Layer Growth Mechanism Research Articles

Growth of GaN Single Crystals Using Ca−Li3N Composite Flux

A Ca−Li3N composite flux was introduced to grow GaN single crystals from Ga melts. The obtained GaN crystals were colorless and up to 4 mm at 800 °C under N2 pressure of about 2 atm. The effects of introducing the composite flux on the size distribution, morphologies, and yield of the GaN crystals were studied. It was found that the addition of Ca retarded the homogeneous nucleations to some extent and resulted in a more uniform size distribution of the GaN crystals in comparison with that grown using Li3N as flux only. The layered morphological feature on the GaN surface suggested that the layer growth mechanism was probably enhanced by the composite flux. The yield of the crystals was strongly dependent on the flux compositions. These results provided a possible new route for improving the growth of GaN crystals from melts in the future by optimizing the composite flux.

Residual stresses modelled by MD simulation applied to PVD DC sputter deposition

The molecular dynamic method was extended for use in deposition techniques, therefore the development of dynamic equations, boundary conditions and mesoscopic observable was necessary. By application of the Tight-binding method based on the density functional theory for molecular potential functions, it is possible to analyze and optimize the nucleation and layer growth mechanisms of elementary layer substrate systems. So far, we have deposited five copper atoms on a silicon (111) substrate surface. With a mesoscopic stress observable we have measured a residual tensile stress of − 650 MPa.

Effects of temperature and transport conditions on calcite growth in the presence of Mg 2+: Implications for paleothermometry

This study links direct measurement of Mg-calcite growth kinetics with high-spatial-resolution analysis of Mg contents in experimental crystals, with particular attention to the effects of temperature on growth rate and reactant transport conditions on Mg distribution. In contrast to previous experiments on Mg partitioning into calcite, here the layer-growth mechanism was observed in situ and step speeds precisely measured with fluid cell atomic force microscopy over a range of temperatures, degrees of supersaturation, and solution Mg concentrations. Data collected from 15° to 30°C yield an activation energy for calcite precipitation of 33 kJ/mol for solutions with [Mg] = 5 × 10 −5 molal. Electron microprobe analyses of large hillocks grown at corresponding conditions demonstrate that Mg has a strong preference for incorporation at negative (acute) step edges, rather than at positive (obtuse) edges when growth rate is limited by surface reactions. This preference is reversed when growth is instead limited by diffusion of reactants through a boundary layer at the mineral-solution interface. These findings show that temperature is not the only strong control on the extent of Mg incorporation and distribution in calcite; transport conditions during mineral growth may also be a first-order factor governing the compositions of natural calcite samples.

Flux growth of (Pb,La)(Zr, Sn,Ti)O 3 single crystals and their characterization

Lead lanthanum zirconate titanate stannate (PLZST) single crystals have been successfully grown by a flux method using KF as flux. Well-developed single crystals with sizes of 2.0 mm×2.0 mm×1.5 mm were obtained for the first time. The crystal morphology was studied and related to a layer growth mechanism controlled by two-dimensional growth. Crystal structure measured by X-ray diffraction technique was single perovskite. The chemical composition of as-grown crystal was analyzed by an energy dispersive X-ray spectrometer and inductively coupled plasma atomic emission spectrometry. The maximum dielectric constant was determined to be 2400. The Curie temperature was found to be about 170 °C, similar to the polycrystalline data.

