1,721,104 research outputs found
Crystalline structure of ZnSe and ZnSSe epilayers grown on (100)GaAs by metalorganic vapour-phase epitaxy
No full textThe crystalline properties of ZnSe/(100)GaAs and ZnSSe/(100)GaAs structures are presented. ZnSe and ZnSSe were grown by MOVPE using dimethyldiselenide and diethyldisulphide along with dimethylzinc:triethylammine. Double-crystal X-ray diffraction (DC-XRD) patterns in the vicinity of the (4 0 0) reciprocal lattice points of ZnSe and ZnSSe were studied. No macroscopic misorientations of the ZnSe crystal with respect to GaAs were detected, suggesting an isotropic defect distribution along the [0 1 1] in-plane directions. The generation/propagation of defects due to the epilayer strain relaxation causes a mosaic-like structure which yields a characteristic broadening of the DC-XRD curves. The line shape of ZnSe and ZnSSe DC-XRD patterns was analysed by a statistical XRD theory under the kinematical approximation to determine the epilayer structural parameters. Simulations of ZnSe/GaAs DC-XRD curves gave a mean mosaic block radius of 140-150 nm and a random mean (microscopic) misorientation angle between the blocks which decreases from 1.3 to 0.4 mrad by increasing the ZnSe thickness from 0.3 to 1.85 um. ZnSSe layers having comparably small (tensile) lattice mismatches showed a lower crystal quality, in agreement with the higher defect density expected for ZnSSe
Residual and thermal strain of ZnS epitaxial layers grown on (100)-GaAs by vapour phase epitaxy
No full textIn this work, ZnS epitaxial layers grown by vapour phase epitaxy
on [100]-oriented GaAs substrates are investigated by x-ray diffraction. The residual strain status of the as-grown samples was determined by high-resolution double-crystal x-ray diffraction measurements. Eleven diffraction curves were
recorded in the vicinity of the (400), (422) and (531) Bragg reflections in different diffraction geometries and for several azimuth angles. The analysis of the experimental data was performed by using a general model, which relates the angular distances between diffraction peaks and strain tensor components in the second-order approximation. This model considers the lowest crystallographic symmetry (triclinic) for the lattice distortion of a cubic unit cell. Our results indicate that the crystallographic symmetry of the distorted ZnS unit cell is orthorhombic. In order to determine the strain contribution due to the different thermal expansion coefficients of ZnS and GaAs (thermal strain) the temperature variation of the residual strain was measured between 25°C and the growth temperature (650°C) by using a single crystal x-ray diffractometer. From our temperature-dependent measurements we determined the thermal misfit between ZnS and GaAs and the linear thermal expansion coefficient of ZnS
Structural properties of MOVPE-grown ZnMgSe epilayers and ZnSe/ZnMgSe MQWs on (100)GaAs
No full textThe structural characterisation of MOVPE-grown ZnMgSe and ZnSe/ZnMgSe multiple quantum wells (MQWs) is reported. Zn0.83Mg0.17Se epilayers and 6 periods ZnSe/Zn0.83Mg0.17Se MQWs having 4.5 nm thick ZnSe wells and 10.5 nm thick alloy barriers were deposited on (100)GaAs after a thin pseudomorphic ZnSe buffer. No macroscopic misorientations between ZnMgSe epilayers and GaAs lattice has been observed, but a broadening of the alloy (400) peak indicates the occurrence of extended defects. Reciprocal space map (RSM) measurements of a Zn0.83Mg0.17Se epilayer show a slight asymmetry of the alloy peak along the growth direction, ascribed to an inhomogeneous relaxation. Diffraction patterns recorded in asymmetrical configuration indicate almost no relaxation of the MQWs structure, within the technique sensitivity. RSMs in the vicinity of the (400) lattice point show a characteristic diffuse scattering around the MQW satellite peaks, indicating that the MQWs structure is beyond the onset of plastic relaxation
MOVPE growth of MgSe and ZnMgSe on (100)GaAs
