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Abstract The present work shows that thermal expansion experiments can be used to measure the γʼ-solvus temperatures of four Ni-base single-crystal superalloys (SX), one with Re and three Re-free variants. In the case of CMSX-4, experimental results are in good agreement with numerical thermodynamic results obtained using ThermoCalc. For three experimental Re-free alloys, the experimental and calculated results are close. Transmission electron microscopy shows that the chemical compositions of the γ- and the γʼ-phases can be reasonably well predicted. We also use resonant ultrasound spectroscopy (RUS) to show how elastic coefficients depend on chemical composition and temperature. The results are discussed in the light of previous results reported in the literature. Areas in need of further work are highlighted. Graphical abstract

Fluorescence from carbon nanodots has been of keen interest over the past decade; however, exploration of radiative pathways including delayed fluorescence (ɑS1) from nanodots have hitherto seen little investigation, thus limiting the scope of nanodot applications. Simple synthesis strategies for ɑS1-emitting nanodots therefore remain necessary. Herein we report an inexpensive, quasi-spherical nanodot structure from combustion-based synthesis exhibiting strong excitation-dependent ɑS1 emission upon aggregation to 100 nm sizes. Interestingly, ɑS1 emission intensity is found to increase with increased temperature. The nanodots also exhibit photostability over multiple heating and cooling cycles, suggesting the possible use of these carbon nanodots as temperature-sensitive luminescence probes.

V2O3 doped with 1.1% Cr is investigated at its isostructural correlation-driven metal-insulator transition near room temperature in its paramagnetic state with X-ray magnetic circular dichroism (XMCD) spectroscopy in external magnetic fields. A relative XMCD amplitude of about 2 permille is observed at the L-2,L-3 absorption edges of vanadium as expected for magnetic moment per mass values of the order of 1 J T-1 kg(-1) from magnetometry and the literature. Across the metal-insulator transition, the vanadium XMCD spectral shape significantly changes. According to atomic multiplet simulations, these changes are due to a changing orbital occupation indicating a changing phase composition. According to estimates used in this study, the dipole moment of the spin density distribution 7⟨Tz⟩ in the bulk increases such that the effective vanadium spin moment increases by a few percent with temperature in the two-phase region. Thereby, it partially compensates for the decrease in the relative XMCD amplitude due to a decreasing alignment of the paramagnetic moments. After a few minor temperature cycles, the sample is in a two-phase state in which the XMCD and X-ray linear dichroism spectra hardly depend on the temperature, and the specific electrical resistance is intermediate, showing only a weak sign of the metal-insulator transition. Early access: The article have been peer-reviewed and accepted for publication. The article content has been finalized, but it has not been assigned to an issue yet.

Polyoxopalladates hosting 3d, 4d or 4f central ions form a new class of molecular materials with well-defined, tunable morphology and ions coordination to be considered as model systems for basic research. We investigated the isotropic X-ray absorption near-edge structure (XANES), X-ray magnetic circular dichroism (XMCD), and X-ray magnetic linear dichroism (XMLD) to monitor the influence of symmetry changes on electronic and magnetic properties. After a brief introductory review about recent results on 3d and 4d transition metal ions, we (i) show that a change of the coordination symmetry from cubic eightfold (tetrahedral) to sixfold (octahedral) reduces the magnetic moment of Co2+ significantly and (ii) investigate whether axial distortions can modify the 4f states of trivalent lanthanide ions. While for Gd3+ there is no effect visible within experimental uncertainties, changes were observed for Dy3+ and Ho3+. The results are compared to atomic multiplet calculations including the intra-atomic spin-dipole term 〈 T z 〉 . Furthermore, several experimental pitfalls are discussed that may lead to misinterpretation of X-ray absorption data but can be excluded in the present study.

A well-known feature of the ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As--As bond. Using detailed x-ray scattering and spectroscopy, we find that Rh-doped ${\mathrm{Ca}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As--As bonds. Our density functional theory calculations reveal that the Fe--As bond geometry is critical for stabilizing magnetism and the pressure-induced drop in the $c$ lattice parameter observed in pure ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$ is mostly due to a constriction within the FeAs planes. The collapsed tetragonal phase emerges when covalent bonding of strongly hybridized Fe $3d$ and As $4p$ states completely wins out over their exchange splitting. Thus the collapsed tetragonal phase is properly understood as a strong covalent phase that is fully nonmagnetic with the As--As bond forming as a by-product.

