In situ monitoring of CdTe nucleation on GaAs (100) using spectroscopic ellipsometry

In situ spectroscopic ellipsometry was used to monitor the nucleation behavior of CdTe grown on vicinal GaAs (100) substrates by organometallic vapor phase epitaxy. CdTe was grown on GaAs (100) substrates of exact and 2° off towards 110 orientations. A spectroscopic ellipsometer was used to collect in situ data at 44 wavelengths from 4000–7000 Å. The Bruggeman's effective medium approximation was employed to determine the variation of the epilayer volume fraction with thickness, which was an indirect way of monitoring the expected island growth behavior. The Stranski-Krastonov (layer plus island) mode of growth was clearly observed for CdTe growth. The growth on the 2° off substrate was also “denser” than that on exact (100), which implied that coalescence of the islands occurred at lower thickness. This was expected since island nucleation is most favored along the ledges on the surface whose spacing decreases with increasing misorientation. A simple nucleation model, assuming cylinder-like islands, was able to fit the experimental data quite well, lending support to the island growth model.     

Growth mechanism of stearic acid single crystals

Micro-morphological observation of the growth patterns on the (001) surfaces of large single crystals of stearic acid grown from solution under moderate growth conditions was carried out by optical and electron microscopy. Two typical growth features, namely, growth spiral steps and two-dimensional (2-D) nucleation-like steps, were identified optically. Then, these patterns were observed in more detail by the replica method of electron microscopy and it was found that the steps are usually two to four times the length of the bimolecular growth unit. These growth units also revealed spiral growth and 2-D nucleation growth patterns.     

SEM studies on hydrothermally grown VSZ-3 and VSZ-4 crystals

Morphology and surface features of two varieties of hydrothermally grown zeolite microcrystals viz: VSZ-3 and VSZ-4 are studied by employing SEM. VSZ-3 crystals have prismatic and prismatic pyramidal habits, whereas VSZ-4 crystals are only prismatic with hexagonal cross-section. In light of the scanning electron microscopic studies the plausible mode of nucleation and growth of these zeolite crystals are proposed.     

Nuleation and Growth of Thin Ag Films Vacuum-deposited onto W

An attempt to analyse the nucleation and growth phenomenon in a typical “Stranski-Krastanov” (SK) growth system - Ag/W (PAUNOV , MICHAILOV 1979; MICHAILOV ), in terms of island growth theory was made. Ag-layers were analysed by the use of scanning electron-and optical microscopes. A strong dependence of the nucleation kinetics on supersaturation and grain orientation was found to exist. On the basis of known rate equations, the number of atoms in the critical cluster i and the value of nucleation potential barrier E were estimated. The comparison between the experimentally obtained size-, first nearest neighbour- and radial distributions and theory shows that: 1) nucleation and growth processes proceed through a surface diffusion of the atoms; 2) no coalescence between the crystallites and no crystal mobility exist.     

Studies on the Growth Rates of Crystals

The crystal growth rates are calculated from the nucleation rate of two-dimensional square shaped nucleus in four different models. In all these models, the growth is initiated by the birth of the critical nuclei. Two limiting values of the lateral spreading velocities perpendicular to the side of the growing island are considered in first two models and finite spreading velocities are treated in other two models.     

Uniformity improvement of selectively-grown InGaAs micro-discs on Si

Dislocation-free InGaAs micro-disks, having a diameter of approximately 5 μm and a thickness of approximately 0.2 μm, have been grown on Si(111) substrate patterned with SiO₂, with excellent uniformity in shape. This shape uniformity was dependent on the initial growth of InAs on Si windows. For the growth of well-controlled lateral islands, it is mandatory to cover the openings completely with a single-domain InAs crystal prior to the subsequent growth of InGaAs, the Ga content of which is essential for lateral growth. Moreover, the shape uniformity of InAs islands strongly affected the uniformity of the InGaAs islands' shape. On the basis of these observations, the diameter of Si openings was reduced to 1 μm to make it easier to fill the openings with InAs without failure. A two-step growth of InAs was devised to achieve void-less nucleation and lateral growth at the same time. It was also found that the recess at the boundary between Si and the SiO₂ mask played a vital role to limit the lateral size of InAs islands: when an InAs island hit that boundary, the lateral growth was suppressed and a stable {?110} facet emerged, and the growth seemed to wait for all the {?110} sidewalls to emerge. This mechanism compensated variation in the progress of InAs growth due to scattered timing of InAs nucleation. Therefore, for the purpose of improving the shape uniformity of InGaAs micro-disks, transition from InAs to InGaAs growth should be attempted after all the sidewalls are covered with {?110} facets and before vertical growth starts.     

Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer

Nonpolar (1 1–2 0) a-plane GaN films have been grown using the multi-buffer layer technique on (1–1 0 2) r-plane sapphire substrates. In order to obtain epitaxial a-plane GaN films, optimized growth condition of the multi-buffer layer was investigated using atomic force microscopy, high resolution X-ray diffraction, and transmission electron microscopy measurements. The experimental results showed that the growth conditions of nucleation layer and three-dimensional growth layer significantly affect the crystal quality of subsequently grown a-plane GaN films. At the optimized growth conditions, omega full-width at half maximum values of (11–20) X-ray rocking curve along c- and m-axes were 430 and 530 arcsec, respectively. From the results of transmission electron microscopy, it was suggested that the high crystal quality of the a-plane GaN film can be obtained from dislocation bending and annihilation by controlling of the island growth mode.     

Influence of initial growth parameters on the structural and optical properties of GaAs on (001) Si

The structural and optical properties of GaAs on (001) Si substrates were investigated by transmission electron microscopy (TEM) and low-temperature photoluminescence (PL). It was found that the success of the two-step growth technique is controlled by the quality (morphology and defect density) of the low-temperature grown AlGaAs nucleation layer. GaAs epilayers grown on low V/III ratio AlGaAs nucleation layers exhibit improved surface morphologies and structural properties. These results were confirmed by optical measurements where it was shown that the best PL response was obtained from GaAs epilayers in which the initial AlGaAs nucleation layers were deposited at a low V/III ratio.     

In situ study of the growth and degradation processes in tetragonal lysozyme crystals on a silicon substrate by high-resolution X-ray diffractometry

The results of an in situ study of the growth of tetragonal lysozyme crystals by high-resolution X-ray diffractometry are considered. The crystals are grown by the sitting-drop method on crystalline silicon substrates of different types: both on smooth substrates and substrates with artificial surface-relief structures using graphoepitaxy. The crystals are grown in a special hermetically closed crystallization cell, which enables one to obtain images with an optical microscope and perform in situ X-ray diffraction studies in the course of crystal growth. Measurements for lysozyme crystals were carried out in different stages of the crystallization process, including crystal nucleation and growth, developed crystals, the degradation of the crystal structure, and complete destruction.     

Temperature control of protein crystal nucleation

The thermal variant of the classical nucleation‐growth‐separation principle is shown, both theoretically and experimentally, to be a reliable tool for studying protein crystal nucleation. The classical nucleation theory is used to elucidate the temperature dependence of crystal nucleus size. A one‐to‐one ratio of the number density of nuclei formed to crystals grown to visible size is achieved using the nucleation‐growth‐separation method. The experiments conducted in such a way show that new nuclei are prevented from appearing while avoiding any crystal loss due to dissolution. The same method is used to study experimentally the interval of growth temperatures where the number density of (nucleated) crystals is relatively insensitive to the growth temperature. It is argued that this temperature interval corresponds to the width of the so‐called metastable zone.     

The as-Grown SiC (0001) Surface as Observed by Reflection Electron Microscopy

Reflection electron microscopy (REM), capable of imaging surfaces in high resolution, reveals that the risers and treads in the terrace growth surfaces of vapor grown SiC(0001) bulk crystals are characterized on a microscopic scale by growth steps. At the risers microscopic growth occurs by the ‘step flow mode’. At the treads, growth occurs by island formation and by operation of growth spirals.     

Spatiotemporal protein crystal growth studies using microfluidic silicon devices

Fundamental investigations of protein crystallization using miniaturized microfluidic silicon devices were presented towards achieving spatiotemporal nucleation and subsequent post-nucleation growth. The developed microfluidic silicon device was typically composed of crystal growth cell, reservoir cell, and optionally of heater elements for supersaturation control. A specific fine pattern area in the growth cell which was fabricated on the silicon substrate with doped p- and n-type silicon layers, served as spatially selective nucleation site of dissolved protein molecules through electrostatic attractive force. In a model material, hen egg white lysozyme, a large number of crystals were grown on the defined nucleation site evenly spaced from each other, whereas parasitic crystal growth positioned around the selective nucleation site, was suppressed by the effects of electrostatic repulsive force between the doped silicon surface and charged protein molecules. A possible crystallization mechanism of describing the heterogeneous nucleation during the initial stage and during the growth of the crystal at the electrolyte–semiconductor silicon surface is proposed and discussed.     

Exaggerated grain growth in bixbyite via fast diffusion along planar defects

Naturally occurring bixbyite single crystals from the Maynard claim at Thomas Mountain Range in Utah, USA, show pronounced grain growth, when compared to bixbyite crystals from other locations in close vicinity. These enlarged single crystals reveal characteristic linear surface features, which were initially interpreted as twin boundaries. HRTEM studies in conjunction with EDS analysis and electron diffraction, however, confirmed that these features originate from thin interlayers of braunite, Mn₇SiO₁₂, epitaxially grown within the host crystal. A model for the observed exaggerated grain growth is presented, which is based on the assumption that fast diffusion occurs along the braunite/bixbyite phase boundaries in three dimensions and, more importantly, promote nucleation at surface edges. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

Characterization of α and γ polymorphs of glycine crystallized from water‐ammonia solution

