Spiral Hillocks Research Articles

Metal–Organic Chemical Vapor Deposition of n‐AlGaN Grown on Strain‐Relaxed Distributed Bragg Reflector Buffer Layers

The strain relaxation, surface morphology, and reflectivity of AlGaN‐distributed Bragg reflectors (DBRs) grown via metal–organic chemical vapor deposition on AlN/Al2O3 templates are investigated. Strain relaxation begins in a 10‐period Al0.50Ga0.50N (27 nm)/Al0.75Ga0.25N (29 nm) DBR, and the degree of strain relaxation (DSR) increases with the number of DBR periods. The 30‐period DBR exhibits a peak reflectivity of 0.82 at 279 nm, with a stopband of 12 nm. The DSR of n‐Al0.62Ga0.38N on the 30‐period DBR increases from 70% to 100% as the n‐Al0.62Ga0.38N thickness increases from 0.4 to 2.5 μm. Although the surface of a DBR comprises numerous spiral hillocks, n‐Al0.62Ga0.38N grown on an AlGaN DBR exhibits a step‐flow growth. A DSR of 100% with threading screw dislocations of 2.0 × 108 cm−2 and threading edge dislocations of 1.2 × 109 cm−2 is obtained for a 2.5 μm‐thick n‐Al0.62Ga0.38N on a 30‐period AlGaN DBR.

Elimination of spiral hillocks in AlGaN grown on high-temperature annealed AlN templates by adopting an AlN/AlGaN stress modulation multilayer

The formation of spiral hillocks during AlGaN growth is investigated by modulating the residual strain in the underlying AlN templates. It is demonstrated that the high-density hillocks are directly related to the compressive stress, in particular for AlGaN on high-temperature annealed (HTA) AlN. AlN/AlGaN stress modulation multilayer is then introduced and optimized before AlGaN growth, which is revealed to be helpful to release the compressive stress in AlGaN as well as to suppress the hillocks. Eventually, the hillocks in AlGaN/HTA-AlN are eliminated, leading to a great improvement of yield for deep-ultraviolet light-emitting diode (DUV-LED) wafers. This work will definitely promote further industrial development and application of DUV-LEDs.

Morphology evolution of homoepitaxial growth of aluminum nitride by hydride vapor phase epitaxy

Free-standing aluminum nitride (AlN) crystals prepared by physical vapor transport (PVT) have been utilized as substrates for the homoepitaxial growth of AlN thick layers by using hydride vapor phase epitaxy (HVPE). The morphology evolution has been investigated in details under various growth conditions. The surface conditions from step flow to step bunching growth mode are governed by growth rates and V/III ratios, which is attributed to the vapor supersaturation. The presence of microcrystal inclusions, formation of spiral hillocks and occurrence of cracks could cause severe surface degradation. Additionally, the substrate misorientation also contributes to the morphology variation to some extent. By optimizing key growth parameters, a 300-μm thick homoepitaxial AlN layer with a smooth surface and high deep UV transparency has been successfully prepared by HVPE method.

Origins of epitaxial macro-terraces and macro-steps on GaN substrates

Localized lattice distortions in GaN substrates can serve as nucleation sites for epitaxial macro-steps and macro-terraces. These detrimental macro-scale features give rise to optically hazy homoepitaxial GaN surfaces. After nucleating, these macro-features grow laterally along the surface and coalesce, leading to significant coverage of the wafer surface. Dot-core GaN substrates consisting of a periodic array of cores were used as a defect-engineered system, where dislocations are intentionally concentrated at the cores. The high density of threading dislocations at the cores induced localized lattice distortions. These distortions are associated predominantly with lattice tilt on the order of hundreds of arcsec across ∼0.5 mm laterally along the wafer surface. The resulting macro-features that nucleated at these localized distorted sites were made up of macro-terraces with lengths ranging ∼30–∼150 μm and macro-step heights ranging ∼200–∼400 nm. Another source of localized distortion was threading screw dislocations or GaN nanopipes that resulted in spiral growth and hillock formation. Based on x-ray topography and optical microscopy measurements, we speculate that the coalescence of hillocks evolves into macro-terraces and macro-steps. While previous studies focused on the substrate miscut as a means to control macro-feature formation, we show that localized lattice tilt from defects is another important contributor to macro-feature formation.

