Etch Pit Morphology Research Articles

Electrical and optical properties and defects of (100)- and (001)-oriented V-doped β-Ga2O3 crystals grown by EFG

Altering the n-type conductivity and optical properties of ultra-wide bandgap β-Ga2O3 by impurity doping has been a topic of research interest in the semiconductor field. Simultaneously, β-Ga2O3 with a monoclinic structure has exhibited interesting anisotropy. In this study, (100) and (001) V-doped β-Ga2O3 crystals were synthesized by the edge-defined film-fed growth (EFG) method, and their structural, electrical and optical properties, and defects were systematically investigated. V-doped β-Ga2O3 crystals with a (100) plane have a better crystalline quality and flatter surface than those with a (001) plane. Compared with undoped crystals (∼1016 cm−3), the carrier concentration of the (100) and (001) V-doped β-Ga2O3 substrates only increased to 4.30 × 1017 cm−3 and 1.92 × 1017 cm−3, respectively, which was related to the VxOy clusters formed by excess V as trapping traps and the carbon DX center as an electron capture trap formed by the relaxation of C atoms in the direction opposite to that of the C–O bond. The generation of absorption peaks and flat regions in the transmittance spectra was attributed to the introduction of V doping. The etch pit morphology and Raman peak intensity on the surfaces of the (100) and (001) V-doped β-Ga2O3 substrates were different owing to the anisotropy of the monoclinic lattice structure. These results contribute to the understanding of the influence of V and anisotropy on β-Ga2O3 crystals and expand their applications in electrical and optical fields.

Defect evaluation in strain-relaxed Ge0.947Sn0.053 grown on (001) Si

Defects in Ge0.947Sn0.053 layers grown using molecular beam epitaxy on (001) Si substrates with 4.9% mismatch are investigated using optical, scanning, and transmission electron and atomic force microscopies. It is shown that the strain relaxation occurs via the introduction of 90° misfit dislocations of short length, at the Ge0.947Sn0.053/Si interface. An irregular morphology in the form of mounds is observed on the surface of epitaxial Ge1−xSnx (0.031 ≤ x ≤ 0.093) and is found to be associated with carbon impurities at the hetero-interface. A low-cost and fast defect selective wet etching technique is described to determine the etch pit density in epitaxial Ge1−xSnx with a low Sn content (≤5.3%). On the basis of etch pit morphology, different defects, e.g., dislocations, stacking faults, and crystal originated particles, are distinguished.

Kikuchi pattern analysis of noncentrosymmetric crystals

Different models of Kikuchi pattern formation are compared with respect to their applicability to noncentrosymmetric crystals, and the breakdown of Friedel's rule in experimental electron backscatter diffraction (EBSD) patterns is discussed. DifferentAIIIBVsemiconductor materials are used to evaluate the resulting asymmetry of Kikuchi band profiles for polar lattice planes. By comparison with the characteristic etch pit morphology on a single-crystal surface, the polar character of the measured lattice planes can be assigned absolutely. The presented approach enables point-group-resolved orientation mapping, which goes beyond the commonly applied Laue group analysis in EBSD.

In situ AFM study on barite (0 0 1) surface dissolution in NaCl solutions at 30 °C

