Dislocation Pits Research Articles

Observations of viscous magnetization in multidomain magnetite

The viscous behavior of multidomain magnetite has been directly observed in both natural and synthetic samples using Bitter pattern imaging. A computer‐controlled fully automated microscope fitted with a heating stage, field coils, and digital camera was used to record viscous acquisition and decay sequences as a function of time, temperature, and field. Domain walls (DW) were observed to move continuously through a series of quasi‐static states over many hours rather than instantaneously. Viscosity was observed only on grains oriented near to the {111} surface. Generally, DWs moved perpendicular to their surface in the direction of the applied field; however, because observed domains respond primarily to the movement of main domains underneath the surface, occasionally, DWs moved in the opposite direction to the applied field. Small variations in temperature were found to strongly influence the viscosity, supporting the idea that viscosity is thermally activated. Viscous and nonviscous domain structures were examined using magnetic force microscopy. These images revealed that the domains displaying viscous behavior tended to be narrow (∼2 μm in width). Larger domains on grains oriented near the {111} surface did not display viscosity, reflecting the greater energy required to move larger domain structures. This may explain why no viscosity was observed on the {110} surfaces, as the domains were wider, that is, ∼6–10 μm. A complex spiraling vortex‐like magnetic domain structure was imaged. Etch pit analysis found a corresponding dislocation pit at the same location. It is suggested that this corresponds to the microstructure around a screw dislocation line.

Optical and structural investigation of GaN xP 1−x/GaP structures for light emitting diodes

A new alloy GaN x P 1− x with different nitrogen content ( 0.67 % < x N < 2.3 % ) dedicated for LED fabrication was studied by optical absorption, photocurrent and electro-luminescence measurements as well as structural parameters employing SEM and SIMS. By increasing the nitrogen level up to 3% the nature of the fundamental band gap changes from an indirect-gap transition in GaP to the quasi-direct transition in GaN x P 1− x alloy therefore the electroluminescence emission is shifted from green to the orange–red range of the visible spectrum. Strong compositional dependence on band gap was confirmed from optical measurements. Measurement of photoluminescence intensity indicates an optimal nitrogen content in GaN x P 1− x alloy around 1.35%. This nitrogen content also exhibits smooth surface morphology, lower appearance of dislocation pits and better homogeneity of nitrogen incorporation revealed by SEM and cathodoluminescence images.

SiH4 exposure of GaN surfaces: A useful tool for highlighting dislocations

AbstractFast-turnaround, accurate methods for the assessment of threading dislocation densities in GaN are an essential research tool. Here, we present an in-situ surface treatment for use in MOVPE (metal-organic vapour phase epitaxy) growth, in which GaN is exposed to a SiH4 flux at 860 °C in the presence of NH3. Subsequent characterisation by atomic force microscopy shows that the treatment is effective in increasing edge and mixed/screw dislocation pit sizes on both n- and p-type material, and on partially coalesced GaN layers.

Faceted dislocation surface pits

Surface pits are commonly formed where threading dislocations emerge from a thin film. Observations of facetted pits are common in complex materials where the Burgers vector tends to be large. Such materials also commonly exhibit hollow-core dislocations/micropipes. In this presentation, we analyze the shapes of surface pits in anisotropic materials. New analytical results will be presented for the isotropic surface energy case that extends the classical analyses. We show that faceting is dominated by surface energy, while the elastic energy associated with the dislocation largely controls the size of the pits. In cases when the temperature is well below the roughening point, the fully faceted Wulff shape will lead to fully faceted pit shape. Analytical predictions for the pit shape and size are made. Application of this method to interpret observations of dislocation pits in GaN and InGaN/GaN superlattices will be presented.

