On the nature of growth hillocks on sodium chlorate crystals

Optical examination of as-grown {100} surfaces of sodium chlorate crystals grown from aqueous solution revealed the presence of elliptical growth hillocks. The hillocks were present on both enantiomorphous forms and originated from dislocations, inclusions, and microcrystals attached to the growing surface. The value of the surface entropy factor equal to 4.55 at 313 K suggests that crystals grow via/or with the participation of dislocation mechanism, and the hillocks are dislocation growth centers. Compound mechanism controlled growth of some crystals because edge nucleation and dislocation centers operated simultaneously on the same surfaces.     

Microstructures of gel-grown ammonium hydrogen tartrate single crystals

Single crystals of ammonium hydrogen tartrate (AHT) have been grown in silica gels by employing the controlled reaction between ammonium chloride and tartaric acid. Transparent AHT crystals upto 24 × 4 × 3 mm³ in size have been grown at room temperature. Optical and electron-optical studies have been made on the various surface structures of {010} faces of the grown crystals. A variety of growth striations and growth hillocks have been observed. Growth layers modified by the presence of misaligned microcrystals have been illustrated. It has been suggested that two-dimensional nucleation, spreading and pilling up of growth layers is mainly responsible for the growth of these crystals and the implications are discussed.     

Surface growth mechanisms and structural faulting in the growth of large single and spherulitic titanosilicate ETS-4 crystals

Morphological, surface and crystallographic analyses of titanosilicate ETS-4 products, with diverse habits ranging from spherulitic particles composed of submicron crystallites to large single crystals, are presented. Pole figures revealed that crystal surfaces with a-, b- and c- axes corresponded to 110, 010 and 001 directions, respectively. Thus, technologically important 8-membered ring pores and titania chains in ETS-4 run along the b-axis of single crystals and terminate at the smallest crystal face. Height of the spiral growth steps observed on 1 0 0 and 0 0 1 surfaces corresponded to the interplanar spacings associated with their crystallographic orientation, and is equivalent to the thickness of building units that form the ETS-4 framework. Data suggest that the more viscous synthesis mixtures, with a large driving force for growth, increased the two- and three-dimensional nucleation, while limiting the transport of nutrients to the growth surface. These conditions increase the tendency for stacking fault formation on 1 0 0 surfaces and small angle branching, which eventually results in spherulitic growth. The growth of high quality ETS-4 single crystals (from less viscous synthesis mixtures) occurred at lower surface nucleation rates. Data suggest that these high quality, large crystals grew due to one-dimensional nucleation at spiral hillocks, and indicate that under these conditions un-faulted growth is preferred.     

Röntgenographische Untersuchungen zur Kristallperfektion von GaAs-Einkristallen und -Epitaxieschichten. Will Kleber zum Gedenken

The defect structure of bulk GaAs and homoepitaxial layers has been characterized by means of reflection methods of x-ray diffraction topography (doublecrystal topography, Berg-Barrett method) and in addition by rocking-curve measurements. Besides of dislocations, long-range distortions of the single crystals of GaAs are investigated and determined quantitatively. The measured widths of the rocking curves approach those expected for perfect crystals. Single and double twinning in growth hillocks on epitaxial GaAs is found in connection with a higher degree of imperfection. The perfection of single-crystal layers is similar to that of the bulk crystals. Based on contrasts observed in double crystal topographs and energetic discussions, statements are made one defect geometry of stacking-fault tetrahedra in epitaxial layers.     

Microstructures on {111} faces of stannic iodide single crystals

Single crystals of stannic iodide (SnI₄) havebeen grown using the controlled reaction between SnCl₂ and KI by diffusion process in silica gel medium. Orange to reddish octahedral stannic iodide crystals up to 3–4 mm in size have been grown at room temperature. Optical studies have been made on the various surface structures of {111} faces of the asgrown crystals. On octahedral faces of these crystals, triangular-shaped hillocks with growth layers in the 〈110〉 directions have been observed. Occasionally, growth spirals on octahedral faces have also been reported. Close loops of growth fronts have been investigated and have been interpreted. It has been suggested that two-diemensional nucleation, spreading and pilling up of triangular growth layers is mainly responsible for the growth and occasionally the growth is due to screw dislocations. The implications are discussed.     

