Growth Terraces Research Articles

Growth-sector dependence of morphological, structural and optical features in boron-doped HPHT diamond crystals

Semiconducting boron-doped diamond single crystals of cubo-octahedral habit with prevalent development of octahedron {111} faces and insignificant area of cube {001}, rhombo-dodecahedron {110} and tetragon-trioctahedron {311} faces were obtained using solution-melt crystallization at high pressure 6.5 GPa and temperatures 1380…1420 °C. Using the Fe-Al solvent, which allows controlled incorporation of boron dopant between 2·10–4…10–2 at.% made it possible to vary the electro-physical properties of the crystals. Methods of micro-photogrammetry, atomic force microscopy, and micro-Raman spectroscopy were applied to reveal sectorial inhomogeneity of impurity composition and morphology of different crystal faces. The obtained crystals were shown to have high structural perfection and boron concentration ranging approximately from 1·1017 up to 7·1018 cm–3. An increase in boron concentration increases the area of {111} faces relatively to the total crystal area. Nanoscale morphological features like growth terraces, step-bunching, dendrite-like nanostructures, columnar substructures, negative growth pyramids on different crystal faces are shown to reflect peculiarities of carbon dissolution at high pressures and temperatures. The changes in the crystals’ habit and surface morphology are discussed in relation to inhomogeneous variation of thermodynamic conditions of crystal growth and dissolution at different boron concentrations.

Kinetic Monte Carlo method for epitaxial 3C-SiC (0001) growth on vicinal surfaces

A kinetic Monte Carlo model has been developed to predict island nucleation and step flow growth on vicinal surface during epitaxial growth of 3C-SiC. In the model, the crystal lattice is represented by a structured mesh which retains fixed atom positions and bond partners of the 3C-SiC crystal lattice. The events considered in the model are the deposition, evaporation and diffusion of atoms, and the attachment and detachment of adsorbed atoms. Both the incorporation energy barrier and Ehrlich-Schwoebel (ES) barrier are taken into account in the model when an adatom migrates to a step or island. The growth parameters such as growth temperature, coverages and terrace width can be selected freely, and the cross process of step flow growth and island nucleation can be simulated. The simulation results show that lower growth temperatures lead to a larger number of islands forming on the vicinal surfaces. There is also an observed coupling between island nucleation and steps at higher temperature. For smaller coverages, the islands and step patterns grow independently, and the steps and islands begin eventually to coalesce over the time. Furthermore, a decrease in terrace width leads to adatoms preferentially attaching to steps and the system to go to a step flow growth system. The results are consistent with predictions for the case of the epitaxial growth on vicinal surfaces.

The synergistic role of azeotropic solvent mixtures and atactic polystyrene on the morphology, crystallization and field effect mobility of thin film 6,13-bis(triisopropylsilylethynyl)-pentacene based semiconductors

The effect of anisole/decane binary solvent mixture and the subsequent addition of low wt% aPS on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) thin film morphology is investigated by optical microscopy, AFM and UV–Vis measurements. We show that, over the composition range anisole/decane of 96/4 to 85/15 wt%, the solution maintains an azeotropic composition with the boiling point of the binary mixture remaining constant at 152 °C, and the solvent composition remaining constant during evaporation and drying. It was found that addition of up to 20 wt% decane has little impact on micro-scale crystal morphology but has a significant influence on the growth mode and terrace roughness. The formation of large crystals is explained in terms of the change in solvent, increase in decane content, weakening the solute–solvent interactions and promoting efficient nucleation of the favoured H-aggregates of TIPS-pentacene. The effect of the TIPS-pentacene—aPS ratio up to 20 wt% for drop-cast thin film was similarly investigated. It is found that addition of aPS has a significant effect on both macroscopic crystal properties such as surface coverage, unity of orientation and long range order. It also changes the surface morphology and layer ordering on the nano-scale.

