Vertical Growth Rate Research Articles

Simulating the effects of surface diffusion on electron beam induced deposition via athree-dimensional Monte Carlo simulation

The effects that adsorbed precursor surface diffusion has on electron beaminduced deposition are explored via a three-dimensional Monte Carlo simulation.Initially the growth rate and resolution are compared for a common set ofdeposition conditions with a variable surface diffusion coefficient ranging from 0 to1 × 10−8 cm2 s−1. The growth rate and resolution are shown to both be enhanced as the growthchanges from a mass transport limited regime to a reaction rate limited regime. Thecomplex interplay between the vertical growth rate, the lateral growth rate, theinteraction volume and the adsorbed and diffused precursor species are discussed.A second scenario is also simulated in which only gas diffused from a constantsource at the perimeter of the simulation boundary is assumed (no gas phaseadsorption). At low diffusion coefficients, the diffusing gas is consumed by secondary andbackscattered electrons and experimentally observed ring-like structures are generated. Athigher diffusion coefficients, the diffusion length is sufficient for the precursoratoms to diffuse to the center (and up the pillar sidewalls) to generate nanowires.

Understanding the Kinetics and Nanoscale Morphology of Electron‐Beam‐Induced Deposition via a Three‐Dimensional Monte Carlo Simulation: The Effects of the Precursor Molecule and the Deposited Material

The electron-beam-induced deposition of silicon oxide from tetraethyorthosilicate and tungsten from tungsten hexafluoride is simulated via a Monte Carlo simulation. Pseudo one-dimensional nanopillars are grown using comparable electron-beam parameters and a comparison of the vertical and lateral growth rate and the pillar morphology is correlated to the precursor and deposited material parameters. The primary and secondary electrons (type I) are found to dominate the vertical growth rate and the lateral growth rate is dominated by forward and secondary electrons (type II). The resolution and morphology of the nanopillars are affected by the effective electron interaction volume and the resultant surface coverage of the precursor species in the effective electron interaction region. Finally, the simulated results are compared to previously reported experimental results.

X‐ray diffraction studies of selective area grown InGaN/GaN multiple quantum wells on multi‐facet GaN ridges

AbstractThe structural properties of InGaN/GaN multiple quantum wells (MQW) were studied using synchrotron based high resolution x‐ray diffraction (HRXRD). MQW structures were grown on the top and sidewall facets of triangular and trapezoidal shaped GaN ridges by metalorganic vapour phase epitaxy (MOVPE) in the regime of selective area growth (SAG). Period and strain variations as a function of oxide mask width were determined for both the sidewall and the top facet growth. Oxide mask widths ranged between 2 and 20 µm with openings between adjacent masks of 4 and 6 µm. Analysis of the x‐ray diffraction curves revealed a sidewall / vertical growth rate ratio of ∼0.3 through a comparison of the top to sidewall facet MQW periods. Masks orientated along the 〈11‐20〉 crystallographic direction showed stronger growth enhancement along with large global strain for MQW growth on the top (0001) plane. Interpreting our results within the framework of vapour phase diffusion revealed that inter‐facet migration of group‐III species needs to be taken into account. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Revelation of nuclear tracks and dislocations: A Monte Carlo simulation of mineral etching