Kinetics and energetics of pharmacolite mineralization via the classic crystallization pathway

Pharmacolite (CaHAsO4·2H2O) mineralization is an important process controlling arsenate mobility in carbonate areas but is poorly understood in terms of crystallization mechanism and kinetics. The present study intends to investigate pharmacolite formation initiated by heterogenous nucleation through in situ observations of the mineral–water interface. Experiments were performed by exposing gypsum substrate to solutions supersaturated with respect to pharmacolite followed by atomic force microscopy imaging to record changes in surface morphology and topography. The data are used to determine growth mode, step speed, and the rate of step birth. Experimental results show the crystallization is achieved by direction-specific mono-molecular layer growth via 2D surface nucleation or spiral hillock development in accord with the classical crystallization model. Subsequent theoretical analyses allow to determine step energies and kinetic coefficients in major growth directions on (010) and indicate that [001] steps are energetically the most stable and morphologically the most prominent while [100] and [101] steps are controlled either by thermodynamic or kinetics. Gypsum plays an important role in aiding pharmacolite formation through epitaxy as the two minerals share a range of structural commonalities. The notably reduced supersaturation needed in substrate-assisted pharmacolite crystallization relative to bulk solution nucleation suggests gypsum may significantly reduce the energy barrier for the mineralization reactions and hence may find applications in As remediation practice.

Estimation of supersaturation at steps during chemical vapor deposition of 4H-SiC () from reported growth rate and cross-sectional profile of spiral hillock

Based on the Burton–Cabrera–Frank (BCF) and Cabrera–Levine theories, supersaturation at steps (σ step) during chemical vapor deposition of 4H-SiC (0001̄) at 1570 °C under the condition of C/Si ratio of unity was estimated from the reported growth rate and cross-sectional profile of a spiral hillock, as a function of surface diffusion length of C2H2 molecules (λC2H2). The previously assumed zero σ step in the BCF theory was found to be a possible source of error in the case λC2H2 > 0.63 μm because of the estimated ratio of σ step to the surface supersaturation exceeding 0.1.

Surface kinetics in halide vapor phase epitaxial growth of GaN layers on GaN (0001) freestanding substrates

The effects of halide vapor phase epitaxial (HVPE) growth conditions such as input V/III ratio and substrate off-cut angle on the surface morphology of n-type GaN layers grown on GaN (0001) freestanding substrates were investigated to develop a model for growing smooth surfaces. The spiral hillock density increased with increasing input V/III ratio and/or decreasing off-cut angle. The critical off-cut angle between the spiral growth and the step-flow growth depended on the vapor supersaturation calculated by thermodynamic analysis. To understand the transition of the growth mode, we proposed a Burton–Cabrera–Frank-theory-based model considering the effect of spiral growth, which was utilized to explain the obtained experimental results. The developed growth model can be effective for predicting the HVPE growth mode between the spiral growth and the step-flow growth.

Calcite Kinetics for Spiral Growth and Two-Dimensional Nucleation.

Calcite crystals grow by means of molecular steps that develop on {10.4} faces. These steps can arise stochastically via two-dimensional (2D) nucleation or emerge steadily from dislocations to form spiral hillocks. Here, we determine the kinetics of these two growth mechanisms as a function of supersaturation. We show that calcite crystals larger than ∼1 μm favor spiral growth over 2D nucleation, irrespective of the supersaturation. Spirals prevail beyond this length scale because slow boundary layer diffusion creates a low surface supersaturation that favors the spiral mechanism. Sub-micron crystals favor 2D nucleation at high supersaturations, although diffusion can still limit the growth of nanoscopic crystals. Additives can change the dominant mechanism by impeding spiral growth or by directly promoting 2D nucleation.