High growth rate GaN on 200 mm silicon by metal-organic vapor phase epitaxy for high electron mobility transistors

It is increasingly important to reduce the cycle time of epitaxial growth, in order to reduce the costs of device fabrication, especially for GaN based structures which typically have growth cycles of several hours. We have performed a comprehensive study using metal-organic vapor phase epitaxy (MOVPE) investigating the effects of changing GaN growth rates from 0.9 to 14.5 µm/h. Although there is no significant effect on the strain incorporated in the layers, we have seen changes in the surface morphology which can be related to the change in dislocation behaviour and surface diffusion effects. At the small scale, as seen by AFM, increased dislocation density for higher growth rates leads to increased pinning of growth terraces, resulting in more closely spaced terraces. At a larger scale of hundreds of µm observed by optical profiling, we have related the formation of grains to the rate of surface diffusion of adatoms using a random walk model, implying diffusion distances from 30 µm for the highest growth rates up to 100 µm for the lowest. The increased growth rate also increases the intrinsic carbon incorporation which can increase the breakdown voltage of GaN films.Despite an increased threading dislocation density, these very high growth rates of 14.5 µm/hr by MOVPE have been shown to be appealing for reducing epitaxial growth cycle times and therefore costs in High Electron Mobility Transistor (HEMT) structures.

Effect of ion irradiation on surface morphology and superconductivity of BaFe2(As1−xPx)2 films

We have irradiated epitaxial thin films of BaFe2(As1−xPx)2 with x≃0.2 (optimal doping) with Au ions having an energy of 250MeV. We have used two different fluences, Φ1=2.4×1011cm−2 and Φ2=7.3×1011cm−2, and we have studied the effects of irradiation on the surface morphology, on the resistivity and on the critical temperature. We have found that irradiation progressively destroys the very clear and interconnected growth terraces typical of the pristine surface, leading first to their smoothening – accompanied by the appearance of localized defects – and then to a completely disordered surface. The residual resistivity increases by almost 60%, but the critical temperature decreases very little (i.e. by about 2%) on going from the pristine film to the most irradiated one. The possible role of the substrate in these results is discussed.

Quaternary hinterland evolution of the active Banda Arc: Surface uplift and neotectonic deformation recorded by coral terraces at Kisar, Indonesia

Coral terrace surveys and U-series ages of coral yield a surface uplift rate of ∼0.5m/ka for Kisar Island, which is an emergent island in the hinterland of the active Banda arc–continent collision. Based on this rate, Kisar first emerged from the ocean as recently as ∼450ka. These uplifted terraces are gently warped in a pattern of east–west striking folds. These folds are strike parallel to more developed thrust-related folds of similar wavelength imaged by a seismic reflection profile just offshore. This deformation shows that the emergence of Kisar is influenced by forearc closure along the south-dipping Kisar Thrust. However, the pinnacle shape of Kisar and the protrusion of its metamorphic rocks through the forearc basin sediments also suggest a component of extrusion along shear zones or active doming.Coral encrusts the island coast in many locations over 100m above sea level. Terrace morphology and coral ages are best explained by recognizing major surfaces as mostly growth terraces and minor terraces as mostly erosional into older terraces. All reliable and referable coral U-series ages determined by MC-ICP-MS correlate with marine isotope stage (MIS) 5e (118–128ka). The only unaltered coral samples are found below 6m elevation; however an unaltered Tridacna (giant clam) shell in growth position at 95m elevation yields a U-series age of 195±31ka, which corresponds to MIS 7. This age agrees with the best-fit uplift model for the island. Loose deposits of unaltered coral fragments found at elevations between 8 and 20m yield U-series ages of <100years and may represent paleotsunami deposits from previously undocumented tectonic activity in the region.