Here, we document a detailed characterisation of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U‐Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean 206Pb/238U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure‐related parameters correspond well with the calculated alpha doses of 1.48 × 1018 g−1 (GZ7) and 2.53 × 1018 g−1 (GZ8), respectively, and the (U‐Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U mass fractions are 680 μg g−1 (GZ7) and 1305 μg g−1 (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U‐Pb geochronology. In addition, GZ7 (Ti mass fractions 25.08 μg g−1 ± 0.18 μg g−1; 95% confidence uncertainty) may prove useful as reference material for Ti‐in‐zircon temperature estimates., Geostandards and Geoanalytical Research, 42 (4), ISSN:1639-4488, ISSN:1751-908X

In this article we report on the synthesis and multiferroic properties of cobalt ferrite (CoFe2O4)–barium titanate (BaTiO3) biphasic composites. The initial composite nanopowder was synthesized by a combination of co-precipitation and organosol methods. A ceramic sample with (3–0) connectivity, i.e. BaTiO3 grains in a CoFe2O4 matrix was obtained by a combination of spark plasma sintering and annealing. In order to understand the correlations between morphology, electric properties, and magnetization, we present a detailed study at different preparation steps and compare it to the properties of a conventionally sintered sample with the traditional (0–3) connectivity, i.e. CoFe2O4 grains in a BaTiO3 matrix. We observe that the (3–0) sample shows improved magnetic properties in comparison to the conventionally sintered composite of the same composition. In spite of relatively large leakage current for the (3–0) sample compared to the traditional (0–3) one, it exhibits a converse magnetoelectric effect that follows the Hdc dependence of the piezomagnetic coefficient. The magnetic field-dependence of electric polarization at the surface was investigated utilizing X-ray absorption spectroscopy and its associated linear and circular dichroisms.

In this Letter we characterize both the photophysical and plasmonic enhanced photophysical properties of carbon nanodots produced by a combustion method. In addition, we have found that max entropy analysis of luminescence decay reveals inhomogeneous distribution of radiative lifetimes. Further, while quantum yields of intrinsic carbon nanodots emission are low [

Magnetization reversal of thin Fe, Co and Ni layers deposited on single-crystalline layers of V2O3 is shown to be governed by various contributions to the magnetic anisotropy of the ferromagnetic layers, which reflect the sensitivity of the systems to the crystallographic and magnetic moment orientation or the structural morphology of the interfaces. The complex behaviour observed is related to the rich phase diagram of the oxide, which undergoes a transition from a paramagnetic metallic to an insulating antiferromagnetic phase as the temperature is decreased to below ∼150 K. Focussing on the anisotropy contributions that result from the exchange interaction across the interface with antiferromagnetic V2O3, we have found a remarkable behaviour of the Fe overlayers which grow in a textured BCC(1 1 0) structure. The exchange anisotropy, probed as a bias of the ferromagnetic hysteresis loops for field-cooled samples, is absent for the interface with the ( 1 1 2 ¯ 0 ) surface of antiferromagnetic V2O3, but exists for the interface with the V2O3 (0 0 0 1) surface. This points to a very different electronic interaction at the two interfaces. The behaviour is at variance with what is observed for the Co and Ni overlayers which show an exchange anisotropy for both surfaces of the oxide. Measurements of X-ray magnetic circular dichroism reveal the presence of a magnetically dead layer in Fe at the interface with V2O3 ( 1 1 2 ¯ 0 ) . It is likely to disrupt the exchange coupling, and thus may be at the origin of the missing exchange anisotropy of Fe on this surface. The different evolution of the spin magnetic moment of Fe with the overlayer thickness suggests a different growth morphology on the two surfaces of the oxide. As a result of the interaction with Fe, the V2O3 layers acquire a small magnetic polarization at the interface at room temperature for both orientations.

Thick GaN layers have been grown by hydride vapor phase epitaxy (HVPE) on different GaN templates grown by metalorganic vapor phase epitaxy. Crack formation could be reduced by using a hydrogen/nitrogen carrier gas mixture. We found strongly different HVPE layer quality when templates grown by different groups have been used, although these templates showed very much the same basic properties. Over 100 µm thick crack-free HVPE layers with extremely flat surface could be grown on optimized templates. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The magnetization of ultrathin Fe deposited in situ onto a V(1 1 0) single crystal has been studied by means of X-ray magnetic circular dichroism measurements at the L 2,3 absorption edges of Fe and V as a function of the Fe coverage. The Fe magnetization sets on at an Fe thickness of 2 ML in accordance with an earlier Polarized Neutron Reflectometry investigation. It increases rapidly with Fe thickness and asymptotically reaches the bulk magnetization at 5 ML. In the critical Fe thickness regime between 2 and 5 ML it significantly increases by cooling from room temperature to 110 K due to the reduced Curie temperature of the ultrathin Fe film. In contrast, the onset of the Fe magnetization at 2 ML is barely influenced by cooling which may be due to the coalescence of the Fe islands observed earlier with low-energy electron diffraction. Residual O contamination on the V surface reduces the Fe magnetization due to the formation of Fe oxide.

