The roughness and intermixing at the heterointerfaces in InAs/GaSb superlattice's have been studied by both high-resolution transmission electron microscopy. There is interest in InAs/AlSb/GaSb heterostructures for fundamental studies and device applications. Solid-source molecular-beam epitaxy (MBE) is a widely used technique for the growth of InAs/AISb/GaSb heterostructures. An important issue for the growth of group III arsenide-antimonide heterostructures is the unintentional incorporation of As into the antimony-containing layers from the As background in the MBE growth chamber. The heteroepitaxial growth of InAs/AlSb/GaSb heterostructures on GaAs substrates opens a new, quite uncommon possibility for the structural characterization, applying far-infrared spectroscopy. Regarding energies of optical interband transitions, interface states in the forbidden gap may also play a role acting as initial or final states of such transitions. Photoluminescence provides information on the energy of the effective band gap in type II heterostructures provided that indeed band-to-band transitions are observed.

Copper formate tetrahydrate, Cu(HCOO)(2)center dot 4H(2)O, undergoes upon cooling below -38 degrees C a phase transition from a paraelectric (PE) into an antiferroelectric (AFE) phase, whereby its monoclinic point group 2/m is preserved and its lattice parameter c doubled. The boundary between the phases PE and AFE and the antiphase boundaries between the AFE domains have been imaged by X-ray diffraction topography (Lang technique), using a rotating-anode generator with Ag-K alpha radiation. A simple cooling chamber allowed the adjustment of the transition isotherm (i.e. the phase boundary) across the plate-shaped crystal and its stepwise movement across the probe. This has been done for slices of orientation (010) and (001) with several directions of the temperature gradient. The phase boundary appears on the topographs by strong kinematical contrast, indicating considerable lattice strain in the transition region. For (001) plates the X-ray diffraction positions of the PE and AFE phase regions on both sides of the phase boundary are different so that the two regions had to be imaged in separate exposures with a small angular readjustment between them. As expected, the domain boundaries, imaged by section topography, show fringe contrast typical for antiphase boundaries. The various findings are described and discussed in detail.

We report on the structural characterization of InAs/(GaIn)Sb superlattices (SL) grown by solid-source molecular-beam epitaxy. SL periodicity and overall structural quality were assessed by high-resolution X-ray diffraction and Raman spectroscopy. Spectroscopic ellipsometry was found to be sensitive to the (GaIn)Sb alloy composition.

PurposeA recent study confirmed that the particle size distribution of a metallic powder material has a major influence on the density of a part produced by selective laser melting (SLM). Although it is possible to get high density values with different powder types, the processing parameters have to be adjusted accordingly, affecting the process productivity. However, the particle size distribution does not only affect the density but also the surface quality and the mechanical properties of the parts. The purpose of this paper is to investigate the effect of three different powder granulations on the resulting part density, surface quality and mechanical properties of the materials produced.Design/methodology/approachThe scan surface quality and mechanical properties of three different particle size distributions and two layer thicknesses of 30 and 45 μm were compared. The scan velocities for the different powder types have been adjusted in order to guarantee a part density≥99.5 per cent.FindingsBy using an optimised powder material, a low surface roughness can be obtained. A subsequent blasting process can further improve the surface roughness for all powder materials used in this study, although this does not change the ranking of the powders with respect to the resulting surface quality. Furthermore, optimised powder granulations lead generally to improved mechanical properties.Practical implicationsThe results of this study indicate that the particle size distribution influences the quality of AM metallic parts, produced by SLM. Therefore, it is recommended that any standardisation initiative like ASTM F42 should develop guidelines for powder materials for AM processes. Furthermore, during production, the granulation changes due to spatters. Appropriate quality systems have to be developed.Originality/valueThe paper clearly shows that the particle size distribution plays an important role regarding density, surface quality and resulting mechanical properties.

An abstract is not available for this content so a preview has been provided. Please use the Get access link above for information on how to access this content.

Structural defects and their impact on the performance, lifetime and reliability of electronic devices are of permanent interest for crystal growers and device manufacturers. This is especially true for epitaxial (Al, Ga)N/GaN based high electron mobility transistor (HEMT) structures on 4H-SiC (0001) substrates. This work points out how micropipes, dislocations and grain boundaries present in a 4H-SiC (0001) wafer and subsequently overgrown with an (Al, Ga)N–GaN-HEMT layer sequence show up in X-ray topographic images and two-dimensional XRD maps. Using X-ray topography in transmission geometry, micropipes and other structural defects are localized non-destructively below structured metallization layers with a spatial resolution of a few tens of micrometers.

