Dislocation Outcrops Research Articles

Deformation twin traces on gold surfaces: A pathway to tailored epitaxial growth of 1D semiconductors.

The field of one-dimensional semiconducting materials holds a wide variety of captivating applications, such as photovoltaic cells, electronic devices, catalysis cells, lasers, and more. The tunability of electrical, mechanical, or optical attributes of a semiconductor crystal relies on the ability to control and pattern the crystal's growth direction, orientation, and dimensions. In this study, we harvest the unique properties of crystallographic defects in Au substrates, specifically twin boundaries, to fabricate selective epitaxial growth of semiconducting nanocrystals. Different crystallographic defects were previously shown to enhance materials properties, such as, screw dislocations providing spiral crystal growth, dislocation outcrops, and vacancies increasing their catalytic activity, dislocation strengthening, and atomic doping changing the crystal's electrical properties. Here, we present a unique phenomenon of directed growth of semiconductor crystals of gold(I)-cyanide (AuCN) on the surface of thin Au layers, using traces of deformation twins on the surface. We show that emergence of deformation twins to the {111} Au surface leads to the formation of ledges, exposing new {001} and {111} facets on the surface. We propose that this phenomenon leads to epitaxial growth of AuCN on the freshly exposed {111} facets of the twin boundary trace ledges. Specific orientations of the twin boundaries with respect to the Au surface allow for patterned growth of AuCN in the <110> orientations. Nano-scale patterning of AuCN semiconductors may provide an avenue for property tuning, particularly the band gap acquired.

Mechanism Study of Stress Corrosion Behavior under Tensile and Compressive Stresses for Welded Joint Used in Nuclear Turbine Rotor.

Stress corrosion damage containing pitting and cracking was investigated for NiCrMoV steel-welded joint used in nuclear turbine rotor in 3.5% NaCl solution. The U-bend specimen containing tensile face and compressive face was adopted which was conducive to study the effects of stress on pitting and stress corrosion cracking. On tensile surface of U-bend specimen, pit grew in open environment and was converted into crack covered with passivated film. Interestingly, corrosion pit was also found on compressive surface, which might attribute to this location creating enclosed environment causing Cl- to diffuse hardly. Then, pit grew under the occluded oxide crust composed of special crystalline corrosion products. When pit approached to critical size, crack initiated from it. The critical size of pit for crack initiation from pit was 35 μm on tensile surface and 95 μm on compressive surface. The sensitivity of crack initiation in tensile surface was higher than that in compressive surface. Then, cracking on compressive surface was controlled by slip dissolution mechanism, that is, dislocation outcrop generated through plastic deformation during manufacturing and absorbed Cl- in enclosed environment to accelerate metal dissolution and film rupture. Thereupon, the stress corrosion cracking on compressive surface was able to be maintained. The findings compared corrosion damage modes caused by the two kinds of stress and emphasized the nonignorable role of compressive stress on stress corrosion damage.

In Situ Liquid-Phase AFM Observation of the Molecular Step Spiral Generation on the (1−01) Surface of Calcium Oxalate Monohydrate Crystal

Calcium oxalate monohydrate (COM) crystal is the major crystalline component of human kidney stones. Its growth event at the nanometer and micrometer scales, i.e., the growth of the COM molecular steps generated from the dislocation outcrop on the crystal surface and its inhibition by associated acidic organic molecules such as citrate, is now well understood by studies conducted using in situ liquid-phase atomic force microscopy (AFM). Yet, the detailed assessment of the evolution of the COM molecular steps at the dislocation outcrop has been poorly conducted. Herein, in situ liquid-phase AFM was used to primarily investigate how those COM molecular steps are generated on a COM broadest crystal surface (1−01) and influenced by a model acidic peptide, L-aspartic acid 6mer (L-Asp6) adsorbed onto the emerging steps and terraces on the surface. Like many other mineral crystals, a segment of the pseudo-triangle-shaped step spiral, in the process of its birth from the dislocation outcrop, starts to move after reaching the critical step length. When the budding step spiral got adsorption of L-Asp6 to it, it appeared rather with ellipse-like hexagonal morphology—which is reflected in the bulk crystal morphology—implying changes in orientation-dependent step edge energy and much-delayed spiral generation time.