We report on a new planetary hotwall research system with independent control of the ceiling and susceptor temperatures. This allows a study of the influence of an actively heated ceiling during a MOCVD process. It was found that the temperature profile across the substrate becomes more uniform when the ceiling temperature is increased while maintaining the same susceptor surface temperature. The parasitic coating on the ceiling was found to be stable and no source of particles up to 1050 °C. The influence of the ceiling temperature on the depletion profile and the possibility to manipulate it was investigated. Good thickness homogeneity of 1.35% (1·σ) on a 4″ sapphire wafer was achieved with excellent XRD results of 206 arcsec for the (0002) reflex and 272 arcsec for the (10-12) reflex for a 4.7 µm thick GaN film. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microelectronics manufacturers continue to subject a wide range of products or representative test vehicles to accelerated thermal cycling tests. Most such testing is focused on comparisons, whether among alternatives or to an established requirement. However, more often than commonly recognized such comparisons may not reflect the relative performances in service. In fact, most current models have been shown to fail to account for important systematic trends as well as being inconsistent with our current understanding of the failure rate controlling damage mechanism. An alternative mechanistically justified model for the thermal fatigue life of SnAgCu solder joints has been proposed. Damage and failure occurs by recrystallization of the large Sn grains across the high strain region of the joint, followed by crack growth along the resulting network of high angle grain boundaries. The recrystallization was shown to be the damage rate controlling mechanism, except for extremely high strain assemblies and/or harsh cycling conditions, i.e. if we can predict the recrystallization we can predict the number of cycles to failure. So far the model accounts for a variety of important trends and offers guidance as to the interpretation and generalization of accelerated test results. General extrapolations towards service conditions will, however, require the specific functional dependence of the rate of recrystallization on the stress and the precipitate distributions. Another potential difficulty is that constitutive relations are extracted from single sided creep experiments while the dislocation cell structures built up under cyclic loading are certain to be different. Furthermore, the repeated `annealing' during the high temperature dwell affects the hardening. Even if these effects could be ignored the ongoing evolution of the constitutive relations would still effectively prevent the extraction of the above-mentioned functional dependence from comparisons between thermal cycling results and FEM. A special experiment is ongoing in which stresses and strains on the solder joints can be controlled and measured directly, allowing the testing of individual assumptions underlying the proposed model. Preliminary results are presented and compared to results of thermal cycling across different temperature ranges.

cited By 14; International audience; In this work we investigate interfacial effects in bilayer systems integrated by La2/3Sr1/3MnO3 (LSMO) thin films and different capping layers by means of surface-sensitive synchrotron radiation techniques and transport measurements. Our data reveal a complex scenario with a capping-dependent variation of the Mn oxidation state by the interface. However, irrespective of the capping material, an antiferromagnetic/insulating phase is also detected at the interface, which is likely to originate from a preferential occupancy of Mn 3d 3z2-r2 eg orbitals. This phase, which extends approximately to two unit cells, is also observed in uncapped LSMO reference samples, thus pointing to an intrinsic interfacial phase separation phenomenon, probably promoted by the structural disruption and inversion symmetry breaking at the LSMO free surface/interface. These experimental observations strongly suggest that the structural disruption, with its intrinsic inversion symmetry breaking at the LSMO interfaces, plays a major role in the observed depressed magnetotransport properties in manganite-based magnetic tunneling junctions and explains the origin of the so-called dead layer.

Silicon Carbide is presently gaining much attention as a material for high temperature, high speed and high power devices. However, fabricating epitaxial SiC or GaN films is still a challenge since very high growth temperatures (up to 1600°C) must be used. This requires a carefully adapted design of reactors to ensure laminar flow conditions and a controlled depletion of the reactants inside the reactor. A second class of materials that is also playing a more and more important role today are the III nitrides (AIN, GaN, InN and alloys consisting of these). These materials are also grown at high deposition temperatures (up to 1300°C). Furthermore, these materials are grown on special buffer layers grown at much lower temperatures and are typically part of complex heterostructures which require abrupt changes of the growth temperature to realize sophisticated optoelectronic and electronic devices. As an example our high quality InGaN layers were grown at about 700°C and the AIGaN cladding layer of a laser or a LED require temperatures higher than 1100°C. In general, both nitrides and SiC are similar in their challenges to the growth equipment. This study uses a family of high temperature reactors to grow SiC and Nitrides. We describe the use of high temperature reactors to grow SiC and Nitrides. The load capacity ranges from single wafer machines to multiple wafer mass production reactors. All these reactors have a two flow injection system allowing a separated inlet of the various reactants. To achieve maximum uniformity of the growth, the Gas Foil Rotation (r) Principle is applied. The multiwafer reactors are Planetary Reactors with double rotation of substrates. Extensive modeling has been used in order to find the optimum reactor geometries. Gas flow dynamics, temperature management and the physical and chemical properties of the precursors were considered in these models. Thus an optimization of uniformity and efficiency and a minimization of undesired parasitic reactions has been obtained. This contribution will work out the differences and the similarities of the high temperature growth of both material systems in AIXTRON reactors already established in production. Results on the growth of SiC and GaN will be presented.

Since the invention of the Planetary Reactors® a reliable tool for mass production of various III–V compounds has existed. These reactors have proven to grow extremely uniform films together with a highly efficient utilization of the precursors. Now a new generation of Planetary Reactors® is introduced: the so-called G3 systems. Their main features are: an inductive heating system with extremely low thermal mass for precise and fast heating, high flexibility in the reactor size (15×2″, 35×2″ to 9×4″ wafers per load so far, further enlargement possible) and the option to use a fully automated cassette-to-cassette wafer loading system. The benefits of this new design are very short cycle times, extreme run-to-run stability and even further reduced cost of ownership. The performance of this reactor will be discussed in conjunction with the well established AIX 2400 reactor with a set up of 15×2″ or 5×4″ wafer. Uniformity of thickness, luminescence intensity and composition of the most important III–V compounds such as GaInP, GaInAsP and AlGaInP are shown.

