V-shaped Defects Research Articles

Morphologic features of the fetal mandibular condyle: Layers, canals and microvascular pattern

During organogenesis the mandibular condyle is divided by a fibrovascular septum, the persistence of which in the growing cartilage can lead to a bifid condyle. In this study we have evaluated the morphology of 3rd trimester human fetal temporomandibular (TMJ) specimens in order to determine the pattern of the vascular morphology associated with the layers and vascular canals (VCs) of the developing condyle (covering layers and condyle proper). Eleven human fetuses of 27-38cm crown-rump length were used for histological (hematoxylin-eosin, Van Gieson stain) and immunohistochemical evaluation (antibodies for bcl2 and CD34) and another two of 24 and 31cm, for TMJ microvasculature studies after black ink injections. With increasing fetal age, the intermediate loose lamina (LL) of the condylar proliferative layer evolves from a vascular-mesenchymal to a fibrillar pattern, via a transitory stage of a clear space that may be misdiagnosed as lower joint cavity (LJC). Within the condyle proper VCs may be present on its entire sagittal length, deepening variably towards the erosive zone and opened superiorly in the LL loose layer. Vessels of the evolving LL enter the condyle, directly or through the VCs; these vessels retract peripherally with increasing age and the intrinsic vessels of the condyle supplied from the erosive zone become prevalent. Vascular morphogenesis at the level of the LL seems comparable to that at the level of the LJC where characteristic glomeruli regress with increasing age. Lack of vascular regression and closure of central V-shaped defects of the condyle, as observed in 2/22 condyles, may represent a developmental substrate for condylar bifidism or a predisposing condition weakening the condyle, and making it more sensitive to trauma in childhood.

Accelerated Degradation of High Power Light-Emitting Diode Resulted from Inhomogeneous Current Distribution

The uniformity of current spreading is one of the key points to inspect the reliability and endurance of InGaN-based light-emitting diodes. In this paper, we propose an effective circuit model to analyze the phenomenon of premature turn-on diodes in the active layer. With the aging tests and novel investigation of failure analyses, the simulating results are good agreement with experimental data. It is found that an inhomogeneous distribution of forward current in light-emitting diode chips, as measured by conductive atomic force microscopes, provides the evidence that V-shaped defects and the associated threading dislocations are electrically active. Furthermore, the results of emission microscopy images and transmission electron microscope reviews support the assumption that the threading dislocations associated with V-shaped defects behave as very small shunt resistors connected across p–n junction. This study provides a degradation mechanism to realized one of the failure modes of high power light-emitting diodes.

Characteristics of efficiency droop in GaN-based light emitting diodes with an insertion layer between the multiple quantum wells and n-GaN layer

We have studied the characteristics of efficiency droop in GaN-based light emitting diodes (LEDs) with different kinds of insertion layers (ILs) between the multiple quantum wells (MQWs) layer and n-GaN layer. By using low-temperature (LT) (780 °C) n-GaN as IL, the efficiency droop behavior can be alleviated from 54% in reference LED to 36% from the maximum value at low injection current to 200 mA, which is much smaller than that of 49% in LED with InGaN/GaN short-period superlattices layer. The polarization field in MQWs is found to be smallest in LED with InGaN/GaN SPS layer. However, the V-shape defect density, about 5.3×108 cm−2, in its MQWs region is much higher than that value of 2.9×108 cm−2 in LED with LT n-GaN layer, which will lead to higher defect-related tunneling leakage of carriers. Therefore, we can mainly assign this alleviation of efficiency droop to the reduction of dislocation density in MQWs region rather than the decrease of polarization field.

Demonstration and Study of Photovoltaic Performances of InGaN p-i-n Homojunction Solar Cells

We reported on the InGaN p-i-n homojunction solar cells with In contents of 0.02 and 0.12. Under the same illumination of Xe lamp, In0.02 Ga0.98N cell exhibited a fill factor (FF) of 67.8%, a higher open-circuit voltage (Voc) of 2.15 V while In0.12Ga0.88N cell showed a FF of 64.8% and a lower Voc of 1.35 V. The measurements of atomic force microscopy (AFM) images and leakage currents revealed that V-shaped defects, which are known to cause the increase in reverse current, were the main factor dominating the Voc of In0.12Ga0.88N cells. Short-circuit current versus open-circuit voltage (Jsc - Voc) curves were consistent with the characteristics expected from the current-voltage equation. Relative external quantum efficiencies (EQEs) were also measured and showed strong dependence on V-shaped defects. These results indicate that defects in bulk InGaN has a negative impact on the photovoltaic performances of solar cells based on III-nitrides.

