Search Results - (Author, Cooperation:X. H. Wu)

2015-01-17

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

Acclimatization ; Agriculture/*history ; *Altitude ; Archaeology ; Climate ; History, Ancient ; Humans ; Radiometric Dating ; Temperature ; Tibet

2018-12-04

American Physical Society (APS)

0556-2813

1089-490X

Physics

Nuclear Structure

2015-10-16

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

1089-7550

AIP Digital Archive

Physics

Defect structures were investigated by transmission electron microscopy for GaN/Al2O3 (0001) epilayers grown by metal-organic chemical vapor deposition using a two-step process. The defect structures, including threading dislocations, partial dislocation bounding stacking faults, and inversion domains, were analyzed by diffraction contrast, high-resolution imaging, and convergent beam diffraction. GaN film growth was initiated at 600 °C with a nominal 20 nm nucleation layer. This was followed by high-temperature growth at 1080 °C. The near-interfacial region of the films consists of a mixture of cubic and hexagonal GaN, which is characterized by a high density of stacking faults bounded by Shockley and Frank partial dislocations. The near-interfacial region shows a high density of inversion domains. Above ∼0.5 μm thickness, the film consists of isolated threading dislocations of either pure edge, mixed, or pure screw character with a total density of ∼7×108 cm−2. The threading dislocation reduction in these films is associated with cubic to hexagonal transformation of the nucleation layer region during high temperature growth. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Extended defect reduction in GaN grown by lateral epitaxial overgrowth (LEO) on large-area SiO2/GaN/Al2O3 wafers by low pressure metalorganic chemical vapor deposition is characterized using transmission electron microscopy and atomic force microscopy. The laterally overgrown GaN (LEO GaN) has a rectangular cross section with smooth (0001) and {112¯0} facets. The density of mixed-character and pure edge threading dislocations in the LEO GaN (〈5×106 cm−2) is reduced by at least 3–4 orders of magnitude from that of bulk GaN (∼1010 cm−2). A small number of edge dislocations with line directions parallel to the basal plane are generated between the bulk-like overgrown GaN and the LEO GaN regions as well as at the intersection of adjacent merging LEO GaN stripes. The edge dislocations are most likely generated to accommodate the small misorientation between bulk-like GaN and LEO GaN regions as well as between adjacent single-crystal LEO GaN stripes. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the "V-defect") initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {101¯1} planes) and an open hexagonal inverted pyramid which is defined by the six {101¯1} planes. Thus, in cross section this defect appears as an open "V". The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({101¯1} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the following characteristics: (i) its intensity is correlated with the side-wall quantum wells; (ii) the temperature independence of the full width at half maximum strongly supports a localized exciton recombination process. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Basal plane sapphire is a common substrate for the heteroepitaxy of GaN. This presents a challenge for fabrication of cleaved-facet GaN lasers because the natural cleavage planes in (0001) α-Al2O3 are not perpendicular to the wafer surface. This letter describes a method for achieving perpendicular cleaved facets through wafer fusion that can potentially be used to fabricate GaN based in-plane lasers. We demonstrate successful fusion of GaN to InP without voids or oxide at the interface and fabricate optically flat cleaved GaN facets that are parallel to the crystallographic planes of the host InP. I–V measurements have been performed across the n-N fused interface. These results show that the fused junction exhibits a barrier of several electron volts for electrons passing from the InP to the GaN and ohmic conduction of electrons moving in the opposite direction. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The structure and morphology of low growth temperature GaN nucleation layers have been studied using atomic force microscopy (AFM), reflection high energy electron diffraction (RHEED), and transmission electron microscopy (TEM). The nucleation layers were grown at 600 °C by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire. The layers consist of predominantly cubic GaN (c-GaN) with a high density of stacking faults and twins parallel to the film/substrate interface. The average grain size increases with increasing layer thickness and during the transition from low temperature (600 °C) to the high temperatures (1080 °C) necessary for the growth of device quality GaN. Upon heating to 1080 °C the nucleation layer partially converts to hexagonal GaN (h-GaN) while retaining a high stacking fault density. The mixed cubic-hexagonal character of the nucleation layer region is sustained after subsequent high-temperature GaN growth. © 1996 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The structure, morphology, and optical properties of homoepitaxial GaN layers grown by molecular beam epitaxy on metalorganic chemical vapor deposition (MOCVD)-grown GaN "template" layers were investigated as a function of the group III/group V flux ratio during growth. GaN layers grown with a low III/V ratio (N-stable growth) displayed a faceted surface morphology and a tilted columnar structure with a high density of stacking faults. In contrast, films grown with a high III/V ratio (Ga-stable growth) displayed comparable structure to the underlying MOCVD-grown template. The transition from N-stable to Ga-stable growth modes was found to occur over a narrow range of Ga fluxes at a growth temperature of 650 °C. Evidence of Ga accumulation and step-flow growth was observed for films grown under Ga-stable conditions, leading to the formation of spiral growth features at the surface termination of mixed edge/screw dislocations. Photoluminescence measurements indicate that the deep-level (∼550 nm) emission is increased relative to the near-band edge emission for films grown under N-stable conditions. © 1997 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

In these studies, we have investigated the role of low-temperature growth in the reduction of threading dislocation (TD) densities in large mismatch heteroepitaxy. Low- and high-temperature (LT) and (HT) GaAs growths on highly mismatched substrates were used to find the mechanism of enhanced TD reduction in LT grown (250 °C) GaAs. LT templates have symmetric (equal) TD subdensities on the {111}A and {111}B planes, whereas HT templates have asymmetric TD subdensities. A model based on TD reactions was applied to the experimental results and confirmed the beneficial role of symmetric TD subdensities in LT GaAs TD reduction. A ductile-to-brittle transition in dislocation behavior was observed at ∼400 °C. © 1999 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The growth of epitaxial MgO films on Sb-passivated (001)GaAs using pulsed laser deposition has been investigated. The temperature at which the Sb-passivation layer was desorbed was found to have a significant effect on the interfacial properties of MgO/GaAs heterostructures. Heating the substrates to 350–380 °C in vacuum resulted in a (1×3) GaAs surface reconstruction suitable for the growth of epitaxial MgO films. However, residual Sb was found to persist on the GaAs surface at temperatures as high as 500 °C. MgO growth after Sb desorption at 350–380 °C resulted in a nonuniform interfacial layer which varied in thickness from ∼0.1 to 1.5 nm, whereas substrates heated to 500 °C prior to MgO growth displayed abrupt interfaces. Capacitance–voltage measurements indicated a qualitative difference in the interfacial electronic properties for the two cases. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The structural evolution of epitaxial GaN layers grown on basal plane sapphire has been studied by atomic force microscopy (AFM), x-ray diffraction, and transmission electron microscopy (TEM). High-temperature growth (1050–1080 °C) on optimized nucleation layers leads to clear, specular films. AFM on the as-grown surface shows evenly spaced monatomic steps indicative of layer by layer growth. AFM measurements show a step termination density of 1.7×108 cm−2 for 5 μm films. This value is in close agreement with TEM measurements of screw and mixed screw-edge threading dislocation density. The total measured threading dislocation density in the 5 μm films is 7×108 cm−2. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The early stages of hydride vapor phase epitaxy (HVPE) of GaN on sapphire were studied using atomic force microscopy, field-emission scanning electron microscopy, cross-sectional transmission electron microscopy, and x-ray diffraction rocking curves. At the high growth rate used (∼33 nm/s), the films appear to be fully coalesced for growth periods as short as 1 s. A distinct surface and subsequent bulk transformation were observed, resulting in significantly smoother film surfaces and improved bulk morphology. The growth of thick (i.e., 300 μm) GaN films using HVPE offers a promising technique for the deposition of high-quality substrates for GaN homoepitaxy. © 1998 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

In this letter we demonstrate that the anomalously low (002) x-ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation (TD) geometry. Epitaxial GaN films were grown on c-plane sapphire by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a horizontal flow reactor. Films were grown with (002) rocking curves (ω-scans) widths as low as 40 arcsec and threading dislocation densities of ∼2×1010 cm−2. The threading dislocations in this film lie parallel to the [001] direction and within the limit of imaging statistics, all are pure edge with Burgers vectors parallel to the film/substrate interface. These TDs will not distort the (002) planes. However, distortion of asymmetric planes, such as (102), is predicted and confirmed in (102) rocking curve widths of 740 arcsec. These results are compared with films with (002) rocking curves of ∼270 arcsec and threading dislocation densities of ∼7×108 cm−2. © 1996 American Institute of Physics.

Electronic Resource

1089-7666

AIP Digital Archive

Physics

How to describe vorticity creation from a moving wall is a long standing problem. This paper discusses relevant issues at the fundamental level. First, it is shown that the concept of "vorticity flux due to wall acceleration'' can be best understood by following fluid particles on the wall rather than observing the flow at fixed spatial points. This is of crucial importance when the time-averaged flux is to be considered. The averaged flux has to be estimated in a wall-fixed frame of reference (in which there is no flux due to wall acceleration at all); or, if an inertial frame of reference is used, the generalized Lagrangian mean (GLM) also gives the same result. Then, for some simple but typical configurations, the time-averaged vorticity flux from a harmonically oscillating wall with finite amplitude is analyzed, without appealing to small perturbation. The main conclusion is that the wall oscillation will produce an additional mean vorticity flux (a fully nonlinear streaming effect), which is partially responsible for the mechanism of vortex flow control by waves. The results provide qualitative explanation for some experimentally and/or computationally observed phenomena.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

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[Auszug] The molluscan shell is a microlaminate composite of mineral and biopolymers exhibiting exceptional nanoscale regularity3, and a strength 3,000 times greater than that of the crystals themselves11. Although the integral proteins typically comprise 〈 2 wt% of the shell, they determine the ...

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