Deep levels in a-plane, high Mg-content MgxZn1-xO epitaxial layers grown by molecular beam epitaxy

Deep level defects in n-type unintentionally doped a-plane MgxZn1−xO, grown by molecular beam epitaxy on r-plane sapphire were fully characterized using deep level optical spectroscopy (DLOS) and related methods. Four compositions of MgxZn1−xO were examined with x = 0.31, 0.44, 0.52, and 0.56 together with a control ZnO sample. DLOS measurements revealed the presence of five deep levels in each Mg-containing sample, having energy levels of Ec − 1.4 eV, 2.1 eV, 2.6 V, and Ev + 0.3 eV and 0.6 eV. For all Mg compositions, the activation energies of the first three states were constant with respect to the conduction band edge, whereas the latter two revealed constant activation energies with respect to the valence band edge. In contrast to the ternary materials, only three levels, at Ec − 2.1 eV, Ev + 0.3 eV, and 0.6 eV, were observed for the ZnO control sample in this systematically grown series of samples. Substantially higher concentrations of the deep levels at Ev + 0.3 eV and Ec − 2.1 eV were observed in ZnO compared to the Mg alloyed samples. Moreover, there is a general invariance of trap concentration of the Ev + 0.3 eV and 0.6 eV levels on Mg content, while at least and order of magnitude dependency of the Ec − 1.4 eV and Ec − 2.6 eV levels in Mg alloyed samples

Single phase a-plane MgZnO epilayers for UV optoelectronics: substitutional behaviour of Mg at large contents

High quality 1 μm thick a-plane MgxZn1−xO layers were produced by molecular beam epitaxy with Mg contents higher than 50%. Resonant Rutherford backscattering spectrometry combined with ion channeling revealed a uniform growth in both composition and atomic order. The lattice-site location of Mg, Zn and O elements was determined independently, proving the substitutional behaviour of Mg in Zn-sites of the wurtzite lattice. X-Ray diffraction pole figure analysis also confirms the absence of phase separation. Optical properties at such high Mg contents were studied in Schottky photodiodes

Demonstrating the decoupling regime of the electron-phonon interaction in a quantum dot using chirped optical excitation

Excitation of a semiconductor quantum dot with a chirped laser pulse allows excitons to be created by rapid adiabatic passage. In quantum dots this process can be greatly hindered by the coupling to phonons. Here we add a high chirp rate to ultra-short laser pulses and use these pulses to excite a single quantum dot. We demonstrate that we enter a regime where the exciton-phonon coupling is effective for small pulse areas, while for higher pulse areas a decoupling of the exciton from the phonons occurs. We thus discover a reappearance of rapid adiabatic passage, in analogy to the predicted reappearance of Rabi rotations at high pulse areas. The measured results are in good agreement with theoretical calculations.Comment: Main manuscript 5 pages and 4 figures, Supplementary Information 5 pages and 3 figures, submitted to PR

1.4669, a new lean duplex stainless steel with improved toughness and machinability

Among duplex stainless steels, the lean duplex family is a quite new family, still in expansion. It allows a goodcorrosion resistance, most of the time over that of a standard 4301 austenitic grade, to high mechanicalproperties, far higher than that of a 4301, and to a low amount of expensive alloying elements such as nickelcompared to that of a 4301.But when these grades are produced in high diameter bars, they often present a bad toughness and a poormachinability, these properties being critical when trying to use such high diameter bars in variousapplications. For example, the well-known 4062 and 4162 grades present an impact strength below 100 J at20°C and below 50 J at - 46°C on ?73mm bars. Moreover, their machinability in terms of tool wear and chipbreakability is below that of 4301 grades, especially when these last ones are of an improved machinabilityversion, such as UGIMA®. The poor machinability of these lean duplex grades is mainly due to their highmechanical properties which induce high cutting forces on the tools during a machining operation, thus, rapidtool wear, and is also due to their very low sulphur content (less than 10ppm) which does not help the chipbreaking contrarily to what happens on a 4301 grade with 0,025%S.It is the reason why UGITECH developed these last few years the 1.4669, a new lean duplex grade with a lowernitrogen content and a higher copper content in order to improve the toughness of this kind of grades and tolower the tool wear rates when machining them via a decrease of the cutting forces on the tools. Moreover, acontrol of the inclusions in the grade was performed in order to improve the chip breakability of the gradewhen machined. Of course, this new grade keeps a corrosion resistance over that of a standard 4301.INTRODUCTIONAmong duplex stainless steel, the lean duplex family is a quitenew family, still in expansion. Themost known are the 1.4062 andthe 1.4162 Lean Duplex Stainless Steels (LDSS). Compared to the1.4362, these new LDSS have a lower amount of expensive alloyingelements such as nickel (between 1,5 and 3% compared tothe 4,5% of the 1.4362). To keep a good ratio between ferrite andaustenite (not so far from 50/50) in these two LDSS, their N wasraised from around 0,12% to 0,2% and, in 1.4162, chromium wasslightly decreased (from 22-23% for 1.4362 to 21% for 1.4162).The consequences of these chemical analysismodification aremultipleas detailed in paragraphs 1 to 3: loss in toughness, in machinability(in terms of tool wear rates), and in some cases in corrosionresistance compared to the standard 1.4362. So the questionis: how can we improve the machinability and toughnessof a LDSS, without too expensive alloying elements and keep, atthe same time, a corrosion resistance equal of over that of a 1.4301austenitic SS?TOUGHNESS OF 1.4062 AND 1.4162 LDSSDifferent ? 73mm bars of 1.4062, 1.4162 and 1.4362 were industriallyproduced in order to compare their toughness, corro-Nicolas Renaudot, Eric Chauveau, Marc MantelUGITECH research centre, FrancePaper presented at the 7th European Stainless Steel Conference -Como, 21-23 September 2011sion resistance and machinability. The chemical analysi

Fiche de lecture. Alice de Poncheville, Charles Castella, Le Hêtre vivant

Note de lectur

Interacting Multiple Try Algorithms with Different Proposal Distributions

We propose a new class of interacting Markov chain Monte Carlo (MCMC) algorithms designed for increasing the efficiency of a modified multiple-try Metropolis (MTM) algorithm. The extension with respect to the existing MCMC literature is twofold. The sampler proposed extends the basic MTM algorithm by allowing different proposal distributions in the multiple-try generation step. We exploit the structure of the MTM algorithm with different proposal distributions to naturally introduce an interacting MTM mechanism (IMTM) that expands the class of population Monte Carlo methods. We show the validity of the algorithm and discuss the choice of the selection weights and of the different proposals. We provide numerical studies which show that the new algorithm can perform better than the basic MTM algorithm and that the interaction mechanism allows the IMTM to efficiently explore the state space

Cleaved-facet violet laser diodes with lattice-matched Al0.82In0.18N/GaN multilayers as n-cladding

Electrically injected, edge-emitting cleaved-facet violet laser diodes were realized using a 480 nm thick lattice matched Si doped Al0.82In0.18N/GaN multilayer as the cladding on the n-side of the waveguide. Far-field measurements verify strong mode confinement to the waveguide. An extra voltage is measured and investigated using separate mesa structures with a single AlInN insertion. This showed that the electron current has a small thermally activated shunt resistance with a barrier of 0.135 eV and a current which scales according to V-n, where n similar to 3 at current densities appropriate to laser operation. (C) 2011 American Institute of Physics. (doi:10.1063/1.3589974