V-shaped Defects Research Articles

Observation of V-shaped defects in the growth of In 0.8Al 0.2Sb/InSb layers: Temperature and V/III flux ratio dependences

In 0.8Al 0.2Sb/InSb layers were grown under various growth temperature and V/III flux ratio conditions on semi-insulating (0 0 1)-oriented GaAs substrates by molecular beam epitaxy. The crystalline qualities of In 0.8Al 0.2Sb/InSb layers were improved by decreasing the V/III flux ratio at a fixed temperature (380 °C) and increasing the growth temperature of the fixed V/III ratio (6). Dislocations were only observed in the In 0.8Al 0.2Sb/InSb layers grown at the optimal condition. On the other hand, many planar defects, including microtwins, were observed when the In 0.8Al 0.2Sb/InSb layers were grown in the Sb-rich condition. Specifically, V-shaped defects drawn boundaries in the microtwins were observed in the In 0.8Al 0.2Sb/InSb layer.

A transmission electron microscopy study of defects formed through the capping layer of self-assembled InAs∕GaAs quantum dot samples

Plan-view and cross-sectional transmission electron microscopy have been used for a detailed study of the defects formed in capped InAs∕GaAs quantum dot (QD) samples. Three main types of defects, V-shaped defects, single stacking faults, and stacking fault pyramids, were found to form under growth conditions that led to either very large, indium enriched, or coalesced islands. All three types of defects originate at the buried quantum dot layer and then travel through the GaAs cap to the surface on the {111} planes. The V-shaped defects were the most common and typically consisted of two pairs of closely spaced 60° Shockley partials with a ⟨211⟩ line direction. The two pairs originate together at the buried QD layer and then travel in “opposite” directions on different {111} planes. The second type of defect is the single stacking fault which consists of a single pair of partial dislocations separated by an ≈50nm wide stacking fault. Finally, both complete and incomplete stacking fault pyramids were observed. In the case of the complete stacking fault pyramid the bounding dislocations along the [110], [11¯0], [101¯], and [101] directions were identified as stair rods. A possible mechanism for the stacking fault pyramid formation, which can also account for the creation of incomplete stacking fault pyramids, is presented.

Growth of InGaN HBTs by MOCVD

The design and growth of GaN/InGaN heterojunction bipolar transistors (HBTs) by metalorganic chemical vapor deposition (MOCVD) are studied. Atomic-force microscopy (AFM) images of p+InGaN base layers (∼100 nm) deposited under various growth conditions indicate that the optimal growth temperature is limited to the range between 810 and 830°C due to a trade-off between surface roughness and indium incorporation. At these temperatures, the growth pressure must be kept above 300 Torr in order to keep surface pit density under control. An InGaN graded-composition emitter is adopted in order to reduce the number of V-shaped defects, which appear at the interface between GaN emitter and InGaN base and render an abrupt emitter-base heterojunction nearly impossible. However, the device performance is severely limited by the high p-type base contact resistance due to surface etching damage, which resulted from the emitter mesa etch.

Room temperature mobility above 2100 cm 2 /Vs in Al 0.3 Ga 0.7 N/AlN/GaN heterostructures grown on sapphire substrates by MOCVD

Al0.3Ga0.7N/AlN/GaN HEMT structures with significantly high mobility have been grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. At room temperature (RT) a Hall mobility of 2104 cm(2)/Vs and a two-dimensional electron gas (2DEG) density of 1.1x10(13) cm(-2) are achieved, corresponding to a sheet resistance of 277.8 Omega/sq. The elimination of V-shaped defects were observed on Al0.3Ga0.7N/AlN/GaN HEMT structures and correlated with the increase of 2DEG mobility. (c) 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim.

V-shaped pits formed at the GaN/AlN interface

A high-quality GaN layer has been achieved using an AlN buffer layer directly grown on a sapphire substrate at high temperature. The microstructure was investigated by high-resolution transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX). A high density of V-shaped pits, with sizes of 20–40 nm, was observed at the interface between AlN and GaN layers. Unlike the previously reported V-shaped defects in nitride-based ternary alloys, these V-shaped pits were formed on top of the AlN layer due to H 2 etching of the surface during a high-temperature growth interruption between AlN and GaN layers. The pits were preferentially formed around the surface terminations of threading dislocations (TDs) in the AlN, similar to chemical etching-induced surface pits. The TD density of the AlN layer is generally very high, leading to a high density of V-shaped pits. Subsequent growth of GaN resulted in the bending over of the pre-existing TDs, significantly reducing the threading dislocation density in the overlaying GaN. The high crystal quality is further proved by highly improved photoluminescence intensity, compared to the case of conventional GaN.