Homo-epitaxial GaN growth on exact and misoriented single crystals: suppression of hillock formation

GaN single crystals were used as substrates for MOCVD growth. The ( ) plane of the substrate crystals was polished to obtain off-angle orientations of 0, 2, and 4° towards the [ ] direction. The highest misorientation resulted in a reduction of the hexagonal hillock density by nearly two orders of magnitude as compared with homo-epitaxial films grown on the exact ( ) surface. The features that are still found on the 4° off-angle sample after growth can be explained by a model involving the interaction of steps, introduced by the misorientation, and the hexagonal hillocks during the growth process. Following from this explanation it could be concluded that surface diffusion is found to be not important during growth on the N-side. The material quality of the N-side was examined by photoluminescence (PL) measurements. The PL spectrum measured at 5 K shows dominant donor bound excitons with a FWHM of 1.4 meV as well as free excitonic transitions.     

Micromorphology of pure and PbI2‐doped CdI2 dendritic single crystals

Undoped and PbI₂‐doped dendritic single crystals were grown by vapour growth technique. The basal surfaces of the as grown crystals were examined by optical and electron microscopy to observe wide variety of growth and defect features. Apart from typical features of dendritic growth, features of overgrowth, slip bands, growth steps and their bunching, etc. were observed. The basal surfaces of the crystals were then etched by controlled condensation of water vapour, after optimizing the etching condition, and the microscopic studies were repeated. Etch pits of hexagonal and triangular shape, both symmetric and asymmetric, and of different density, were observed in the case of undoped and doped crystals, respectively. In some cases, crystallographic hillocks were also observed. The crystals were also examined by X‐ray diffraction for their polytypism and related behaviour. The results are analyzed to elicit information on the correlation of structure, defects and surface features of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spiral growth of potassium bichromate crystals

Some observations of growth spirals and hillocks on {001} surfaces of potassium bichromate crystals grown from aqueous solutions are presented and discussed. It is shown that a spiral pattern in fact represents a macrospiral developed as a result of the cooperative action of a group of dislocations.     

In situ atomic force microscopy studies of surface morphology,growth kinetics,defect structure and dissolution in macromolecular crystallization

Surface morphologies of thaumatin, catalase, lysozyme and xylanase crystals were investigated using in situ atomic force microscopy. For thaumatin, lysozyme and xylanase crystals, growth steps having a height equal to the unit cell parameter were produced both by screw dislocations and two-dimensional nuclei. Growth of catalase crystals proceeded in alternating patterns exclusively by two-dimensional nucleation and the successive deposition of distinctive growth layers, each having a step height equal to half the unit cell parameter. The shapes of islands on successive layers were related by 2-fold rotation axes along the 〈0 0 1〉 direction. Experiments revealed that step bunching on crystalline surfaces occurred either due to two- or three-dimensional nucleation on the terraces of vicinal slopes or as a result of uneven step generation by complex dislocation sources. Growth kinetics for thaumatin and catalase crystals were investigated over wide supersaturation ranges. Strong directional kinetic anisotropy in the tangential step growth rates in different directions was seen. From the supersaturation dependencies of tangential step rates and the rates of two-dimensional nucleation, the kinetic coefficients of the steps and the surface free energy of the step edge were calculated. Adsorption of impurities which formed filaments on the surfaces of catalase and thaumatin crystals was recorded. Cessation of growth of xylanase and lysozyme crystals was also observed and appeared to be a consequence of the formation of dense impurity adsorption layers. Crystal growth resumed upon scarring of the impurity adsorption layer and clearing of the crystal surface with the AFM tip. Adsorption of three-dimensional clusters, which consequently developed into either properly aligned multilayer stacks or misaligned microcrystals was recorded. For catalase crystals, incorporation of misoriented microcrystals as large as 50×3×0.1 μm³ produced elastic deformations in growth layers of ≈0.6%, but did not result in the defect formation. Etching experiments on catalase crystals revealed high defect densities.     