This paper reports in situ observations on barite (001) surface dissolution behavior in 0.1–0.001M NaCl solutions at 30°C using atomic force microscopy (AFM). The step retreating on barite (001) surfaces changed with increasing NaCl solution concentrations. In solutions with a higher NaCl concentration (⩾0.01M), many steps showed curved or irregular fronts during the later experimental stage, while almost all steps in solutions with a lower NaCl concentration exhibited straight or angular fronts, even during the late stage. The splitting phenomenon of the initial 〈hk0〉 one-layer steps (7.2Å) into two half-layer steps (3.6Å) occurred in all NaCl solutions, while that of the initial [010] one-layer steps observed only in the 0.1M NaCl solution. The step retreat rates increased with an increasing NaCl solution concentration. We observed triangular etch pit and deep etch pit formation in all NaCl solutions, which tended to form late in solutions with lower NaCl concentrations. The deep etch pit morphology changed with increasing NaCl solution concentrations. A hexagonal form elongated in the [010] direction was bounded by the {100}, {310}, and (001) faces in a 0.001M NaCl solution, and a rhombic form was bounded by the {510} and (001) faces in 0.01M and 0.1M NaCl solutions. An intermediate form was observed in a 0.005M NaCl solution, which was defined by {100}, a curved face tangent to the [010] direction, {310}, and (001) faces: the intermediate form appeared between the hexagonal and rhombic forms in solutions with lower and higher NaCl concentrations, respectively. The triangular etch pit and deep etch pit growth rates also increased with the NaCl solution concentration. Combining the step and face retreat rates in NaCl solutions estimated in this AFM study as well as the data on the effect of water temperature on the retreat rates reported in our earlier study, we produced two new findings. One finding is that the retreat rates increase by approximately two-fold when the NaCl solution concentration increases by one order of magnitude, and the other finding is that the retreat rate increase due to a one order of magnitude increase in the NaCl concentration corresponds to an increase of approximately 8°C in water temperature. This correlation may help to understand and evaluate increasing dissolution kinetics induced by the different mechanisms where barite dissolution is promoted by the catalytic effect of Na+ and Cl− ions (through an increase in the NaCl solution concentration) or by an increase in the hydration of Ba2+ and SO42− (through an increase in water temperature).

A comprehensive stochastic model of phyllosilicate dissolution: Structure and kinematics of etch pits formed on muscovite basal face

Accurate modeling of phyllosilicate dissolution kinetics is a complex problem involving recognition of the influence of structure, chemical composition, lattice defects, and surface topography on local and global dissolution mechanisms. Previous research has provided a wealth of experimental observations that illustrate the dominant role of etch pits in formation of the steps on the basal face of mica during the dissolution reaction. The shape of the etch pits bears important information on surface reactivity and dissolution rate anisotropy at given environmental conditions. In order to understand the influence of various kinetic factors on etch pit and step morphology, as well as the overall dissolution mechanisms, we have developed a new Kinetic Monte Carlo model simulating dissolution of these minerals. The model considers the effects of chemical composition, structural position, the number of first and second-order nearest neighbors and the steric hindrance of surface atoms on the etch pit morphology and step reactivity. We describe several complexity levels of the model which are characterized by the different ranges of the effects considered. These levels were developed in order to find the most optimal model capable of predicting experimentally observed etch pit morphologies. Our simulation results show that the models based on the sole consideration of the first coordination sphere in general can predict etch pit shape and orientation, while recognition of the site reactivity difference imposed by the steric factors helps us to explain the geometry of monolayer pit superposition. However, the distinction of the ledge and kink sites at all the experimentally observed surface steps is possible only with the use of the second-order neighbors. Based on these findings, we propose a mechanistic scheme explaining the role of the first and second-order coordination numbers in step stabilization and correct prediction of the etch pit structure.

Extreme Monolayer-Selectivity of Hydrogen-Plasma Reactions with Graphene

We study the effect of remote hydrogen plasma on graphene deposited on SiO₂. We observe strong monolayer selectivity for reactions with plasma species, characterized by isotropic hole formation in the basal plane of monolayers and etching from the sheet edges. The areal density of etch pits on monolayers is 2 orders of magnitude higher than on bilayers or thicker sheets. For bilayer or thicker sheets, the etch pit morphology is also quite different: hexagonal etch pits of uniform size, indicating that etching is highly anisotropic and proceeds from pre-existing defects rather than nucleating continuously as on monolayers. The etch rate displays a pronounced dependence on sample temperature for monolayer and multilayer graphene alike: very slow at room temperature, peaking at 400 °C and suppressed entirely at 700 °C. Applying the same hydrogen plasma treatment to graphene deposited on the much smoother substrate mica leads to very similar phenomenology as on the rougher SiO₂, suggesting that a factor other than substrate roughness controls the reactivity of monolayer graphene with hydrogen plasma species.

Molecular Modulation of Calcite Dissolution by Organic Acids

Dissolution of the calcite (104) surface in aqueous solution in the presence of 10 organic acids has been studied using fluid-cell atomic force microscopy (AFM) in vitro. Etch pit morphology varies...