Etch pit formation on β-Ga2O3 by molten KOH+NaOH and hot H3PO4 and their correlation with dislocations

The chemical etch pitting method is a powerful technique to reveal dislocations in semiconductor materials; however, a reliable etchant has not been established for β-Ga2O3. In this work, we performed a comparative study of two etchants, molten KOH+NaOH and hot H3PO4, to reveal dislocations from the (−201) surface. Using transmission electron microscopy, it was determined that KOH+NaOH could reveal all grown-in dislocations. The dislocation outcrops were located at the pit cores. The pit shape, specifically, its symmetry in the [010] direction, had a close correlation with the line direction of the dislocations but there was no simple correlation with the Burgers vector. On comparing H3PO4 with KOH+NaOH on the same sample, the results indicated that both etchants could reveal dislocations with a large angle to the (−201) surface but H3PO4 could not reveal dislocations nearly parallel to this surface. Surface damage induced by polishing produced etch pits in both etchants with smaller dimensions than those of the grown-in dislocations.

Carrier Diffusion in GaN : A Cathodoluminescence Study. III. Nature of Nonradiative Recombination at Threading Dislocations

We investigate the impact of threading dislocations with an edge component ($a$ or $a+c$ type) on carrier recombination and diffusion in $\mathrm{Ga}\mathrm{N}$(0001) layers close to the surface as well as in the bulk. To this end, we utilize cathodoluminescence imaging of the top surface of a $\mathrm{Ga}\mathrm{N}$(0001) layer with a deeply buried (In,$\mathrm{Ga}$)N quantum well. Varying the acceleration voltage of the primary electrons and comparing the signal from the layer and the quantum well enables us to probe carrier recombination at depths ranging from the close vicinity of the surface to the position of the quantum well. Our experiments are accompanied by fully three-dimensional Monte Carlo simulations of carrier drift, diffusion, and recombination in the presence of the surface, the quantum well, and the dislocation, taking into account the dislocation strain field and the resulting piezoelectric field at the dislocation outcrop. Near the surface, this field establishes an exciton dead zone around the dislocation, the extent of which is not related to the carrier diffusion length. However, reliable values of the carrier diffusion length can be extracted from the dipolelike energy shift observed in hyperspectral cathodoluminescence maps recorded around the dislocation outcrop at low acceleration voltages. For high acceleration voltages, allowing us to probe a depth where carrier recombination is unaffected by surface effects, we observe a much stronger contrast than expected from the piezoelectric field alone. This finding provides unambiguous experimental evidence for the strong nonradiative activity of edge threading dislocations in bulk $\mathrm{Ga}\mathrm{N}$ and hence also in buried heterostructures.

Self-catalytic growth of one-dimensional materials within dislocations in gold

Dislocations in metals affect their properties on the macro- and the microscales. For example, they increase a metal's hardness and strength. Dislocation outcrops exist on the surfaces of such metals, and atoms in the proximity of these outcrops are more loosely bonded, facilitating local chemical corrosion and reactivity. In this study, we present a unique autocatalytic mechanism by which a system of inorganic semiconducting gold(I) cyanide nanowires forms within preexisting dislocation lines in a plastically deformed Au-Ag alloy. The formation occurs during the classical selective dealloying process that forms nanoporous Au. Nucleation of the nanowire originates at the surfaces of the catalytic dislocation outcrops. The nanowires are single crystals that spontaneously undergo layer-by-layer one-dimensional growth. The continuous growth of nanowires is achieved when the dislocation density exceeds a critical value evaluated on the basis of a kinetic model that we developed.

Sublimation Anisotropic Etching of Silicon Carbide in Aluminum Nitride Vapors

Specific features of sublimation etching of crystalline silicon carbide (SiC) in the presence of aluminum nitride (AlN) vapors have been reported. The etching is shown to be essentially anisotropic. Abnormally high speed of sublimation in a lateral direction, which weakly depends on surface orientation and polarity, is revealed through increasing artificial holes and flattening out surface elevations prepared in the shape of rectangular mesa structures. A high-rate etching is observed at outcrops of micropipes and closed-core screw dislocations, which leads to the formation of pits on () C-terminated faces. During annealing, the pits evolve into holes that might provoke the formation of pores inside free-standing AlN layers grown in one process with the sublimation of SiC substrates. The sublimation kinetics of a Si-terminated face is evidently different from that of a C-terminated face. It is found that a liquid phase is formed on the etched surfaces and it plays a key role in interpreting the etching anisotropy on SiC crystal faces of different orientations. It is shown that the sublimation of mesa-shaped SiC substrates can be used with the aim of optimizing the fabrication of thick freestanding AlN layers.