Influences of gallium and nitrogen partial pressure on step-bunching and step-meandering morphology of InGaN quantum barrier layer

As the templates for the growth of the quantum well layer, the morphology of the quantum barrier layer is crucial. Through a cross-comparison among samples grown under different conditions, the effect of TMGa flux, NH3 flux, indium content, and temperature on morphology of InGaN quantum barrier grown by Metal-Organic Chemical Vapor Deposition (MOCVD) is studied. It is found that TMGa and NH3 flux increments are counterbalancing each other in terms of adatom diffusion. Without balanced V/III fluxes, the surface roughens either by spiral hillocks or by strong step meandering. The morphology can be quite different even under a similar V/III ratio. LED samples using the V/III unbalanced quantum barriers, compared to the V-III balanced ones, present inferior optical properties.

The effect of nanoindentation on crystal growth rate fluctuations investigated by in-situ atomic force microscopy

In this article the results of direct observations of elementary processes of crystal growth using AFM are presented. A method of statistical processing of AFM data was developed, which made it possible to compile an array of average tangential and average normal growth rates and their fluctuations. Nanoindentation was applied to a growing spiral hillock; intensified fluctuations and simultaneous growth and dissolution were recorded within a local area. The behavior of average rates, characterized as self-oscillations, suggests self-organization of the system. Comparison of the data obtained in the experiment with those obtained from the other experiments where no nanoindentation was applied indicates that the phenomena of intensified fluctuations and simultaneous growth and dissolution are caused by the nanoindentor action.

Investigation of Factors Influencing the Occurrence of 3C-Inclusions for the Thick Growth of on-Axis C-Face 4H-SiC Epitaxial Layers.

In this study, we grew homoepitaxial layers on 3-inch on-axis carbon-face 4H-silicon carbide substrates and attempted to suppress the generation of 3C-inclusions. It was found that the 3C-inclusion density decreased with increasing time spent on reaching an objective flow rate for the precursors. It is suggested that 3C-SiC nucleation occurred on large terraces of the on-axis substrates, which existed before the substrates were covered with spiral hillocks. This nucleation was suppressed owing to the decrease in the degree of supersaturation at the initial growth stage. Moreover, we found that the 3C-inclusions were also generated owing to contamination in the form of graphite products. Furthermore, we succeeded in growing a thick on-axis 4H-SiC homoepitaxial layer on a 3-inch substrate and demonstrating its free-standing epitaxial layer with a thickness of 182 μm and a 3C-inclusion density of 2.0 cm−2.

Carbon related hillock formation and its impact on the optoelectronic properties of GaN/AlGaN heterostructures grown on Si(111)

The integration of GaN on Si as large scale substrates still faces many hurdles. Besides the large difference in the lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-electron-mobility transistor heterostructures grown on Si(111) by metal-organic chemical vapor deposition. The presence of these morphological and structural defects is attributed to the presence of localized contamination at the AlN/Si interface. These carbon-based defects cause highly defective regions in the AlN seed layer, which propagate through all the AlGaN buffer layers inducing the formation of V-shaped pits at the AlGaN interfaces. In hillock regions of the wafers, Raman spectroscopy indicates disturbed two-dimensional electron gas characteristics resulting from GaN/AlGaN interface roughness and a decreased amount of free carriers in the potential well. Energy-dispersive x-ray spectroscopy reveals Ga accumulation inside the V-pits and in nanopipes below, which is responsible for defective areas and the increased GaN growth rate resulting in hillock formation. Photoluminescence measurements confirm the presence of Ga-rich material reducing the inherent gallium vacancy concentration. Here, the reduced amount of Ga-vacancies acting as a shallow acceptor suppresses the ultraviolet luminescence band from donor–acceptor pair transition.