Structural defects and their impact on the performance of electronic devices are of permanent interest for crystal growers and device manufacturers. This is especially true for epitaxial (Al,Ga)N/GaN-based high electron mobility transistor structures on 4H-SiC (0001) substrates. This work concentrates on the recognition and imaging of defects in (Al,Ga)N/GaN/4H-SiC(0001) heterostructures accomplished non-destructively by three X-ray diffraction techniques. X-ray topography and X-ray Bragg angle mapping are compared with respect to the spatial resolution of the defects. X-ray curvature measurements are used to quantify long-ranging stresses in the heterostructure during device fabrication.

The mosaic structure of an (Al,Ga)N layer grown on (0001) sapphire showing natural ordering was studied by high-resolution X-ray diffraction (HRXRD) reciprocal-space mapping. The direction-dependent mosaicity of the layer has been elaborated using maps of symmetrical and asymmetrical reflections. The reciprocal-lattice points show significant broadening depending on the direction in reciprocal space, the diffraction order and the reflection type (fundamental or superstructural). The evaluation followed two paths: (i) a procedure based on the Williamson–Hall plot and (ii) a new approach based on the statistical diffraction theory (SDT). Here, the transformed Takagi equations were implemented for the simulation of the reciprocal-space maps (RSM) for symmetrical and asymmetrical reflections. The reconstruction comprised the mosaic block size, their average rotation angle and the spatial distribution of some components of the microdistortion tensor. The results based on the SDT modelling agree well with those obtained by the Williamson–Hall method, while providing a higher degree of precision and detail.

X-ray topography is a nondestructive technique that permits the visualization of internal defects in the crystal lattice of a gemstone, especially diamond, which is highly transparent to X-rays. This technique yields a unique "fingerprint" that is not altered by gem cutting or treatments such as irradiation and annealing. Although previously a complicated and time-consuming procedure, this article presents a simplified X-ray topographic routine to fingerprint faceted diamonds. Using the table facet as a point of reference, the sample is crystallographically oriented in a unique and reproducible way in front of the X-ray source so that only one topograph is necessary for fingerprinting. Should the diamond be retrieved after loss or theft, even after recutting or exposure to some forms of treatment, another topograph generated with the same routine could be used to confirm its identity unequivocally.

Quaternary InxGa1−xAsyN1−y (x>0.53, 1−y<0.03) QWs grown between In0.52Al0.48As barriers on InP substrate were analyzed by SIMS depth profiling. For the determination of the indium and nitrogen calibration curves, InxGa1−xAs (0≤x≤1) standards were used which were partly implanted with nitrogen at an energy of 75keV and a dose of 5×1015N2molecules/cm2. The QW structures were grown by molecular beam epitaxy with a nitrogen plasma source. MCs+ secondary ions (M=Al, Ga, In, As and N) were used for depth profiling. Nitrogen is found incorporated in InxGa1−xAs layers in concentrations needed for the intended laser applications. The nitrogen concentration can be reliably assessed by SIMS. Thickness and compositional data agree with the nominal data and the data determined by high resolution X-ray diffraction. The In-content is apparently not influenced by the incorporation of nitrogen and vice versa.

The gas-sensing characteristics and the morphology of vanadium pentoxide thin films have been investigated. The thin films were prepared by reactive electron beam evaporation of vanadium on surface-oxidised silicon wafers and additional thermal oxidation. Structural and morphological analyses of the V 2 O 5 thin films in the thickness range of 100-200nm were performed. The polycrystalline monophase V 2 O 5 films consist of grains with surface areas in the range of 100 nm to 1 μm square. Gas measurements were carried out with single-chip thin-film sensor arrays in synthetic air with 50% humidity. The sensors are analytically suitable as they are sensitive to ammonia, methane, carbon monoxide and nitric dioxide. Particularly for NO 2 , a distinctive temperature dependence of the gas reaction has been observed.

Quaternary pseudomorphically strained GaInAsN films and double-quantum wells were grown by plasma assisted molecular-beam epitaxy on an InP substrate. The In content ranged from 53% to 70% while the N content was varied between 0% and 2.4%. A reduction of compressive strain and a low-energy shift of photoluminescence (PL) peak position was observed with increasing N concentration, accompanied by a reduction in PL peak intensity and increase in linewidth. The net effect of N incorporation on the GaInAsN band gap energy was calculated from the measured PL peak energies. The thus obtained composition dependent GaInAsN band gap energy was fitted using the band anticrossing model, yielding values for the interaction parameter CMN for high In-containing GaInAsN being only slightly smaller than that reported for low In-content GaInAsN on GaAs.