Growth mechanism and thermoelectric properties of PbTe/SnTe/PbTe heterostructures

An electron microscopy study of the mechanism of SnTe and PbTe layer growth in PbTe/SnTe/PbTe heterostructures prepared by thermal evaporation in vacuum onto a KCl substrate was performed. It is established that PbTe and SnTe grow on one another in a layer-by-layer fashion with the introduction of misfit dislocations on the interface at a critical thickness dc≈2 nm. The experimentally determined dependence of the elastic stress in a growing layer (either PbTe or SnTe) on the layer thickness and the critical thickness are in good agreement with those calculated theoretically. The dependences of the thermoelectric properties of PbTe/SnTe/PbTe heterostructures on the SnTe layer thickness (dSnTe=5–100 nm) at fixed thicknesses of the PbTe layers were studied at room temperature. It was found that in the thickness range of dSnTe≈(10–15) nm, an inversion of the dominant carrier sign from n to p takes place. The d-dependences of the thermoelectric properties were interpreted within the framework of a three-layer model, treating a PbTe/SnTe/PbTe heterostructure as three conductors connected in a parallel fashion, each characterized by its specific electrophysical parameters.

Hydrothermal synthesis of titanosilicate ETS-10 using Ti(SO 4) 2

The ternary reaction composition diagram in the SiO 2–Na 2O–TiO 2–KF–H 2SO 4–H 2O system was developed for static ETS-10 synthesis at 473 K utilizing Ti(SO 4) 2 and N-brand sodium silicate solutions as sources of Ti and Si, respectively. For KF/TiO 2 = 1.5 and H 2O/TiO 2 = 300, pure, crystalline (⩾95%) ETS-10 products were obtained in a narrow reaction composition zone (Na 2O/SiO 2 = 0.790–0.825, SiO 2/TiO 2 = 5.30–5.90, initial pH = 10.22–10.95). The presence of islands and terraces with heights of ∼1.2–1.7 nm suggests a layer growth mechanism via two-dimensional surface nucleation. The aspect ratio R = c/ a of typical ETS-10 crystals ( c is the axial length of the bi-pyramid and a is the linear dimension of the square basal plane shared by the two pyramids) and the roughness of crystal pyramidal side surface decreased with increasing initial synthesis mixture pH. The Si/Ti ratios in the ETS-10 products were close to the theoretical value of 5. The crystallization curves for this pure ETS-10 showed that the induction time decreased substantially from 36 to 19 h, whereas crystallization rate increased only slightly from 19% to 27% h −1 upon increasing initial pH. Increasing dilution of the initial synthesis mixture with water (5.50SiO 2:4.46Na 2O:1.00TiO 2:1.5KF:3.35H 2SO 4:450–750H 2O) resulted in progressively longer crystallizations, producing bi-pyramidal crystals of monotonically increasing dimensions. The results of experiments using NaF instead of either KF or KOH (i.e., conducted without K + and F +) suggested that in unseeded, organic-free ETS-10 syntheses, K + was necessary for the nucleation of ETS-10, while F − was not.

Enhanced flux pinning in (Bi, Pb)-2223 superconductor by Nd addition

Bi1.8Pb0.35Sr2.0Ca2.1Cu3.1NdxOy (x = 0.00,0.010, 0.020 and 0.030) superconductors were prepared in bulk form. Phase analysis byXRD, microstructural examination by SEM, superconductivity measurements at 77 K, etcwere carried out to evaluate the relative performance of the samples. Both the self-fieldJC and JC in the presence of an applied magnetic field of the Nd-added samples are found to be muchbetter than those of the undoped one. Also the peak value of the bulk pinning force density,Fp max, of Nd-added samples get shifted towards higher magnetic fields, indicativeof enhanced flux pinning strength in these samples. For the sample withNd = 0.030, small fractions of (Bi, Pb)-2212 were found even after the last stage of sintering. Accordinglythe microstructure of this sample shows considerable difference from the other samples,wherein the natural layered growth mechanism of (Bi, Pb)-2223 is disrupted by graingrowth and coarsening in random directions. It is likely that Nd ions are substituting at theCa site, and the point defects thus created act as flux pinners, enhancing the self-fieldJC as wellas the JC–B characteristics of the Nd-added samples.