Suppression of dislocation-induced spiral hillocks in MOVPE-grown AlGaN on face-to-face annealed sputter-deposited AlN template

AlGaN films were grown on face-to-face annealed sputter-deposited AlN/sapphire (FFA Sp-AlN) templates via metalorganic vapor phase epitaxy (MOVPE), and the growth behavior of the AlGaN films was investigated. The sapphire substrates with small off-cut exhibited poor surface flatness of AlGaN grown on the FFA Sp-AlN templates owing to the formation of large hillock structures. To understand the origin of these hillock structures, the crystallinity and surface morphology of conventional fully MOVPE-grown AlN/sapphire (MOVPE-AlN) templates and the FFA Sp-AlN template were comprehensively studied. The screw- and mixed-type threading dislocation density of the FFA Sp-AlN template was estimated to be approximately 1.8 × 106 cm−2, which was two orders of magnitude lower than that of the MOVPE-AlN template. Consequently, the uniquely observed growth of the hillock structures in the FFA Sp-AlN templates was attributed to their low density of screw- and mixed-type threading dislocations. The large surface off-cut sapphire substrates suppressed the hillock structures on the FFA Sp-AlN templates. The improvement in surface flatness resulted in better optical properties of multiple quantum wells grown on the AlGaN layer. These results demonstrate a promising method for achieving highly efficient and cost effective AlGaN based deep ultraviolet light-emitting diodes.

Equilibrium Shape of 2D Nuclei Obtained from Spiral Hillocks on {010} Form of Gypsum

Growth patterns on the {010} form of gypsum crystals were investigated, under controlled temperature and supersaturation, by atomic force microscope. With the morphology of hillocks formed by growth spirals generated by screw dislocations, the two-dimensional (2D) equilibrium morphology on the {010} form of gypsum was calculated. Edge energies of the [001], [102], [101], [201], and [100] steps were obtained by measuring the spacing between hillocks steps running in the same direction. Their values range between 8 × 10–7 and 2.3 × 10–6 erg/cm. The calculated critical size of 2D nucleus on {010} form is quite large (∼3400 nm2) and witnesses the low supersaturation value at which experiments were done.

Suppression of Polytype Transformation with Extremely Low-Dislocation-Density 4H-SiC Crystal in Two-Step Solution Method

We achieved the growth of extremely-high quality SiC crystal with two-step solution method with specially-designed seed crystals. The two-step growth consists of 1st step growth on Si-face for the reduction of threading dislocations and 2nd step growth on C-face for the reduction of basal plane dislocations and thickening. In this method, we can make the dislocation density extremely low, while the polytype easily changes during growth due to the absence of spiral hillocks originating from threading screw dislocation (TSD). In this study, we prepared specially designed seed crystals for both 1st and 2nd growth steps to provide steps continuously. In the seeds, a few TSDs exist at the upper-side of the step structure. Consequently, we demonstrated the suppression of the polytype transformation during the C-face growth with extremely low-dislocation-density crystal. Accordingly, we successfully obtained extremely low-dislocation density 4H-SiC with TSD, TED and BPD density of 11, 385 and 28 cm-2.

Study of spiral growth on 4H-silicon carbide on-axis substrates

We grew epitaxial layers on on-axis carbon-face 4H-silicon carbide substrates and investigated the growth conditions for the generation of spiral growth. We discovered that spiral growth occurs in regions where the local off-angle is less than 0.05° and when the spiral hillocks have a tilt angle of 0.06°. Moreover, we found that each spiral hillock coalesced without causing dislocation in the areas where the spiral growth occurred. Our results indicate that spiral growth is dominant when the spiral hillocks have a tilt angle greater than the off-angle of the substrate. Step-flow growth is overcome by spiral growth because the rate of spiral growth is greater than that of step-flow growth.

Structure, Energetics, and Dynamics of Screw Dislocations in Even n-Alkane Crystals.

