Pit Defects Research Articles

The origin of shallow etch pit defects in low dislocation density GaP crystals

The defect corresponding to shallow etch pits (S-pits) on GaP wafer has been revealed. It is a vacancy-type Frank dislocation loop which surrounds a stacking fault. It is proposed that this loop is formed by the condensation of excess gallium vacancies during cooling of the GaP crystal. The phosphorus precipitates on the dislocation loop result from the condensation of the emitted phosphorous interstitials accompanied by the growth of the loop. Stoichiometry control is necessary to suppress the formation of S-pit defects in low dislocation density compound semiconductor crystals.

Articular surface defects in the third metatarsal and third cuneiform: Nonosseous tarsal coalition

Frequencies of articular surface defects on the third metatarsal and third cuneiform, seen as pits of varying sizes on the plantar one third of the tarsometatarsal articular face, were investigated in skeletal populations from North America and Japan, as well as in gibbon, orangutan, chimpanzee, and gorilla skeletons. The apes did not exhibit the defects, although the number of observed specimens of each type was small. The newly presented human frequencies corresponded well with those from other published sources. The defects appeared both unilaterally and bilaterally, with no apparent sex or side biases. Statistical tests between the various populations found that, in general, geographically close populations had more similar frequencies of the defect. Possible etiologies for the defect were investigated, including biomechanical influences, degenerative arthritis, infection, trauma, and a developmental condition known as tarsal coalition, which proved to be the best explanation. Tarsal coalition results from the failure of a joint space to form properly during fetal growth. It can occur between any two adjacent bones of the foot. Several clinically important coalitions, whose presence interferes with normal walking, are known. However, coalition between the third metatarsal and third cuneiform has not been reported in the clinical literature, suggesting that the defect causes little or no foot dysfunction. Tarsal coalition is thought to have a strong genetic component, suggesting that the pit defect may be useful as a skeletal nonmetric trait, as others have stated. Am J Phys Anthropol 109:53–65, 1999. © 1999 Wiley-Liss, Inc.

Articular surface defects in the third metatarsal and third cuneiform: nonosseous tarsal coalition.

Frequencies of articular surface defects on the third metatarsal and third cuneiform, seen as pits of varying sizes on the plantar one third of the tarsometatarsal articular face, were investigated in skeletal populations from North America and Japan, as well as in gibbon, orangutan, chimpanzee, and gorilla skeletons. The apes did not exhibit the defects, although the number of observed specimens of each type was small. The newly presented human frequencies corresponded well with those from other published sources. The defects appeared both unilaterally and bilaterally, with no apparent sex or side biases. Statistical tests between the various populations found that, in general, geographically close populations had more similar frequencies of the defect. Possible etiologies for the defect were investigated, including biomechanical influences, degenerative arthritis, infection, trauma, and a developmental condition known as tarsal coalition, which proved to be the best explanation. Tarsal coalition results from the failure of a joint space to form properly during fetal growth. It can occur between any two adjacent bones of the foot. Several clinically important coalitions, whose presence interferes with normal walking, are known. However, coalition between the third metatarsal and third cuneiform has not been reported in the clinical literature, suggesting that the defect causes little or no foot dysfunction. Tarsal coalition is thought to have a strong genetic component, suggesting that the pit defect may be useful as a skeletal nonmetric trait, as others have stated.

Effect of Buffer Layer and III/V Ratio on the Surface Morphology of Gan Grown by MBE

The surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and “loop” defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than 1 nm RMS.

Template Electropolymerization of Polypyrrole Nanostructures on Highly Ordered Pyrolytic Graphite Step and Pit Defects

Polypyrrole nanostructures with diameters ⩽ 10 nm have been electropolymerized using step and pit defects on highly ordered pyrolytic graphite (HOPG) as templates for electropolymerization. Step defects were naturally occurring, and pits were formed via oxidation of freshly cleaved surfaces of an HOPG wafer by heating at ∼640°C. Underpotential deposition of approximately ∼80 mV caused polypyrrole to form only on the step and pit edges of HOPG at and not on the basal plane. The size of these nanostructures could be controlled by limiting the pyrrole polymerization time at anodic potentials. Recent modeling results allow the morphology of the deposition to be inferred, and we find the electrochemical data consistent with wire‐shaped growth for up to 30 s at constant potential, after which the growth changes morphology. Scanning tunneling microscopy data confirm this result. Preliminary studies show that these polypyrrole nanostructures can be removed by sonication.

A grand canonical Monte Carlo study of adsorption on graphitic surfaces with defects

A new hybrid model for defects in graphitic surfaces is proposed. The model allows a wide range of defects to be simulated efficiently. Grand canonical Monte Carlo simulations of the adsorption of nitrogen on graphitic surfaces containing pit defects have been carried out within the new model. The simulations indicate that while adsorption is enhanced at low pressure the high-pressure adsorption is reduced. In contrast to adsorption at 77 K on non-defective surfaces, nitrogen molecules are ordered in the pits.

Effect of Buffer Layer and III/V Ratio on the Surface Morphology of Gan Grown by MBE

AbstractThe surface morphology of GaN is observed by atomic force microscopy for growth on GaN and AlN buffer layers and as a function of III/V flux ratio. Films are grown on sapphire substrates by molecular beam epitaxy using a radio frequency nitrogen plasma source. Growth using GaN buffer layers leads to N-polar films, with surfaces strongly dependent on the flux conditions used. Flat surfaces can be obtained by growing as Ga-rich as possible, although Ga droplets tend to form. Ga-polar films can be grown on AlN buffer layers, with the surface morphology determined by the conditions of buffer layer deposition as well as the III/V ratio for growth of the GaN layer. Near-stoichiometric buffer layer growth conditions appear to support the flattest surfaces in this case. Three defect types are typically observed in GaN films on AlN buffers, including large and small pits and “loop” defects. It is possible to produce surfaces free from large pit defects by growing thicker films under more Ga-rich conditions. In such cases the surface roughness can be reduced to less than l nm RMS.