Electroluminescence (EL) testing of Light Emitting Diode (LED) structures is usually done at the chip level. Assessing the optical and electrical properties of LED structures at the wafer scale prior to their processing would improve the cost effectiveness of producing LED-lamps. A non-destructive method for studying the luminescence properties of the structure at the wafer-scale is photoluminescence (PL). However, the relationship between the on-wafer PL data and the final device EL can be less than straightforward (Y. H Aliyu et al., Meas. Sci. Technol. 8, 437 (1997) [1]) as the two techniques employ different carrier injection mechanisms. This paper provides an overview of some different techniques in which palladium is used as a contact in order to obtain on-wafer electroluminescence information which could be used to screen wafers prior to processing into final devices. Quick mapping of the electrical and optical characteristics was performed using either palladium needle electrodes directly, or using the latter in conjunction with evaporated palladium contacts to inject both electrons and holes into the active region via the p-type capping layer of the structure. For comparison, indium was also used to make contact to the n-layer so that electrons could be directly injected into that layer. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metalorganic chemical vapor deposition (MOCVD) has been established as the most favoured method for the processing of (Ba, Sr)TiO3, Pb(Zr, Ti)O3 and SrBi2Ta2O9 thin films. Due to good step coverage, uniformity of thickness and composition as well as throughput MOCVD will certainly be the choice for the mass production of future electroceramic thin film based devices such as volatile and non-volatile memories, electrooptic devices, microactuators and sensors. Since many groups showed in a laboratory scale that electroceramic thin films deposited by MOCVD techniques are suitable for future applications in terms of electrical and mechanical properties, the need for production worthy tools is rapidly increasing. In this paper we present a large scale manufactor tool (capacity up to 4 × 300 mm) which meets all demands for an automized mass production of electroceramic thin films.

For the fabrication of epitaxial films of silicon carbide or the Group III nitrides, high growth temperatures (up to 1700°C) and fast heating and cooling of the growth environment have been found to be necessary. A range of production systems meeting these requirements has been designed with different loading capacities. In this paper, we present results from various machines showing the high quality and excellent homogeneity obtainable for 3C-SiC on Si and on 6H-SiC, as well as GaN on sapphire.

Photoluminescence (PL) and reflection spectra of undoped and Mg-doped GaN single layers grown on sapphire substrates by metalorganic vapor phase epitaxy (MOVPE) were investigated in a wide range of temperatures, excitation intensities, and doping levels. The undoped layers show n-type conductivity (μ=400 cm2/Vs, n=3×1017 cm−3). After annealing at T=600–700°C, the Mg-doped layers showed p-type conductivity determined by the potential-profiling technique. A small value of the full width at half maximum (FWHM=2.8 meV) of the excitonic emission and a high ratio between excitonic and deep level emission (≈5300) are evidences of the high layer quality. Two donor centers with activation energies of 35 and 22 meV were observed in undoped layers. A fine structure of the PL band with two narrow lines in the spectral range of the donor-acceptor pair (DAP) recombination was found in undoped layers. An anomaly was established in the temperature behavior of two groups of PL lines in the acceptor-bound exciton and in donor-acceptor pair regions in Mg doped layers. The lower energy line quenched with increasing temperature appreciably faster than the high energy ones. Our data does not agree with the DAP recombination model. It suggests that new approaches are required to explain the recombination mechanisms in undoped and Mg-doped GaN epitaxial layers.

State-of-the-art GaN devices more and more make use of low-dimensional effects. Both light emitting diodes (LEDs) and laser diodes consist of quantum wells in their active regions. This requires an extremely precise control of layer thickness and uniformity of alloy composition during MOVPE growth. For mass production of these structures the requirements are even tougher. Wafer uniformity within one run and reproducibility from run to run are most critical issues here. In this paper we present a class of multiwafer MOVPE-reactors which have proven to produce high quality GaN based material. We will also present data on bulk layer uniformity as well as the excellent interface sharpness of quantum well structures.

The electronic structure of Cu films on Co(100) in the $k$-space region associated with the short-period oscillation of the interlayer coupling in Cu/Co(100) multilayers has been examined by spin- and angle-resolved photoemission and by ab initio calculations. The photoemission intensity near the Fermi energy exhibits a rapid oscillatory behavior as a function of film thickness with a period close to that of the interlayer exchange coupling. The photoemission intensity and polarization reflect a modulation of the density of states induced by spin-polarized quantum-well states, in good agreement with ab initio calculations of quantum-well states.