Variation of Electrostatic Discharge Robustness Induced by the Surface Morphology of High Power Light-Emitting Diodes

The capability of high-power nitride-based light-emitting diodes (HPLED) to withstand electrostatic discharge (ESD) is very important key index due to the horizontal structure of the insulating property of the sapphire substrate. Accordingly, the investigation of ESD failure mechanisms is a beneficial topic. However, it is difficult to real-time monitor the damage caused by the ESD stress because it occurred in a very short period. Before the series ESD stress, atomic force microcopy (AFM) and conductive AFM (C-AFM) were applied to explore the correlation between surface morphology and electrical properties of LED chips. Furthermore, after the series ESD stress, transmission electron microscopy (TEM) was used to investigate the failure modes and compare to the distribution of the surface current observed by C-AFM. These findings suggest that the V-shaped defect and surface morphology are strong correlate to the endurance of ESD stress.

Cavitation erosion behavior of nanocomposite Ti–Si–C–N and Ti/Ti–Si–C–N coatings deposited on 2Cr13 stainless steel using a Plasma Enhanced Magnetron Sputtering process

Abstract In the work presented in this paper, the liquid droplet erosion resistance of two nanocomposite Ti–Si–C–N single layer coatings (SMR1 and SMR3) and one multilayer structured nanocomposite Ti/Ti–Si–C–N coating (SMR2) on 2Cr13 stainless steel has been investigated by means of cavitation erosion. Three sets of 2Cr13 samples were deposited using a Plasma Enhanced Magnetron Sputtering (PEMS) process using different parameters to 35 µm, 68 µm and 59 µm thick. The cavitation erosion resistance of the coatings was evaluated with a vibratory cavitation apparatus in distilled water and in a 3 wt.% NaCl solution; the latter was used to simulate an aggressive environment. The surface and cross-sectional morphologies of the samples were studied using scanning electron microscopy (SEM) before and after the cavitation erosion tests. The results from the investigation indicate that the three coatings exhibited excellent cavitation erosion resistance in both media compared with the uncoated substrate. The cavitation erosion resistance of the thicker single layer coating SMR3 and the multilayer coating SMR2 was especially pronounced, while SMR1 was not so satisfactory because microcracks extended to the substrate due to its small thickness. The SMR2 coating effectively prevented the propagation of the microcracks in a manner of layer-by-layered spallation, owing to its multilayer structure. Even though the coatings improved the erosion resistance of the substrate, the SEM study revealed that there were many micro-droplets and V-shaped defects in the coatings, which formed the nucleation sites for the coating spallation. Therefore, if the size and the number density of the micro-droplets can be reduced and the coatings can be made more homogeneous, without V-shaped defects, the cavitation erosion resistance of coated 2Cr13 steel should be further improved.

Structural defects and device electrical behaviour in AlGaN/GaN heterostructures grown on 8° off-axis 4H-SiC

This paper reports on the influence of material defects on the electrical behaviour of AlGaN/GaN heterostructures grown onto off-axis 4H-SiC. A structural characterization revealed the presence of near-surface V-shaped defects, mostly oriented along the miscut direction [11-20]. High electron mobility transistors with the channel oriented along this direction showed a preferential conduction, while a significant reduction of the drain current occurred only along the orthogonal direction. An electrical analysis allowed us to demonstrate the anisotropy of the mobility of the two-dimensional electron gas.

Study of InGaN/GaN quantum dot systems by TEM techniques and photoluminescence spectroscopy

InGaN/GaN multilayer quantum dot structures produced by MOCVD techniques on c-plane sapphire were studied by transmission electron microscopy (TEM) and photoluminescence (PL) techniques. Indium fluctuations ranging from 1-4 nm were observed with both energy filtered TEM (EFTEM) and high angle annular dark field (HAADF) scanning TEM. The existence of V-shaped defects with nucleation centres at the termination of threading dislocation were observed in HAADF images. There was also evidence of the formation of large quantum dots at low densities from lattice HRTEM images. This was further confirmed by PL measurements through the observation of a single sharp line at low power with the typical saturation behaviour at higher power excitation.