Intrinsic Kinetics of Gypsum and Calcium Sulfate Anhydrite Dissolution: Surface Selective Studies under Hydrodynamic Control and the Effect of Additives

The intrinsic dissolution activity of the basal (010) and edge (001) surfaces of gypsum and polycrystalline calcium sulfate anhydrite crystals has been investigated, under far from equilibrium conditions, via the channel flow cell (CFC) method with off-line inductively coupled plasma-mass spectrometry (ICP-MS) for the measurement of dissolved Ca2+ from the crystal surface. This approach allows measurements to be made over a wide range of flow rates so that the importance of mass transport versus surface kinetics can be elucidated. Complementary quantitative modeling of the dissolution process was carried out by formulating convective–diffusive equations that describe mass transport in the CFC, coupled to a boundary condition for dissolution of the crystal surface. We found that a linear rate law applied, and intrinsic dissolution fluxes were deduced. The following dissolution fluxes, J0 = kdiss × ceq were measured, where kdiss is the dissolution rate constant and ceq the calcium sulfate concentration in saturated solution: 5.7 (±1.4) × 10–9 mol cm–2 s–1 for basal plane gypsum and 4.1 (±0.7) × 10–9 mol cm–2 s–1 for calcium sulfate anhydrite. Edge plane gypsum, under the experimental conditions applied, was found to dissolve at a mass transport-controlled rate. The effects of l-tartaric acid, d-tartaric acid, and sodium trimetaphosphate (STMP) as important potential additives of the dissolution process of basal plane gypsum were investigated. It was found that the tartaric acids had little effect but that STMP significantly retarded gypsum dissolution with J0 = 1.6 (±0.6) × 10–9 mol cm–2 s–1 (5 mM STMP solution). The mode of action of STMP was further elucidated via etch pit morphology studies.

An integrated study of uranyl mineral dissolution processes: etch pit formation, effects of cations in solution, and secondary precipitation

Abstract Understanding the mechanism(s) of uranium-mineral dissolution is crucial for predictive modeling of U mobility in the subsurface. In order to understand how pH and type of cation in solution may affect dissolution, experiments were performed on mainly single crystals of curite, Pb2+ 3(H2O)2[(UO2)4O4(OH)3]2, becquerelite, Ca(H2O)8[(UO2)6O4(OH)6], billietite, Ba(H2O)7[(UO2)6O4(OH)6], fourmarierite Pb2+ 1−x(H2O)4[(UO2)4O3−2x(OH)4+2x] (x= 0.00–0.50), uranophane, Ca(H2O)5[(UO2)(SiO3OH)]2, zippeite, K3(H2O)3[(UO2)4(SO4)2O3(OH)], and Na-substituted metaschoepite, Na1−x[(UO2)4O2−x(OH)5+x] (H2O)n. Solutions included: deionized water; aqueous HCl solutions at pH 3.5 and 2; 0.5 mol L−1 Pb(II)-, Ba-, Sr-, Ca-, Mg-, HCl solutions at pH 2; 1.0 mol L−1 Na- and K-HCl solutions at pH 2; and a 0.1 mol L−1 Na2CO3 solution at pH 10.5. Uranyl mineral basal surface microtopography, micromorphology, and composition were examined prior to, and after dissolution experiments on micrometer scale specimens using atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Evolution of etch pit depth at different pH values and experimental durations can be explained using a stepwave dissolution model. Effects of the cation in solution on etch pit symmetry and morphology can be explained using an adsorption model involving specific surface sites. Surface precipitation of the following phases was observed: (a) a highly-hydrated uranyl-hydroxy-hydrate in ultrapure water (on all minerals), (b) a Na-uranyl-hydroxy-hydrate in Na2CO3 solution of pH 10.5 (on uranyl-hydroxy-hydrate minerals), (c) a Na-uranyl-carbonate on zippeite, (d) Ba- and Pb-uranyl-hydroxy-hydrates in Ba-HCl and Pb-HCl solutions of pH 2 (on uranophane), (e) a (SiOx(OH)4−2x) phase in solutions of pH 2 (uranophane), and (f) sulfate-bearing phases in solutions of pH 2 and 3.5 (on zippeite).