A simple atomistic Monte Carlo simulation suggests that there are up to four stages in the evolution of an etch pit in the (0 0 1)-surface of an idealised regular lattice. During the first stage, the etch pit is an inverted pyramid; its horizontal and vertical dimensions increase at a constant rate; the apparent horizontal ( v h) and vertical ( v d) growth rates are faster than during all subsequent stages but nevertheless less than the step retreat rate ( v s) on account of surface etching ( v v). The pyramid apex is truncated in the second stage; it is thereafter bounded by an expanding bottom plane and shrinking lateral walls; this is accompanied by a gradual decrease of v h; v d drops to a negative value indicating a slow decrease of the etch-pit depth; the bottom plane acquires a concave-up curvature; the outward curvature of the walls, initiated during the first stage, increases. During the third stage the etch pit consists of a single concave-up bottom plane; v h and v d decrease at declining rates; consecutive etch-pit profiles are scalable in the horizontal direction. The hypothetical fourth stage is inferred but not documented by the simulations; it sets in when v h is reduced to zero; unless this corresponds to an as yet unidentified steady-state condition, the etch pit from here on forth shrinks until it eventually disappears altogether. The sole cause for this succession is the process of stochastic rounding of confined steps and faces. The triangular footprint of recoil-track, fission-track, ion-track and dislocation etch pits in trioctahedral mica and its compliance with the monoclinic symmetries implies that the relevant periodic bond chains are O–Mg/Fe–O chains in the octahedral layer. The size distribution of etched recoil tracks is due to (1) depth variations resulting from the size distribution of the latent tracks, (2) the random truncation of the surface tracks, (3) the variable rate of etch-pit enlargement and (4) the fact that new tracks are exposed at the surface due to surface etching. The greater size of dislocation, fission-track and ion-track etch pits is due to their greater extent below the surface. The increase of the number of etched tracks with etching time due to bulk etching is non-linear because the bulk etch rate v v is not constant. The evolution of etch-pit shape with continued etching can also cause loss of tracks due to observation effects related to loss of contrast.

Spiral Growth and Step Edge Barriers

The growth of spiral mounds containing a screw dislocation is compared to the growth of wedding cakes by two-dimensional nucleation. Using phase field simulations and homoepitaxial growth experiments on the Pt(111) surface we show that both structures attain the same large scale shape when a significant step-edge barrier suppresses interlayer transport. The higher vertical growth rate of the spiral mounds on Pt(111) reflects the different incorporation mechanisms for atoms in the top region and can be formally represented by an enhanced apparent step-edge barrier.

Effect of interfacial strain on shape and composition of MOCVD grown III-Nitride nanostructures

AbstractIt is determined that the interfacial strain due to lattice mismatch between the substrate (template) and the overgrown nanostructure has substantial influence on the final morphology and faceting of nanostructures. AlGaN and GaN nanostructures were grown on different substrates/templates, namely Sapphire, AlN/Sapphire, Al0.5Ga0.5N/Sapphire and GaN/Sapphire, where interfacial strain for overgrown nanostructures changes from compressive to tensile respectively. GaN nanostructures grown by metalorganic chemical vapor deposition (MOCVD) using identical growth parameters exhibit pyramidal morphologies under low or no mismatch strain, while highly strained structures with compressive strain evolve into prismatic shapes when grown in the c-crystallographic direction. In addition, it is shown that interfacial strain also affects the growth rate and alloy composition of ternary AlGaN nanostructures produced under identical conditions on different templates/substrates. AlGaN nanostructures grown under compressive strain show higher vertical and lower lateral growth rates when compared to those under tensile strain. Scanning electron microscopy, transmission electron microscopy and cathodoluminescene were used to characterize growth rates, shape and composition of the nanostructures. Observed variation in morphologies, growth rates and compositions are explained in terms of the strain dependency of adatom diffusion barriers on strained surfaces and Ehrlich-Schwoebel barriers across different crystallographic planes. Density functional (DFT) calculations were performed to determine the dependency of adatom diffusion on strained GaN and AlN surfaces.

Drought Responses of Kentucky Bluegrass and Creeping Bentgrass as Affected by Abscisic Acid and Trinexapac-ethyl