The impact of substrate nitridation temperature and buffer design and synthesis on the polarity of GaN epitaxial films

The polarity of GaN epitaxial layers grown on GaN and AlN buffer layers was investigated and found to be dependent on nitridation temperature over the range of 200–700°C. When low temperature (LT), 500°C, GaN buffer layers are used, GaN epitaxial layers grown on 200°C nitrided sapphire have a higher density of N-polar inversion domains. However, a high density of dislocation pits was observed on GaN epitaxial layers based on AFM morphology when GaN epitaxial layers were grown on LT GaN buffer of 700°C nitrided sapphire substrate. With high temperature (HT), 850°C, AlN buffer layers, the density of N-polar inversion domains in GaN epitaxial layers depends on the thickness of AlN buffer layer. The structural quality of Ga-polar GaN epitaxial layer is dramatically improved when LT GaN and HT AlN buffer layers are combined with an optimized annealing time. The measured full-widths at half-maximum of (0 0 0 2) symmetric and (1 0 .4) asymmetric reflections are 68 and 246 arcsec, respectively, for 1.0 μm GaN epitaxial layers. The results presented here can be implemented to produce low dislocation density, single Ga-polar GaN epitaxial layers.

A surface characterisation and microstructural study by scanning electron microscopy of the N-methyl-n-alkylpyrrolidinium tetrafluoroborate organic salts

A microstructural characterisation of the family of N-methyl-N-alkylpyrrolidinium tetrafluoroborate organic salts was carried out by observation of powder surface morphologies with the aim of extending the microstructure-property correlation. Inherent difficulties limiting extensive studies of organic solids by SEM, including volatility under vacuum, charging due to electron beam irradiation, and air-sensitivity were overcome with the use of a Field Emission SEM and cryostage attachment. This technique, providing considerable improvements in image quality at low accelerating voltages, enabled direct observation of complex microstructural features in samples exhibiting high temperature plastic crystalline phases (N,N-dimethylpyrrolidinium tetrafluoroborate [P11BF4]; N-methyl-N-ethylpyrrolidinium tetrafluoroborate [P12BF4]; N-methyl-N-propylpyrrolidinium tetrafluoroborate [P13BF4]). Extensive lattice imperfections including grain boundaries, slip planes and dislocation pits were observed within particles of approximately 200 μm diameter. The N-methyl-N-butylpyrrolidinium tetrafluoroborate (P14BF4) sample in this series revealed columnar single crystals with high aspect ratios. The origin of plastic flow properties is discussed using single crystal and polycrystalline slip observations and a relationship proposed between defect characteristics and transport properties.

Electrical and luminescence properties of (CdZn)Te single crystals prepared by the vertical gradient freezing method

Cd 1− x Zn x Te ( x = 0.04–0.1) single crystals with a diameter up to 100 mm and maximum height of 35 mm were prepared in a vertical arrangement by gradual cooling of the melt. The crystals grew in most cases in 〈 1 1 1 〉 orientation and they are twin-free. The average density of dislocation pits is 5 × 10 4 cm −2. The FWHM values ranging from 25 to 8 arcsec were obtained on the (1 1 1)A face even near the edge of the ingot. Optical transmittance reaches 63% at 10 μm in most cases. The typical single crystals are p-type with a carrier concentration in the range of 10 14–10 16 cm −3 and mobility of 50–100 cm 2/V s at 300 K. An important acceptor level influencing carrier concentration was found to be 140 meV above the valence band. Diffusion length of both minority electrons and holes was determined to be typically 3–10 μm. In the luminescence spectra, free and bound exciton recombination bands were resolved together with donor- and acceptor- type defect features.

Atomic Force Microscopy of Atomic-Scale Ledges and Etch Pits Formed During Dissolution of Quartz

The processes involved in the dissolution and growth of crystals are closely related. Atomic force microscopy (AFM) of faceted pits (called negative crystals) formed during quartz dissolution reveals subtle details of these underlying physical mechanisms for silicates. In imaging these surfaces, the AFM detected ledges <1 nanometer (nm) high that were spaced 10 to 90 nm apart. A dislocation pit, invisible to optical and scanning electron microscopy measurements and serving as a ledge source, was also imaged. These observations confirm the applicability of ledge-motion models to dissolution and growth of silicates; coupled with measurements of dissolution rate on facets, these methods provide a powerful tool for probing mineral surface kinetics.