We present results on the growth of Ga–In–Al–N films in multiwafer Planetary Reactors® with 7×2 in. wafer capacity. The unique design of these reactors allows us to combine high efficiency (Ga efficiency in GaN around 30%) and excellent uniformity. Thickness uniformities better than ±5% on a 2 in. wafer are routinely obtained. Compositional uniformity of GaInN is better than 2 nm. Wafer-to-wafer and run-to-run reproducibility concerning thickness and composition is within the 1% range. Besides this, the material grown also has a high purity. Hall mobilities higher than 500 cm 2 V −1 s −1 are obtained for GaN. Furthermore, the design of the reactors is dedicated to the growth of complex nitride heterostructures. Fast switching manifolds for various group III precursors and different carrier gases allow a flexible design of the growth runs including nucleation, bulk layers (doped/undoped) and quantum wells. The extremely low thermal mass of the reactor permits very rapid changes of the deposition temperature between room temperature and 1300°C (optional: 1600°C). Of course p- and n-type doping is also possible. Doping uniformities are below 1% indicating extremely uniform wafer temperatures. The results demonstrate that Planetary Reactors® are reliable and efficient tools for the mass production of blue-green optoelectronic devices.

Various Al-Ga-In Nitride alloys have been grown in AIXTRON Planetary Reactors®. GaN is grown with an excellent optical quality and very good thickness uniformity. GaInN with photoluminescence emission wavelengths in the visible blue region have also been produced. AlGaN is demonstrated as the material with the widest bandgap. Furthermore, we will present data on doped material. Characterization of the heterostructures by photoluminescence and sheet resistivity measurements is presented as well.

The need for ferroelectric capacitors and FRAMs is rapidly increasing as world wide communication networks are being erected and a higher integration of memory chips will be introduced in the next few years. To meet the demand for economical production of the raw material - thin film structures of ferroelectrics - we developed a metalorganic vapor phase epitaxy (MOVPE) reactor for the deposition of the material films on a large substrate area. We expanded the existing Planetary Reactor® to be able to take charges of nine six inch wafers or the equivalent (total substrate area of 0.25 square meters). The growth process can be predicted by numerical simulations rather than by growth runs in the real reactor. The reactor is proven in mass production environments for the growth of multicompound semiconductors. A major achievement is the compensation of the depletion of the gas phase which can result in poor uniformity of the films. By rotating the wafers, the growth rate along the direction of flow i...

In this study a range of MOVPE systems for high temperature epitaxial growth processes, such as SiC and group III nitrides, is presented. It is shown that extensive modelling of the heat transfer, gas flow and reactant depletion has led to highly efficient reactors that provide the capability for uniform growth of binary and ternary material systems at temperatures up to 1600 C in single- and multi wafer configurations. Experimental results of various layers are presented to prove the availability of state-of-the-art process technology in production-scale systems.

The worldwide demand for ultra-high-brightness blue and green LEDs has driven the development of MOCVD for Al-Ga-In-N alloy systems towards efficient multiwafer technology. This new MOCVD approach has been developed using a unique growth initiation cycle which provides material of superior quality and increased lateral thickness uniformities in the 7 in × 2 in to 15 in × 2 in Planetary Reactor ® , which is to our knowledge the largest Nitride MOCVD reactor in the field of successful production of blue LEDs. For proper growth initiation for blue-green LED material it is essential that heating from room temperature to 1000 °C can be done in less than 2 min and the cool down from 1000 to 500 °C takes place in 3 min. The growth initiation cycle has been demonstrated to yield device quality GaN with X-ray FWHM of 30 arcsec and excellent PL uniformity. The key to the excellent results is the high flexibility of this unique MOCVD process which can be used between 10 and 1000 mbar, a variety of total flow rates and extremely precise temperature control and uniformity across the entire reactor and the substrates. Using all these flexible parameters in the appropriate way allows the required growth rate to be adjusted and the necessary control on In composition in InGaN with the successful demonstration of blue LEDs to be obtained.

Copyright (c) 1997 Elsevier Science S.A. All rights reserved. The worldwide demand for ultra-high-brightness blue and green light emitting devices (LEDs) has driven the development of metalorganic chemical vapor deposition (MOCVD) for Al−Ga−In−N alloy systems towards efficient multiwafer technology. This new MOCVD approach has been developed using a unique growth initiation cycle which provides material of superior quality and increased lateral thickness uniformities in the 7×2 to 15×2 Planetary Reactor®, which is to our knowledge the largest Nitride MOCVD reactor in the field of successful production of blue LEDs. The MOCVD process, based on a laminar radial two-flow approach will be discussed in detail. The design provides material with abrupt interfaces, also while using different substrates like Al 2 O 3 , SiC, Si. Wafer rotation is performed by means of Gas Foil Rotation®. For proper growth initiation for blue-green LED material it is essential that heating from room temperature to 1000°C can be done in less than 2 min and the cool down from 1000 to 500°C takes place in 3 min. The growth initiation cycle has been demonstrated to yield device quality GaN with X-ray full width at half maximum (FWHM) of 30 arcsec and excellent photoluminescence (PL) uniformity. Key to the excellent results is the high flexibility of this unique MOCVD process that can be used between 10 and 1000 mbar, a variety of total flow rates and extremely precise temperature control and uniformity across the entire reactor and the substrates, by means of a new multicoil heater system. Using all these flexible parameters in the appropriate way allows to adjust the required growth rate and to obtain the necessary control on In composition in InGaN with the successful demonstration of blue LEDs.

