Composition analysis of semiconductor quantum wells by energy filtered convergent-beam electron diffraction

We show how energy-filtered convergent-beam electron diffraction (EFCBED) patterns can be used to determine the chemical composition of buried semiconductor strained quantum wells. Our method is based on a quantitative analysis of the intensities of high-order Bragg lines in the transmitted disc of EFCBED patterns taken from plan-view samples. This analysis makes it possible to determine the displacement vector R introduced between the top and bottom parts of the matrix by the deformation of the quantum well and consequently to determine its composition. This is illustrated in the case of an In(x)Ga(1-)(x)As quantum well buried in a GaAs matrix. A detailed analysis of the effect of experimental parameters on Bragg lines intensity is performed. In particular, the importance of the choice of the diffraction vector is pointed out. The relative uncertainty on the measurement of the indium content x is found to be lower than 5% and a possible occurrence of slight compositional fluctuations in the (001) growth plane is pointed out.     

Impact of N on the atomic-scale Sb distribution in quaternary GaAsSbN-capped InAs quantum dots

The use of GaAsSbN capping layers on InAs/GaAs quantum dots (QDs) has recently been proposed for micro- and optoelectronic applications for their ability to independently tailor electron and hole confinement potentials. However, there is a lack of knowledge about the structural and compositional changes associated with the process of simultaneous Sb and N incorporation. In the present work, we have characterized using transmission electron microscopy techniques the effects of adding N in the GaAsSb/InAs/GaAs QD system. Firstly, strain maps of the regions away from the InAs QDs had revealed a huge reduction of the strain fields with the N incorporation but a higher inhomogeneity, which points to a composition modulation enhancement with the presence of Sb-rich and Sb-poor regions in the range of a few nanometers. On the other hand, the average strain in the QDs and surroundings is also similar in both cases. It could be explained by the accumulation of Sb above the QDs, compensating the tensile strain induced by the N incorporation together with an In-Ga intermixing inhibition. Indeed, compositional maps of column resolution from aberration-corrected Z-contrast images confirmed that the addition of N enhances the preferential deposition of Sb above the InAs QD, giving rise to an undulation of the growth front. As an outcome, the strong redshift in the photoluminescence spectrum of the GaAsSbN sample cannot be attributed only to the N-related reduction of the conduction band offset but also to an enhancement of the effect of Sb on the QD band structure.     

Combined AFM, XPS, and contact angle studies on treated indium-tin-oxide films for organic light-emitting devices

In the present work, surface modifications were performed on the indium-tin-oxide (ITO) substrates and the treated ITO surface properties were investigated by different characterization techniques. AFM and XPS methods were applied to measure surface roughness and chemical composition, respectively. Standard goniometry was used to determine contact angle and to calculate surface energy. Experimental results show that the ITO surface properties are subjected to the treatment methods which lead the surface to a certain degree of changes. Wettability of the modified surfaces was then monitored as a function of time elapsed after treatment and quantified. Furthermore, the polymer light-emitting electrochemical cells (LECs) with the differently treated ITO substrates as device electrodes were fabricated and characterized. We observe that the electrical and optical performances of the polymer LECs are affected by the treatment methods on the ITO surface which result in the modification of interface formation and electrical contact of the ITO substrate with the polymer blend in the polymer LECs.     

Studies on the synthesis, structure, growth and physical properties of an organic NLO crystal: 2-Amino-5-nitropyridinium Phenolsulfonate

A noncentrosymmetric crystal was prepared from 2-amino-5-nitropyridine (2A5NP) and p-Phenolsulfonic acid, which was designed for second harmonic generation. Good quality single crystals of 2-amino-5-nitropyridinium Phenolsulfonate (2A5NPP) were successfully grown by the slow evaporation method with dimensions 10 × 4 × 3 mm³. The unit cell dimensions were determined from single crystal X-ray diffraction studies. The structural perfection of the grown crystals has been analyzed by high-resolution X-ray diffraction (HRXRD) rocking curve measurements. Fourier transform infrared (FTIR) spectral studies have been performed to identify the functional groups. The optical transmittance window and the lower cutoff wavelength of the 2A5NPP have been identified by UV-vis-NIR studies. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study its thermal properties. Powder test with Nd:YAG laser radiation shows a high second harmonic generation. The laser induced surface damage threshold for the grown crystal was measured using Nd:YAG laser.     