The plant growth regulators abscisic acid (ABA) and trinexapac-ethyl (TE) may affect turfgrass responses to drought stress through regulating shoot growth and water relations. The objectives of this study were to investigate the effects of foliar application of TE and ABA on turf growth of two cool-season turfgrass species, Kentucky bluegrass (Poa pratensis L.) and creeping bentgrass (Agrostis stolonifera L.) exposed to drought stress, and to examine water relations associated with changes in drought tolerance due to TE or ABA treatment. ‘L-93’ creeping bentgrass and ‘Brilliant’ Kentucky bluegrass plants were foliar sprayed with 0.904 mL·ha−1 a.i. TE five times before exposure to drought or with 6.75 mL/week of ABA at 100 μm before and after exposure to drought in growth chambers. Drought stress was imposed by withholding irrigation until plants were permanently wilted. Foliar application of TE or ABA maintained higher soil volumetric water content, leaf relative water content, and turf quality for a longer period of time during 28 days of stress exposure for Kentucky bluegrass and creeping bentgrass compared with the untreated control. Leaves of TE-treated and ABA-treated plants in both species also had lower ψS at 28 days of drought stress than the untreated control. Creeping bentgrass treated with TE or ABA and Kentucky bluegrass treated with TE exhibited significantly lower shoot vertical growth rates at the initiation of drought stress, but maintained higher growth rates during prolonged drought compared with the untreated control. Turf treated with TE or ABA also showed higher levels of photochemical efficiency than the untreated control for both species. Our results suggest that TE or ABA application could prolong the survival of turfgrass under conditions of drought stress by suppressing shoot vertical growth and lowering water use during the early phase of drought and by sustaining growth and photosynthetic activity during prolonged periods of drought stress through osmotic adjustment for retaining cellular hydration.

Pulsed selective epitaxial growth of hexagonal GaN microprisms

Hexagonal GaN microprism structures were fabricated by pulsed selective epitaxial growth (SEG) under conditions that suppressed lateral overgrowth. Unlike previously reported pulsed SEG processes in which the precursor gas flow was modulated by valves, the approach reported here utilizes a flow geometry that produces a significant deposition rate only over the downstream portion of the 2 in wafer, which yields a pulsed deposition process when combined with substrate rotation. Hexagonal GaN microprism structures were grown on a GaN/sapphire substrate with a dielectric (SiO x ) mask of varying thickness (100, 150, and 200 nm) with circular openings ranging from 4 to 20 μm in diameter. Optimal structures were obtained with a SiO x mask thickness of 200 nm and circular openings of 6 and 8 μm. The vertical growth rate of the hexagonal prism structures was ∼50 nm/min, corresponding to the deposition of approximately six GaN bilayers (∼1.7 nm) per cycle. This pulsed deposition process has advantages over etching processes for fabricating hexagonal microprisms for optical cavity devices, including elimination of etching damage and formation of crystallographic facets with a high degree of parallelism.

Effect of gas flow on the selective area growth of gallium nitride via metal organic vapor phase epitaxy

The effect of gas flow on the selective area growth (SAG) of gallium nitride (GaN) grown via metal organic vapor phase epitaxy (MOVPE) has been investigated. In this study, the SAG of GaN was carried out on a silicon dioxide striped pattern along the GaN 〈 1 1 ¯ 0 0 〉 direction. SAG was initiated with the striped pattern oriented parallel and normal to the incoming gas flow in a horizontal reactor. The orientation of the pattern did not impact cross section of the structure after re-growth as both orientations resulted in similar trapezoidal structures bounded by the (0 0 0 1) and { 1 1 2 ¯ n } facets ( n ≈ 1.7 - 2.2 ). However, the growth rates were shown to depend on the orientation of the pattern as the normally oriented samples exhibited enhanced vertical and cross-sectional growth rates compared to the parallel oriented samples. All growths occurred under identical conditions and therefore the difference in growth rates must be attributed to a difference in mass transport of species.

Quantitative simulation of in situ reflectance data from metal organic vapour phase epitaxy of GaN on sapphire

Multi-wavelength in situ reflectometry at normal incidence has been applied to monitoring metal organic vapour phase epitaxy of GaN films on sapphire (0 0 0 1) substrates. A new quantitative analysis model has been developed to incorporate time-dependent light scattering by a rough surface, and a time-dependent vertical growth rate during growth on a rough surface, into the virtual interface model that has previously been applied to multilayer structures with optically smooth surfaces and interfaces. It is shown that the vertical growth rate increases as the surface roughness decreases in the early stage of high-temperature GaN growth, reaching a limiting value when the surface becomes optically smooth. The time dependence of growth rate is correlated with microscopic crystal growth mechanisms on the rough surface, which involve mass transport on the facets and/or mass exchange between the growing surface and gas-phase ambient. Our optical modelling is supported by direct morphological investigations of films from growths terminated at various stages, using atomic force microscopy. High-temperature optical constants of GaN layers extracted from the simulations are well matched to literature values.