Comparison of the growth mechanism of TiO 2-coated SiO 2 particles prepared by sol–gel process and water-in-oil type microemulsion method

TiO 2-coated SiO 2 particles were synthesized by sol–gel process and microemulsion method, respectively, in order to compare the growth mechanism of TiO 2 particles coated on SiO 2 core particles. Firstly, monodispersed SiO 2 core particles were prepared and then the SiO 2 particles were allowed to coat and grown by further addition of TEOT as a precursor of TiO 2 particles. In this work, Nielsen's chronomal analysis and Overbeek's theory were applied to elucidate the growth mechanism of TiO 2 particles coated on the surface of SiO 2 core particles. As a result, it was found that the growth mechanism of all of the TiO 2-coated SiO 2 particles obtained by sol–gel process and microemulsion method was presumed to be surface nucleation growth in which the rate-determining step was a first-order polynuclear layer growth mechanism. In the case of particle size distribution, however, sol–gel process was more advantageous than microemulsion method. In addition, the growth rate constant of TiO 2-coated SiO 2 particles prepared by sol–gel process (2.95 × 10 −6 cm/s) was much faster than that by microemulsion method (6.06 × 10 −8 cm/s). So, sol–gel process was suitable to increase the particle size of TiO 2-coated SiO 2 particles.

Surface morphological study of flux grown lead doped rare-earth manganite single crystals

Rare-earth manganites with divalent doping have attracted attention because of their interesting physical phenomena. Manganites, doped with divalent lead, are grown from solvent PbO–PbF 2. Single crystals of R 1− x Pb x MnO 3 and mixed rare-earth (R y R′ 1− y ) 1− x Pb x MnO 3, where R′, R=La, Nd and Pr are grown with different values of x using eutectic composition of PbO: PbF 2 as solvent. Growth temperature can then be significantly reduced. Optimized charge-to-flux ratio 1:6 yields large crystals with dimension up to 4×3×2 mm 3. A variety of surface features such as hillocks, micro and macro step patterns, dendrites as well as clear featureless surfaces are observed on as-grown crystals. Clear surfaces reveal that growth occurred predominantly by two-dimensional layer growth mechanism. The hillocks might have originated at the site of screw dislocation, which suggests the growth mechanism at low supersaturation. Hopper growth observed on a few crystals is suggestive of unstable growth. Optimum growth parameters are employed to achieve better yield and good quality crystals.

ATOMIC FORCE MICROSCOPY STUDIES ON GROWTH STEPS AND 2D NUCLEI ON THE {100} FACES OF DEUTERATED L-ARGININE PHOSPHATE CRYSTALS

Growth steps and 2D nuclei of the {100} faces of the deuterated L-arginine phosphate (DLAP) crystals have been studied using ex-situ atomic force microscopy (AFM). Straight steps along the b direction as well as meandered steps are detected. The bunched steps have wider terraces than the elementary ones, which are supposed to result from the slower growth rate of the former than the latter. Many 2D nuclei exist on the step terraces and edges acting as the growth sources. Occasionally, 2D islands generated by 2D nuclei could also be observed. In conclusion, the crystal grows by layer growth mechanism.

Anisotropic behaviour of semiconducting tin monosulphoselenide single crystals

Single crystals of ternary mixed compounds of group IV-VI in the form of a series, SnSxSe1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), have been grown using direct vapour transport technique. The grown crystals were characterized by the X-ray diffraction analysis for their structural parameter determination. All the grown crystals were found to be orthorhombic. The microstructure analysis of the grown crystals reveals their layered type growth mechanism. From the Hall effect measurements Hall mobility, Hall coefficient and carrier concentration were calculated with all crystals showingp-type nature. The d.c. electrical resistivity measurements perpendicular toc-axis (i.e. along the basal plane) in the temperature range 303–453 K were carried out for grown crystals using four-probe method. The d.c. electrical resistivity measurements parallel to c-axis (i.e. perpendicular to basal plane) in the temperature range 303–453 K were carried out for the same crystals. The electrical resistivity measurements showed an anisotropic behaviour of electrical resistivity for the grown crystals. The anisotropic behaviour and the effect of change in stoichiometric proportion of S and Se content on the electrical properties of single crystals of the series, SnSxSe1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), is presented systematically.