Distinguishing between EL2 and dislocation formation mechanisms in GaAs by mapping topographies

Several models have been reported to investigate the relationships between the optically active defect level, EL2, and dislocation formation mechanisms. For undoped LEC grown GaAs, the comparison between EL2 and dislocation has been made using mapping capabilities. In this presentation, we aim to distinguish the formation mechanisms using three different types of mapping topographies on a broad base of materials including undoped and Si doped GaAs wafers grown by LEC and HB/GF techniques. The mapping topography methods used are photo-chemically etched growth striation, near infrared transmittance determined EL2, and molten KOH etched dislocation pits. Comparisons of different topographies are made on the same wafer in all cases. In brief, the EL2 distribution has shown good correlation with growht striation where EL2/growth striation have had only moderate to no correlation with dislocation distribution for undoped and Si doped GaAs regardless of growth techniques. The results suggest strongly that the formation mechanism for EL2 relates directly to solidification. The results also indicate that EL2 as a defect complex is only secondary in degree of importance in formation. The point defects and plastic deformation, when present, are dominant.

Tracing of dislocations in ammonium hydrogen tartrate single crystals

AbstractSparingly soluble ammonium hydrogen tartrate (AHT) crystals are grown by the gel method, derived from the diffusion of ammonium chloride into the set gel containing tartaric acid. Crystals up to 23 × 5 × 3 mm3 in size are grown at room temperature. AHT crystals are cleaved along (010) planes and the cleavage surfaces are studied by using multiple beam interferometry. The interferograms have revealed that the cleavages are quite flat. An attempt is made to trace the trajectory of dislocations of isolated as well as matched pairs of (010) cleavages of AHT when etched in a mixture of formic acid and methyl alcohol (2:1) and 1.0 M SrCl2 solutions. Optical and transmission electron micrographs of dislocations show oblique, parallel and continuous line characteristics. Rows of equally spaced dislocation pits are observed and the implications of this are discussed.

Etch pit studies in CdTe crystals

Etch pit formation has been studied on (111)A, (111)B, and (100)CdTe surfaces using a nitric acid/potassium bichromate etch with varying Ag+ ion concentrations. The results can best be explained using an adsorption poisoning model originally proposed to explain etch pit formation in InSb, with modifications to take into account the higher ionicity of CdTe. Although the results were fairly reproducible, the etch pit density was consistently lower than that obtained with the Nakagawa etch. Aβ dislocation pits were seen for the first time in CdTe.

A Defect Etchant for InGaAsP

A defect revealing etching technique for grown on <100> by LPE is reported. A chemical solution used with illumination produces protrusions in thin quaternary layers, which correlate well with dislocation pits in underlying and overlying layers. Using the new etchant, an increase in defect density in the active layer over the defect density in the buffer layer has been detected in lattice matched (≲0.04%) LED structures.

Deformation by scratches and frictional properties of MgO crystals

The subsurface structure of MgO crystals frictionally damaged on their (001) faces by scratching with a diamond stylus in the 〈100〉 direction was investigated using a scanning electron microscope in both the cathodoluminescence and the secondary electron modes. The friction is independent of the load below a critical value, and above this value it depends on the load W according to μ ∝ Wn where μ is the coefficient of friction. The subsurface beneath the sliding contact consists of four characteristic material zones: zone I is severely deformed with a high defect density which causes quenching of the luminescence; zone II is a plastically deformed intermediate zone showing enhanced luminescence; zone III is a slipped zone which on etching shows a dislocation pit pattern; zone IV consists of non-deformed crystalline material. The normalized depth da, where d is the depth of each deformed zone and a is the track half-width, is independent of the load. The frictional behaviour was examined by using empirical relations for the damage to estimate the plastic work done during the scratching process.