Crystal structure and characterization of a novel organic crystal: 4-Dimethylaminobenzophenone

Single crystals of a novel organic material, dimethylaminobenzophenone were grown from aqueous solution employing the technique of controlled evaporation. Dimethylaminobenzophenone belongs to the monoclinic system, with a = 12.5755(7) Å, b = 7.9749(4) Å, c = 13.0946(7) Å, α = 90°, β = 111.6380(10)° and γ = 90°. Fourier transform infrared study has been performed to identify the functional groups. The transmittance of dimethylaminobenzophenone has been used to calculate the refractive index n; the extinction coefficient K and both the real ?_r and imaginary ?_i components of the dielectric constant as functions of photon energy. The optical band gap of dimethylaminobenzophenone is 2.9 eV. The structural prefection of the grown crystals has been analyzed by high-resolution X-ray diffraction rocking curve measurements. Thermo gravimetric analysis and differential thermal analysis have also been carried out, and the thermal behavior of dimethylaminobenzophenone crystal has been studied. The dielectric properties and mechanical properties have been investigated.     

Growth and characterization of pure and potassium chloride-doped Zinc Tris-thiourea Sulphate (ZTS) single crystals

Single crystals of pure and potassium chloride-doped Zinc Tris-thiourea Sulphate (ZTS) were grown from aqueous solution by slow evaporation technique. The grown crystals were subjected to various studies such as XRD, FTIR, atomic absorption, SHG and TGA-DTA studies. The melting point and density of the grown crystals were also measured. The various studies revealed the incorporation of the impurity (potassium chloride) into ZTS crystals and the investigations indicated that the impurity played an important role in the changes of the spectral and structured properties of ZTS crystals.     

Growth conditions of the cubic phase cBN in boron nitride films

The nucleation and the subsequent coalescence period of the cubic phase cBN in sputter deposited BN-films is characterized by a shrinking of the film thickness. This is due to the transition of hBN into the denser cBN-phase which occurs inside a highly textured hBN base layer. The corresponding variation of the film thickness with the deposition time is described by a quantitative model. Full BN-stoichiometry in the hBN base layer is shown to be a mandatory condition for the nucleation process and the following growth of the cubic BN-phase. An increase of the substrate temperature fosters the incorporation of nitrogen into the growing film and, thus, the achievement of the stoichiometry condition.     

Micro-Raman characterization of plasma nitrided Ti6Al4V-ELI

Titanium nitride surface coatings have been extensively used as wear resistant and biocompatible layers on titanium alloys. Plasma nitriding is a suitable method to obtain these coatings that have to be characterized with phase sensitive techniques, because hardness and wear resistance depend critically on crystallography and stoichiometry. Many of these analytical techniques have very stringent vacuum and sample geometry requirements that frequently do not allow the characterization of surface films ontop of biomedical components (bone or dental implants). Microprobe Raman spectroscopy can be applied without vacuum requirements on samples of variable size and shape. In this work, this technique is used to investigate the TiN layer on a Ti6Al4V-ELI alloy, plasma nitrided at different substrate temperatures, in order to observe the changes in stoichiometry. Changes of the relative intensities of acoustic and optical modes were related to TiN composition, comparing with the results of Nuclear Reaction Analysis, using the ¹⁵N(p, αγ)¹²C reaction induced with 429 keV protons. A linear relationship between the Raman peak area ratio and nitrogen content was observed, that can be used as calibration curve for further Raman measurements of unknown samples.     

Development of oxides dispersion strengthened steels for high temperature nuclear reactor applications

By introducing a dispersion of nanosized yttrium oxides particles into a steel matrix, the upper temperature limit in mechanical creep strength can be enhanced in temperature by 100 K at least. Production routes for the production of a new class of oxides dispersion strengthened (ODS) steels are investigated within this work. Preliminary results obtained when doping pure iron matrix phase with two types of yttrium oxides (Y₂O₃) nanoparticles (commercial as well as laboratory fabricated nanopowder) are presented. The twofold purpose of this work is firstly to obtain a comparative analysis between the commercial and the laboratory fabricated Y₂O₃ nanopowder used to produce the doped iron, and secondly to demonstrate the feasibility of new production route by observing the nanostructure of the first test batches with pure iron. Observations are carried out with transmission electron microscopy (TEM) to determine the size distribution of the particles in the powder, while glow discharge optical emission spectroscopy (GDOES) and high resolution-scanning electron microscopy (HR-SEM) are used to analyze the chemical composition and the homogeneity of the produced doped iron. It is demonstrated, that even with small size particles nanopowder fabricated in the laboratory, the distribution is fairly homogeneous compared to the one obtained with a relatively large particles commercial nanopowder, confirming the feasibility of the new production route.     