An extension of Bond's method for determining precision lattice parameters serves to determine the lattice parameters of (strained) unit cells of a film from the peak positions of the film alone. We call this measurement and evaluation procedure the ‘Extended Bond Method’ (EBM). The procedure avoids recurrence to possibly unreliable or unavailable lattice parameters of the substrate; it is successful, irrespective of whether a layer is pseudomorphically strained, partially relaxed, or completely relaxed, as long as the strain can be described as a uniaxial distortion parallel to the growth direction. Using a mathematical/graphical evaluation procedure, the chemical composition of a layer is obtained, once its strained lattice parameters have been determined. The technique is of particular value for the determination of the strain and the composition of group-III nitride layers.

A new type of band-edge aligned carrier barrier is introduced into InGaAs-AlGaAs single quantum-well (SQW) high-power diode laser structures in order to prevent thermionic emission and the overflow of carriers at elevated operating temperatures. These barriers, which are located in the direct vicinity of the active zone of the laser, are undoped to avoid free-carrier absorption. An InGaAs-AlGaAs SQW laser structure with a 10-nm-thick AlGaAsSb electron-blocking layer on the p-side of an In/sub 0.2/Ga/sub 0.8/As quantum well was realized. The composition of this layer was adjusted so that its valence-band edge matches that of the adjacent AlGaAs waveguide layer. This is to prevent any additional voltage drop or series resistance due to the injection of holes into the quantum well through the electron blocking layer. These lasers show a high characteristic temperature T/sub 0/ of about 225 K for 1500-/spl mu/m-long as-cleaved devices, which is about 60 K higher than the same laser structure without the blocking layer. Simultaneously low internal losses (/spl alpha//sub i//spl ap/1.5 cm/sup -1/ at 20/spl deg/C) and high internal quantum efficiencies (/spl eta//sub i//spl ap/93% at 20/spl deg/C) are achieved. No additional voltage drop or series resistance was measured. The higher temperature stability is mainly attributed to the suppression of carrier leakage and a reduced free-carrier absorption at elevated temperatures.

We report on full wafer and small area photoluminescence topography investigations of VGF GaAs:Si wafers. The wavelength-specific images exhibit various correlations and anti-correlations. Intensity variations due to competitive radiative and non-radiative recombination processes are mainly due to stoichiometric fluctuations and can be distinguished from those generated by the variation of the silicon dopant concentration. X-ray transmission topograms allow to identify grown-in defects like precipitates and dislocations and to correlate these with the observed macro- and microscopic luminescence variation patterns.

The breaking strength of single crystalline silicon beams depends strongly on the fabrication process. We compared the breaking strength of KOH processed silicon beams with the theoretical breaking strength calculated with FEM simulations. (C) 2002 Elsevier Science Ltd. All rights reserved.

The optimization of MBE growth conditions and layer structures for room temperature operation of 2.26 mum AlGaAsSb/GaInAsSb laser structures is investigated. Index guided triple quantum well large optical cavity diode lasers with 64 mum x 1000 mum cavities and high reflection/antireflection coated facets reveal a cw output power of 350 mW at T = 280 K. An internal quantum efficiency eta (i) of 69%, internal losses ri of 7.7cm(-1) and a threshold current density for infinite cavity length of j(infinity) = 144 A/cm(2) are obtained for this structure. (C) 2001 Elsevier Science B.V. All rights reserved.

The optoelectronic properties of short-period InAs/(GaIn)Sb superlattices (SLs) grown by molecular beam epitaxy on GaSb substrates are discussed. We report on the optimization of the SL materials properties with special emphasis on the use for infrared detection devices. The materials quality is evaluated by using high resolution x-ray diffraction, atomic force microscopy, and photoluminescence spectroscopy. In-plane magneto-transport investigations were performed applying mobility spectrum analysis. The SL diodes were analyzed performing standard electro-optical measurements. The observation of resonances in the I-V curves in the regime of Zener-tunneling due to Wannier-Stark localization opens a new tool for the electrical investigation of photodiodes with low band gap energy. The status of the processing technology is reported demonstrating the feasability for the fabrication of 256 x 256 focal plane arrays operating in the 8-to-12 mum atmospheric window. In addition, results are given for mid-infrared SL-diodes, grown with lattice matched AlGaAsSb barriers inserted in the binary InAs/GaSb SL system.