Lateral Growth Front Research Articles

Effects of inorganic seed promoters on MoS2 few-layers grown via chemical vapor deposition

In the last years, transition metal dichalcogenides (TMDs), especially at the two-dimensional (2D) limit, gained a large interest due to their unique optical and electronic properties. Among them, MoS2 received great attention from the scientific community due to its versatility, workability, and applicability in a large number of fields such as electronics, optoelectronics and electrocatalysis. To open the possibility of 2D-MoS2 exploitation, its synthesis over large macroscopic areas using cost-effective methods is fundamental. In this study, we report a method for the synthesis of large-area (∼cm2) few-layers MoS2 via liquid precursor CVD (L-CVD), where the Mo precursor (i.e. ammonium heptamolybdate AHM) is provided via a solution that is spin-coated over the substrate. Given the capability of organic and inorganic molecules, such as alkaline salts, to enhance MoS2 growth, we investigated the action of different inorganic salts as seed promoters. In particular, by using visible Raman spectroscopy, we focused on the effect of Na(OH), KCl, KI, and Li(OH) on the thickness, morphology, uniformity and degree of coverage of the grown MoS2. We optimized the process tuning parameters such as the volume of spin-coated solution, the growth temperature, and the seed promoter concentration, to synthesise the lowest possible thickness which resulted to be 2 layers (2L) of the highest quality. We witnessed that the addition of an inorganic seed promoter in the solution improves the extension of the grown MoS2 promoting lateral growth front, and therefore the degree of coverage. From this study, we conclude that, amongst the investigated seed promoters, K-based salts proved to grant the growth of high-quality two-layer MoS2 with optimal and uniform coverage of the SiO2/Si substrate surface.

Roles of specific hydrogen-bonding interactions in the crystallization kinetics of polymers

Polyamides hold specific hydrogen-bonding interactions as the hierarchical thermodynamic driving forces for crystallization. Their roles in crystallization kinetics are worthy of further investigation. We performed dynamic Monte Carlo simulations to compare crystallization kinetics among 16-mer melts holding hydrogen-bonding interactions with variable strengths specifically assigned to 2, 4 and 8 monomers on the chain, respectively. The results demonstrate higher densities and strengths of hydrogen-bonding interactions generally accelerating the cooling and isothermal crystallization as well as the lateral growth of lamellar crystals, consistent with the experimental observations of polyamide crystallization. Too strong hydrogen-bonding interactions will slow down polymer diffusion and thus bring retardation to crystallization kinetics. Furthermore, hydrogen-bonding interactions at the central 8–9 monomers make higher crystal growth rates than those at the evenly distributed 5–12 monomers, demonstrating the dominant roles of specific interactions in the hierarchical parallel packing of polymer chains at the lateral growth front of lamellar crystals.

Double-lamella phenomenon of once-folding polymer crystallization raised by chain-end hydrogen-bonding interactions

Poly (ethylene glycol) (PEG) holding molecular weights around 7000 Da exhibits a double-lamella phenomenon on the surface replicas of bulk once-folding crystallization, whose mechanism is still unclear. Concerning to its specific hydrogen-bonding interactions at chain-ends, we performed dynamic Monte Carlo simulations to investigate the effects of specific chain-end interactions on once-folding crystallization of 16-mers. The results showed that the chain-end hydrogen-bonding interactions bring little effects to both kinetics of primary nucleation and lateral growth of chain-folded lamellar crystals but make a more significant terrace at the lateral growth front with a step of metastable once-folding towards the next step of stable chain-extending, raising the double-lamella phenomenon of their surface replicas. We found that the chain-end hydrogen-bonding interactions are rich at the surfaces of once-folded lamellar crystals, which thus enhance their metastability and retard their thickening into the chain-extended crystals. In addition, intra- and inter-molecular secondary nucleation at corresponding chain-folded and extended growth fronts show different growth rate dependences on the specific interaction strengths. Our results revealed the role of specific chain-end interactions in PEG crystallization.

Lateral and vertical growth of Mg-doped GaN on trench-patterned GaN films

Growth of Mg-doped GaN on trench-patterned GaN films consists of competing lateral and vertical growth fronts that result in regions with different electronic properties. Under typical growth conditions, lateral growth from the trench sidewall occurs at a faster rate than vertical growth from the trench base. When the trench width is sufficiently narrow, the growth fronts from opposite sidewalls coalesce and lead to eventual planarization of the top surface. Secondary electron imaging and cathodoluminescence mapping are used to correlate the morphology and the optical properties of regions resulting from lateral and vertical growth. For our growth conditions, the lateral-to-vertical growth rate ratio is found to be about 2.