Treatment strategies in the management of jejunoileal and colonic atresia

<b>BACKGROUND/PURPOSE:</b> The purpose of this prospective study was to review the operative findings, treatment strategies, as well as the results of management of 46 consecutive cases of jejunoileal and colonic atresia, managed over a 2-year period.<br><b> MATERIALS AND METHODS:</b> There were 42 patients with jejunoileal atresia (JIA) and 4 with colonic atresia (CA). The 4 group types were: type I-membranous (n=20), type II- blind ends separated by a fibrous cord (n=6), type IIIa- blind ends with a V-shaped mesenteric defect (n=10), type IIIb- apple-peel atresia (n=4) and type IV- multiple atresias (n=6). Primary surgery for JIA consisted of resection with a single anastomosis (n=37), anastomosis after tapering jejunoplasty (n=3), multiple anastomosis (n=1) and a Bishop-Koop ileostomy (n=1). For CA, resection with primary anastomosis was performed. A single end-to-oblique anastomosis after adequate resection of dilated proximal bowel, was the preferred surgical procedure. In the absence of facilities for administering TPN, early oral/nasogastric (NG) tube feeding was encouraged. In patients with anastomotic dysfunction, conservative treatment of the obstruction followed after its resolution by gradually increased NG feeds, was the preferred treatment protocol. <br><b> RESULTS:</b> Late presentation or diagnosis with hypovolemia, electrolyte imbalance, unconjugated hyperbilirubinemia (n=25) and sepsis (n=6), were significant preoperative findings. After resection and anastomosis, significant shortening of bowel length was seen in 16 patients (34.7%). Postoperative complications included an anastomotic leak (n=3), a perforation proximal to the anastomosis in 1 and anastomotic dysfunction in 5 patients. Full oral or NG tube feeding was possible only by the 13th to 31st postoperative day (POD), after the primary surgery in patients with anastomotic dysfunction and those undergoing reoperation. Overall, 38 patients survived (82.6%). Mortality was highest in patients with type IIIb or type IV JIA.<br><b> CONCLUSIONS:</b> Despite lack of ideal facilities for neonatal intensive care and administration of TPN, good results were achieved in the management of JIA and CA by following these principles: (1) adequate preoperative resuscitation, (2) meticulous surgical technique and a standardized surgical protocol, (3) early recognition of postoperative complications and their management by a uniform protocol and (4) wherever possible, early institution of oral or NG feeds, preferably by breast milk.<br>

ESD engineering of nitride-based LEDs

GaN-based light emitting diodes (LEDs) with p-cap layers grown at various temperatures were fabricated. It was found that the LED with 900/spl deg/C-grown p-cap layer could only endure negative 1100 V electrostatic discharge (ESD) pulses while the LED with 1040/spl deg/C-grown p-cap layer could endure ESD pulses as high as negative 3500 V. It was also found that the ESD performances of the LEDs with 900 and 1040/spl deg/C-grown p-cap layers were limited by the V-shape defects and the bonding pad design, respectively.

V-defects and dislocations in InGaN/GaN heterostructures

In the growth of InGaN/GaN multi-quantum well (MQW) heterostructures by metal organic chemical vapor deposition, V-defects attached to threading dislocations have been observed and investigated. Energy-dispersive X-ray analysis and conventional transmission electron microscopy studies were carried out in order to determine the In composition and investigate the behavior of the dislocations. The V-defects are limited by {101¯1} lattice planes, they are attached to threading dislocations and may start at the third quantum well. The associated dislocation runs up into the overgrown GaN layer. Some ( a+ c) dislocations were shown to decompose inside the multi-quantum well, giving rise to a misfit segment in the c-plane and a V-shape defect.

Nature of V-shaped defects in HgCdTe epilayers grown by molecular beam epitaxy

The microstructure and micromorphology of V-shaped defects in (301) and (112) HgCdTe layers grown by molecular beam epitaxy (MBE) have been investigated in detail on the consecutive stages of its formation by transmission and high-resolution electron microscopy (TEM and HREM) and atomic force microscopy (AFM). The nature of V-shaped defects in epilayers grown by MBE is reported. It was shown that the nucleation of a V-shaped defects occurs on the relief perturbations and its growth is stimulated by non-optimal growth conditions. The formation of V-shaped defect begins from the capture of excess Te along the growth front at the steady-state stages of HgCdTe growth and along the irregularities on the surface of the CdTe buffer layer at the initial stages. High-resolution observations established the occurrence of elemental Te and misoriented HgCdTe grains along the macrosteps on the growth HgCdTe surface. The captured Te initiates the formation of walls with disordered structure (DS walls) extended along the growth direction and containing misoriented HgCdTe grains, Te precipitates and dislocations observed by TEM. While growing, vertical columns packed with stacking faults and bounded by DS walls are formed. We find that these columns can be considered as the basic part of the V-shaped defect in HgCdTe grown by MBE. Due to the continual Te nucleation and twinning inside the defect area, polycrystalline growth can occur.