Influence of growth temperature on GaN:Cr incorporation and structural properties in MOVPE

The influence of the growth temperature in metal organic vapor-phase epitaxy (MOVPE) on intentional Cr incorporation during GaN growth is studied in the range 900–1125 °C. The structural properties are investigated and compared to those obtained from undoped layers deposited at the same growth conditions and using the same growth procedure; Whereas, the best surface morphology and the best crystal quality are found at 1125 °C for undoped GaN layers, the best structural and morphological properties are obtained for the Cr-doped GaN (GaN:Cr) layers grown at 950 °C. GaN:Cr layers deposited at this temperature additionally exhibit the highest Cr concentration in the series as well as a Cr accumulation in the beginning stage of GaN:Cr growth. The accumulation may be responsible for the changed growth mode and the improved structural characteristics as well as for the observed V-shaped defects usually associated with strain relaxation. Thermo-remanent magnetization and a hysteresis loop were observed even above room temperature. The magnetic properties correlate to the structural properties of GaN:Cr layers.

First-Trimester Diagnosis of Familial Split-Hand/Split-Foot Malformation

Split-hand/split-foot malformation (SHFM), also called lobster claw deformity or ectrodactyly, is a rare congenital defect expressed as a median cleft of hands, feet, or both due to the absence of the central rays. The reported incidence is approximately 1 per 90,000. The anomaly results from abnormal development of the hand and foot plates during the seventh week of gestation.' The malformations can be isolated to the hands and feet with variable expression or more commonly may be seen as part of a syndrome. Typical and atypical forms are recognized. The typical variety has a V-shaped central defect, whereas the atypical form, usually sporadic, has a U-shaped cleft. 2 Advancements in screening sonography have led to the potential for detecting this rare malformation at mid gestation. 3-6 We report a case of familial SHFM diagnosed by transvaginal sonography at 11 weeks' gestation.

Origin and suppression of V-shaped defects in the capping of self-assembled InAs quantum dots on graded Si1−xGex/Si substrate

Self-assembled InAs quantum dots (QDs) on graded Si1−xGex/Si substrate were observed to adopt a truncated pyramidal shape when capped with GaAs. Our results suggest that atomic migration from the QD apex to the GaAs cap layer contributes to the formation of the symmetrical V-shaped defects near the top edges of the truncated pyramidal QDs. By employing an In0.1Ga0.9As cap, material migration and strain around the QDs were reduced, hence suppressing the occurrence of the V-shaped defects. Furthermore, photoluminescence thermal quenching rate of the QDs is significantly slower compared to the QDs with GaAs cap. Finally, 1.3 μm room-temperature photoluminescence was observed.

Analyses of early events during chondrogenic repair in rat full-thickness articular cartilage defects

In this study we investigated the cellular events that occur during the onset of chondrogenic differentiation during the repair of full-thickness defects of articular cartilage. The V-shaped full-thickness cartilage defects (width 0.7 or 1.5 mm; depth 0.8 mm; length 4 mm) were created in the femoral patellar groove of rats using a custom-built twin-blade device. The time course of the repair response in these cartilage defects was examined using a semi-quantitative histological grading scale. Cartilaginous repair responses failed to occur in the larger 1.5 mm defects, which was covered only by fibrous scar tissue. In contrast, hyaline-like articular cartilage was regenerated concomitantly with the repair of the subchondral bone by 4 weeks in smaller 0.7 mm width defects. Cells in the reparative regions were then characterized by immunohistochemistry and in situ hybridization. Undifferentiated mesenchymal cells migrate into the defects and fill the cavities within 4 days of their creation. The expression of PCNA, N-cadherin, and PTH/PTHrP receptors was induced in cells at the center of the defects, where type II collagen-positive polygonal-shaped cells also begin to appear at day 7. Marrow-derived mesenchymal cells acquire higher levels of proliferative activity in induced cartilage cavities after their initial migration and filling of the smaller 0.7 mm defects. During the regenerative repair of articular cartilage in the rat, there is a distinctive step that appears to be analogous to the precartilaginous condensation that is pivotal during chondrogenesis in development.