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

HYDJET++ is a Monte Carlo event generator for simulation of relativistic heavy ion AA collisions considered as a superposition of the soft, hydro-type state and the hard state resulting from multi-parton fragmentation. This model is the development and continuation of HYDJET event generator (Lokhtin and Snigirev, EPJC 45 (2006) 211). The main program is written in the object-oriented C++ language under the ROOT environment. The hard part of HYDJET++ is identical to the hard part of Fortran-written HYDJET and it is included in the generator structure as a separate directory. The soft part of HYDJET++ event is the “thermal” hadronic state generated on the chemical and thermal freeze-out hypersurfaces obtained from the parameterization of relativistic hydrodynamics with preset freeze-out conditions. It includes the longitudinal, radial and elliptic flow effects and the decays of hadronic resonances. The corresponding fast Monte Carlo simulation procedure, C++ code FAST MC (Amelin et al., PRC 74 (2006) 064901; PRC 77 (2008) 014903) is adapted to HYDJET++. It is designed for studying the multi-particle production in a wide energy range of heavy ion experimental facilities: from FAIR and NICA to RHIC and LHC.

Program summary

Program title: HYDJET++, version 2Catalogue identifier: AECR_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AECR_v1_0.htmlProgram obtainable from: CPC Program Library, Queen's University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 100 387No. of bytes in distributed program, including test data, etc.: 797 019Distribution format: tar.gzProgramming language: C++ (however there is a Fortran-written part which is included in the generator structure as a separate directory)Computer: Hardware independent (both C++ and Fortran compilers and ROOT environment [1] (http://root.cern.ch/) should be installed)Operating system: Linux (Scientific Linux, Red Hat Enterprise, FEDORA, etc.)RAM: 50 MBytes (determined by ROOT requirements)Classification: 11.2External routines: ROOT [1] (http://root.cern.ch/)Nature of problem: The experimental and phenomenological study of multi-particle production in relativistic heavy ion collisions is expected to provide valuable information on the dynamical behavior of strongly-interacting matter in the form of quark-gluon plasma (QGP) [2-4], as predicted by lattice Quantum Chromodynamics (QCD) calculations. Ongoing and future experimental studies in a wide range of heavy ion beam energies require the development of new Monte Carlo (MC) event generators and improvement of existing ones. Especially for experiments at the CERN Large Hadron Collider (LHC), implying very high parton and hadron multiplicities, one needs fast (but realistic) MC tools for heavy ion event simulations [5-7]. The main advantage of MC technique for the simulation of high-multiplicity hadroproduction is that it allows a visual comparison of theory and data, including if necessary the detailed detector acceptances, responses and resolutions. The realistic MC event generator has to include maximum possible number of observable physical effects, which are important to determine the event topology: from the bulk properties of soft hadroproduction (domain of low transverse momenta pT?1 GeV/c) such as collective flows, to hard multi-parton production in hot and dense QCD-matter, which reveals itself in the spectra of high-pT particles and hadronic jets. Moreover, the role of hard and semi-hard particle production at LHC can be significant even for the bulk properties of created matter, and hard probes of QGP became clearly observable in various new channels [8-11]. In the majority of the available MC heavy ion event generators, the simultaneous treatment of collective flow effects for soft hadroproduction and hard multi-parton in-medium production (medium-induced partonic rescattering and energy loss, so-called “jet quenching”) is lacking. Thus, in order to analyze existing data on low and high-pT hadron production, test the sensitivity of physical observables at the upcoming LHC experiments (and other future heavy ion facilities) to the QGP formation, and study the experimental capabilities of constructed detectors, the development of adequate and fast MC models for simultaneous collective flow and jet quenching simulations is necessary. HYDJET++ event generator includes detailed treatment of soft hadroproduction as well as hard multi-parton production, and takes into account known medium effects.Solution method: A heavy ion event in HYDJET++ is a superposition of the soft, hydro-type state and the hard state resulting from multi-parton fragmentation. Both states are treated independently. HYDJET++ is the development and continuation of HYDJET MC model [12]. The main program is written in the object-oriented C++ language under the ROOT environment [1]. The hard part of HYDJET++ is identical to the hard part of Fortran-written HYDJET [13] (version 1.5) and is included in the generator structure as a separate directory. The routine for generation of single hard NN collision, generator PYQUEN [12,14], modifies the “standard” jet event obtained with the generator PYTHIA 6.4 [15]. The event-by-event simulation procedure in PYQUEN includes