Growth and Carbon Metabolism of Tall Fescue and Zoysiagrass as Affected by Deficit Irrigation

Understanding turfgrass physiological responses to deficit irrigation will help explain potential effects of this practice on turf quality and subsequent stresses. The objective of this study was to investigate the influence of deficit irrigation growth and physiology of ‘Falcon II’ tall fescue (Festuca arundinacea Schreb) and ‘Meyer’ zoysiagrass (Zoysia japonica Steud). Turf was subjected to deficit irrigation levels of 20%, 40%, 60%, 80%, and 100% of actual evapotranspiration (ET) from June to Sept. 2001 and 2002 in Manhattan, Kans. In an earlier study, minimum deficit irrigation levels required to maintain acceptable quality (MDIL) were determined. We compared growth and physiological parameters at these MDIL with turf irrigated at 100% ET. Tall fescue had a lower canopy vertical growth rate (30% lower), canopy net photosynthesis (Pn, 14% lower), and whole-plant respiration (Rw, 11% lower) in 1 of 2 years when irrigated at the MDIL compared with 100% ET; tiller number was not reduced at the MDIL. Water use efficiency (μmol CO2 per mmol H2O) in tall fescue increased by 15% at the MDIL relative to turf receiving 100% ET in 1 of 2 years. In zoysiagrass, the MDIL had no effect on any of the growth or physiological parameters measured. Reductions in canopy vertical growth rate at the MDIL in tall fescue during deficit irrigation would likely reduce mowing requirements. Across all deficit irrigation levels, Pn was more sensitive to deficit irrigation in both grasses than was Rw, which could potentially contribute to declines in canopy vertical growth rate, tiller number, and turf quality. Zoysiagrass exhibited higher water use efficiency than tall fescue, particularly at irrigation levels 60% or more ET.

Mean Flow Field of a Nonbuoyant Rectangular Surface Jet

The flow field for a low Froude number nonbuoyant rectangular surface jet is studied using laser induced fluorescence and laser Doppler velocimetry. It is shown that a surface current develops in turbulent, but not in laminar jets. The existence of a self-similar regime is established. The lateral growth rate of the jet below the surface is about 4.8 times its vertical growth rate, while that of the surface current is 8.8 times. The vertical growth rate is about half that of a plane surface jet and less than one third that of a free axisymmetric jet. While the horizontal profiles of the streamwise velocity are similar to those observed in free, submerged, and wall jets, the vertical distributions are distinctly flatter.

Power-law scaling during shadowing growth of nanocolumns by oblique angle deposition

The authors have investigated the power-law behaviors of various morphological parameters during the shadowing growth of ruthenium (Ru) nanocolumns by an oblique angle sputter deposition technique with substrate rotation. Particularly, wavelength and column number density were measured at different column heights (h). The exponents associated with the wavelength (pλ) and column number density (pn), correlated by the geometrical relationship pλ≈−12pn, were measured by atomic force microscopy to be ∼0.5 and ∼−1.0, respectively. Using a one-dimensional facet growth model based upon the principle of evolutionary selection under oblique angle deposition, they showed that the exponents associated with the column number density and wavelength can be predicted. The authors also illustrated that the exponent value associated with column number density originates from the competitive growth among columns that have different vertical growth rates. The simulated exponent values are independent of the shape of the facet, which indicates the universality of these power-law exponents.

Spatial Variability of Bomb 14C in an Upland Peat Bog

As part of a study investigating the carbon balance of a blanket bog, we made an assessment of the spatial variation of radiocarbon concentrations in the surface layers of a small area of peatland in the north of England. The peat depth at which bomb-14C content was the highest varied considerably between cores sampled from across the site. At several sampling locations, 14C levels >100% Modern were confined to the surface 8 cm, whereas bomb 14C was evident at 1 site, located only meters away, to a depth of at least 12–16 cm. Using the layer where 14C levels first exceeded 100% Modern as a chronological reference layer, we estimated the carbon accumulation rate over the last 50 yr for the surface peat at each site (range ∼20 to ∼125 g C m2 yr-1). Our results show that although carbon accumulation over the last 50 yr was similar across the site, variation in the depth to which bomb 14C was evident implied considerable variation in the vertical peat growth rate.