Crystallization of metastable α and stable γ polymorphs of glycine was carried out from aqueous solution in the presence of ammonia. Pure aqueous solution and solution with lower concentration of ammonia yield α nucleation and solution with a critical concentration of ammonia yield γ nucleation. Variation in the solubility of glycine in double distilled water and pH of the resulting solution due to the ammonia incorporation was studied in a range of temperatures. The induction period for the α and γ nucleation in the solution was determined and its variation due to the ammonia incorporation was also studied. Single crystals of both the polymorphs were grown by slow evaporation method. Effect of ammonia concentration and the resultant pH of the solution on the nucleation, growth and morphology of the grown polymorphs were investigated. The unidirectional growth of the γ polymorph along the polar axis was revealed. X‐ray powder diffraction method was employed to distinguish both the polymorphs structurally. Their thermal stability above room temperature was studied by differential scanning calorimetry which revealed that the as‐grown γ polymorph transforms to α at 179.6 °C while the as‐grown α retains its phase until melting. The optical transmittance of the grown γ polymorph was studied in the UV‐Vis‐Near IR region. The second harmonic generation (SHG) efficiency of the grown γ polymorph was studied with a Nd:YAG laser source and is about 6.8% higher than that of the inorganic standard KDP. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Atomic Force Microscopy Studies on Growth Mechanisms and Defect Formations on {110} Faces of Cadmium Mercury Thiocyanate Crystals

Growth mechanisms and defect formations on {110} faces of cadmium mercury thiocyanate crystals grown at 30°C (σ=0.24) were investigated by using atomic force microscopy (AFM). It was found that, under this condition, spiral dislocation controlled mechanism and 2D nucleation mechanism operates simultaneously and equally during growth, which is completely different from the traditional 2D nucleation and dislocation source controlled mechanisms. A number of 2D nucleus are formed at the large step terraces generated by dislocation sources, leading to the unequal growth rates of the elementary steps and thereby “step bunches” arecaused. Various defects are formed under this growth condition, which is assumed to result from the incongruence between the steps generated by different sources. A new kind of 2D defect, corresponding to one growth layer in height, was observed for the first time.     

Investigations on Flux Grown Cadmium Sulphide Crystals

Observations on surface microtopography of flux grown CdS crystals are reported. Crystals resulting with cooling rate above 30 °C/hr are reported to be dendritic crystals, while those with cooling rate below 30 °C/hr are reported to be normal crystals like mineral greenockite as reported in standard books on mineralogy. Reasoning for dendritic growth is briefly discussed. Growth patterns and etch pits formation on them are interpreted. It is established that these crystals grow by two-dimensional nucleation mechanism and by spreading and piling of growth layers.     

Modeling of the shape of polyethylene oxide single crystals and determination of the kinetic crystallization parameters: Theoretical and experimental approaches

The curvature of faces of polymer single crystals is described by the system of Mansfield equations, which is based on the Frank-Seto growth model. This model assumes the velocity of nucleus steps to be the same for their propagation to the right and left and is valid only for symmetric crystallographic planes. To describe the shape of polyethylene oxide single crystals grown from melt and limited by the {100} and {120} folding planes, it is assumed that the layer velocities to the right and left are different on {120} faces. This approach allows modeling, with a high accuracy, of the observed shapes of polymer single crystals grown at different temperatures, which makes it possible to determine unambiguously the fundamental crystallization parameters: the dimensionless ratio of the secondary homogeneous nucleation rate to the average velocities of nuclei along the crystallization planes and the ratio of nucleus velocities to the right and left. In addition, it was found that a known macroscopic single-crystal growth rate can be used to determine the absolute values of the secondary homogeneous nucleation rate and the velocities of nuclei along the growth plane.     

ZnWO4单晶衬底上ZnO薄膜的晶核发育与形貌分析

ZnWO4单晶的a晶面与氧化锌c晶面晶格匹配很好,是制备氧化锌薄膜的优良衬底.本文采用溶胶-凝胶法在ZnWO4单晶衬底上制备出透明的ZnO薄膜.通过光学显微镜对薄膜晶核发育过程和形貌进行了详细的分析.实验结果表明:在结晶刚开始,系统将经历成核--长大的过程,随着生长过程的进行,在主晶轴上(一次轴)上又长出二次轴、三次轴等等,最后逐渐形成树枝状晶核.由于ZnO晶核是在非平衡条件下生长的,故在晶核发育过程中又出现了三种不同的生长形态--成核生长、枝晶生长和分形生长.     

Fabrication of a GaN lateral polarity junction by metalorganic chemical vapor deposition

A fabrication process for growth of GaN lateral polarity junctions consisting of Ga-polar and N-polar domains grown simultaneously side-by-side on c-plane sapphire was developed using the polarity control scheme. An ammonia-annealing step following deposition and patterning of a thin low-temperature AlN nucleation layer played a crucial role in avoiding mixed-polarity growth of the remaining AlN nucleation layer, as well as in nitriding the bare sapphire surface to facilitate growth of N-polar GaN. The achievement of both polar domains, free from inversion domains within a contiguous domain, led to Ga-polar domain exhibiting featureless morphology with highly resistive characteristics, while N-polar domains exhibited hexagonally faceted morphology and were highly conductive.