The suitability of an N 2 carrier in LP-MOVPE of GaInAs InP device structures and for the growth of (Al)GaInP is investigated for the first time. Al-free GaInAs InP HEMTs and MSM photodetectors exhibit cutoff frequencies of f t = 135 GHz and f max = 200 GHz and a bandwidth of 16 GHz and responsivity of 0.27 A/W, respectively. AlGaInP and GaInP layers deposited using the optimized growth conditions showed excellent structural, optical and homogeneity properties. For example X-ray diffractograms with FWHMs as low as 15–16 arcsec for 1 μm thick layers and 300 K photoluminescence mappings over full 2 inch wafers with standard deviations of ±0.23 and ±0.26 nm were obtained for both materials.

X-ray circular magnetic dichroism, polarized neutron diffraction, ac susceptibility, and Seebeck effect have been measured for several members of the RCo2 series (R = Ho, Tm, Er) as a function of temperature and applied magnetic field. The experimental results show robust parimagnetism (a general behaviour along the RCo2 series with R being a heavy rare earth ion) and two reversal temperatures in some systems, which is an unexpected result. Polarised neutron diffraction show differences between results obtained on single crystals or polycrystalline ingots. We propose an interpretation of parimagnetic RCo2 as a Griffiths phase of the high temperature, magnetically ordered, amorphous RCo2 phase., Finantial support from MINECO MAT2011/23791 and DGA IMANA projects, partially funded by the FEDER program and the European Social Fund is acknowledged. C.M. Bonilla acknowledges a FPI predoctoral grant.

The magnetism of the $c(2\ifmmode\times\else\texttimes\fi{}2)$ Mn/Ni(100) surface alloy on Ni films of 3.2 to 4.0 monolayer thickness on a Cu(100) crystal is studied element specifically as a function of temperature using magnetic circular x-ray dichroism. The Mn signal yields information about the surface of the system, whereas the Ni signal contains information which is averaged over the whole thickness of the Ni film. As a consequence of the ferromagnetic Ni-Mn coupling, Mn and Ni have identical critical temperatures ${T}_{c}$. The empirical exponent $\ensuremath{\beta}$ is slightly enhanced at the surface.

In optoelectronics as well as in electronic devices, the application of low dimensional heterostructures is a core element for obtaining certain effects (two-dimensional electro gas, quantum size) or improvements in performance. In this contribution, we will present low pressure metal-organic vapour phase epitaxy (MOVPE) processes and the related reactor design for medium-, large-, and very-large-scale production of III-V semiconductor structures. The application for low-dimensional structure growth will be addressed with examples from industrial and research applications. A vital example for multiple application of low dimensional structures is vertical cavity surface emitting lasers (VCSELs). The design of the Bragg-reflectors, the cavity, and accurate and reproducible growth are crucial for the laser performance. The control of composition in the active part of the laser as well as the film thickness of the quantum wells used influence the emission wavelength and threshold current of the lasers. A variety of AlGalnP-based VCSEL structures will be discussed with particular emphasis on the effects of the low dimensions of the structures while extremely high reproducibility within the vertical design of the structures is required. The MOVPE process technology has been scaled up successfully from the single 2 in wafer growth chamber to multiple 4 in substrate size reactor. The application of the Planetary Reactor® technology in superlattice and quantum well growth has already been industrialized by various laboratories and production facilities in GaAs/AlGaAs and GalnP/AlGalnP laser technology.

The exchange bias effect refers to the shift of the hysteresis loop of a ferromagnet in direct contact to an antiferromagnet. For applications in spintronics a robust and tunable exchange bias is required. Here we show experimental evidence for a perpendicular exchange bias in a prototypical ferrimagnetic spin valve consisting of DyCo(5)/Ta/Fe(76)Gd(24), where the DyCo(5) alloy has the role of a hard ferrimagnet and Fe(76)Gd(24) is a soft ferrimagnet. Taking advantage of the tunability of the exchange coupling between the ferrimagnetic layers by means of thickness variation of an interlayer spacer, we demonstrate that perpendicular unidirectional anisotropy can be induced with desirable absolute values at room temperature, without making use of a field-cooling procedure. Moreover, the shift of the hysteresis loop can be reversed with relatively low magnetic fields of several hundred Oersteds. This flexibility in controlling a robust perpendicular exchange bias at room temperature may be of crucial importance for applications.

We report on the use of surface-sensitive techniques (x-ray absorption spectroscopy and x-ray magnetic circular dichroism) to investigate interfacial effects in sputtered manganite thin films, La2/3Ca 1/3MnO3 (LCMO) and La2/3Sr 1/3MnO3 (LSMO), with different capping layers [MgO, LaAlO3 (LAO), SrTiO3 (STO), NdGaO3 (NGO), and Au]. To ensure surface sensitivity, data were acquired by using the total electron yield detection mode. It is found that LSMO and LCMO films exhibit similar behavior when capped with oxide layers but dissimilar effects when a metallic capping is used. Almost bulklike Mn valence at the interface was observed in the case of LAO capping. However, a notorious increase of the Mn oxidation state was detected for both MgO and NGO capping layers. In contrast, metallic Au and STO capping promotes a reduction of the Mn oxidation state. These results are correlated with the x-ray magnetic circular dichroism data, showing a concomitant decrease of the saturation magnetization at the interface in those cases where a modification of the Mn oxidation state is observed. © 2011 American Physical Society., We acknowledge financial support from the Spanish MEC (MAT2009-08024), CONSOLIDER (CSD2007-00041), and FEDER program. The research leading to these results has received funding from the European Community’s Seventh Framework Program (FP7/2007-2013) under Grant No. 226716. Z.K. thanks the Spanish MEC for the financial support through the RyC program.