EPD investigation in LEC‐grown silicon‐doped gallium arsenide

AbstractThe EPD distribution is investigated in silicon‐doped LEC‐grown gallium arsenide crystals, 30 ÷ 40 mm in mainbody diameter and weighing within 300 ÷ 500 grams. The silicon‐doping‐dependent hardening effect, previously observed in S‐doped and Bridgman grown silicon‐doped gallium arsenide is here confirmed. The EPD decreasing with silicon‐doping increase is observed to be less pronounced than in the case of S‐doped and Bridgman grown silicon‐doped samples and possible reasons for this different behaviour are discussed. In any case, a large microprecipitate density together with the EPD reduction appears as a clustering of shallow‐pits around dislocation pits.

DISLOCATION ETCH PITS ON THE {111} CRYSTAL PLANE OF InSb FORMED BY HCl-Fe+++ SOLUTION

The fine structure of the dislocation pits, which are formed by HCl-Fe+++ etching solution, is observed experimentally. The correlation between structure features of the dislocation pits and the directions as well as slip planes of the dislocations is investigated. We discussed the kinetic processes in which the etch pits are formed. The results indicated that different rate of nueleation and velocity of lateral motion on the dissolution steps of 〈112〉and〈112〉 crystal orientations is the main reason which makes the dislocation pits have two kinds of dissolute edges with different features.

Low dislocation density GaSb single crystals grown by LEC technique

Dislocation density in LEC-grown GaSb single crystals was studied. It was observed that a thermal shock due to seed-contact to a melt contributed to the generation of dislocations in the crystal. By melting back the dislocation-free seed after contacting the melt, approximately 20–25 mm GaSb single crystals with only a few dislocation pits can be successfully grown.

The effect of annealing on residual stress and dislocation propagation in silicon slices with damaged layer induced by scribing

The effect of annealing on residual stress and dislocation propagation in silicon slices with a damaged layer induced by diamond scribing, laser scribing and diamond blade cutting was studied by infra-red photoelastic measurements and dislocation pit observations. Residual stress and dislocation propagation both showed clear annealing temperature dependence at temperatures above 500° C, although the residual stress was greatly reduced by a small degree of dislocation propagation. The experimental results can be explained using the stress recovery theory by the model of the damaged layer with a mosaic crystal layer and a single crystal layer with micro-cracks and dislocations.

Etch Pit Pattern of GaAs Crystals Made by Light Irradiated Electrolytic Etching

A light-irradiated electrolytic etching technique was developed for GaAs crystals. This technique, which employs a chemical echant consisting of AgNO3, CrO3, HF and H2O, realize clear etch pit patterns in a thinly etched layer of GaAs. A square pattern and a triangular pattern appear on the {100} plane and {111} As plane, respectively. Etching depth is about one tenth that of traditional electrolytic etching without light. This makes application to accurate epitaxial layer evaluation possible. The patterns obtained by this method are shown to be dislocation pits by comparison with X-ray topography patterns. The clear patterns obtained can be made even when the etchant is at room temperature. The results open the way for practical evaluation of thin epitaxial layers.

Correlation between Dislocation Pits in GaP LPE Layers and LEC Substrates

In order to improve the efficiency of a GaP green-emitting diode, one essential point is to decrease the D-pit density in the epitaxial layer. Two types of D-pits on the LPE (111) phosphorus surfaces characterized by their respective sizes were studied to determine their correlation with LEC crystal characteristics. Both types of D-pits were found to have origins in the LEC substrates. Small-size D-pits in the LPE layer originate from the substrate dislocations which from a `star' pattern on the LEC wafer. On the other hand, large-size D-pits have an origin whose distribution is very different from that of small D-pits. The large-size D-pit density correlates with the roughness of the RC etched LEC surfaces, and its density levels differ on a large scale among the LEC crystals used. As both types of D-pits act as a shunt path for minority carriers, both origins should be eliminated in the LEC crystals.

Correspondence between Nonradiative Dark Spots, Microplasma Emissions, and Dislocation Pits in GaP : N Light‐Emitting Diodes

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