Etch Pit Studies Research Articles

Dislocation-based crack initiation and propagation in single-crystal SrTiO3

Understanding the irreversible deformation (dislocation activation and crack formation) of functional oxides, based on which various advanced electronic devices are fabricated, is critical for the optimal structural design and mechanical reliability. Here, we demonstrate the dislocation-based crack initiation and propagation in a model perovskite, single-crystal SrTiO3, at small scale. Using nanoindentation tests with spherical tips and etch pit study, we identify the sequence of the irreversible deformation events occurred in single-crystal (001) SrTiO3. For a locally stressed volume that is free of pre-existing cracks, the material undergoes the following processes: (1) purely elastic deformation; (2) dislocation activation on primary slip planes that are 45° inclined to the surface; (3) dislocation activation on secondary slip planes perpendicular to the surface; (4) crack initiation by dislocation pile-up; followed by (5) concurrent crack propagation and dislocation multiplication and motion at higher loads. Specifically, we focus on the crack formation caused by the dislocation pile-up beneath the spherical indenter. We also identify the favorable crack propagation planes to be {110} when the (001) surface is indented. These findings in cubic SrTiO3 are believed to be applicable for other ceramic materials with cubic structure.

Nanoscale to microscale reversal in room-temperature plasticity in SrTiO3 by tuning defect concentration

Incipient room-temperature plastic deformation in single-crystal SrTiO3 at the nanoscale and the microscale is contrasted by applying a scale-dependent indentation technique. Using nanoindentation pop-in, nanoindentation creep, and an evaluation of dislocation spacing via etch pit study, we demonstrate a reversal of yield stress for crystals with different vacancy concentrations and pre-existing dislocation densities. A competing mechanism between dislocation nucleation and dislocation motion on crystal plasticity at different length scales is highlighted. This finding enables us to complete the understanding of dislocation-mediated plasticity for ceramic oxides at the nanoscale as compared to the microscale and macroscale.

Spatial distribution of structural defects in Cz-seeded directionally solidified silicon ingots: An etch pit study

The PV properties of wafers processed from Cz-seeded directionally solidified silicon ingots suffer from variable structural defects. In this study, we draw an overview on the types of structural defects encountered in the specific case of full 〈1 0 0〉 oriented growth. We found micro twins, background dislocations, and subgrains boundaries. We discuss the possible links between thermomechanical stresses and growth processes with spatial evolution of both background dislocation densities and subgrain boundaries length.

Study of dislocations in HgCdTe epilayers at (1 1 1)B and (1 1 0) surfaces using modified defect etchant

A new etchant, based on the modification of standard Chen etchant has been developed for revealing etch pits in HgCdTe epilayer at (111)B and (110) planes. We have optimized the oxidizing and dissolving agents in the solution by systematic experimentation to obtain the new etchant. By using this etchant we revealed the etch pits reliably and consistently in several HgCdTe epilayers grown on (111)B face CdZnTe substrates using LPE (liquid phase epitaxy) technique, The new etchant is not sensitive to the sample orientation and works equally well for both (111) and (110) planes. The dislocations revealed by the modified etchant were investigated by the study of etch pits at (111)B surface and at the cleaved plane, i.e., (110) face. Different types of etch pits have been observed and investigated. The etch pits were found to be dependent on the CdZnTe substrate and the stress induced during the growth itself.

Etch Pit Formation on Germanium Alloys by in situ Gaseous Hydrogen Chloride Etching