Channel and landscape dynamics in the alluvial forest mosaic of the Carmanah River valley, British Columbia, Canada

The highly diverse shifting-mosaic of forest patches of an alluvial forest within the Carmanah River valley on the west coast of Vancouver Island, British Columbia was studied to examine the hydrogeomorphic disturbance regime that structures it. We used a landscape-scale analysis to quantify historical channel migrations and changes in the extent of specific forest types. This GIS-based analysis using a 70-year aerial photographic record was complemented by field-based research. Thirty-eight plots containing 4509 trees were sampled for forest structure, age, and elevation above the contemporary channel. These data, including a vegetation chronosequence spanning over 500years, were used to examine channel and landscape dynamics. Our findings support a general conceptual model that describes cycles of patch development and destruction in unconfined alluvial forests of the Pacific Coastal Ecoregion. Over the past century, Carmanah River has eroded nearly 30% of the alluvial forest in this study area, and approximately 65% over the past 500years. At least 80% of the 2007 channel was forested area within the past 70years. Younger landforms were disturbed more frequently than mature forest patches, which suggest that as biogeomorphic succession progresses the likelihood of future disturbance decreases. Estimated half lives of landforms ranged from 24years for pioneer bars to over 1500years for old growth terraces. Years of regional high magnitude floods resulted in a net loss of floodplain forest area indicating that disturbance was climate driven in this pluvial watershed, whereby rain events result in flood disturbance that converted forests to channel. These events initiate a subsequent course of vegetation succession and geomorphic development, and often result in the deposition of large wood that modifies the channel environment and contributes to channel avulsion and further hydrogeomorphic disturbance. The composition of the landscape is a reflection of the balance between the disturbance rate and successional development. We also observed a relationship between landscape composition and watershed size. Specifically, the ratio of mature to developing alluvial forests was higher in this smaller watershed compared to larger watersheds in the region. Results imply that larger flood events predicted to occur with climate change may change the disturbance regime of floodplain forests and alter landscape composition.

Crystal structure and surface morphology of magnetron sputtering deposited hexagonal and perovskite-like YbMnO3 thin films

Abstract Thin YbMnO 3 films with thickness 50–100 nm were grown by RF-magnetron sputtering on NdGaO 3 (0 0 1), SrTiO 3 (0 0 1), SrTiO 3 (1 1 0), LaAlO 3 (0 0 1) and on buffer layered Pt(1 1 1)/SrTiO 3 (1 1 1) substrates. X-ray analysis reveals that the platinum buffer layer grown on a single crystalline SrTiO 3 (1 1 1) substrate as well as the hexagonal YbMnO 3 film with c -axis normal to the substrate plane grow epitaxially. The orthorhombic (perovskite-like) modification of YbMnO 3 , which otherwise forms in a bulk hexagonal structure, has been epitaxially stabilized on NdGaO 3 , SrTiO 3 and LaAlO 3 single crystal substrates. The surface topography of the films was studied by atomic force microscopy. The surface of the hexagonal films reveals spiral-shaped growth terraces with a step height of half the c -lattice parameter. The perovskite-like films have a smooth surface with small 50–100 nm islands. Piezoresponse force microscopy was used to demonstrate the ferroelectric behavior of the hexagonal films. Thin films of a compound with a structure different from the bulk one can also show different physical properties. In the case of multiferroics it can lead to enhancing of magnetoelectric coupling.

High temperature solution growth, chemical depotassiation and growth mechanism of KxRhO2 crystals

Single-phase KxRhO2 single crystals (x = 0.70, 0.60 and 0.50) have been grown with a spontaneous nucleation method. More potassium deficient KxRhO2 single crystals (x = 0.39 and 0.25) were also obtained by chemical de-intercalations from the as-grown crystal but two-phases exist, which is proved by an electrochemical method. Microstructure characterizations by scanning electron, atomic force microscopy and transmission electron microscopy reveal the existence of a growth step, growth terrace and tri-junction on the as-grown KxRhO2 crystals. These results suggest that the growth of KxRhO2 follows a two-dimensional nuclei layered growth mechanism, which is also consistent with the Jackson factor extracted from the differential scanning calorimetric result. This work provides crucial information to optimize the growth conditions of KxRhO2 single crystals.