Pulsed epitaxial lateral overgrowth of GaN by metalorganic vapour phase epitaxy

A mixed pulsed and normal GaN epitaxial lateral overgrowth (ELO-GaN) by epitaxy metalorganic vapour phase epitaxy (MOVPE) is reported in this study. Monitoring by using an in situ spectroscopic reflectometer has shown that a varying vertical growth rate during the pulsed growth was observed as in the normal ELO-GaN growth process, however, the growth rate was dramatically reduced in pulsed growth. Cross-section scanning electron microscope (SEM) images have shown that a lateral to vertical growth ratio (LTVGR) of 7 was obtained under a set of growth conditions on a template with a GaN trench and SiO 2 mask width of 5 and 15 μ m , respectively, and with the stripes aligned in the GaN 〈 1 ¯ 1 0 0 〉 crystallographic direction. Two types of growth instability associated with pulsed growth were observed under some growth conditions. One is the formation of large steps on the ELO-GaN stripes before coalescence; the other is the formation of hexagonal pyramids on the coalesced surface. The origin of pyramidal formation was found exactly on the coalescence boundaries. A mixed pulsed and normal ELO-GaN growth technique has been established to eliminate the large steps, and formation of pyramids can be avoided by switching to normal growth conditions before ELO-GaN stripes coalesce. The thickness of ELO-GaN has been successfully controlled below 1 μ m before coalescence, and below 3 μ m for a fully coalesced ELO-GaN film by this technique. Atomic force microscope (AFM) has confirmed that ELO-GaN films grown by this technique are of high structural quality.

Responses of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline

AbstractFrom 2001 to 2004 we experimentally warmed 40 large, naturally established, white spruce [Picea glauca(Moench) Voss] seedlings at alpine treeline in southwest Yukon, Canada, using passive open‐top chambers (OTCs) distributed equally between opposing north and south‐facing slopes. Our goal was to test the hypothesis that an increase in temperature consistent with global climate warming would elicit a positive growth response. OTCs increased growing season air temperatures by 1.8°C and annual growing degree‐days by one‐third. In response, warmed seedlings grew significantly taller and had higher photosynthetic rates compared with control seedlings. On the south aspect, soil temperatures averaged 1.0°C warmer and the snow‐free period was nearly 1 month longer. These seedlings grew longer branches and wider annual rings than seedlings on the north aspect, but had reduced Photosystem‐II efficiency and experienced higher winter needle mortality. The presence of OTCs tended to reduce winter dieback over the course of the experiment. These results indicate that climate warming will enhance vertical growth rates of young conifers, with implications for future changes to the structure and elevation of treeline contingent upon exposure‐related differences. Our results suggest that the growth of seedlings on north‐facing slopes is limited by low soil temperature in the presence of permafrost, while growth on south‐facing slopes appears limited by winter desiccation and cold‐induced photoinhibition.

Pressure effect of growing with electron beam‐induced deposition with tungsten hexafluoride and tetraethylorthosilicate precursor

Electron beam-induced deposition (EBID) provides a simple way to fabricate submicron- or nanometer-scale structures from various elements in a scanning electron microscope (SEM). The growth rate and shape of the deposits are influenced by many factors. We have studied the growth rate and morphology of EBID-deposited nanostructures as a function of the tungsten hexafluoride (WF6) and tetraethylorthosilicate (TEOS) precursor gas pressure and growth time, and we have used Monte Carlo simulations to model the growth of tungsten and silicon oxide to elucidate the mechanisms involved in the EBID growth. The lateral radius of the deposit decreases with increasing pressure because of the enhanced vertical growth rate which limits competing lateral broadening produced by secondary and forward-scattered electrons. The morphology difference between the conical SiO(x) and the cylindrical W nanopillars is related to the difference in interaction volume between the two materials. A key parameter is the residence time of the precursor gas molecules. This is an exponential function of the surface temperature; it changes during nanopillar growth and is a function of the nanopillar material and the beam conditions.