Study of the Elementary Growth Processes During Vapor-Phase Epitaxy of Semiconducting III–V Compounds

A review of studies performed at the V. D. Kuznetsov Siberian Physical-Technical Institute at Tomsk State University and aimed at obtaining detailed information on the elementary growth processes proceeding at the crystallization front during vapor-phase epitaxy of semiconducting III–V compound films is presented in the paper. The general approach to the problem and methods of its solution are described. Results of investigations of the adsorption layer composition, surface diffusion processes, and incorporation of growth components into a crystal are presented. The mechanism of epitaxial layer growth in semiconducting III–V compounds is discussed.

Growth of In 2O 3(ZnO) 5 platelet micro-crystals by flux methods

Growth of In 2O 3(ZnO) 5 micro-crystals was investigated by flux methods. In 2O 3(ZnO) 5 platelet micro-crystals were synthesized in sulfate and CaCl 2 salts fluxes at 1150°C and 1050°C, respectively. The former showed a narrow size distribution with a median size of 2.6 μm; the latter showed a bimodal size distribution with a peak of primary particles around 26 μm and another peak of their agglomerates around 600 μm. The crystal morphology of crystals from CaCl 2 flux was related to a layer growth mechanism controlled by two-dimensional nucleation. Lattice-constants refinement and ICP-AES elemental analysis revealed that Ca tends to be incorporated into the In 2O 3(ZnO) 5 lattice probably at the Zn site.

Fabrication and Dielectric Properties of Multilayer Ta2O5/Al2O3 Nanostructures

Multilayer Ta2O5/Al2O3 nanostructures are produced on Si(100) and Al by molecular layering, and their dielectric properties are investigated. The mechanisms of layer growth are discussed in terms of surface reactions.

Corrosion behaviour of the amorphous Mg 65Y 10Cu 15Ag 10 alloy

The corrosion behaviour of the amorphous Mg 65Y 10Cu 15Ag 10 alloy as well as of its crystalline multiphase counterpart was studied in alkaline electrolytes and compared with that of the amorphous Mg 65Y 10Cu 25 alloy. Electrochemical investigations were carried out in 0.3 M H 3BO 3/Na 2B 4O 7 buffer solution with pH=8.4 and in 0.1 M NaOH solution with pH=13. Tafel plots were recorded and cyclic potentiodynamic polarisation tests were conducted, transients were measured at anodic potentials. Potentiostatically formed surface layers were characterised by Auger electron spectroscopy and atomic force microscopy. Changes in the corrosion behaviour were noticed which are attributed to the presence of silver. The passive layers formed in the two electrolytes were quite different in the composition as well as in morphology. The layer growth mechanisms also showed some variation presumably mainly due to the presence of silver, though copper still seems to play a dominant role in the passivation of this alloy in the weakly alkaline solution. The amorphous alloys displayed superior corrosion behaviour compared to the crystalline alloy, because of the absence of the heterogeneties existing in crystalline alloys.

Growth mechanism and electrical properties of Pb[(Zn 1/3Nb 2/3) 0.91Ti 0.09]O 3 single crystals by a modified Bridgman method