Calcite surface structure and reactivity: molecular dynamics simulations and macroscopic surface modelling of the calcite–water interface

Calcite-water interactions are important not only in carbon sequestration and the global carbon cycle, but also in contaminant behaviour in calcite-bearing host rock and in many industrial applications. Here we quantify the effect of variations in surface structure on calcite surface reactivity. Firstly, we employ classical Molecular Dynamics simulations of calcite surfaces containing an etch pit and a growth terrace, to show that the local environment in water around structurally different surface sites is distinct. In addition to observing the expected formation of more calcium-water interactions and hydrogen-bonds at lower-coordinated sites, we also observed subtle differences in hydrogen bonding around acute versus obtuse edges and corners. We subsequently used this information to refine the protonation constants for the calcite surface sites, according to the Charge Distribution MUltiSite Ion Complexation (CD-MUSIC) approach. The subtle differences in hydrogen bonding translate into markedly different charging behaviour versus pH, in particular for acute versus obtuse corner sites. The results show quantitatively that calcite surface reactivity is directly related to surface topography. The information obtained in this study is not only crucial for the improvement of existing macroscopic surface models of the reactivity of calcite towards contaminants, but also improves our atomic-level understanding of mineral-water interactions.

Direct imaging of the structural domains in the iron pnictidesAFe2As2(A=Ca,Sr,Ba)

The parent compounds of recently discovered iron-arsenide superconductors, $A{\text{Fe}}_{2}{\text{As}}_{2}$ with alkaline earth $A=\text{Ca},\text{Sr},\text{Ba}$, undergo simultaneous structural and magnetic phase transitions at a temperature ${T}_{SM}$. Using a combination of polarized light microscopy and spatially resolved high-energy synchrotron x-ray diffraction we show that the orthorhombic distortion leads to the formation of $45\ifmmode^\circ\else\textdegree\fi{}$-type structural domains in all parent compounds. Domains penetrate through the sample thickness in the $c$ direction and are not affected by crystal imperfections such as growth terraces. The domains form regular stripe patterns in the plane with a characteristic dimension of $10--50\text{ }\ensuremath{\mu}\text{m}$. The direction of the stripes is fixed with respect to the tetragonal (100) and (010) directions but can change by $90\ifmmode^\circ\else\textdegree\fi{}$ on thermal cycling through the transition. This domain pattern may have profound implications for intrinsic disorder and anisotropy of iron arsenides.

Surface kinetics and thermal instability of N-face InN grown by plasma-assisted molecular beam epitaxy

The role of thermal instability and In surface coverages on the growth kinetics has been investigated for N-face InN films grown by plasma-assisted molecular beam epitaxy. Film thickness analysis using scanning electron microscopy combined with In desorption measurements by quadrupole mass spectrometry demonstrated significant thermal decomposition starting at ∼560°C and inhibiting growth completely beyond ∼635°C. Within this temperature region two decomposition pathways were identified: a low-temperature regime characterized by In droplet accumulation and a high-temperature regime with direct desorption from bulk InN. A growth diagram has been constructed, exhibiting three characteristic growth structures for different In∕N flux ratios and growth temperatures: a dry no-adlayer terminated surface under N-rich conditions, an In adlayer terminated surface, and a surface, consisting of an In adlayer and droplets under In-rich conditions. Smooth step-flow growth terraces were observed in films grown under In-rich and surprisingly also under N-rich conditions at temperatures of thermal decomposition. Such high adatom diffusivity resulted from the autosurfactant action of the In adlayer, with a saturated coverage of 1 ML as determined from the reflection high energy electron diffraction patterns during the consumption of adsorbed In by active nitrogen.

Homogeneous spectroscopic properties in manganite films

The correlation between topographic features and spectroscopic properties was studied by means of room-temperature scanning tunnelling microscopy on single-crystalline fully strained La 0.77 Ca 0.23 MnO 3 films. Growth terraces are atomically flat and present similar semiconducting-like tunnel current–voltage characteristics. The STM-estimated gap is in agreement with the polaron binding energy obtained from the insulating-phase resistivity. Conductance maps at different voltages do not present recognizable spatial patterns. In conclusion, ‘extrinsic’ phase separation is not likely in the manganite films studied.