Mechanism of Various Growth Morphologies of Kutch Quartz Crystals

Locations and estimated amounts of deposits of quartz crystals found in the Kutch district of Gujarat State, India are given. A report about various morphologies of these crystals in geodes is presented. Influence of supersaturation on the mechanism of formation of the initial, intermediate and final growth morphologies has been assessed. It is suggested that the changing growth morphology is a function of supersaturation value. Trapezium-shaped and semi-circular hillocks observed on the prism faces are described and discussed. It is proposed that these hillocks develop and grow in a sequential manner and like crystals have initially triangular form.     

Surface morphology and growth mechanisms of argyrodite-type Cu6PS5 hal crystals

Single crystals of Cu₆PS₅Hal (Hal = Cl, Br, I) have been grown by dissociative sublimation and by chemical vapour transport with P, S, Hal, CuHal and combinations thereof as transporting agents in the temperature range 800 to 1000 K. Microscopic observation of the tetrahedral faces revealed several growth features. Macrosteps, closed loop steps and surface protuberances are always accompanied with the presence of liquid CuHal/Cu₂S droplets on the growing crystal surface. The phenomena are characteristic for a two-dimensional Vapour-Liquid-Solid growth mechanism. In the absence of CuHal/Cu₂S liquid phase, direct growth from the gas phase (VS growth) takes place leading to hillocks and growth spirals with low step heights (∼ 5–10 Å).     

Effect of zinc doping on pentaerythritol tetranitrate single crystals

In this study, the effect of zinc impurity on the organic high explosive pentaerythritol tetranitrate (PETN) single crystal has been investigated with optical microscopy and ex situ atomic force microscopy (AFM). The optical images show that the crystal shape has a transition with a predictable trend from long crystal to compact one as the zinc concentration is increased. Also, the 2‐dimentional (2‐D) growth hillocks are observed clearly on (110) face with contact AFM. The crystal growth occurs on monomolecular steps generated by 2‐D nucleation and followed by layer‐by‐layer expansion, and the macro‐steps formed onto the surface before spreading laterally as step bunches. The zinc ions are incorporated in growth steps as the zinc concentration is increased. The mechanism of inorganic impurity on molecular crystallization growth is still unclear. However, the incorporation of impurities may significantly affect growth kinetics of defect structure, and the bulk properties of molecular crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth morphology of {100} faces of L‐Arginine phosphate monohydrate single crystals investigated by atomic force microscopy

Morphology of the {100} faces of L‐arginine phosphate monohydrate (LAP) single crystal grown at 25 °C at a supersaturation of 0.32 has been discussed. The rectangular dislocation growth hillocks elongate along the b direction, which manifests the fast growth due to the strong Period Bond Chain (PBC) bonds along this direction. Apart from that, the growth hillocks are consistent with the macro‐morphology of the crystal grown at the pH value of about 4.2. The lopsided shapes of the hillocks result from step bunching. Triangular pits are assumed to form during the process of the steps getting across the impurities. The hollow cavities existing on the growth hillocks also elongate along the b direction and they can lead to the formation of other defects such as dislocations. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Role of microcrystals in the growth and development of prism faces of cultured quartz (II.) Detached microcrystals

Results of microtopographical investigations on prism faces of cultured quartz crystals grown by hydrothermal crystallization techniques, undertaken with a view to determine the influence of microcrystals which get attached to the growing host prism face and then get detached, at some stage of growth, from or still remain attached to the host face, with special reference to the role played by them in the growth and development of host prism faces in particular and the quartz crystals in general, are discussed. The study is an extension of, and obviously to be dealt in continuation with, the earlier one (Part-I, see JOSHI , KOTRU ). Polygonal cavities of various shapes and structures observed on the prism faces are described. Mechanism of formation of the polygonal cavities is explained. Also described are pyramidal depressions, striations, growth hillocks, and etch pits within the cavities on the growing host prism faces. Some of the observed structures within the cavities are attributed to the imprints of the surface structures on the faces of detached microcrystals of quartz. Evidence of growth within the cavities is offered and the implications of such a growth are discussed. Evidence of micro-etching in the cavities is suggestive of the possibilities of some of the grown cultured quartz crystals to get etched in the autoclave. Two different suggestions are made for the interpretation of the origin of etch pits within the cavities, and there is support for the view that they are as a result of etching, only at a time when the supply quartz gets exhausted, of some of the grown crystals where the growing chamber is the hottest.     