1.

generation of initial parton spectra with PYTHIA and production vertexes at given impact parameter;

2.

rescattering-by-rescattering simulation of the parton path in a dense zone and its radiative and collisional energy loss;

3.

final hadronization according to the Lund string model for hard partons and in-medium emitted gluons.

Then the PYQUEN multi-jets generated according to the binomial distribution are included in the hard part of the event. The mean number of jets produced in an AA event is the product of the number of binary NN subcollisions at a given impact parameter and the integral cross section of the hard process in NN collisions with the minimum transverse momentum transfer . In order to take into account the effect of nuclear shadowing on parton distribution functions, the impact parameter dependent parameterization obtained in the framework of Glauber-Gribov theory [16] is used. The soft part of HYDJET++ event is the “thermal” hadronic state generated on the chemical and thermal freeze-out hypersurfaces obtained from the parameterization of relativistic hydrodynamics with preset freeze-out conditions (the adapted C++ code FAST MC [17,18]). Hadron multiplicities are calculated using the effective thermal volume approximation and Poisson multiplicity distribution around its mean value, which is supposed to be proportional to the number of participating nucleons at a given impact parameter of AA collision. The fast soft hadron simulation procedure includes

1.

generation of the 4-momentum of a hadron in the rest frame of a liquid element in accordance with the equilibrium distribution function;

2.

generation of the spatial position of a liquid element and its local 4-velocity in accordance with phase space and the character of motion of the fluid;

3.

the standard von Neumann rejection/acceptance procedure to account for the difference between the true and generated probabilities;

4.

boost of the hadron 4-momentum in the center mass frame of the event;

5.

the two- and three-body decays of resonances with branching ratios taken from the SHARE particle decay table [19].

The high generation speed in HYDJET++ is achieved due to almost 100% generation efficiency of the “soft” part because of the nearly uniform residual invariant weights which appear in the freeze-out momentum and coordinate simulation. Although HYDJET++ is optimized for very high energies of RHIC and LHC colliders (c.m.s. energies of heavy ion beams and 5500 GeV per nucleon pair, respectively), in practice it can also be used for studying the particle production in a wider energy range down to per nucleon pair at other heavy ion experimental facilities. As one moves from very high to moderately high energies, the contribution of the hard part of the event becomes smaller, while the soft part turns into just a multi-parameter fit to the data.Restrictions: HYDJET++ is only applicable for symmetric AA collisions of heavy (A?40) ions at high energies (c.m.s. energy per nucleon pair). The results obtained for very peripheral collisions (with the impact parameter of the order of two nucleus radii, b∼2RA) and very forward rapidities may be not adequate.Additional comments: Accessibility http://cern.ch/lokhtin/hydjet++Running time: The generation of 100 central (0-5%) Au+Au events at (Pb+Pb events at ) with default input parameters takes about 7 (85) minutes on a PC 64 bit Intel Core Duo CPU @ 3 GHz with 8 GB of RAM memory under Red Hat Enterprise.References:[1] I.P. Lokhtin, A.M. Snigirev, Eur. Phys. J. C 46 (2006) 211.[2] N.S. Amelin, R. Lednicky, T.A. Pocheptsov, I.P. Lokhtin, L.V. Malinina, A.M. Snigirev, Iu.A. Karpenko, Yu.M. Sinyukov, Phys. Rev. C 74 (2006) 064901.[3] N.S. Amelin, I. Arsene, L. Bravina, Iu.A. Karpenko, R. Lednicky, I.P. Lokhtin, L.V. Malinina, A.M. Snigirev, Yu.M. Sinyukov, Phys. Rev. C 77 (2008) 014903.