide an unprecedented level of external control over nanostructure self-assembly.

Performance improvement of highly mismatched GaSb layers on GaAs by interfacial-treatment-assisted chemical vapor deposition

Strain-relieved GaSb quantum dots on GaAs can be achieved by either periodic interfacial misfit (IMF) or the conventional Stranski–Krastanov (SK) growth modes by changing the growth parameters. In this study, the Sb interfacial treatment was employed to improve the GaSb crystal quality including low defect density, smooth surface morphology, and high hole mobility. This technique yields two-dimensional (2D) islands with a height as low as 1.7 nm and width up to 190 nm in the IMF growth mode. In contrast to the interfacial treatments conventionally employed in the initial strain relaxation of GaSb/GaAs hererostructure, the Sb treatment promotes the formation of strong Ga-Sb bonds on the surface of the grown island, which effectively reduces the interfacial free energy and thus promotes the formation of 2D islands. With the Sb interfacial treatment, a high-relaxation 100-nm GaSb epilayer was grown on the GaAs substrate, the epilayers was strain relaxed and exhibited enhanced electrical properties with a high hole mobility of ~667 cm2 V−1 s−1 and with superior optical properties as evidenced by the photoluminescence B-line peak. The results of this study demonstrate an effective interfacial-treatment growth technique to relax the initial strain for the highly mismatched GaSb layers grown on a GaAs substrate.

Strain-induced growth instability and nanoscale surface patterning in perovskite thin films.

Despite extensive studies on the effects of epitaxial strain on the evolution of the lattice and properties of materials, considerably less work has explored the impact of strain on growth dynamics. In this work, we demonstrate a growth-mode transition from 2D-step flow to self-organized, nanoscale 3D-island formation in PbZr0.2Ti0.8O3/SrRuO3/SrTiO3 (001) heterostructures as the kinetics of the growth process respond to the evolution of strain. With increasing heterostructure thickness and misfit dislocation formation at the buried interface, a periodic, modulated strain field is generated that alters the adatom binding energy and, in turn, leads to a kinetic instability that drives a transition from 2D growth to ordered, 3D-island formation. The results suggest that the periodically varying binding energy can lead to inhomogeneous adsorption kinetics causing preferential growth at certain sites. This, in conjunction with the presence of an Ehrlich-Schwoebel barrier, gives rise to long-range, periodically-ordered arrays of so-called “wedding cake” 3D nanostructures which self-assemble along the [100] and [010].

Investigation of the growth mechanism of SrRuO3 thin films fabricated by pulsed laser deposition

SrRuO3 thin films were prepared on (100) SrTiO3 substrates by pulsed laser deposition (PLD), and the dependences of film morphology and growth mode on the defocusing distance, repetition, oxygen pressure and substrate temperature were systematically investigated. Our results suggest that the growth mode transforms from three-dimensional (3D) island formation to two-dimensional (2D) layered growth as the defocusing distance increases. Under high laser repetition, adatoms do not have enough time to diffuse to step edges to promote step advancement; consequently, adatoms aggregate to form islands. The effective diffusivity was estimated as D* = 5 × 104 nm2/s, which represents the average diffusion including adatoms and small clusters. Decreasing oxygen pressure leads to a change in growth mode from 2D layered to 3D island formation. The X-ray diffraction (XRD) patterns show that under low oxygen pressure, the (00l) peaks disappear when l is odd, indicating that oxygen vacancies are preferentially produced in the SrO plane. With decreasing substrate temperature, the growth mode changes from step flow to multilayered growth. The relationship between step width and substrate temperature was investigated, and an activation energy of Ea = 0.8 eV was obtained by fitting.