Morphological classification and quantitative analysis of etch pits

Etch pits created by hydrochloric and phosphoric acid on cleaved CaF2(111) are investigated by scanning force microscopy (SFM). A geometric and dimensional analysis of the etch pits reveals two distinctly different types. Type-I etch pits evolve at dislocation defects, are pointed and their size and eccentricity is related to the angle between the dislocation and the surface. Type-II etch pits result from defects below the surface, are flat-bottomed for longer etching times and exhibit a characteristic ratio of depth and edge length depending on the type of etchant. An analysis of etch pit morphology allows an identification of the origin of an etch pit and a characterization of the associated defect structure.

Effects of magnesium ions on near-equilibrium calcite dissolution: Step kinetics and morphology

Dissolution kinetics at the aqueous solution-calcite ( 1 0 1 ¯ 4 ) interface at 50 °C were investigated using in situ atomic force microscopy (AFM) to reveal the influence of magnesium concentration and solution saturation state on calcite dissolution kinetics and surface morphology. Under near-equilibrium conditions, dissolved Mg 2 + displayed negligible inhibitory effects on calcite dissolution even at concentrations of 10 - 4 molal ( m ) . Upon the introduction of 10 - 3 m Mg 2 + , the solution saturation state with respect to calcite, Ω calcite ( = a Ca 2 + · a CO 3 2 - K sp ( calcite ) ) , acted as a “switch” for magnesium inhibition whereby no significant changes in step kinetics were observed at Ω calcite < 0.2 , whereas a sudden inhibition from Mg 2 + was activated at Ω calcite ⩾ 0.2 . The presence of the Ω -switch in dissolution kinetics indicates the presence of critical undersaturation in accordance with thermodynamic principles. The etch pits formed in solutions with 10 - 3 m Mg 2 + exhibited a unique distorted rhombic profile, different from those formed in Mg-free solutions and in de-ionized water. Such unique etch pit morphology may be associated with the anisotropy in net detachment rates of counter-propagating kink sites upon the addition of Mg 2 + .

Dissolution kinetics of calcite at 50–70 °C: An atomic force microscopic study under near-equilibrium conditions

Direct measurements of calcite ( 10 1 ¯ 4 ) faces were performed using in situ atomic force microscopy (AFM) to reveal the dissolution processes as a function of solution saturation state and temperature. Time-sequential AFM images demonstrated that step velocities at constant temperature increased with increasing undersaturation. The anisotropy of obtuse and acute step velocities appeared to become more significant as solutions approached equilibrium and temperature increased. At saturation state Ω > 0.02, a curvilinear boundary was formed at the intersection of two acute steps and the initially rhombohedral etch pit exhibited a nearly triangular shape. This suggests that the [ 4 ¯ 41 ] a and [ 48 1 ¯ ] a steps may not belong to the calcite-aqueous solution equilibrium system. Further increase in the saturation state ( Ω ⩾ 0.3) led to a lack of etch pit formation and dissolution primarily occurred at existing steps, in accordance with Teng (2004). Analysis of step kinetics at different temperatures yielded activation energies of 25 ± 6 kJ/mol and 14 ± 13 kJ/mol for obtuse and acute steps, respectively. The inconsistencies in etch pit morphology, step anisotropy, and step activation energies from the present study with those of studies far-from-equilibrium can be explained by increased influence of the backward reaction, or growth, near-equilibrium. We propose that the backward reaction occurs preferentially at the acute–acute kink sites. The kinetics and effective activation energies of near-equilibrium calcite dissolution presented in this work provide accurate experimental data under likely CO 2 sequestration conditions, and thus are crucial to the development of robust geochemical models that predict the long-term performance of mineral-trapped CO 2.