Identification of fine structures at the surface of epi-ready GaN wafer observed by confocal differential interference contrast microscopy

Confocal differential interference contrast microscopy detected dots and line contrasts on the surface of epi-ready (0001) GaN wafers. Large and small dots consisted of pits, ∼1.4 μm in width and ∼7 nm in depth, and ∼0.5 μm in width and ∼2.7 nm in depth, respectively; the pits mainly formed on the outcrops of dislocations, with Burgers vectors ≥ c and b = a, respectively, via inadequate chemical mechanical polishing. Lines consisted of scratches induced via polishing. Deep scratches accompanied dense basal plane dislocation loops.

The study of the planar defects of the nano-sized retained austenite in Mn-Si-Cr steel

The morphology and crystallographic features of the nano-sized retained austenite (RA) in low carbon Mn-Si-Cr steel were explored in the present work. The nano-sized RA between the sub-laths inside the martensite laths is perpendicular to the lath boundary and shows particular long-axis direction, i.e. (3 5 1)γ. The stacking faults connected with the dislocation outcrops on the interfaces are observed in the nano-sized RA. The phase transformation stress accompanying the martensitic transformation can induce plastic deformation by the slip of both the stacking faults in the nano-sized RA and the Shockley partial dislocations in the interface.

Determination of the Carrier Diffusion Length inGaNfrom Cathodoluminescence Maps Around Threading Dislocations: Fallacies and Opportunities

We investigate, both theoretically and experimentally, the drift, diffusion, and recombination of excitons in the strain field of an edge threading dislocation intersecting the GaN{0001} surface. We calculate and measure hyperspectral cathodoluminescence maps around the dislocation outcrop for temperatures between 10 to 200 K. Contrary to common belief, the cathodoluminescence intensity contrast is only weakly affected by exciton diffusion, but is caused primarily by exciton dissociation in the piezoelectric field at the dislocation outcrop. Hence, the extension of the dark spots around dislocations in the luminescence maps cannot be used to determine the exciton diffusion length. However, the cathodoluminescence energy contrast, reflecting the local bandgap variation in the dislocation strain field, does sensitively depend on the exciton diffusion length and hence enables its experimental determination.

Sites of Initiation of Explosive-Emission Processes on the Surface of Single-Crystal and Coarse-Grained Polycrystalline Copper

Results of a study of sites of vacuum breakdown initiation on the electrolytically polished surface of single-crystalline and coarse-grained polycrystalline copper are presented. The coincidence of cathode erosion traces with sites of dislocation outcrops has been established.

The mechanism of pitting initiation and propagation at deformation bands intersection of cold-rolled metastable stainless steel in acidic ferric chloride solution

Pitting initiation and propagation at the deformation bands intersection of cold-rolled 14Cr10Mn stainless steel in acidic ferric chloride solution with a pH of 1.3 were investigated by a scanning electron microscope equipped with electron back scatter diffraction and a high-resolution transmission electron microscopy. The experimental results show that the pits with regular morphology are a sort of dislocation etch pit with strict triggering conditions. The pitting corrosion resistance of the etch pits is closely related to the orientations in the increasing order of {100}〈001〉 < {110}〈112〉 < {111}〈110〉. The pit wall will expand to the periphery along the trend of retaining the close-packed plane, leading to an increase in pit depth after the pitting initiation is triggered. We believe that hydrogen ions compete with chloride ions and preferentially adsorbed on the dislocation outcrops in the austenite around α′-martensite and form hydrogen atoms occupying vacancies to prevent chloride ions from eroding the metal matrix. The results also reveal that the vacancy bands formed by the slip of edge dislocations, and the vacancy groups formed by the dissolve of partial dislocation outcrops are patterns for triggering dislocation etch pits.