Etch pit formation is an effective method of crystal quality measurement, especially of heteroepitaxial films. Besides a few well-known wet etching methods, gaseous etch by hydrogen chloride has also been shown to reveal semiconductor crystal defects [1]. In this work we show that after etching in gaseous hydrogen chloride, etch pits can form on epitaxial germanium alloys and reveal threading dislocations in these alloys.The deposition of the epitaxial germanium alloys was conducted on bare Si(001) substrate in a Centura® 300 mm RP Epi chamber from Applied Materials. Commercially available gas sources such as dichlorosilane (SiH2Cl2), germane (GeH4), digermane (Ge2H6), and tin tetrachloride (SnCl4) were used in the thermal chemical vapor deposition of epitaxial germanium, silicon germanium (Si1-xGex), and germanium tin (Ge1-xSnx). Etching in hydrogen chloride was performed in the same chamber at a temperature below 400°C.Before the deposition of germanium alloys, a layer of relaxed germanium was deposited on Si substrate. This layer provides a close lattice match to the germanium alloys to be deposited on top. This also provides a source of threading dislocations for etch pit study. An etch of this relaxed germanium layer in hydrogen chloride shows pitting as revealed by atomic force microscopy (AFM), illustrated in Fig. 1. The pit sizes depend on both etch temperature and time. It is further demonstrated that the locations of threading dislocations in this relaxed germanium layer coincide with etch pits. Fig. 2 shows a plan-view transmission electron microscopy (TEM) image on an etched relaxed germanium surface. The threading dislocations shown by diffraction contrast and the etch pits shown by thickness contrast have a clear overlap. This overlap can only be seen for the large pits with a fairly uniform size of approximately 0.5 μm in Fig. 2(a). Smaller pits, circled in Fig. 2(a), appear uncorrelated with threading dislocations.On an as-deposit relaxed germanium layer, germanium alloyed with either silicon or tin was deposited. Germanium-rich silicon germanium was deposited at ≤ 500°C. A low temperature (≤ 350°C) process was used to deposit homogeneous and compositionally uniform germanium tin. The alloy film thickness is controlled below its critical thickness so that the film is fully strained with respect to the relaxed germanium layer underneath. Therefore the threading dislocation density in the germanium alloy layer is approximately the same as that on the surface of the relaxed germanium layer.High-resolution X-ray diffraction (XRD) measurement shows that both silicon germanium and germanium tin are fully strained before and after etch on relaxed germanium. Fig. 3(a) and 3(b) show the ω/2θ scan and the AFM image, respectively, of a post-etch Si0.14Ge0.86 on relaxed germanium. The density of the large pits appears close to that on an etched relaxed germanium layer. The silicon germanium surface shows more smaller pits than the relaxed germanium surface.Fig. 4(a) and 4(b) show the ω/2θ scan and the AFM image, respectively, of a post-etch Ge0.92Sn0.08 on relaxed germanium. The density of the large pits is again close to that on an etched relaxed germanium layer.The authors thank Dr. Hong Zhang of Precision TEM for the TEM analysis.

Mapping mesoscale heterogeneity in the plastic deformation of a copper single crystal

Part of a ‘multiscale characterization’ study of heterogeneous deformation patterns in metals is reported. A copper single crystal was oriented for single slip in the (111)[] slip system and tested to ∼10% strain in roughly uniaxial compression. The macroscopic strain field was monitored during the test by optical ‘image correlation’. The strain field was measured on orthogonal surfaces, one of which (the x-face) was oriented perpendicular to [] and contained the [] direction of the preferred slip system. The macroscopic strain developed in an inhomogeneous pattern of broad, crossed shear bands in the x-face. One, the primary band, lay parallel to (111). The second, the ‘conjugate’ band, was oriented perpendicular to (111) with an overall () habit that contains no common slip plane of the fcc crystal. The mesoscopic deformation pattern was explored with selected area diffraction, using a focused synchrotron radiation polychromatic beam with a resolution of 1–3 µm. Areas within the primary, conjugate and mixed (primary + conjugate) strain regions of the x-face were identified and mapped for their orientation, excess defect density and shear stress. The mesoscopic defect structure was concentrated in broad, somewhat irregular primary bands that lay nominally parallel to (111) in an almost periodic distribution with a period of about 30 µm. These primary bands were dominant even in the region of conjugate strain. There were also broad conjugate defect bands, almost precisely perpendicular to the primary bands, which tended to bridge primary bands and terminate at them. The residual shear stresses were large (ranging to well above 500 MPa) and strongly correlated with the primary shear bands; interband stresses were small. The maximum resolved shear stresses within the primary bands were oriented out of the plane of the bands, and, hence, could not recover the dislocation structure in the bands. The maximum resolved shear stresses in the interband regions lay predominantly in {111} planes. The results are compared to the mesoscopic defect patterns found in Cu in etch pit studies done some decades ago, which also revealed a mesoscopic dislocation structure made up of orthogonal bands.