Domain switching kinetics in ferroelectric-resistive BiFeO3 thin film memories

We fabricated (00l) BiFeO3 (BFO) thin films in different growth modes on SrRuO3/SrTiO3 substrates using a pulsed laser deposition technique. X-ray diffraction patterns show an out-of-plane lattice constant of 4.03 Å and ferroelectric polarization of 82 µC/cm2 for the BFO thin film in a layer-by-layer growth mode (2D-BFO), larger than 3.96 Å and 51 µC/cm2 for the thin film in the 3D-island formation growth mode (3D-BFO). The 2D-BFO thin film at 300 K shows switchable on/off diode currents upon polarization flipping near a negative coercive voltage, which is nevertheless absent from the above 3D-BFO thin film. From a positive-up–negative-down pulse characterization technique, we measured domain switching current transients as well as polarization–voltage (Pf–Vf) hysteresis loops in both semiconducting thin films. Pf–Vf hysteresis loops after 1 µs-retention time show the preferred domain orientation pointing to bottom electrodes in a 3D-BFO thin film. The poor retention of the domains pointing to top electrodes can be improved considerably in a 2D-BFO thin film. From these measurements, we extracted domain switching time dependence of coercive voltage at temperatures of 78–300 K. From these dependences, we found coercive voltages in semiconducting ferroelectric thin films much higher than those in insulating thin films, disobeying the traditional Merz equation. Finally, an equivalent resistance model in description of free-carrier compensation of the front domain boundary charge is developed to interpret this difference. This equivalent resistance can be coincidently extracted either from domain switching time dependence of coercive voltage or from applied voltage dependence of domain switching current, which drops almost linearly with the temperature until down to 0 in a ferroelectric insulator at 78 K.

Impurity accumulation in an adsorption layer during MBE doping

A theoretical model of capture and surface segregation of an impurity during MBE doping is proposed. It is assumed that an impurity is incorporated in the surface layer of a crystal doped due to both the blocking of the impurity atom by host atoms in kinks on steps and the exchange between the impurity atoms adsorbed and the host atoms in the surface layer. The surface segregation is considered as an accumulation of an impurity in the adsorption layer due to the jumps of impurity atoms adsorbed over the steps and the migration of impurity atoms from the surface layer into the adsorption layer. It is shown that an increase in the supersaturation near a step with a drop in the temperature and an increase in the growth rate suppresses the surface segregation, due to the more effective impurity blocking in the kinks. The formation of 2D islands on the terraces and nonequilibrium kinks at the island edges results in a partial drop of the supersaturation and weakening of the temperature and growth rate effects on the surface segregation. When impurity atoms intensively migrate from the surface layer to the adsorption one, the blurring of the doping profile sharply (superexponentially) increases with the temperature. This is connected with the exponential dependence of the probability of an impurity immuring in the surface layer by a moving step on the rate constant of the impurity migration from the surface layer into the adsorption one. The model reproduces typical dependences of the width of the doping transition’s concentration region on the temperature and growth rate obtained in experiments on Sb-doping of Si.

Tin surface segregation, desorption, and island formation during post-growth annealing of strained epitaxial Ge1−xSnx layer on Ge(0 0 1) substrate

Annealing of strained Ge1−xSnx epitaxial layers grown on Ge(001) substrate results in two distinctive regimes marked by changes in composition and morphology. Annealing at low temperatures (200–300°C or Regime-I) leads to surface enrichment of Sn due to Sn segregation, as indicated by X-ray photoelectron spectroscopy (XPS) results, while the bulk Sn composition (from X-ray diffraction (XRD)) and the surface morphology (from atomic force microscopy (AFM)) do not show discernible changes as compared to the as-grown sample. Annealing at temperatures ranging from 300°C to 500°C (Regime-II) leads to a decrease in the surface Sn composition. While the Ge1−xSnx layer remains fully strained, a reduction in the bulk Sn composition is observed when the annealing temperature reaches 500°C. At this stage, surface roughening also occurs with formation of 3D islands. The island size increases as the annealing temperature is raised to 600°C. The decrease in the Sn composition at the surface and in the bulk in Regime-II is attributed to additional thermally activated kinetic processes associated with Sn desorption and formation of Sn-rich 3D islands on the surface.

Growth of Thick Ge Epilayers on Si(100) Substrates Utilizing Two-Step Approach by Ultrahigh Vacuum Chemical Vapor Deposition

Ge epilayers on low temperature (LT) Ge buffer layers were grown by ultrahigh vacuum chemical vapor deposition using the two-step approach. Effects of the growth temperature and the thickness of the low temperature Ge buffers were studied. It was demonstrated that it was unable to obtain flat LT Ge buffers just through lowering the growth temperature due to the ultraslow grow rate that 3D islands formation couldnt be prohibited. However, the rough LT Ge surface was smoothed by subsequent growth at elevated temperature if the LT Ge layer was thick enough and the compressive strain was relaxed largely, and the detrimental Si-Ge intermixing was effectively prohibited as well. When using proper growth conditions for the low temperature Ge buffer, thick Ge epilayer with a low threading dislocation density and a smooth surface was obtained.