Lateral facet growth of ice and snow – Part 1: Observations and applications to secondary habits

Abstract. Often overlooked in studies of ice growth is how the crystal facets increase in area, that is, grow laterally. This paper reports on observations and applications of such lateral facet growth for vapor-grown ice in air. Using a new crystal-growth chamber, we observed air pockets forming at crystal corners when a sublimated crystal is regrown. This observation indicates that the lateral spreading of a face can, under some conditions, extend as a thin overhang over the adjoining region. We argue that this extension is driven by a flux of surface-mobile molecules across the face to the lateral-growth front. Following the pioneering work on this topic by Akira Yamashita, we call this flux “adjoining surface transport” (AST) and the extension overgrowth “protruding growth”. Further experiments revealed other types of pockets that are difficult to explain without invoking AST and protruding growth. We develop a simple model for lateral facet growth on a tabular crystal in air, finding that AST is required to explain observations of facet spreading. Applying the AST concept to observed ice and snow crystals, we argue that AST promotes facet spreading, causes protruding growth, and alters layer nucleation rates. In particular, depending on the conditions, combinations of lateral- and normal-growth processes can help explain presently inexplicable secondary features and habits such as air pockets, small circular centers in dendrites, hollow structure, multiple-capped columns, scrolls, sheath clusters, and trigonals. For dendrites and sheaths, AST may increase their maximum dimensions and round their tips. Although these applications presently lack quantitative detail, the overall body of evidence here demonstrates that any complete model of ice growth from the vapor should include such lateral-growth processes.

Determination of the Critical Size of Secondary Nuclei on the Lateral Growth Front of Lamellar Polymer Crystals

Nucleation is the rate-limiting first step of crystallization. The energy barrier related to form the critical nucleus predominantly affects the nucleation rate. Determination of the size of the cr...

Non-edge-triggered inversion from Ga polarity to N polarity of c-GaN domains on an SiO2 mask during epitaxial lateral overgrowth

It was previously reported that N-polar c-GaN domains nucleated in window openings on c-plane sapphire were inverted to Ga-polar domains at the edge of an SiO2 mask during epitaxial lateral overgrowth, but it was asserted that polarity inversion of N-polar GaN domains could not occur beyond the edge of the SiO2 mask. However, that assertion was demonstrated only in the case of a-facet-exposed GaN. It is reported here that polarity inversion from Ga polarity to N polarity of m-facet-exposed c-GaN domains occurred during epitaxial lateral overgrowth on the flat region beyond the edge of a circular-patterned SiO2 mask. An increased flow rate of NH3 during the epitaxial lateral overgrowth is thought to induce this type of non-edge-triggered polarity inversion. Further investigation reveals that non-edge-triggered polarity inversion is also possible when the a facet is exposed at the lateral growth front of Ga-polar GaN domains.

Stratigraphy of a diamond epitaxial three-dimensional overgrowth using doping superlattices

The selective doped overgrowth of 3D mesa patterns and trenches has become an essential fabrication step of advanced monolithic diamond-based power devices. The methodology here proposed combines the overgrowth of plasma-etched cylindrical mesa structures with the sequential growth of doping superlattices. The latter involve thin heavily boron doped epilayers separating thicker undoped epilayers in a periodic fashion. Besides the classical shape analysis under the scanning electron microscope relying on the appearance of facets corresponding to the main crystallographic directions and their evolution toward slow growing facets, the doping superlattices were used as markers in oriented cross-sectional lamellas prepared by focused ion beam and observed by transmission electron microscopy. This stratigraphic approach is shown here to be applicable to overgrown structures where faceting was not detectable. Intermediate growth directions were detected at different times of the growth process and the periodicity of the superlattice allowed to calculate the growth rates and parameters, providing an original insight into the planarization mechanism. Different configurations of the growth front were obtained for different sample orientations, illustrating the anisotropy of the 3D growth. Dislocations were also observed along the lateral growth fronts with two types of Burger vector: b011¯=12[011¯] and b112=16[112]. Moreover, the clustering of these extended defects in specific regions of the overgrowth prompted a proposal of two different dislocation generation mechanisms.