Pitting behavior of welded joint and the role of carbon ring in improving corrosion resistance

Pitting behavior and its transition process to cracking were investigated after immersion at 180 °C in pure water and 3.5% NaCl solution for NiCrMoV welded joint manufactured by narrow gap submerged arc welding technology. Dislocation outcrops in the interior of the grains formed small anodes, which mainly attributed to the pit growth for weld metal (WM) and heat-affected zone (HAZ). The serious pitting in WM was induced by strong strain concentration generating from welding thermal stress. Competition behavior between pitting and cracking for WM in pure water and chloride solution was also proposed. Chloride accelerating anodic dissolution caused serious pitting that predominated over cracking in chloride solution, while cracking preferentially formed and replaced mild pitting in pure water. In addition, carbon rings around pits decreased pitting susceptibility for HAZ. The formation of carbon rings generated from carbon diffusion was driven by strain aging and selective corrosion. Moreover, micro-Raman manifested the carbon ring was covered by protective oxide layer that inhibited pit growth and improved pitting resistance for HAZ. This work contributes to the understanding of pit growth and crack initiation for welded joint used in nuclear turbine.

Dislocation etching of diamond crystals grown in Mg-C system with the addition of silicon

The dislocation structure of diamond crystals grown in the Mg-Si-C system at pressure of 7.0–7.5 GPa and temperature of 1800 °C was studied by selective etching. We determined the dislocation density and identified individual features of etching for the growth sectors of {111} and {100} faces. On the {111} faces, all etch pits formed at the outcrops of different dislocations were found to be identical. The feature of grown diamond crystals is the presence of numerous large dislocation loops in the {100} growth sectors. The identified patterns of crystals etching and the internal crystal structure features are associated with the presence of a silicon impurity in the grown diamonds.

Threading dislocations in MBE grown AlInSb metamorphic buffers: Revealed and counted

The authors compare four methods to investigate the threading dislocations (TDs) observed in metamorphic buffers used in the growth of InSb quantum well on GaAs (001) substrates. Three types of buffers with varying number of Al0.24In0.76Sb interlayers (N = 0, 1, and 3) were studied. Cross-sectional scanning transmission electron microscopy (STEM) revealed an effective dislocation filtering by the interlayers. Individual TDs were identified with atomic-force microscopy (AFM) as distinct morphological features of dislocation outcrops on the surface. Threading dislocation density (TDD) is reduced by 1 order of magnitude with three interlayers, consistent with the STEM observation. TDD measured with a scanning electron microscope in electron channeling contrast imaging (ECCI) mode agrees closely with the AFM analysis, except for the N = 0 buffer where the ECCI gives TDD lower by more than a factor of two. The etch pit density of N = 3 buffer, measured with a Nomarski differential interference contrast microscope after defect selective etching (DSE), is almost 1 order of magnitude lower than the TDD measured by AFM and ECCI. Due to the large pit size, the used etching recipe only works well for samples with TDD lower than 107 cm−2. AFM, ECCI, and DSE are excellent alternatives to transmission electron microscopy in the process of metamorphic buffer optimization. The AFM technique offers the additional advantage of high vertical resolution morphology mapping. Such capability is of great importance for the optimization of metamorphic buffers from the perspective of surface smoothness improvement.

Unusual growth macrolayers on {100} faces of diamond crystals from magnesium-based systems

We studied unusual growth macrolayers on the {100} faces of diamond crystals grown in Mg-C and Mg0.9Ge0.1-C systems at 7.0GPa and in the temperature range of 1800–1900°C. The layers were shaped as rectangles highly elongated in one of the [110] directions. The layer elongation (length to width ratio) was different, but did not exceed 23. We found that the ends of the macrolayers in most cases were composed of {111} microfacets, and their growth occurred by layers along these microfacets. Using selective etching, we found that the macrolayers were not associated with outcrops of dislocations and any planar defects on the {100} faces. It is supposed that the formation of these highly elongated layers occurs by two-dimensional nucleation that becomes possible at high carbon supersaturation under conditions of kinetically limited diamond growth in Mg-based systems.