No full textWe report on the MOVPE of MgSe and ZnMgSe on (100)GaAs. Dimethylzinc:triethylammine, ditertiarylbutylselenide and bis(methylcyclopentadienyl)magnesium were used as Zn, Se and Mg sources, respectively. MgSe and ZnMgSe layers were grown either directly on GaAs or after a thin pseudomorphic ZnSe layer. To avoid the hygroscopicity of Mg-based chalcogenides, a ZnSe capping layer was grown on some samples. The crystallographic phase of as-grown layers was determined by both single- and double-crystal X-ray diffraction measurements. MgSe layers turned out to be mosaics made of (1 1 1)- and (1 0 0)-oriented MgSe crystals in their rocksalt phase. No signatures of the zincblend (ZB) phase was observed, irrespective of whether MgSe was grown on (1 0 0)GaAs or after the ZnSe buffer. Zn1-xMgxSe (0.07 < x < 0.45) epilayers were grown on ZnSe/(100)GaAs in the ZB phase. The MOVPE solid/vapour distribution curve of ZnMgSe is presented, showing that the incorporation of Mg into the ternary crystal is less efficient than Zn, a result of the smaller gas phase diffusion coefficient of the Mg alkyl with respect to that of dimethylzinc. 5K cathodoluminescence measurements performed on ZnMgSe showed dominant deep blue near band-edge emission which shifts towards higher energies by increasing the amount of Mg. Weak self-activated bands were also observed in the spectra
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
On hydrogen transport VPE-grown CdTe epilayers for fabrication of 1-100 keV X-ray detectors
No full textThe hydrogen transport vapour-phase epitaxy growth of CdTe on ZnTe/GaAs hybrid substrates for the realisation of detector-grade epilayers is reported. Growth rates close to 10 um/h were achieved by growing CdTe at around 770°C and with the source temperature at 827°C. The crystalline structure of the as-grown epilayers was investigated by double crystal X-ray diffraction measurements. Despite the large epilayer/substrate lattice misfit, the crystalline perfection of CdTe layers rapidly improves with the thickness; indeed, FWHM values of the (40 0)CdTe peak down to 59 arcsec are obtained for similar to 30 um thick samples; this value compares well with results published in the literature for best epitaxial CdTe. Correspondingly, mirror-like and nearly featureless surfaces were observed. CdTe samples grown below 650°C are p-type and low resistive [rho less than or equal to 20 ohm.cm at room temperature (RT)], whilst at temperatures > 650°C the layers are n-type. In this case, resistivity similar to 10(6) ohm.cm are obtained for growth between 675°C and 700°C, but lower values result above 700°C, However, resistivity in the 10^(4)-10^(5) ohm.cm range are achieved in the latter case by increasing the epilayer thickness up to 30 um. This behaviour is ascribed to the occurrence of Ga-related donors in CdTe, the latter originating by the diffusion of Ga atoms from the GaAs substrate. These donors are almost compensated by residual accepters in CdTe, giving rise to the medium/high resistivity values observed for some of the samples. The detection capability of the material has been demonstrated by time-of-flight (TOF) measurements performed on a device made by an n-CdTe epilayer. The analysis of the TOF collected charge as a function of the applied reverse voltage gives mu.tau approximate to 4.9x10^(-6) cm^(2)/V for this material. Such low value arises from a large electron trapping in CdTe epilayers, consistently with the similar to 10^(18) cm^-(3) density of compensated (positively charged) donors
Effects of ad-atom stoichiometry and stacking faults nucleation on the structure and morphology of MOVPE-grown (100)ZnTe homoepitaxial layers
No full textZnTe with its 2.26 eV (548.5 nm) direct gap at RT, is an ideal semiconductor for fabrication of efficient light emitting diodes and lasers operating in the pure green region of the visible light (540-580 nm). Until few years ago, the lack of high-quality ZnTe single crystal wafers forced the growth of ZnTe and related alloys on III-V substrates (namely GaAs) leading to severe structural problems, preventing the realisation of device-quality heterostructures. This limitation has been recently overcome, as high quality ZnTe substrates, grown by VGF method, became available [1]. Still, substrate preparation and MOVPE growth conditions remain critical steps in the fabrication of high quality ZnTe homoepitaxial layers. We have previously reported on the effects of in-situ H2 annealing of the ZnTe substrate surface on the overall crystalline quality of the ZnTe epilayers. In the present work we further demonstrate that the micro-crystalline structure and surface morphology of ZnTe epilayers grown by atmospheric pressure MOVPE on high quality VGF-grown (100)ZnTe:P wafers result from the complex interplay between surface ad-atom stoichiometry during the growth and stacking faults (SFs) nucleation at the epilayer-substrate homo-interface
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