Relaxor-based piezoelectric Pb[(Zn 1/3Nb 2/3) 0.91Ti 0.09]O 3 (PZNT 91/9) single crystals 28 mm in diameter and 30 mm in length were grown by a modified Bridgman technique with PbO flux using an allomeric seed crystal. The crystals were grown in sealed platinum crucibles at about 1250°C. The obtained crystals are of pure perovskite structure within the sensitivity of X-ray diffraction measurement and the as-grown crystals have three appearing faces with slight deviation in angle from the pseudocubic (0 0 1) cub face. The growth mechanism of the PZNT crystals could be explained from the viewpoint of the formation of growth units and the incorporation of growth units into different interfaces of the crystal lattice. The growth rate of various crystal faces is related to the assembling modes of the growth units into different interfaces. Assembling of [BO 6] octahedral growth unit into (1 1 1) cub interface has the strongest bonding force, and this crystal direction has the fastest growth rate; therefore, (1 1 1) cub crystal face disappears, assembling of [BO 6] into (0 0 1) cub interface has the smallest bonding force, and this crystal face has the smallest growth rate; therefore, (0 0 1) cub crystal face appears. The growth steps aligning approximately along the 〈0 0 1〉 cub direction reveal that (0 0 1) cub face proceeds by a layer growth mechanism controlled by two-dimensional nucleation starting at crystal corners or edges. Dielectric measurement proves that the quality of the obtained PZNT crystals is good. Domain observation shows that both the rhombohedral and tetragonal orientation state domain coexist in the morphotropic PZNT 91/9 single crystals.

Nucleation, growth and properties of thin layers of iron phosphides

The relationship between iron phosphide layer growth parameters, their structure and properties were considered. The layers were generated on a base of Armco iron and 0.4% C, 1.1% Cr steel as a result of annealing in a mixture of argon and phosphorus vapours. Mechanism and kinetics of iron phosphide layer growth was described. It is possible to obtain a layer of Fe 3P+Fe 2P phosphides and a solid solution of phosphorus in iron. Iron phosphide layer increases the hardness and the friction wear resistance of iron alloys.

Effect of thiourea and saccharin on the roughness of electrodeposited ultrathin nickel and cobalt layers

Ni and Co films were produced by electrodeposition from plating baths containing either thiourea or saccharin. The effect of the organic additives on surface roughness was studied for the Ni-thiourea, Ni-saccharin, Co-thiourea and Co-saccharin systems. Layer thicknesses were varied from 1 to 10 nm and the additive concentration was varied from about 1 μM to 1 mM. Contact mode atomic force microscopy was used to measure both the root mean square (RMS) peak height (nm) and the areal peak density (μm−2) of each film. Although the RMS peak height and areal density were both influenced by film thickness and by additive concentration, the two roughness measures provide complementary information on the layer morphology and growth mechanisms. The four systems studied responded differently to changes in the concentration of the added organic. For example, for films 8 nm thick, addition of 1.22 × 10−3 thiourea reduced the Ni peak density about 80%; in contrast, the Co peak density was increased over 180% by addition of 1.25 × 10−3 thiourea. Use of peak height and peak density data to infer growth mechanisms for ultrathin films is discussed.

Growth models in microporous materials

Crystallisation pathways in framework materials, such as zeolites, have been monitored using a combination of atomic force microscopy, high-resolution electron microscopy and modelling. The principle conclusions of this study is that many open-framework crystals grow via a layer growth mechanism. The layers have a thickness which is related to the unit cell parameter in the direction of growth. The development of each layer can be sub-divided into a number of individual growth processes including nucleation of a new layer (or terrace), growth at edge sites and kink sites. Modelling of atomic force micrographs yields relative probabilities for these individual growth processes and reveals that growth at kink sites is favoured with nucleation of new terraces the slowest process. The layer mechanisms proposed explain the incorporation of defects in framework structures and further elucidate the role of structure directing agents.

Nb:BST: Crystal growth and ferroelectric properties

Nb substituted barium strontium titanate single crystals were grown by the high-temperature solution growth technique with different Nb concentrations. The growth conditions were optimized to grow good quality large sized single crystals. Nb doping reduced the twin formation considerably and yielded bulk single crystals. A surface morphology observation indicates layer and hopper growth mechanisms. Powder X-ray diffraction studies show an increase in c/a ratio with Nb content, and ferroelectric studies revealed a decrease in curie temperature and a sharp increase in dielectric constant and spontaneous polarization.