Spiral hillocks on n-alkane crystal surfaces were observed immediately after Frank recognized the importance of screw dislocations for crystal growth, yet their structures and energies in molecular crystals remain ill-defined. To illustrate the structural chemistry of screw dislocations that are responsible for plasticity in organic crystals and upon which the organic electronics and pharmaceutical industries depend, molecular dynamics was used to examine heterochiral dislocation pairs with Burgers vectors along [001] in n-hexane, n-octane, and n-decane crystals. The cores were anisotropic and elongated in the (110) slip plane, with significant local changes in molecular position, orientation, conformation, and energy. This detailed atomic level picture produced a distribution of strain consistent with linear elastic theory, giving confidence in the simulations. Dislocations with doubled Burgers vectors split into pairs with elementary displacements. These results suggest a pathway to understanding the mechanical properties and failure associated with elastic and plastic deformation in soft crystals.

Evolution of calcite growth morphology in the presence of magnesium: Implications for the dolomite problem

The effect of magnesium on calcite growth morphology was known to occur as step rounding in some cases and surface segmentation in others. What remains unknown are the conditions for and the relations between the different effects, suggesting a lack of comprehensive understanding of the fundamental cause. Here we investigated the evolution of spiral hillock morphology on calcite cleavage surfaces in solutions with increasing Ca to Mg ratios and supersaturation levels using in situ atomic force microscopy. We isolated the effects of Mg and saturation by conducting experiments under conditions of constant pH, ionic strength, and Ca2+/CO32−. Our results revealed three types of morphological variations, ranging from step rounding in one direction (type I), to all directions (type II), and finally to a mosaic-like surface segmentation associated with monolayer buckling and step bunching (type III). These results suggest that the effect of magnesium on calcite growth depends upon multiple parameters including the concentration of Mg in solution, the step speed, as well as the extensiveness of Mg for Ca substitution in calcite lattice. We propose that the morphological variation may be understood by a model taken into consideration of (1) the lifespan and flux size of Mg ions at kinks in comparison to step kinetics, and (2) the diffusion and alignment of point defects created by the substitution of Mg for Ca in the crystal lattice. Stress calculations show that the maximum amount of Mg which calcite lattice can sustain before plastic deformation is ∼40%, suggesting that lattice stress due to the mismatch between MgCO3 and CaCO3 is likely the ultimate cause for the difficulty of ambient condition dolomite crystallization.

First Direct Observation of Impurity Effects on the Growth Rate of Tetragonal Lysozyme Crystals under Microgravity as Measured by Interferometry

The normal growth rates R and apparent step velocities (lateral growth rates of a spiral hillock) V of tetragonal hen egg-white lysozyme (HEWL) crystals were for the first time measured by Michelson interferometry in the international space station (as part of the NanoStep project) using commercialized HEWL samples containing 1.5% impurities. A significant increase in V under microgravity was confirmed compared to step velocities Vstep on the ground, while a decrease in R was also confirmed compared to that in the purified solution under microgravity as expected. Because of exact measurement of growth rates, kinetic analyses of R were conducted as a function of supersaturation, σ (σ ≡ ln(C/Ce), where C is the concentration; Ce is the solubility), using a spiral growth model and a two-dimensional (2D) nucleation growth model. For both models over a wide range of σ, R in the impure solution was significantly lower than that in the purified solution. The degree of the suppression of impurity effects was also...

Dislocation-Actuated Growth and Inhibition of Hexagonal l-Cystine Crystallization at the Molecular Level

Crystallization of l-cystine is a critical process in the pathogenesis of kidney stone formation in cystinuria, a disorder affecting more than 20 000 individuals in the United States alone. In an effort to elucidate the crystallization of l-cystine and the mode of action of tailored growth inhibitors that may constitute effective therapies, real-time in situ atomic force microscopy has been used to investigate the surface micromorphology and growth kinetics of the {0001} faces of l-cystine at various supersaturations and concentrations of the growth inhibitor l-cystine dimethylester (CDME). Crystal growth is actuated by screw dislocations on the {0001} l-cystine surface, producing hexagonal spiral hillocks that are a consequence of six interlacing spirals of anisotropic molecular layers. The high level of elastic stress in the immediate vicinity around the dislocation line results in a decrease in the step velocities and a corresponding increase in the spacing of steps. The kinetic curves acquired in the ...