Suppression of cracks and V-shaped defects, and improvement of reflectivity of GaN/AlGaN distributed Bragg reflectors by insertion of multiple interlayers

GaN/AlxGa1–xN distributed Bragg reflectors (DBRs) for the near-ultraviolet region have been grown on sapphire substrates by metal-organic chemical vapor deposition. In order to suppress the generation of cracks and V-shaped defects on the surface of the DBRs, AlGaN/AlN superlattices and step-graded multiple AlGaN layers were inserted prior to or during the growth of the DBRs. By optimizing those layers, the reflectivity of a 40-pair GaN/Al0.35Ga0.65N DBR was increased up to 96% without any cracks and V-shaped defects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of electrochemical discharge machining technology for slicing non-conductive brittle materials

The electrochemical discharge machining (ECDM) has been proved to be a potential process for the machining of high-strength non-conductive materials. Traveling wire electrochemical discharge machining (TW-ECDM), a newly developed technology, is used to slice the small size (10–30 mm diameter) optical glass and quartz bars. The electrical–thermal etching effect and its feasibility are investigated. The energy release intensities and their physical phenomena under different sizes of discharge wires, power source modulations and methods of electrolyte supply are discussed. The pulsed dc power proves better spark stability and more spark energy release proportion than constant dc power. The input power is modulated to obtain the appropriate frequencies and duty factors for machining glass and quartz materials. The ion translation rate, the electrolyte immersing depth and the concentration of the alkali are found to be the dominant factors of bubbles reaction. Based on the SEM photographs of the workpiece surface, it is noted that the more purple the sparks from the mixed gases of hydrogen and vapor, the better the etching effect is. The V-shape defect occurring at the cut-in and cut-out can be significantly reduced by rotating the workpiece. Finally the appropriate cutting conditions for the quartz and borosilicate optical glass materials are concluded.

Study of electrochemical discharge machining technology for slicing non-conductive brittle materials

The electrochemical discharge machining (ECDM) has been proved to be a potential process for the machining of high-strength non-conductive materials. Traveling wire electrochemical discharge machining (TW-ECDM), a newly developed technology, is used to slice the small size (10–30 mm diameter) optical glass and quartz bars. The electrical–thermal etching effect and its feasibility are investigated. The energy release intensities and their physical phenomena under different sizes of discharge wires, power source modulations and methods of electrolyte supply are discussed. The pulsed dc power proves better spark stability and more spark energy release proportion than constant dc power. The input power is modulated to obtain the appropriate frequencies and duty factors for machining glass and quartz materials. The ion translation rate, the electrolyte immersing depth and the concentration of the alkali are found to be the dominant factors of bubbles reaction. Based on the SEM photographs of the workpiece surface, it is noted that the more purple the sparks from the mixed gases of hydrogen and vapor, the better the etching effect is. The V-shape defect occurring at the cut-in and cut-out can be significantly reduced by rotating the workpiece. Finally the appropriate cutting conditions for the quartz and borosilicate optical glass materials are concluded.

Investigations of V-shaped defects and photoluminescence of thin GaN-rich GaNP layers grown on a GaN epilayer by metalorganic chemical vapor deposition

Structural defects in high-temperature GaN-rich GaNP layers grown on a thick GaN epilayer by metalorganic chemical vapor deposition were investigated by means of transmission electron microscopy and atomic force microscopy. It is found that there are inverted hexagonal pyramid V-shaped defects in the GaNP layers with a higher phosphorus (P) composition. These defects are generally related to threading dislocations propagating from the GaN layer beneath. Consequently, the dislocation density in the GaNP layers is dramatically decreased to 5–8×107 cm−2. Photonluminescence (PL) measurements show that the PL wavelength of GaNP redshifts and the integrated emission intensity significantly increases, with respect to that from GaN layers grown under identical conditions. The emission enhancement is attributed to lowering of the dislocation density and enlarging the escape cone of photons by the rough surface, which result from the V-shaped defects formed in the GaNP layer.

Helical-type surface defects in GaN thin films epitaxially grown on GaN templates at reduced temperatures

Abstract Surface pits in the form of v-shaped defects and resulting surface roughening, previously associated solely with InGaN films, were observed and investigated using atomic force microscopy on GaN films grown at 780°C via metalorganic vapor phase epitaxy on conventionally and pendeo-epitaxially deposited GaN thin film templates. The density of the v-shaped defects was similar to the density of threading dislocations of ∼3×10 9 cm −2 (that originate from the heteroepitaxial interface between the GaN template layer and the SiC substrate). Moreover, the v-defect density was diminished with decreases in the dislocation density via increases in the template layer thickness or the use of pendeo-epitaxial seed layers. A concomitant reduction in the full-width half-maxima of the X-ray rocking curves was also observed. A qualitative model is presented that describes the formation of v-shaped defects as a result of interactions between the movement of surface steps, screw-type dislocation cores, and clusters of atoms on the terraces that form under conditions of high surface undercooling.

Engineering of an insulating buffer and use of AlN interlayers: two optimisations for AlGaN–GaN HEMT-like structures

The semi-insulating character of GaN epitaxial layers can be achieved by the control of the early stages of growth on the substrate. Adding two low temperature (LT) AlN interlayers is a technique enough powerful to reduce threading dislocation densities by up to one order of magnitude. A compressive strain as high as 2.8 x 10 -3 is induced in the uppermost GaN epilayer. The global structure is kept semi-insulating so that it is a perfect template for undoped AlGaN-GaN HEMTs (High Electron Mobility Transistors). HEMTs with interlayers present better two dimensional electron gas (2DEG) properties: up to 20% higher carrier density (n s ) and 40% higher mobility. Typically n s is as high as 1.7 x 10 13 cm -2 for a record mobility of 1200 cm 2 /Vs. The improvement of the mobility can be correlated to the reduction of nano-scale V-shaped defects in the AlGaN (less morphological-relaxation). The improvement of n s could be explained by the higher piezo-doping resulting from GaN extra-compression and AlGaN weaker relaxation. As a consequence, the DC transistors characteristics are improved: in 2 μm gate transistors, the maximum current and transconductance are increased by up to 80% and 20%, respectively, and could be extrapolated to values as high as 1500 mA/mm and 250 mS/mm for 0.2 μm gate devices.

Metalorganic Chemical Vapor Deposition Growth of a GaN Epilayer on an Annealed GaN Buffer Layer

We investigated the growth of an epitaxial layer deposited on an annealed buffer layer using transmission electron microscopy. It is shown that crystalline quality of the epitaxial layer depends on the annealing conditions of the buffer layer. Dislocations and formation of V-shaped defects are observed and may originate from the structure and morphology of the buffer layer.

Interactions between inversion domains and InGaN/GaN multiple quantum wells investigated by transmission electron microscopy

The propagation properties of inversion domains (IDs) in InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been investigated by transmission electron microscopy (TEM). The majority of the IDs, originating from the sapphire and/or buffer layer, propagate through the MQWs with normal wurtzite structure retaining their original structural features. Some of IDs could induce V-shaped pits in the MQW structures proposing a new formation mechanism for the so-called V-shaped defects. Detailed measurements show that a few IDs are found to be stopped in abnormal MQW regions, where In droplets appear due to phase separation. We presented direct evidence of pure In-phase droplets by means of high-resolution TEM. The above results provide new information on the structural defects in InGaN/GaN-based materials.

Structural analysis of Si-doped AlGaN/GaN multi-quantum wells

The effect of Si doping on the structural properties of AlGaN/GaN multi-quantum well layers grown on GaN was investigated. V-shaped defect was formed by Si-doping. PL intensity gradually increases with Si doping. But when the Si concentration exceeds 4.2×10 19 cm −3, PL intensity rapidly decreased with the formation of V-shaped defect.

The effect of mounting-induced strain and defect on the properties of AlGaAs 808 nm LDs

The effects of the mounting process on the properties of broad-area AlGaAs SQW 808 nm high-power laser bar were investigated using electroluminescence microscopy (ELM), micro-photoluminescence (μ-PL), photoluminescence microscopy (PLM) and micro-photocurrent (μ-PC) spectroscopy. Local strain and V-shaped defects were observed as a function of position by μ-PL and PLM, respectively. ELM images from each emitter were taken as a function of bar injection current, allowing the determination of the “apparent” external differential quantum efficiencies ( η ext). The results suggest that there is a strain threshold, above which η ext decreases with increased strain. μ-PC measurements show that the emitters with defects have a larger PC tail in the sub-bandgap region. The peak position of the first derivative (d I PC/d E) max of the PC spectra displays a blue shift with the increase of strain. The magnitude of the (d I PC/d E) max has a weak inverse correlation with strain. The results suggest that with the increase of strain up to threshold, defect-related non-radiative recombination centers form and accumulate. The increased non-radiative recombination results in the decrease of η ext of the emitter, i.e., the degradation of the emitters and the bar.

V-shaped defects connected to inversion domains in AlGaN layers

Thick AlGaN layers and AlGaN/GaN superlattices have been grown on GaN using metalorganic chemical vapor deposition. Cross-sectional transmission electron microscopy has been used to show that V-shaped surface pits on these samples differ from similar features observed in the InGaN system. Inversion domains and segregated Al are found in the middle of each V pit, and superlattice layers are observed to follow the pit sidewalls.