We studied the (001/2) diffraction peak in the low-temperature phase of magnetite (Fe3O4) using resonant soft x-ray diffraction (RSXD) at the Fe-L2,3 and O-K resonance. We studied both molecular-beam-epitaxy (MBE) grown thin films and in-situ cleaved single crystals. From the comparison we have been able to determine quantitatively the contribution of intrinsic absorption effects, thereby arriving at a consistent result for the (001/2) diffraction peak spectrum. Our data also allow for the identification of extrinsic effects, e.g. for a detailed modeling of the spectra in case a "dead" surface layer is present that is only absorbing photons but does not contribute to the scattering signal., Comment: to appear in Phys. Rev. B

We show both theoretical and experimental evidences of the appearance of ferromagnetism in MgO thin films. First principles calculations allow predicting the possibility of the formation of a local moment in MgO, provided the existence of Mg vacancies which create holes on acceptor levels near the O 2p-dominated valence band. Magnetic measurements evidence of the existence of room-temperature ferromagnetism in MgO thin films. High-resolution transmission electron microscopy demonstrates the existence of cation vacancies in our samples. Finally, by applying the element specificity of the x-ray magnetic circular dichroism technique, we also demonstrate that the magnetic moments of the system arise from the spin polarization of the 2p electrons of oxygen atoms surrounding Mg vacancies.

Depletion mode InGaAs microwave power MISFETs with 0.7 mu m gate lengths and 0.2 mm gate widths have been fabricated using an epitaxial process. The devices employed a plasma deposited silicon dioxide gate insulator. The RF power performance at 18 GHz, 20 GHz, and 23 GHz is presented. An output power density of 1.04 W/mm with a corresponding power gain and power-added efficiency of 3.7 dB and 40%, respectively, was obtained at 18 GHz. This is the highest output power density obtained for an InGaAs based transistor on InP at K-band. Record output power densities for an InGaAs MISFET were also demonstrated to the stable within 3% over 17 hours of continuous operation at 18 GHz. >

We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias., 4 pages, 3 figures

Ga x In1−x As y P1−y alloys lattice matched to InP substrates are currently used to fabricate optoelectronic and integrated optics devices. To achieve devices with high performances and high fabrication yield, the uniformity and reproducibility of the Ga x In1−x As y P1−y epitaxial layers (composition, thickness, doping, etc.) have become key parameters. These problems have been addressed in the frame of ESPRIT project 2518 and are presented in this paper. Several aspects have been considered starting from the optimization of InP substrates, the MOVPE growth of uniform GalnAsP layers, the material characterization to the validation of material uniformity on passive optical waveguides. Both scanning photoluminescence analysis and waveguide losses measurements performed on 2 inch wafers with a high lateral resolution have shown that high quality uniform GalnAsP layers can be obtained reproducibly on 2″ InP substrates using a commercially available LP-MOCVD growth process. In particular, more than 60% of 36 mm long, 3μm wide and 100μm spaced rib waveguides exhibit losses below 0.8dBcm−1.

We have employed Soft and Hard X-ray Resonant Magnetic Scattering and Polarised Neutron Diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial Ni$_{81}$Fe$_{19}$(111)/CoO(111) exchange biased bilayer. The combination of these scattering tools provides unprecedented detailed insights into the still incomplete understanding of some key manifestations of the exchange bias effect. We show that the several orders of magnitude difference between the expected and measured value of exchange bias field is caused by an almost anisotropic in-plane orientation of antiferromagnetic domains. Irreversible changes of their configuration lead to a training effect. This is directly seen as a change in the magnetic half order Bragg peaks after magnetization reversal. A 30 nm size of antiferromagnetic domains is extracted from the width the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and x-ray scattering. A reduced blocking temperature as compared to the measured antiferromagnetic ordering temperature clearly corresponds to the blocking of antiferromagnetic domains. Moreover, an excellent correlation between the size of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio is found, providing a comprehensive understanding of the origin of exchange bias in epitaxial systems., 8 pages, 5 figures, submitted

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Exchange bias in layered CoO/Fe structures is investigated by x-ray resonant magnetic reflectivity (XRMR) measurements. Element-specific hysteresis loops are obtained from x-ray magnetic circular dichroism effects in the XRMR spectra. Evidence is provided for the existence of different types of uncompensated moments in the antiferromagnetic material. Explanations are given for the microscopic nature of these moments and the complex exchange interactions that determine the magnetization reversal in exchange bias systems.