Seeded growth of AlN on N- and Al-polar [formula omitted] AlN seeds by physical vapor transport

We demonstrated seeded growth of AlN on large-area Al- and N-polar <0 0 0 1>-oriented AlN seeds using the physical vapor transport method (PVT). In both cases, crystals having a diameter of 15 mm were obtained from 5 mm seeds. Based on growth step and terrace width analyses, it was found that the N-polar face was suitable for growth within a large window of growth parameters while the Al-polar seeds yielded high-quality crystals only at low supersaturation.

Spatial distribution and environmental preferences of the piassaba palm Aphandra natalia (Arecaceae) along the Pastaza and Urituyacu rivers in Peru

Aphandra natalia (Balslev and Henderson) Barfod, an economically important fibre producing palm, is common in rainforest on low terraces along the Pastaza and Urituyacu rivers in Amazonian Peru. The aim of this study was to investigate the spatial distribution and environmental preferences of Aphandra in old-growth terrace forest to which it is limited in this region. Densities of immature and mature individuals were 507 ± 212 (S.D.) ha −1 and 19 ± 8 ha −1, respectively, in 11 (5 × 500 m) transects placed in old growth terrace forest near four villages and 739 ± 188 ha −1 and 96 ± 49 ha −1, respectively, in six irregular transects placed in what the local villagers considered dense Aphandra stands. We examined environmental and spatial correlates of Aphandra occurrences using stepwise multiple autologistic regressions. Site, soil moisture, slope inclination, and topographic position influenced the spatial distribution of Aphandra. Furthermore, the distribution was strongly clumped, independently of environmental factors, with particularly the concentration of immature individuals around adults pointing to dispersal limitation as the likely causal mechanism.

Spiral growth of grossular under hydrothermal conditions

Grossular (Grs 99 Adr 01 ) crystals were synthesized in hydrothermal experiments by the reaction wollastonite + calcite + anorthite = grossular + CO 2 (400 MPa; 695-750 °C; X CO₂ = 0.1; solid:fluid = 2:1) using natural minerals as starting materials. Growth spirals are visible due to silica decorations. The crystals are bounded by {110} and {211}. If crystallized at T ≤ 730 °C, they are birefringent and show sector twinning; grown at T ≥ 730 °C, they are optically isotropic. The crystal growth rates in the experiments ranged from about 1 to &gt;100 μm/d. The edges of the growth terraces moved by 5-200 nm/s. The morphology of the growth steps is interpreted against the background of supersaturation with respect to grossular in the fluid. The highest supersaturation occurs during the initial stage of the reaction (~1 h), ultimately at the wollastonite surfaces. In this initial stage, grossular crystallizes with only {111} faces present covered by round growth spirals. When a steady state with respect to dissolution and precipitation rates is reached (after ~24 h), all grossular crystals show {110} and {211} with polygonal spirals on {110} and parallel growth steps on {211}. All straight growth-step edges parallel [11-1]. When the solid phases assume equilibrium with the fluid (after calcite in calcite-deficient experiments is used up completely), round growth spirals appear on {211}. The spirals are elongated parallel to [11-1] and compete against parallel steps in that same direction. The surface morphology of the crystals synthesized under steady state conditions mimicks that of natural grandite crystals, and their growth rates equal the estimated growth rates of grandite crystals from natural hydrothermal systems.