Novel blue continuous-wave current-injected light source

Vertical-cavity surface-emitting lasers (VCSELs) contain two high-reflectivity distributed-Bragg reflectors (DBRs) as well as a short cavity characterized by a small optical-mode volume. They emit a single longitudinal mode in a circularly symmetric beam with little beam divergence. Both characteristics are superior to those achieved with edge-emitting lasers and desirable for many practical applications in, for example, high-density optical storage and laser printing. In addition, the strong field in the microcavity facilitates exploration of cavity quantum-electrodynamics effects, such as single-photon and polariton emission, controlled spontaneous emission, and low threshold (or thresholdless) lasing. Over the past decade, two developments have affected the realization of electrically pumped gallium nitride (GaN)-based VCSELs. First, high-reflectivity, high-quality DBRs composed of AlxGa1−xN and GaN exhibit a large lattice mismatch that forms cracks in the epitaxially grown DBR structure. These cracks could result in a reduction in optical reflectivity and become a leakage-current path. The second is the fabrication of novel, high-transparency, high-conductivity contacts for current injection, which was triggered by the difficulties encountered in constructing low-resistance p-type GaN layers. In 1996, GaN-based edge-emitting laser diodes were first realized at room temperature by Nakamura’s team.1 Arakawa and co-workers subsequently fabricated an In0.1Ga0.9N VCSEL and observed lasing at 77K for the first time in 1998.2 We recently fabricated a crack-free high-reflectivity DBR by incorporating AlN/GaN superlattice layers between the DBR structures to reduce V-shaped defects and act as strain buffers.3 Figure 1 shows the device structure employing hybrid mirrors. It consists of 29 pairs of AlN/GaN DBRs and a 5λ (where λ refers to the operating wavelength) active region with a Figure 1. Schematic diagram of an electrically pumped vertical-cavity surface-emitting laser. DBR: Distributed-Bragg reflector. GaN, AlN: Gallium, aluminum nitride. MQW: Multiquantum well.

Thermal annealing of SiC thin films with varying stoichiometry

Thin films of amorphous hydrogenated silicon carbide (a-SiC:H) with varying stoichiometry of silicon and carbon were prepared by Plasma Enhanced Chemical Vapour Deposition (PECVD). After deposition a subsequent rapid thermal annealing was carried out at 900, 1100 and 1300 °C. The as-deposited and annealed SiC thin films were investigated by X-ray diffraction (XRD), cross-section and plane view transmission electron microscopy (TEM), Fourier-Transformed Infrared (FTIR) and Raman spectroscopy. The deposited films were amorphous directly after deposition and became nanocrystalline after annealing at 1300 °C. This was verified by XRD measurements. Microvoids were found in all films investigated by TEM. Densification and crystallisation as well as cooling introduced tensile stresses within the annealed SiC film and lead eventually to crack formation in the film. The annealed stoichiometric SiC film shows v-shaped extended defects at the Si–SiC interface likely formed during the annealing at high temperatures. FTIR and Raman spectroscopy showed a strong influence of initial film properties such as hydrogen content and binding structure on the internal structure of the SiC thin films after annealing.

Enhancing the quantum efficiency of InGaN green light-emitting diodes by trimethylindium treatment

This work demonstrates the effectiveness of using trimethylindium (TMIn) treatment to improve the luminescence efficiency of InGaN∕GaN quantum wells grown by metal-organic vapor-phase epitaxy. Photoluminescence, x-ray diffraction, atomic force microscopy, and high-resolution transmission electron microscopy indicate that the treatment leads to a smoother InGaN surface and InGaN∕GaN interface with substantial decrease in V-shape defects density, compared to the samples without treatment. Green light-emitting diodes prepared by this method exhibit higher output power than the control device. These improvements are attributed to the surface smoothing process in TMIn ambient, resulting in an abrupt InGaN∕GaN interface.

Defects in nanostructures with ripened InAs/GaAs quantum dots

InAs/GaAs quantum dot (QD) structures were grown by molecular beam epitaxy (MBE) with InAs coverages θ continuously graded from 1.5 ML to 2.9 ML. A critical coverage of 2.23 ML is found, above which the islands undergo ripening, which causes a fraction of quantum dots to increase in size and to eventually relax through the formation of pure, edge-type misfit dislocations which propagate towards the surface in the form of V-shaped defects. Concomitant with ripening, extended-defect related traps with activation energies of 0.52 and 0.84 eV were observed, and regarded as the cause of the significant worsening of the optical and electrical properties in high coverage structures. Their relationship with the observed dislocations is discussed.

The chondrogenic repair response of undifferentiated mesenchymal cells in rat full-thickness articular cartilage defects

The aim of this study is to develop a rat model of full-thickness articular cartilage defects that is suitable for detailed molecular analyses of the regenerative repair of cartilage. The V-shaped full-thickness defects (width: 0.7 mm; depth: 0.8 mm; and length: 4mm) were created in the femoral patellar groove of 6 weeks old male rats using a custom-built twin-blade device. Prior to starting the repair experiments, our device was examined for its accuracy and reliability in generating defects. Then, the time course of the repair response in these cartilage defects was examined using a semi-quantitative histological grading scale. The expression of chondrogenic differentiation markers in the reparative regions was examined with immunohistochemistry and in situ hybridization. Our device creates full-thickness articular cartilage defects uniformly. In these defects, undifferentiated mesenchymal cells filled the defect cavities (4 days) and initiated chondrogenic differentiation at the center of the defect (7 days). Cartilage formation was observed in the same region (2 weeks). Finally, hyaline-like articular cartilage and subchondral bone layers were reconstituted in their appropriate locations (4 weeks). We have successfully developed a rat model containing identically sized full-thickness defects of articular cartilage that can undergo chondrogenic repair in a reproducible fashion.