Microscopic investigations of morphological structures on the pyramidal faces of KDP and DKDP single crystals

Studying a number of {101} faces of KDP and DKDP single crystals in the as-grown state we found in most cases one, rarely few, clearly stepped growth hillocks covering the whole face. The geometry of these growth hillocks was determined from photomicrographs; the step heights were measured with a profilometer. To prove our results we carried out X-ray topographic investigations and in-situ observations on KDP crystals. In addition we studied the as-grown faces of ADP and LiJO₃ single crystals.     

Micromorphology of as-grown surfaces of crystals

This paper describes and discusses the micromorphology of as-grown surfaces of single crystals. After a brief introduction to the methods of observation of surfaces, the mechanism of growth and development of crystals is first outlined. Here the formation of bunches and macrosteps, the relationship between the growth mechanism and the surface entropy factor, and the effect of impurities on the surface morphology are described. Common structures observed on the as-grown surfaces are then explained in relation to growth conditions of the crystals. The following growth structures are described: elementary spirals, macrospirals, hillocks of dislocation and nondislocation origin, interlacing and slip patterns, macrosteps, inclusions, block structures, growth striations and impurity striations.     

Surface topographical studies of prism faces of ammonium tartrate single crystals

A study of surface structures of Prism faces (110) of ammonium tartrate single crystals grown by gel technique is described and illustrated. Vertical striations closly and widely spaced are observed. Isolated as well as densely populated rectangular elongated hillocks are reported and mechanism of the growth of these faces is assessed in the light of observed surface structure.     

Effect of organic impurities on the surface morphology and growth mechanism of KBP crystals (C8H5O4K)

The surface morphology of the (010) face of potassium biphthalate (KBP) crystals grown from aqueous solutions under the supersaturation ranging within 0.029–0.04 has been studied by the methods of optical and electron microscopies. It was revealed that the (010) surface has polygonal growth macrohills of the dislocation nature, small hillocks developing by the mechanism of successive two-dimensional nucleation, and numerous two-dimensional nuclei. The density of small hillocks (10⁴–10⁵ cm^?2) exceeds the dislocation density in KBP crystals by one to two orders of magnitude. It is shown that at low supersaturations, the (010) face grows simultaneously by the dislocation mechanism and the mechanism of successive two-dimensional nucleation. It is also established that the tangential velocity of growth-step motion on the (010) face increases in the presence of organic impurities. This effect can be used as one of the factors increasing the growth rates of crystal faces at low impurity concentrations (the so-called catalytic effect of impurities).     

In-situ observation of crystal growth and the mechanism

The spatial and time resolution in the measurements of growth rates and the observation of surface morphologies and the associated transport phenomena reflecting their growth mechanism have been developed because advanced microscopes and interferometers have attained nano-scale resolution. The first part covers the historical background how in-situ observation of crystal growth at molecular-level by optical and other scanning methods had been developed for understanding of crystal growth by measuring crystal growth rates and by observing surface nano-topographies, such as growth steps and spiral hillocks, with the same vertical resolutions comparable to that of the scanning probe microscopic techniques. The potential of recently developed interferometric techniques, such as Phase-Shift Interferometry (PSI) is then reviewed with the principle of the optics. Capability of measuring growth rates of crystals as low as 10⁻⁵ nm/s (1 µm/year) is introduced. Second part of the article emphasizes basic interferometric technique for the understanding of crystal growth mechanism by measuring growth rate vs supersaturation. Utilization of these techniques not only in fundamental crystal growth fields but also in environmental sciences, space sciences and crystallization in microgravity would briefly be introduced. At the end, we select a few examples how growth mechanism was analyzed based on these kinetic measurements.     

Surface morphology and quality of potassium bichromate crystals obtained from aqueous solutions in relation to growth conditions

The characteristics of macrospiral layers on growth surfaces of potassium bichromate crystals and their quality are examined in relation to growth conditions. It was observed that the layers become thick and rounded, more inclusions are trapped and the crystal quality deteriorates with an increase in supersaturation, while an opposite effect is noted with an increase in growth temperature. The observations are discussed from the standpoint of crystal growth theories.