Crown development in a pioneer tree, Rhus trichocarpa, in relation to the structure and growth of individual branches

Based on an allometric reconstruction, the structure and biomass-allocation patterns of branches and current-year shoots were investigated in branches of various heights in the pioneer tree Rhus trichocarpa, to evaluate how crown development is achieved and limited in association with height. Path analysis was conducted to explore the effects of light availability, basal height and size of individual branches on branch structure and growth. Branch angle was affected by basal height, whereas branch mass was influenced primarily by light availability. This result suggests that branch structure is strongly constrained by basal height, and that trees mediate such constraints under different light environments. Previous-year leaf area and light availability showed positive effects on current-year stem mass. In contrast, branch basal height and mass negatively affected current-year stem mass. Moreover, the length of stems of a given diameter decreased with increasing branch height. Therefore the cost of biomass investment for a unit growth in length is greater for branches of larger size and at upper positions. Vertical growth rate in length decreased with increasing height. Height-dependent changes in stem allometry and angle influenced the reduction in vertical growth rate to a similar degree.

Performance in the Transition Zone of Two Hybrid Bluegrasses Compared with Kentucky Bluegrass and Tall Fescue

Hybrid bluegrasses (HBG) are crosses between Kentucky bluegrass (Poa pratensis L.) (KBG) and Texas bluegrass (Poa arachnifera Torr.), which may exhibit better heat tolerance and drought resistance than other cool-season turfgrasses. Two HBG cultivars (‘Thermal Blue’ [HBG1] and ‘Dura Blue’ [HBG2]), one KBG (‘Apollo’), and one tall fescue (Festuca arundinacea Schreb.; ‘Dynasty’ [TF]) were evaluated for two years in northeastern Kansas for establishment rates after seeding, visual quality and growth characteristics, and drought resistance. Irrigation treatments included 60% and 100% evapotranspiration (ET) replacement and a control receiving only natural precipitation. Tall fescue reached full cover 37, 52, and >73 days faster than HBG1, KBG, and HBG2, respectively. In both years, average quality over the growing season ranked: TF > KBG > HBG1 > HBG2; an infestation of bluegrass billbugs (Sphenophorus parvulus Gyllenhal) in 2003 reduced quality among bluegrasses but not TF. Canopy density was lower in HBG2 and higher in TF among treatments. Clipping biomass of TF was 42 to 73% higher than that of the bluegrasses. Vertical growth rates were highest in HBG1 and TF and lowest in KBG. Drought generally reduced quality among bluegrasses, but effects on TF were negligible. Results indicate that TF is better adapted than HBG where soils are deep in the transition zone. Further research is needed using new cultivars of HBG and in areas with different soils.

Structure and dynamics of Langmuir–Blodgett tristearin films: Atomic force microscopy and theoretical analysis

The structure and temporal evolution of tristearin (SSS) monolayers at the air–water interface at 20±1 °C are investigated with the Langmuir method. The deposited Langmuir–Blodgett (LB) layers were investigated with atomic force microscopy (AFM). The LB experiments showed that adsorption isotherms obtained with commonly used compression rates do not correspond to thermodynamic equilibrium. Under isobaric conditions at π ⩾ 10 mN / m , a slow compression was found, corresponding to the formation of crystals on top of the monolayer. The AFM images reveal that SSS initially form trident monolayers at air–water interface. These layers are thermodynamically stable at surface pressure π ⩽ 5 mN / m . The thickness of the trident monolayer was found to be 1.6–1.8 nm, corresponding to tilt angles of the molecule chains varying from τ = 43 ° at π = 10 mN / m to τ = 53 ° at π = 40 mN / m . For π ⩾ 10 mN / m , growth of crystals takes place with a tuning fork conformation of the SSS molecules on top of the trident monolayer. The crystals grow with time, mainly in lateral directions. The growth rate increases with surface pressure. A new model is developed to quantitatively describe the crystal growth process. A lateral growth rate of 2.3 nm/min and a vertical growth rate of 0.005 nm/min were calculated for 1 individual crystal at π = 10 mN / m . The same growth process that was observed on the air–water interface was also observed in transferred monolayers at room temperature, though the growth was much slower.