New observations on high-temperature creep at very low stresses

Creep tests were conducted in compression to evaluate the flow behavior of aluminum at very high temperatures and low stresses. The experiments used two types of specimens: single crystals of 99.999% purity and oligocrystalline samples of 99.97% purity. Results obtained for the single crystals lie consistently between the conventional region of Harper–Dorn creep and the anticipated behavior based on an extrapolation of conventional 5-power creep. Using etch pit studies with the single crystals, it is shown there is no evidence for subgrain formation and the measured dislocation densities are consistent with data extrapolated from the conventional 5-power region. The creep results on single crystals suggest the stress exponent is close to ∼3 at low stresses. It is demonstrated these results are consistent with earlier data including the results of Harper and Dorn when their data are plotted in terms of the true applied stress without incorporating a threshold stress.

Optical inhomogeneities in Li2B4O7 single crystals

AbstractLithium tetraborate, Li2B4O7 (LTB) crystals were grown by Czochralski technique. It belongs to tetragonal system with lattice parameter a = b = 9.479 Å, c = 10.286 Å. Transmission range of this crystal is 170 nm – 3300 nm. This material has attracted much attention because of its application in higher harmonic generation of Nd:YAG laser and radiation dosimetry. The grown crystals were characterized for their optical inhomogeneity using etch pit study, hardness, conoscopy, interferometry, and UV‐VIS transmittance techniques. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical, mechanical and thermal studies of nonlinear optical crystal l-arginine acetate

The experimental results of optical, thermal, mechanical and chemical etch pit studies of nonlinear optical crystal l-arginine acetate (LAA) are presented. Quality of the grown crystal is considerable good as observed from the etch pit density (10 4 cm −2). LAA has a wide optical transmission window between 220 and 1500 nm. Its optical damage threshold and SHG efficiency is comparable with that of KDP. Hardness of LAA is anisotropic in nature depending on both direction and plane of indentation and (0 1 0) 〈2 0 3〉 is the hardest plane and direction combination. The obtained results of Young's modulus calculated from ultrasonic velocity data and microhardness measurement confirmed the applicability of Marshall law for soft crystal category.

Growth and Fabrication of 2 inch Free-standing GaN Substrates via the Boule Growth Method

ABSTRACTHigh quality, single crystal GaN substrates have been demonstrated using a boule growth process. Here we report on the crystalline boules that were formed during the growth process and their material characterization. Using hydride vapor phase epitaxy process, GaN crystals were grown at growth rates greater than 200 μm/hr. Boules greater than 3 mm thick were grown and processed into free-standing substrates. Rocking curve measurements using high-resolution X-ray diffraction were performed on the substrates with FWHM values of 92 and 146 arcsec for the (002) and (102) reflections, respectively. Atomic force microscope images, etch pit studies, and transmission electron micrographs of the GaN material show high quality material quality with a dislocation density in the range of 5×106 to 1×107 cm-2.

Threading dislocation evolution in mega-electron-volt phosphorus implanted silicon

The effect of dose and energy on postannealing defect formation for high energy (mega-electron-volt) phosphorus implanted silicon has been studied using etch pit studies and transmission electron microscopy (TEM). Previous work has shown that after annealing there is a strong dependence of dislocation density threading to the surface on the implanted phosphorus dose and energy. A superlinear increase in threading dislocation density (TDD) with implant energy between 180 and 1500 keV is observed for a dose of 1×1014 cm−2. In addition as a function of ion fluence, there is a maximum in the threading dislocation density at a dose of 1×1014 cm−2 followed by a rapid decrease in TDD. Both the superlinear increase in TDD with increasing energy and the rapid decrease with increasing dose have been further investigated by TEM. A TEM study of these higher doses revealed formation of a strong bimodal loop distribution with small loops averaging <1000 Å and large loops averaging around 1 μm in size. Over the dose range of 1×1014 cm−2 to 5×1014 cm−2, the superlinear decrease in TDD from 1×106 cm−2 to <1×104 cm−2 coincides with the superlinear increase in small dislocation loops from below 1×106 cm−2 to above 1×1010 cm−2. It is suggested that the homogeneous nucleation theory can explain many of the results. However, the chemical presence of phosphorus appears to also play an important role in the formation of the small dislocation loops and possibly threading dislocations.