In situ atomic force microscopy studies of growth mechanisms and surface morphology of zinc thiourea sulfate crystals

In situ atomic force microscopy (AFM) has been utilized in studies of the growth mechanism on the (100) face of zinc tris (thiourea) sulphate (ZTS) crystals growing from solution. The growth on the (100) face of pure ZTS crystal is mainly controlled by two dimensional (2D) nucleation mechanisms, under which the hillock is formed through layer‐by‐layer growth. It is easier to form 2D nuclei at edge dislocation and the apex of steps. The growth of 2D nucleus is in accord with nucleation‐spreading mode. The growth rate along the 〈010〉 direction is faster than that along 〈001〉 direction, both of which increase firstly and then decrease with the spread of nucleus. The kinetic coefficients of one nucleus have been roughly estimated to be 3.6 × 10−4 cm/s and 1.8 × 10−4 cm/s in two directions, while the activation energy E was calculated to be 53.7 kJ/mol and 55.4 kJ/mol, respectively. The 2D nuclei can be generated under lower supersaturation with the addition of EDTA. If there are several hillocks growing together, step bunches will form when the steps moving in the same direction meet each other, while the meeting of steps that move in the inverse direction will result in the separation of steps. The ability of nucleation of edge dislocation outcrops are different even they are close to each other on the same surface. When the nucleus was generated at the edge dislocation sites, it cannot spread speedily until finishes an “incubation period”. Moreover, the detour of microsteps was observed due to the existence of pits. If the microcrystals attached on the surface block the step advancement, or leave the surface or are covered by the macrosteps, the pits are formed. If the macrosteps advanced across the pits, the pits will be covered and the liquid inclusions may form. However, if the microcrystal forming in the pit grow up and expose on the surface, the pit will not be covered by macrosteps. The formation of solid inclusions may be caused by the microcrystals being embedded into the single steps which move layer‐by‐layer.

Microstructure of quenched doped InSb

Magnetic characterization results indicate that, after liquid quenching, InSb samples doped with manganese, manganese + zinc, and manganese + cadmium are magnetic semiconductors. According to microstructural analysis data, polished sections of these materials demonstrate surface order: the grains have the form of wedges directed from the periphery of the section to its center, occupy essentially the entire surface of the section, and have low-angle boundaries with dislocation outcrops on the sample surface. According to X-ray diffraction data, quenched doped InSb samples are free of impurity phases and have preferential crystallographic orientations. Analysis of the present experimental data leads us to conclude that the surfaces of metallographic specimens of doped InSb are sections through textures whose orientation—under given quenching conditions—depends on the dopant composition.

An in situ AFM investigation on the morphology of the (100) growth interface of ZTS crystal

Interface morphology of the (100) face of zinc tris-thiourea sulphate (ZTS) crystals, grown under different supersaturations, at 30°C, was investigated using in situ atomic force microscopy (AFM). It comes out that the (100) face exhibits stepped surface characteristics. At lower supersaturations, nucleation easily occurred at the outcrops of edge dislocations which could act as persistent sources of monomolecular growth steps, and the edge dislocation sites appeared to exhibit a “memory function”. At higher supersaturations, however, poly-nucleation becomes the primary growth mechanism. The rates of step advancement in the [010] and [001] directions of the two-dimensional nuclei were different, as it ensues from the symmetry anisotropy. The activation energy of the two-dimensional nuclei along with the step kinetic coefficients in the [010] and [001] directions, were calculated. We also found that microcrystals on the surface of the crystal could be better integrated into the crystal surface, or otherwise lead to the generation of pits by migration of the microcrystals. Pits could be gradually filled by elementary steps and thus no defect would be formed, or they might also be covered directly by macrosteps, which would result in liquid inclusions.

A microstructural study of a quenched InSb ingot

Samples cut from a quenched ingot of the compound semiconductor InSb have been characterized by precision X-ray diffraction. The results show that the central and top parts of the ingot have [110] texture, whereas its bottom part is polycrystalline, which is interpreted in terms of the heat removal geometry during quenching. The texture plane coincides with the easy cleavage plane (110), typical of the III–V compound semiconductors and due to dislocation pile-ups. The microstructure of the quenched InSb ingot includes characteristic microcracks, dotted with dislocation outcrops on the polished surface. The observed increase in the unit-cell parameters of quenched InSb is tentatively attributed to the high dislocation density in the quenched ingot.