Temperature dependence of a‐plane GaN low angle incidence microchannel epitaxy by ammonia‐based metal‐organic molecular beam epitaxy

AbstractThe growth temperature dependence of a‐plane GaN produced by low angle incidence microchannel epitaxy using NH3‐based metal‐organic molecular beam epitaxy was studied. It was found that the width of the laterally grown region increased with temperature. This was because at low temperatures, {10‐12} side facets were formed, which suppressed the lateral growth process by giving rise to inter‐surface diffusion of Ga adatoms from the side to the top of the growing layer. These facets also caused the lateral growth front to assume a zigzag shape. The formation mechanism of the facets can be understood in terms of the Mullin‐Sekerka instability, by which minute periodic undulations gradually increase with growth time when their period is larger than a certain critical value, which increases with growth temperature. Defects in the epitaxial layers were also studied using transmission electron microscopy. Although the defect density was found to be very low in the laterally grown layer itself, a very large number of dislocations and stacking faults were observed both in the template layer and in the epitaxial regions above the openings. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Systematic Kinetic Analysis on Monolayer Lamellar Crystal Thickening via Chain-Sliding Diffusion of Polymers

Lamellar polymer crystals are metastable due to their limited lamellar thickness. We performed dynamic Monte Carlo simulations of lattice linear polymers to investigate the kinetics of isothermal thickening via chain-sliding diffusion in single lamellar crystals of polyethylene and poly(ethylene oxide). We sorted out three typical cases for controversial experimental observations. The basic case is a continuous increase of lamellar thickness for heavily folded long chains, with a logarithmic time dependence typical at the lateral growth front. Its kinetics is dominated by the activation energy barrier for sliding diffusion with higher speeds at higher temperatures. For integer-folded short chains, however, the lamellar thickness increases discontinuously, and its kinetics is dominated by a free energy barrier for surface nucleation. The latter can be further split into two cases: the thickening in the melt is mainly driven by the bulk free energy, with lower speeds at higher temperatures due to a temperature-sensitive barrier; while the thickening on a solid substrate is mainly driven by the surface free energy, with higher speeds at higher temperatures due to a temperature-insensitive barrier. The simulations facilitate our systematic understanding to the case-by-case microscopic mechanisms for the thickening of monolayer lamellar crystals via sliding diffusion of polymers.

Epitaxial Lateral Overgrowth of (1-100) m-Plane GaN on m-Plane 6H-SiC by Metalorganic Chemical Vapor Deposition

Planar m-plane GaN was grown on (1100) m-plane 6H-SiC substrates using high-temperature AlN nucleation layers by metalorganic chemical vapor deposition. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images showed striated features on the sample surface aligned along the GaN [1120] direction, which are perpendicular to those associated with (1120) a-plane GaN. The epitaxial relationship between the m-GaN and 6H-SiC was analyzed using high-resolution x-ray diffraction (XRD). In order to reduce the defect density, epitaxial lateral overgrowth (ELO) was carried out on an m-GaN template with mask stripes along the GaN [1120] direction, which makes the lateral growth fronts advance along the GaN c-axis. On-axis XRD rocking curves show that the full width at half maximum (FWHM) values for the ELO samples were reduced by nearly half when compared to those of the m-plane template without ELO. Clear atomic steps were observed in the wing regions by AFM. The absence of the striated features that are associated with the template could be indicative of the reduction of basal stacking faults in the ELO wings. Low-temperature photoluminescence (PL) spectra showed an excitonic emission at 3.47eV, a basal stacking fault (BSF)-related emission at 3.41 eV, and other defect-related emissions at 3.29 eV and 3.34 eV.

An examination of facet formation during solid phase epitaxy of line-shaped amorphized regions in (001) and (011) Si

The shapes of the recrystallization fronts observed during solid phase epitaxy (SPE) in line-shaped amorphized Si (a-Si) regions in single-crystal (001) and (011) Si are shown to exhibit a complex time evolution inconsistent with simple models in which regrowth behavior is determined exclusively by the competition between fixed-growth-rate lateral and vertical SPE. We find that the main features of our data for ⟨110⟩-aligned a-Si lines may be explained by the propensity of the lateral and vertical growth fronts to form stable a-Si∕{111} interfaces at their edges. These same a-Si∕{111} interfaces have been previously implicated as the root cause of the trench-edge defects produced during the recrystallization of patterned a-Si regions bounded laterally by oxide-filled trenches. An extension of a nanofacet model recently developed to explain the trench-edge defects is shown to explain many of the observed recrystallization behaviors.

Mechanisms of dislocation reduction in an Al0.98Ga0.02N layer grown using a porous AlN buffer

Greatly improved optical properties of Al0.55Ga0.45N∕Al0.65Ga0.35N multiple quantum well (MQW) structures grown on the low dislocation density Al0.98Ga0.02N layer grown using a porous AlN buffer have been observed by photoluminescence measurement, in comparison with a MQW grown on a normal AlN layer. The mechanism for the dislocation reduction has been explored by transmission electron microscopy. Due to an attraction towards lateral growth fronts, the threading dislocations in the AlN layer bend during the overgrowth, but mainly by small angles, which is different from the 90° bending generally observed in classic epitaxial lateral overgrowth method. The dislocation reduction is realized through two main pathways: enhanced dislocation annihilation due to the small-angle line bending and the termination of dislocations at local free surfaces introduced by the nanoscale voids, produced around the interface between the Al0.98Ga0.02N and AlN layers.