Vapor-phase epitaxial lateral overgrowth of ZnSe on GaAs

We describe and demonstrate a process for ZnSe epitaxial lateral overgrowth (ELOG) on stripe-patterned, oxide-masked (1 1 1)GaAs substrates using a simple near-equilibrium chemical vapor transport process based on a reversible reaction mediated by hydrogen at atmospheric pressure. The ELOG process exhibits excellent selectivity, wherein epitaxial seeding and initial growth is restricted to mask openings, and subsequent lateral epitaxy has aspect ratios as high as 20. We also achieved near-seamless impinging growth of continuous epilayers of ZnSe on the masked substrate. Etch-pit density studies indicate defect reduction in areas of the ZnSe overgrown atop the oxide compared to areas of the ZnSe epilayer seeded directly over the mask openings. The composite GaAs substrate, oxide mask, and ZnSe epilayer structure is under considerable stress due to the thermal expansion mismatch of these materials. Some approaches for reducing thermal stress are discussed.

Scanning Force Microscopy Study of Etch Pits Formed during Dissolution of a Barite (001) Surface in CDTA and EDTA Solutions

Dissolution of the barite (001) surface in aqueous solutions of 0.18 M CDTA (trans-1,2-cyclohexylenediaminetetraacetic acid) and 0.18 M EDTA (ethylenediaminetetraacetic acid) at pH 12 was investigated using ex situ scanning force microscopy. In both solutions, triangular and trapezoidal etch pits developed on the (001) surface and became deeper and longer with increasing dissolution time. The orientation of the etch pits in CDTA and EDTA solutions was elongated along the crystallographic b axis. Furthermore, dissolution of the (001) surface in a layer-by-layer fashion was observed. This resulted in the formation of “alternating” etch pits with heights of one half-unit cell (about 3.6 Å), with the orientations of any two consecutive etch pits pointing oppositely to each other. In CDTA, etch pits within the half-unit cell were frequently bounded along the 〈120〉 and 〈010〉 directions. However, in EDTA, etch pits within the half-unit cell were bounded along the 〈110〉 and 〈010〉 directions. The dissolution behavior of barite in these two solutions is different based on the observed differences in the etch pits geometries as an assay for specific interactions between the crystal surface and organic molecules. Thus, we suggest that CDTA molecules bind to one Ba2+ cation along the 〈120〉 and/or 〈010〉 directions and EDTA molecules bind along the 〈110〉 directions to two Ba2+ cations exposed on the (001) surface.

Scanning Tunneling Microscope Study of Etch Pits on Highly Oriented Pyrolytic Graphite Heated in an Atomic Absorption Electrothermal Analyzer

Scanning tunneling microscopy (STM) was used to elucidate monolayer etch pits that form on highly oriented pyrolytic graphite (HOPG) heated in an electrothermal analyzer. Pits form at elevated temperatures due to reactions between oxygen and exposed carbon edge atoms (defects) and additionally with intraplanar carbon atoms (through abstraction). Samples of HOPG without analyte or matrix modifier were placed in the depression of a pure pyrolytic graphite platform and heated by using standard analysis furnace programs. Under argon stop-flow conditions, pits form in less than a second at atomization temperatures equal to and above 1200 °C. With low argon flow rates (40 mL/min), pits formed at atomization temperatures equal to and greater than 1750 °C in less than a second. Quantitative pit formation rates were used to indicate oxygen partial pressure, which may be as high as ∼ 10−3 atm at 1200 °C. Reaction rates were used to predict surface degradation due to oxygen attack and determine that 1-μm depth normal to the surface would be removed by 200 successive 5-second-period furnace firings at 1200 °C. Implications for increases in surface reactivity and analyte intercalation are discussed.

Direct observations of interface instabilities

Single crystals of lead bromide doped with silver bromide were grown by the vertical Bridgman method. Direct observations were made in order to understand the interfacial instabilities. Numerical studies were carried out to provide a framework for interpreting the observed convective and morphological instabilities. Observations on interfacial instabilities in lead bromide with 500 and 5000 ppm silver bromide impurities supported the numerical results predicted for 1-g conditions. X-ray rocking curves, X-ray contour scans, and etch-pit studies showed that increasing solutal convection deteriorated the crystal quality of the crystals.