Relation between etch-pit morphology and step retreat velocity on a calcite surface in aspartic acid solution

Effects of l-aspartic acid ( l-Asp) on dissolution of calcite were investigated. The step retreat velocity and dissolution rate of calcite were measured simultaneously using an AFM flow-through system. The etch-pit morphology of calcite was observed using confocal laser scanning microscopy. Results show that the etch-pit morphologies changed drastically depending on the l-Asp concentration ([ l-Asp]) in the order of rhomboidal, pentagonal, and triangular (not perfectly, but retaining an extra step). The change in obtuse step directions and appearance of the [0 1 0] step triggered these morphological changes. Addition of l-Asp accelerated all step retreats at [ l-Asp]<0.01 M, which implied the effect of l-Asp on the diffusive barrier. In contrast, at [ l-Asp]>0.01 M, l-Asp inhibited the retreats of obtuse steps and [0 1 0] step, although the retreat velocities of acute steps were constant irrespective of [ l-Asp]. These results suggest that the directional changes and the inhibition of retreat velocities of obtuse steps were attributed to the generation of [ 4 1 1] and [4 5 1] steps caused by l-Asp. Moreover, we confirmed the preferential effects of l-Asp on the [4 8 1] + to [ 4 4 1] ± step edge, and proposed the preferential effects of l-Asp on the [ 4 1 1] to [4 5 1] step edge.

Studies on ion-beam modifications of ADP and KDP crystal surfaces

Experimental investigations on He+ ion (150 keV) beam modifications of the (100) surfaces of ADP and KDP single crystals, relative to asgrown surfaces have been made, to assess the nature and extent of radiation damage. Studies on the surface micromorphology, defect substructure, defect concentration, surface conductivity and microhardness have been made. Studies on chemical etching in both ion irradiated and asgrown surfaces revealed drastic changes in etchpit morphology and defect concentration. Measurement of surface electrical conductivity, employing four-point probe method, showed enhancement in conductivity at all the temperatures of observation which is attributed to enhanced proton migration. Measurement of microhardness and studies on planar plastic anisotropy in asgrown surfaces relative to ion bombarded surfaces have been made employing Knoop indenter and the results are interpreted in terms of the nature of bonding, bond strength and radiation damage on ion bombardment. Plastic anisotropy in these surfaces is found to be in conformity with the (100) plane on which the indentations were made.

Kinetic inhibition of calcite (1 0 4) dissolution by aqueous manganese(II)

This study presents evidence of calcite (1 0 4) dissolution inhibition by dissolved manganese(II) through detailed observations of crystal surface morphology. Rates of dissolution have been quantified by direct measurement of surface-normal retreat and etch pit growth at far-from-equilibrium conditions using vertical scanning interferometry. The approach provides new insight into the control that dissolved inorganic carbon (DIC) exerts on manganese inhibition kinetics. Our results show that at 2×10 −6 molal manganese, the rate of overall calcite dissolution is suppressed to close to zero, but only when sufficient CO 3 2− is available in solution. The near arrest of surface-normal dissolution is brought about by the likely reduction in step retreat, inferred through observed variation of etch pit and surface morphology. The inhibition mechanism likely reflects the strong affinity of Mn 2+ for carbonate ion. In solution, this affinity is expressed by formation of a strong complexing ligand (MnCO 3 0). On the (1 0 4) surface, this affinity may govern formation of a similar surface complex that stabilizes reactive sites, thereby inhibiting step movement and defect nucleation. Comparison with results from other metals suggests that inhibition may be understood in the context of the relative energetics of dehydration versus carbonation reaction steps.

Magnesium inhibition of calcite dissolution kinetics

We present evidence of inhibition of calcite dissolution by dissolved magnesium through direct observations of the (104) surface using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). Far from equilibrium, the pattern of magnesium inhibition is dependent on solution composition and specific to surface step geometry. In CO 2-free solutions (pH 8.8), dissolved magnesium brings about little inhibition even at concentrations of 0.8 × 10 −3 molal. At the same pH, magnesium concentrations of less than 0.05 × 10 −3 molal in carbonate-buffered solutions generate significant inhibition, although no changes in surface and etch pit morphology are observed. As concentrations exceed magnesite saturation, the dissolution rate shows little additional decrease; however, selective pinning of step edges results in unique etch pit profiles, seen in both AFM and VSI datasets. Despite the decreases in step velocity, magnesium addition in carbonated solutions also appears to activate the surface by increasing the nucleation rate of new defects. These relationships suggest that the modest depression of the bulk rate measured by VSI reflects a balance between competing reaction mechanisms that simultaneously depress the rate through selective inhibition of step movement, but also enhance reactivity on terraces by lowering the energy barrier to new etch pit formation.