Morphology evolution in a growing film

A general method for quantitative description of growth and morphology evolution in thin film growing via 3D island mechanism is presented. The method is based on the calculation of the survival probability for any point on (or above) a substrate. The growth law for different island growth mode (hemisphere and paraboloid) and condensation regime is derived in a self-consistent manner at all successive deposition stages, which shows growth acceleration at late stages due to island collisions. A temporal evolution of a lateral growth front perimeter that can be used for experimental determination of the growth law is also given. For surface relief, forming at paraboloid growth, the height–height autocorrelation function (ACF) is derived for different hillock shape and different nucleation mode generating different hillock space distribution. It is found that the hillock shape strongly affects the overall surface roughness and the hillock space distribution influences on both the ACF form and the rms roughness. For hemisphere growth surface morphology evolution during film growth is analyzed in terms of the rms surface roughness and the roughness coefficient and it is shown that both of them can be presented as a universal (independent of a growth regime) unimodal function of either coverage or film thickness with a maximum just prior to the completed film formation.

Beam Induced Lateral Epitaxy: a new way to Lateral Growth in Molecular Beam Epitaxy

We developed a new technique of epitaxial lateral growth without using oxide masks called beam induced lateral epitaxy (BILE). In this technique, molecular beams are directed at a nearly glancing angle with respect to a substrate that has pre-fabricated truncated ridges. By using BILE we grew GaAs laterally from the side of ridges on a GaAs substrate. The growth behavior of BILE strongly depended on both incident angle of the Ga beam and the crystal orientation of the truncated ridges. The formation of facets on the lateral growth front controlled the grown shape of the layers. By using a (111) B substrate with BILE, we grew a smooth, flat (111) B facet on the top of the layer.

Chain folding in polymer melt crystallization studied by dynamic Monte Carlo simulations

The morphological metastability of spontaneous crystallization from the melt of short-chain semiflexible homopolymers was studied through dynamic Monte Carlo simulations of a lattice multiple-chain system. Frictional hindrance for the sliding diffusion of the chains in the crystallites was employed to enhance the metastability of folded-chain crystallites, and distinguish the polymer crystallite from its mesophase, though their phase transitions have the similar driving forces. The integral folding of short chains in the crystallites and the constant linear crystal growth rate were identified with the actual polymers. In addition, the roughness of the local growth front accompanied with the occasional reversals and jumplike advancing was observed, which cannot be explained by current models. The crowding of the dangling ends on the fold surface seems to be the main reason for suppressing the lateral growth front, and the mechanism of chain folding was proposed. Its implications to the special behaviors of polymer melt crystallization, such as the semicrystalline state, the effect of the chain rigidity and molecular weight to crystal growth, the reversible premelting, and molecular segregation are briefly discussed.

Role of lateral growth fronts in the formation of In xGa 1− xAs bridge layer on trench substrates

The role of the lateral growth fronts in the formation of InGaAs bridge layer on trench substrates was investigated. For the edge angles of growth fronts ⩽90°, the bridge layers were formed on the basis of “Berg effect”. The bridge formation of {1 1 1}A growth front was greatly influenced by its neighboring {1 1 1}B growth front.

Structural and vibrational properties of GaN

Structural and vibrational properties of device quality pure GaN substrate grown using a lateral epitaxial overgrowth (LEO) technique were studied using x-ray diffraction, Brillouin, Raman, and infrared spectroscopy. Lattice constants were found to be a=3.1896±0.0002 Å and c=5.1855±0.0002 Å. Comparing the results with those on GaN epilayer directly grown on sapphire substrate, it is shown that the GaN substrate is indeed of high quality, i.e., the lattice is relaxed. However the GaN substrate has a small enough but finite residual strain arising from the pileup of the lateral growth front on SiO2 masks in the course of LEO. It was also found that the elastic stiffness constants C13 and C44, are more sensitive to the residual strain than the optical phonon frequencies. The high frequency and static dielectric constants were found to be 5.14 and 9.04. The Born and Callen effective charges were found to be 2.56 and 0.50.

Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth

Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1μm nucleation zones. Although emission is comprised of both UV (∼365nm) and yellow (∼550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines.

Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth

AbstractGrowth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1 μm nucleation zones. Although emission is comprised of both UV (∼365nm) and yellow (∼550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines.