1,034 research outputs found
Thermal stability of in-grown vacancy defects in GaN grown by hydride vapor phase epitaxy
We have used positron annihilation spectroscopy to study the thermal behavior of different native vacancy defects typical of freestanding GaN grown by hydride vapor phase epitaxy under high pressure annealing at different annealing temperatures. The results show that the VGa‐ON pairs dissociate and the Ga vacancies anneal out from the bulk of the material at temperatures 1500–1700K. A binding energy of Eb=1.6(4)eV can be determined for the pair. Thermal formation of Ga vacancies is observed at the annealing temperatures above 1700K, indicating that Ga vacancies are created thermally at the high growth temperature, but their ability to form complexes such as VGa‐ON determines the fraction of vacancy defects surviving the cooling down. The formation energy of the isolated Ga vacancy is experimentally determined.Peer reviewe
Clustering of vacancy defects in high-purity semi-insulating SiC
Positron lifetime spectroscopy was used to study native vacancy defects in
semi-insulating silicon carbide. The material is shown to contain (i) vacancy
clusters consisting of 4--5 missing atoms and (ii) Si vacancy related
negatively charged defects. The total open volume bound to the clusters
anticorrelates with the electrical resistivity both in as-grown and annealed
material. Our results suggest that Si vacancy related complexes compensate
electrically the as-grown material, but migrate to increase the size of the
clusters during annealing, leading to loss of resistivity.Comment: 8 pages, 5 figure
Electrical compensation and cation vacancies in Al rich Si-doped AlGaN
We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1 x 10 16 cm(-3) to 2 x 10 18 cm(-3) in samples with a high C content, strongly correlated with the Si doping level in the samples ranging from 1 x 10 17 cm(-3) to 7 x 10 18 cm(-3). On the other hand, we find predominantly neutral cation vacancies with concentrations above 5 x 10 18 cm(-3) in samples with a low C content. The cation vacancies are important as compensating centers only in material with a high C content at high Si doping levels.Peer reviewe
Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy
The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.Peer reviewe
Production of muons from heavy-flavour hadron decays in pp collisions at root s=5.02 TeV
Production cross sections of muons from semi-leptonic decays of charm and beauty hadrons were measured at forward rapidity (2.5 <y <4) in proton-proton (pp) collisions at a centre-of-mass energy root s = 5.02 TeV with the ALICE detector at the CERN LHC. The results were obtained in an extended transverse momentum interval, 2 <p(T)<20 GeV/c, and with an improved precision compared to previous measurements performed in the same rapidity interval at centre-of-mass energies root s = 2.76 and 7 TeV. The p(T)- and y-differential production cross sections as well as the p(T)-differential production cross section ratios between different centre-of-mass energies and different rapidity intervals are described, within experimental and theoretical uncertainties, by predictions based on perturbative QCD.Peer reviewe
Vertical-external-cavity surface-emitting lasers and quantum dot lasers
The use of cavity to manipulate photon emission of quantum dots (QDs) has
been opening unprecedented opportunities for realizing quantum functional
nanophotonic devices and also quantum information devices. In particular, in
the field of semiconductor lasers, QDs were introduced as a superior
alternative to quantum wells to suppress the temperature dependence of the
threshold current in vertical-external-cavity surface-emitting lasers
(VECSELs). In this work, a review of properties and development of
semiconductor VECSEL devices and QD laser devices is given. Based on the
features of VECSEL devices, the main emphasis is put on the recent development
of technological approach on semiconductor QD VECSELs. Then, from the viewpoint
of both single QD nanolaser and cavity quantum electrodynamics (QED), a
single-QD-cavity system resulting from the strong coupling of QD cavity is
presented. A difference of this review from the other existing works on
semiconductor VECSEL devices is that we will cover both the fundamental aspects
and technological approaches of QD VECSEL devices. And lastly, the presented
review here has provided a deep insight into useful guideline for the
development of QD VECSEL technology and future quantum functional nanophotonic
devices and monolithic photonic integrated circuits (MPhICs).Comment: 21 pages, 4 figures. arXiv admin note: text overlap with
arXiv:0904.369
Hall Normalization Constants for the Bures Volumes of the n-State Quantum Systems
We report the results of certain integrations of quantum-theoretic interest,
relying, in this regard, upon recently developed parameterizations of Boya et
al of the n x n density matrices, in terms of squared components of the unit
(n-1)-sphere and the n x n unitary matrices. Firstly, we express the normalized
volume elements of the Bures (minimal monotone) metric for n = 2 and 3,
obtaining thereby "Bures prior probability distributions" over the two- and
three-state systems. Then, as an essential first step in extending these
results to n > 3, we determine that the "Hall normalization constant" (C_{n})
for the marginal Bures prior probability distribution over the
(n-1)-dimensional simplex of the n eigenvalues of the n x n density matrices
is, for n = 4, equal to 71680/pi^2. Since we also find that C_{3} = 35/pi, it
follows that C_{4} is simply equal to 2^{11} C_{3}/pi. (C_{2} itself is known
to equal 2/pi.) The constant C_{5} is also found. It too is associated with a
remarkably simple decompositon, involving the product of the eight consecutive
prime numbers from 2 to 23.
We also preliminarily investigate several cases, n > 5, with the use of
quasi-Monte Carlo integration. We hope that the various analyses reported will
prove useful in deriving a general formula (which evidence suggests will
involve the Bernoulli numbers) for the Hall normalization constant for
arbitrary n. This would have diverse applications, including quantum inference
and universal quantum coding.Comment: 14 pages, LaTeX, 6 postscript figures. Revised version to appear in
J. Phys. A. We make a few slight changes from the previous version, but also
add a subsection (III G) in which several variations of the basic problem are
newly studied. Rather strong evidence is adduced that the Hall constants are
related to partial sums of denominators of the even-indexed Bernoulli
numbers, although a general formula is still lackin
Electronic and structural properties of vacancies on and below the GaP(110) surface
We have performed total-energy density-functional calculations using
first-principles pseudopotentials to determine the atomic and electronic
structure of neutral surface and subsurface vacancies at the GaP(110) surface.
The cation as well as the anion surface vacancy show a pronounced inward
relaxation of the three nearest neighbor atoms towards the vacancy while the
surface point-group symmetry is maintained. For both types of vacancies we find
a singly occupied level at mid gap. Subsurface vacancies below the second layer
display essentially the same properties as bulk defects. Our results for
vacancies in the second layer show features not observed for either surface or
bulk vacancies: Large relaxations occur and both defects are unstable against
the formation of antisite vacancy complexes. Simulating scanning tunneling
microscope pictures of the different vacancies we find excellent agreement with
experimental data for the surface vacancies and predict the signatures of
subsurface vacancies.Comment: 10 pages, 6 figures, Submitted to Phys. Rev. B, Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
- …