Effect of In as Surfactant on the Growth of AlN/GaN Distributed Bragg Reflectors by Metal Organic Vapor Phase Epitaxy

AbstractNitride based Distributed Bragg Reflectors (DBRs) have several important applications in current nitride based optoelectronic devices. DBRs can be implemented in resonant cavity light emitting diodes (RCLEDs) to improve light extraction and obtain a more directional emission and in vertical cavity surface emitting lasers (VCSELs) to achieve a lower threshold current. Due the large contrast in refractive index, AlN/GaN DRBs are practical for obtaining high reflectivity and wide bandwidth using relatively few periods. Cracking of the samples is typical for AlN/GaN DRBs due to the tensile strain which results from the 2.4% lattice mismatch and to the difference in thermal expansion coefficients. In addition to cracks, v-shaped defects may also arise in the surface due to surface undulation from stored elastic misfit strain or from threading dislocations that result in scattering and diffraction. Several techniques to reduce the number of cracks and defects have been investigated to obtain smooth surface morphology and high reflectivity, e.g. superlattices to reduce the strain or the growth of AlInGaN/GaN DBRs that offer less lattice mismatch but also lower refractive index contrast. In this work, results of the use of Indium (In) as a surfactant in Metal Organic Vapor Phase Epitaxy (MOVPE) will be discussed. This study addresses AlN/GaN DBR structures designed for peak reflectivity around 465 nm. During the AlN layers’ growth, trimethylindium was introduced to the system and resulted in a reduction of surface cracks. Results of growths at In flow rates are reported and discussed.

Effects of the quantum dot ripening in high-coverage InAs∕GaAs nanostructures

We report a detailed study of InAs∕GaAs quantum dot (QD) structures grown by molecular beam epitaxy with InAs coverages θ continuously graded from 1.5 to 2.9 ML. The effect of coverage on the properties of QD structures was investigated by combining atomic force microscopy, transmission electron microscopy, x-ray diffraction, photoluminescence, capacitance-voltage, and deep level transient spectroscopy. In the 1.5–2.9 ML range small-sized coherent QDs are formed with diameters and densities that increase up to 15nm and 2×1011cm−2, respectively. For θ>2.4 ML large-sized QDs with diameters of 25nm and densities ranging from 2×108to1.5×109cm−2 coexist with small-sized QDs. We explain the occurrence of large-sized QDs as the inevitable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with θ, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. Island relaxation substantially affects the properties of QD structures: (i) free carrier concentration is reduced near the QD plane, (ii) the QD photoluminescence intensity is significantly quenched, and (iii) deep levels show up with typical features related to extended structural defects.

Effects of the Facial Osseous Defect Morphology on Gingival Dynamics After Immediate Tooth Replacement and Guided Bone Regeneration: 1-Year Results

This article describes different scenarios of facial osseous defects when the osseous-gingival relationship exceeds 3 mm and evaluates the effects of the morphology of the compromised facial bone on gingival dynamics after immediate tooth replacement and guided bone regeneration. The implant success rate and peri-implant bone change were also reported. Twenty-three patients treated consecutively with the mean age of 39.5 years (range, 25 to 63 years) underwent immediate tooth replacement and guided bone regeneration in sockets with facial bony defects exceeding 3 mm. Facial bony defects were categorized into V-, U-, and Ultra-U (UU)-shaped. The patients were evaluated clinically and radiographically at 1-year after implant placement. At 1-year, the implant success rate was 100% (23/23). No marginal bone change of greater than 1 mm was observed. Greater than 1.5 mm of facial gingival recessions were noted in 8.3% (1/12) of V-shaped, 42.8% (3/7) of U-shaped, and 100% (4/4) of UU-shaped defects. U- and UU-shaped defects showed significantly higher frequency and magnitude of facial gingival recession (>1.5 mm) when compared with V-shaped defects 1-year after immediate tooth replacement and guided bone regeneration. It is important to identify the type of facial bony defect during diagnosis and treatment planning, so that appropriate treatment can be prescribed. The combination of delayed implant placement after staged reconstruction of unfavorable U- and UU-shaped labial extraction socket defects should be considered in areas of high esthetic concern.