Noncollinear acousto-optic tunable filter: thallium phosphorus selenide system

Single crystals of thallium phosphorous selenide Tl₃PSe₄, were grown from the melt by the Bridgman method. The crystal quality was evaluated by optical and metallographic techniques. The crystals transmit radiation between 0.7 and 14 μm. Etchpit studies show that the crystals are free from inclusions and lamellar twins and exhibit a uniform cross-sectional etchpit density. Thallium phosphorous selenide crystals are biaxial and belong to the orthorhombic crystal class. A detailed design for the noncollinear acousto-optic tunable filter that uses Tl₃PSe₄ crystal is presented. Design of filters from this class of crystals facilitates control of the angular aperture, an important device parameter.

Crack formation during laser cutting of silicon

A proposed semiquantitative model for crack formation during laser cutting of single holes in silicon wafers and ribbons predicts that compressive plastic deformation should occur in an annular region surrounding the hole during cutting. When the annular region cools to room temperature, the residual circumferential stresses are predicted to be tensile in the deformed zone and compressive outside. The predicted residual radial stresses are tensile everywhere and are maximum at the deformed zone boundary. The presence of a plastically deformed zone was confirmed by etch pit studies in originally dislocation-free Czochralski silicon. Radial cracks were observed to terminate at the deformed zone boundary and circumferential cracks were observed to follow the deformed zone boundary. Both of these observations are qualitative confirmation of the proposed model of crack formation. Holographic interferometry was used to measure the residual displacement field relative to the uncut state but no residual displacements were observed. The lack of observable residual displacements was attributed to the relaxation of residual stresses by crack growth.

Etch pit study of different crystallographic faces of L-arginine phosphate (LAP)

Etch pit studies have been made on different crystallographic faces of LAP single crystals. A new etchant has been developed which produces reproducible and good quality pits. The observed etch pits reveal the symmetry of different faces and the etching action somewhat depends upon the relative abundance of the various faces in the crystals grown from aqueous solution.

Morphology and polarity of GaAs needles

GaAs needle crystals with the zincblende structure were grown by reacting a mixture of Ga and Ga 2O 3 with As in a closed quartz tube. The six lateral surfaces of the needles are parallel to the {112} planes. The growth axis is the [111] polar direction. Some of them changed their growth axis from one of the <111 > polar axes to another during growth. Examinations of the crystals by scanning electron microscopy and X-ray diffraction clarified the detailed morphology. The polarity of grown crystals was determined from a study of etch pits revealed on the {111} and {112} planes by an HF+H 2O 2 etchant.

Etch pit study of dislocation formation in Hg1−xCdxTe during array hybridization and its effect on device performance

The relationship between hybridization force, dislocation (etch pit) density, and device performance has been investigated for devices fabricated from Hg1−xCdxTe liquid phase epitaxy (LPE) material. Coldwelded indium column interconnects are the predominant method used to mechanically and electrically attach mosaic arrays to silicon multiplexers. The mismatch in thermal expansion between the Hg1−xCdxTe photodiode arrays and silicon multiplexers stresses the interconnects when the arrays are cooled to their operating temperature (typically 40–120 K) which can lead to failures. Increased hybridization force improves the mechanical stability of the interconnects but can introduce dislocations in the Hg1−xCdxTe. Dislocations have been reported to degrade the performance of some device structures. The dislocation density in the LPE film after growth was ≊8×105/cm2. We have found no significant change in dislocation density at hybridization forces up to ≊3.9×107 N/m2. At forces on the order of 5.9×107 N/m2 (≊10% of the Vickers hardness), the dislocation density in the films increased to 90×105/cm2 in the areas where the indium columns were. The diffusion limited performance of the test structures at 120 K was not significantly degraded by the increased dislocation density. At 80 K, however, where the devices were modeled to be generation–recombination limited, there was an increase in dark current with increased dislocation density. The structures used in the study were fabricated from LPE Hg1−xCdxTe grown on a CdTe substrate. Implantation of boron into the p-type material through a ZnS passivation layer was used to form junctions. A zero-bias junction resistance of 1000 to 7000 Ω cm2 was typical for the test structures which had a cutoff wavelength of 4.7 μm at 120 K.