Thermoelectric properties of Ca3Co4O9 and Ca2.8Bi0.2Co4O9 thin films in their island formation mode

Abstract

Interface science of virtual GaN substrates on Si(111) via Sc2O3/Y2O3 buffers: Experiment and theory

The final film quality of GaN on foreign substrates is known to crucially depend on the initial GaN interface and nucleation characteristics. To shed light on these characteristics of recently pioneered virtual, hexagonal GaN(0001) substrates on Si(111) via step graded Sc2O3(111)/Y2O3(111) buffers, a complex GaN(0001)/Sc2O3(111) interface structure model and the initial nucleation scenario is derived from a combined experimental (reflection high energy electron diffraction and X-ray photoelectron spectroscopy) and theoretical ab initio study. It is shown that the GaN/Sc2O3 interface chemistry is determined by a N-Ga-O-Sc atomic arrangement leading to N-polar GaN films. However, the atomic GaN(0001)/Sc2O3(111) interface configuration is complex and local perturbations might be at the origin of Ga-polar inversion domains in the mainly N-polar GaN films. The initial growth of GaN on Sc2O3 is characterized by an ultrathin N-Ga-O-Sc wetting layer which carries tensile strain and relaxes with increasing thickness. Further GaN deposition results in the formation of 3D islands which fully relax before island coalescence occurs. The implications of the GaN/Sc2O3 interface configuration, the 3D nucleation growth mode, and the coalescence process of misaligned islands are discussed with respect to the defect characteristics (inversion domains, cubic inclusions, threading dislocations) of the final GaN layer.

Comparison of arsenide and phosphide based graded buffer layers used in inverted metamorphic solar cells

The effect of graded buffer layer (GBL) composition on inverted metamorphic 1.1-eV In0.24Ga0.76As single-junction solar cells, with applications as a subcell in high-efficiency multijunction solar cells, is investigated. In experiment A, AlxInyGa(1−x−y)As was used as a GBL to transition from the GaAs substrate lattice constant to that of the In0.24Ga0.76As cell. In experiment B, In1−xGaxP was employed. Both GBLs were deposited using growth conditions optimized for lattice-matched growth. Reciprocal space maps showed that the InGaAs cell was fully relaxed in both experiments. They also revealed that the AlInGaAs GBL relaxed very quickly after the start of growth (<0.1 μm). The InGaP GBL on the other hand remained partially strained throughout its structure and full relaxation was only achieved after growth of the InGaAs cell. Atomic force microscopy of the surface of the AlInGaAs GBL showed typical cross-hatch morphology with a roughness of 8.9 nm. The surface of the InGaP GBL was much rougher at 18.3 nm with unusual morphology, likely due to 3D island formation due to unrelieved strain. These findings were confirmed by transmission electron microscopy where the InGaAs cell of experiment A was largely free of imperfections with a defect density of 1.1 × 106 cm−2. Experiment B had defects readily seen throughout the GBL and the InGaAs cell above had a defect density of 1.5 × 109 cm−2. The hardness of the AlInGaAs GBL surface was measured to be 7.2 GPa and exhibited an indentation size effect. The hardness of the InGaP GBL surface was 10.2 GPa regardless of the depth of penetration of the indenter. The lack of indentation size effect in InGaP is due to the high density of dislocations already present in the material due to unrelieved strain. Solar cells fabricated from experiment A wafers exhibited excellent band gap-voltage offset Woc = (Eg/q) − Voc of 0.414 V. Cells from experiment B exhibited a poor Woc of 0.686 V, most likely due to the threading dislocations acting as non-radiative recombination centers.

Initial Growth Mode, Nanostructure, and Molecular Stacking of a ZnPc:C60 Bulk Heterojunction

AbstractThe initial growth modes of ZnPc films is examined, revealing the previously undescribed nanoscale crystal structure evolution and the nanograins of the ZnPc:C60 mixed layers in the thin films. Initially, the ZnPc molecules are stacked in the preferred γ(200) configuration, similar to the structures of CuPc. The ZnPc thin film growth display 2D planar to 3D island growth after the initial compressive strain had relaxed in films 7–8 MLs thick. 3D island formation decreases the prevalence of the preferred ordering in the γ(200) crystals. The ZnPc films consist of randomly distributed ellipsoid nanograins during the initial growth stages. The ellipsoid nanograins transition to an ordered state later in the growth process. Insertion of C60 changes the preferred molecular stacking of ZnPc, and β(100) forms in the ZnPc:C60 layers fabricated at room temperature, which is usually observed at high annealing temperatures (200 °C) in a single ZnPc film. The ellipsoid ZnPc nanograins also retain their shapes in the ZnPc:C60 mixed layers. The formation of β(100) and the presence of ellipsoid nanograins in the mixed layer are related to improvements relative to planar devices in the organic photovoltaic device performance.