Silicalite Crystal Growth Investigated by Atomic Force Microscopy

Atomic force microscopy has been used to image the various facets of two morphologically distinct samples of silicalite. The smaller (20 microm) sample A crystals show 1 nm high radial growth terraces. The larger (240 microm) sample B crystals show growth terraces 1 to 2 orders of magnitude higher than the terraces on sample A with growth edges parallel to the crystallographic axes. Moreover, the terraces on the (010) face are significantly higher than the terraces on the (100) face - inconsistent with the previously proposed 90 degrees intergrowth structure. Sample A highlights that under certain synthetic conditions, silicalite grows in a manner akin to zeolites Y and A, via the deposition of layers comprising, in the case of silicalite, pentasil chains. It is probable that the rate of terrace advance is identical on the (010) and (100) faces, and it is the rate of terrace nucleation that dictates the overall growth rate of each facet and hence the relative size expressed in the final crystal morphology. Analysis of the growth terraces of sample B and detailed consideration of the structures of both MFI, and a closely related material MEL, lead to the proposal of a generalized growth mechanism for silicalite including the incorporation of defects within the structure. These defects are thought to be responsible for both the relative and the absolute terrace heights observed and may also explain the hourglass phenomenon observed by optical microscopy. The implications of this growth mechanism, supported by results of infrared microscopy, generate a new dimension to the continuing debate on the existence of intergrowths within one of the most important structures relevant to zeolite catalysis.

IN SITU CHARACTERISATION OF CALCITE GROWTH AND INHIBITION USING ATOMIC FORCE MICROSCOPY

Real time in situ monitoring of calcite growth and inhibition on the cleavage plane [Formula: see text] is investigated using atomic force microscopy (AFM). Calcite growth and inhibition were studied using a Molecular Imaging microscope in contact mode, equipped with an in situ fluid cell. As has been reported previously, it is observed that calcite growth from aqueous solution is by motion of mono-molecular steps, and dissolution by a combination of step motion and etch pit expansion. The measured step heights were between 2.7 to 3 Å. Our aim is to study the effects of a range of phosphonate-based crystal growth inhibitors of related structure, in order to provide insight into the mechanism of inhibition as a function of the additive structure. Results of this study demonstrate the effect of inhibitors on the growth steps and terraces on the [Formula: see text] surface of calcite in real time. The organic additives bind to the crystal surface, with selective binding to the step edges particularly evident in some cases. In other cases, it appears that the additive acts by binding to the terraces on the crystal surface, resulting in inhibition by coating the surface. Efforts to correlate the observations made by atomic force microscopy with bulk crystallization experiments have been made.

Observation of curious spiral-like features on spray pyrolysed Pb doped Hg-based HTSC films

The surface morphology of spray pyrolysed Hg(Pb)Ba2Ca2Cu3O8+δ films is investigated by scanning electron microscopy (SEM). By imaging the as-grown surfaces of the films, we have directly observed spiral-shaped growth terraces that emanate from screw dislocations. The high density of screw dislocations (∼106cm−2) observed by SEM micrographs of the films is expected to nucleate during the initial stages of growth due to the bi-phasic nature of the material, providing a continual supply of ledge incorporation sites for the depositing species. A possible mechanism, for the generation of these screw dislocations in terms of incoherent coalescence of growth fronts formed from Hg(Pb):1223 and Hg(Pb):1234 nuclei, is described.

The effect of sputter deposition conditions on the growth mechanism of YBa2Cu3O7- delta thin films studied by scanning tunnelling microscopy

Surface topography of as-grown films of YBa2Cu3O7- delta (YBCO) deposited by off-axis RF magnetron sputtering on MgO and SrTiO3 single-crystal (100) substrates, has been investigated by scanning tunnelling microscopy (STM)/scanning tunnelling spectroscopy (STS) operated in air at ambient temperature. Both c-axis- and a-axis-oriented YBCO films have been investigated. In the case of c axis thin films, we have directly observed spiral-shaped growth terraces, which emanate from screw dislocations of this layered superconducting oxide. The growth steps are generally seen to possess a step height close to or multiples of the unit cell height of the YBCO crystalline structure. In the case of a axis growth, the substrate as well as the deposition temperature determines the grain orientation. Furthermore, our STS data revealed that the surface layer of the film is semiconducting and the tunnelling spectrum varies its shape depending on the tip-to-sample distance. A superconducting gap appears clearly in the spectra when the STM tip is placed closer to the surface than the normal position of the scanning mode. This suggests that the semiconducting layer is confined in the topmost surface region of the as-prepared film, while the layer beneath it is superconducting in character.