Application of orthodox defect-selective etching for studying GaN single crystals, epitaxial layers and device structures

In this communication, results are presented of the application of etching in molten E+M etch (KOH-NaOH eutectic mixture with 10% MgO) for studying defects in GaN. The method was used to study defects on differently oriented cleavage and basal planes of GaN single crystals, MOCVD-, MBE- and HVPE-grown epitaxial layers and LD and LED structures. Dislocations, dislocation loops and stacking faults have been revealed on , and Ga- and N-polar planes. Diversified etch pit morphology was observed depending on the crystallographic orientation of the etched samples and was correlated with the crystallographic symmetry of the GaN lattice. Etching results were calibrated using TEM analysis.

Etch-pit morphology of tracks caused by swift heavy ions in natural dark mica

Ion tracks in solids can be visualized by appropriate etching. During the etching procedure, size and depth of the etch pits grow linearly with time. Their shape is mainly controlled by the crystal structure. For example, in muscovite, ion tracks have a rhombic cross-section after HF etching, whereas in polycarbonate etch pits are circular after NaOH etching. Natural phlogopite (dark mica) may contain fission tracks and alpha-recoil tracks (ART) as latent radiation damage. HF can be used to make them visible by optical, scanning electron and scanning force microscopy (SEM, SFM). ART, generated by collisions of the recoil nuclei with the lattice atoms, provide etch pits, which are triangular at the surface, whereas the fission tracks, created via electronic energy loss (d E/d x), have hexagonal etch pits. After ion irradiation of phlogopite in the electronic d E/d x regime, the etch pits are triangular below 5.7 keV/nm and hexagonal above 8.8 keV/nm in shape. To examine more precisely the shape transition and its relation to the radiation damage, phlogopite from the Kerguelen Islands (French territory, Indian Ocean) was first annealed (500 °C, 3.5 h) and subsequently irradiated at GSI with 58Ni (kinetic energy ∼81 MeV), d E/d x amounting to 10.4 keV/nm (according to SRIM 2000). Using polyethylene terephthalate (PET) foils of seven different thicknesses as a degrader, d E/d x in the sample could be reduced stepwise to 2.4 keV/nm. The irradiated samples were etched with 4% HF at room temperature and afterwards imaged with SEM and SFM. It was observed that the triangles relate to the octahedral sites (represented by OH, O, Fe, Mg and other ions) and the hexagons to the SiO 4-tetahedral positions in the tetrahedral sheet. We interpret our findings as evidence that the d E/d x-dependent etch-pit morphologies are controlled by the lattice structure.

The kinetics and mechanism of MgO dissolution

Reactions and atomic rearrangements at fluid–crystal interfaces play an important role in catalysis and in controlling the kinetics and mechanisms of dissolution. We have studied the attachment and reactions of water molecules at the MgO–water interface by combining measurements of 1 H and 2 D surface penetration and etch pit morphology with ab initio calculations. These studies show that the most common MgO cleavage surface, (001), is thermodynamically unstable when hydrated. Proton rearrangement on such surfaces precedes proton–cation exchange and provides a general mechanism for the detachment of ions during dissolution. The kinetics of dissolution are strongly influenced by the concentration of surface defects and a simple model based on the ab initio results predicts a dissolution rate of 10 −10 mol cm −2 s −1 for a typical surface defect concentration of 0.1.

Studies on the ion-beam modifications in ethylene diamine sulphate

The growth features, defect sub-structure and the influence of ion implantation on the (001) and (100) surfaces of ethylene diamine sulphate (EDS) single crystals have been studied. The surface micromorphology and defect substructure measurements of surface electrical conductivity and microhardness in both as-grown and He+ ion-implanted surfaces of EDS have also been studied. Optical and scanning electron microscopic investigations on the (001) surface of EDS revealed octogonal mother-liquor inclusions and spiral and hopper growth features. Chemical etching of the (100) surface revealed an increase in dislocation density on ion implantation, the etch pit morphology remaining the same. Studies on planar plastic anisotropy in both (001) and (100) surfaces indicated the crystallographic nature of the single crystal. Enhancement of surface electrical conductivity on ion implantation has been attributed to the formation, migration and increase in SO −4 ion concentration relative to the as-grown EDS surfaces.