Rectangular vacancy island formation and self-depletion in Czochralski-grown PbMoO 4 single crystal during heat treatment

Single crystal samples of PbMoO 4 with a 〈100〉 orientation have been investigated by DTA, chemical etching, and thermal treatments at 950°C for various annealing times. Surface morphologies of chemically and thermally etched {;100}; surfaces have been studied in detail in respect of microstructure, etch pit dimension, defect type and density. It is found that both chemical and thermal etching techniques reveal a similar radial defect-distribution pattern on the {;100}; surfaces while DTA shows a small compositional variation along the radial and axial directions. The correlation between two types of etch pits has been addressed. For thermally etched samples, a kinetic study on the rectangular vacancy island (thermal etch pit) formation and self-depletion has been presented. Based on the current work, the formation rate of the vacancy islands can be expressed as D 2 x − D 2 0 = k 0t 1.32 exp( − E a RT ) . Mechanisms regarding the vacancy island formation and self-depletion upon the heat treatment have also been proposed. The current study on the post-growth heat treatment has also identified key processing parameters to improve overall crystal quality.

The dependence of bulk defects on the axial temperature gradient of silicon crystals during Czochralski growth

4″, 6″ and 8″ Cz crystals were grown with different heat shields which protect the growing crystals against radiation emitted from the melt surface and hot graphite parts, thereby determining their thermal environment. It was found that the critical pull rate, at which the oxidation induced stacking fault (OSF) ring vanishes in the wafer centre, varies with the crystal diameter and the type of heat shield. A calculation of the axial temperature gradient at the solid/liquid interface for each crystal diameter/heat shield combination revealed that the critical pull rate is proportional to this axial temperature gradient G, which, in turn, is a function of the crystal diameter and heat shield. Thus, the critical pull rate is entirely determined by the axial gradient G. The OSF ring appears in the wafer centre when the equation V G = C crit = 1.3 × 10 7t-3 cm 2 min −1 K −1 holds ( V is the pull rate). For V G > C crit flow pattern/D-defects are observed, whereas V G < C crit describes the condition for the growth of large pit defects.

Dependence of the Grown-in Defect Distribution on Growth Rates in Czochralski Silicon

As-grown defects in 6-inch-diameter Czochralski-silicon crystals grown under different crystal growth rate conditions (0.4, 0.7, 1.1 mm/min) were studied by means of preferential etching and IR light-scattering tomography (LST). Grown-in defect images were classified into four types as follows: (a) flow patterns (wedge-shaped etch pits), (b) IR-defect images observed by LST, (c) ringlike distributed small pits, and (d) large pits. It was found by secondary ion mass spectrometry that IR defects are oxygen precipitates. Large pit defects were identified by transmission electron microscopy as large dislocation loops with a length of about 30 µm. At growth rates from 0.7 mm/min to 1.1 mm/min, flow pattern defects and IR defects coexist inside a ringlike distributed oxidation-induced stacking fault (ring-OSF) region. However, at growth rates less than 0.7 mm/min, large pit defects were observed in the region outside the ring. Characteristic ringlike distributed small pit defects were observed on the outer periphery of the ring region. Flow pattern defects were annihilated during annealing at 1100°C, while IR defects were stable at 1250°C.

Growth of large SiC single crystals

We have grown 6H-polytype SiC single crystal boules up to 60 mm in diameter by the physical vapor transport process at 2300°C. [0001] oriented substrate wafers prepared from these undoped crystals exhibit resistivities of up to 10 5 Ω cm and etch pit defect densities of 10 4−10 5 cm -2. Epitaxially-grown microwave MESFET structures exhibit 5 GHz cutoff frequency; the highest reported to date.

Fatigue crack initiation and propagation in electron beam vapor-deposited coatings for gas turbine superalloys

Thermal fatigue crack initiation and propagation in NiCoCrAlY-coated nickel base superalloys have been evaluated. At relatively low strain ranges, the cycles required to initiate coating cracking are a significant fraction of the total fatigue life of a component. Crack initiation occurs preferentially at pit and flake defects in the coating. The strain-temperature cycle shape has a significant effect on coating crack initiation; cycle I, which peaks tensile strains at the minimum temperature, is the most severe from a coatings standpoint. Cycle I thermal fatigue crack initiation and propagation in NiCoCrAlY coatings are dependent on the thermal expansion mismatch with the substrate. The rate of coating crack propagation is also dependent on coating thickness.

Role of metallic contamination in the formation of ’’saucer’’ pit defects in epitaxial silicon

Metallic contamination present in silicon wafers prior to epitaxy or introduced during epitaxial processing were found to be responsible for microdefect formation in epitaxial silicon. Such contamination is often due to use of stainless‐steel tweezers. The defects as revealed by Secco etching appeared as shallow saucer pits (s‐pits). High densities of s‐pits were found to affect the surface carrier concentration of the epitaxial layer as measured by the high frequency MOS technique. The s‐pits were found to nucleate oxidation‐induced stacking faults (OSF) on (100) material during subsequent processing. Neutron activation analysis was used to identify the elements causing s‐pits, and transmission electron microscopy analysis was done to identify the defect as a dislocation loop. Several means of reducing s‐pit densities are also presented.