We applied x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at the Mn $2p\text{\ensuremath{-}}3d$ resonances to study the Mn $3d$ electronic configuration and the coupling of Mn $3d$ magnetic moments in various ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films. The homogeneity of the Mn depth profile throughout the ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ film was tested by additional structure-sensitive x-ray resonant reflectivity measurements. In all investigated ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$ films the electronic and magnetic configuration of the Mn impurities varies throughout the Mn-doped layer. This inhomogeneity is caused by the surface segregation of nonferromagnetic Mn in a ${d}^{5}$ configuration. X-ray resonant reflectivity data show that the accumulation of nonferromagnetic Mn near the surface is strongly enhanced by low-temperature annealing. By XMCD we identified the Mn species responsible for the long-range ferromagnetic coupling. It is characterized by an Mn $3{d}^{5}\text{\ensuremath{-}}3{d}^{6}$ mixed-valence acceptor state that is unchanged at all investigated Mn concentrations, ranging from 1% to 6%. Additional nonferromagnetic Mn occurs in the bulk of high-concentration samples. We discuss a model in which the latter is due to antiferromagnetic Mn-Mn nearest-neighbor pairs.

Extended X ray absorption fine structure EXAFS data and magnetic EXAFS MEXAFS data were measured at two temperatures 180 K and 296 K in the energy region of the overlapping L edges of bcc Fe grown on a V 110 crystal surface. In combination with a Bayes Turchin data analysis procedure these measurements enable the exploration of local crystallographic and magnetic structures. The analysis determined the atomic like background together with the EXAFS parameters which consisted of 10 shell radii, the Debye Waller parameters, separated into structural and vibrational components, and the third cumulant of the first scattering path. The vibrational components for 97 different scattering paths were determined by a two parameter force field model using a priori values adjusted to Born von Karman parameters of inelastic neutron scattering data. The investigations of the system Fe V 110 demonstrate that the simultaneous fitting of atomic background parameters and EXAFS parameters can be performed reliably. Using the L2 and L3 components extracted from the EXAFS analysis and the rigid band model, the MEXAFS oscillations can only be described when the sign of the exchange energy is changed compared to the predictions of the Hedin Lundquist exchange and correlation functional

We present an investigation of the growth of InP based materials in a Multiwafer Planetary Reactor (R). A reactor suited for the simultaneous growth of 15/spl times/2", 8/spl times/3" or 5/spl times/4" wafers was used to grow InP, GaInAs and various GaInAsP compositions. As predicted by the theory of the Planetary Reactor (R) principle, excellent uniformities of all layer properties were found. Without any tuning of process parameters, thickness variations of /spl plusmn/1% or better were obtained. The compositional uniformity was also very good. This was confirmed by X-ray diffraction and PL mapping. Emission wavelength uniformities were in the 1 nm range. p- and n-type doping was also investigated. Sheet resistivity measurements on doped samples reveal uniformities around 1%.

The ex situ preparation of chemically synthesized cobalt nanoparticles leads unavoidably to particles coated with organic surfactants and a thin CoO shell. Argon ion $({\mathrm{Ar}}^{+})$ etching is a method often used to remove the surfactant and the oxide shell. Ex situ prepared nanoparticles of $9.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ diameters were investigated as a function of the sputtering time, by means of soft x-ray absorption spectroscopy at the C and O K edges and at the Co ${\mathrm{L}}_{2,3}$ edges. Low-energy $(500\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ ${\mathrm{Ar}}^{+}$ ion etching was effective in removing the organic coating layer of the nanoparticles, while much higher energy and longer sputtering times were required to affect the CoO shell. On the other hand, only a short time using low-energy hydrogen ion $({\mathrm{H}}^{+})$ etching was necessary to completely reduce the oxide shell. Magnetic circular dichroism of soft x-rays was used to measure the magnetic properties of the etched samples. We found that the orbital to spin magnetic moment ratio, ${\ensuremath{\mu}}_{l}∕\ensuremath{\mu}_{s}{}^{\mathrm{eff}}$, increases as a function of the ${\mathrm{Ar}}^{+}$ etching time. Such an increase may be ascribed to the incomplete removal of the CoO shell as well as to surface strain, increased roughness, and an effective reduction of the particle size induced by the high-energy ion etching. A much smaller effect was observed for the low-energy $(500\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and short time ${\mathrm{H}}^{+}$ etched samples, where the magnetic moment ratio after the reducing procedure ${\ensuremath{\mu}}_{l}∕\ensuremath{\mu}_{s}{}^{\mathrm{eff}}=0.12$ is comparable to values obtained for bulk cobalt.

The improvement in wafer yield to increase the amount of usable wafer area per epitaxy run is of paramount interest. One possibility would be to change the precursors just for the active layer of the device to combine the advantages of PH/sub 3/ and TBP in one structure. As TBP is known as already totally decomposed at 600 /spl deg/C as opposed to the case of PH/sub 3/ it could give a less temperature sensitive alternative to grow quaternary material for optoelectronic devices. On the other hand buffer and claddings can still be grown with the cheaper PH/sub 3/. This investigation, focuses on the wafer photoluminescence standard deviation (Std. Div) in a total of 14 laser test structure MOCVD runs. The goal is to check if there could be an advantage to using different precursors in one structure.