To send content items to your account,
please confirm that you agree to abide by our usage policies.
If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account.
Find out more about sending content to .
To send content items to your Kindle, first ensure firstname.lastname@example.org
is added to your Approved Personal Document E-mail List under your Personal Document Settings
on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part
of your Kindle email address below.
Find out more about sending to your Kindle.
Note you can select to send to either the @free.kindle.com or @kindle.com variations.
‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi.
‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
CRESST II is an experiment for direct WIMP search, using cryogenic detectors. the ratio of the two signals (temperature rise and scintillation light) measured for each interaction is an excellent parameter for discrimination of the radioactive background. the main remaining background is the neutron flux present at the experimental site, since neutrons produce the same signals as WIMPs do. Based on Monte Carlo simulations the present work shows how neutrons from different origins affect CRESST and which measures have to be taken into account to reach the sensitivity goal.
We compare in a systematic way spectrometric, photometric and mid-infrared (VLTI/MIDI) interferometric measurements with different types of model atmospheres. Self-consistent dynamic model atmospheres in particular were used to interpret in a consistent way the dynamic behavior of gas and dust. The results underline how the joint use of different kind of observations, as photometry, spectroscopy and interferometry, is essential to understand the atmospheres of pulsating C-rich AGB stars. The sample of C-rich stars discussed in this work provides crucial constraints for the atmospheric structure.
Fundamental aspects of (electro-)luminescence of Cu(In,Ga)Se2 solar cells and modules are investigated by means of spectrally and spatially resolved measurements. The validity of the reciprocity relation between spectrally resolved electroluminescence emission and photovoltaic quantum efficiency is verified for the case of industrially produced ZnO/CdS/Cu(In,Ga)Se2 heterojunction solar cells. Further we find that photo- and electroluminescent emission in these devices obey a superposition principle only in a limited range of the applied electrical or illumination bias. This range depends on the light soaking history of the sample and extends up to an injected current density of approximately 15 mAcm-2 after 3 h of light soaking at a temperature of 400 K. In the state prior to light soaking this range is limited to 4 mAcm-2. At higher bias, a characteristic discrepancy between electroluminescence and electro-modulated photoluminescence appears. We attribute this anomaly to a potential barrier behavior close to the CdS/ Cu(In,Ga)Se2 interface. Metastable defect reactions induced by holes injected into the space charge region partly reduce this barrier. We further find that the luminescence efficiency is enhanced by a factor of 3 by light soaking at 400 K. Spatially resolved electroluminescence measurements conducted during application of voltage or current bias at ambient temperature in the dark are qualitatively compatible with the conclusions drawn from the spectrally resolved measurements.
We describe the current, 9-spacecraft Interplanetary Network (IPN). The IPN detects about
325 gamma-ray bursts per year, of which about 100 are not localized by any other missions.
We give some examples of how the data, which are public, can be utilized.
The Magnetoresistance Measured perpendicular to the plane of the Multilayer, (CPP-MR) has been measured for the Cu/CO and Cu/ (Ni/Fe) systems. The predictions of a two spin-channel model are summarized, and the Cu/CO data are analysed in terms of this theory. The Cu/ (NiFe) data show a more complex behaviour.
The Layer Korringa Kohn Rostoker-Coherent Potential Approximation technique was used to calculate the low temperature Giant Magnetoresistance from first principles for Co|Cu and permalloy|Cu superlattices. Our calculations predict large giant Magnetoresis-tance ratios for Co|Cu and extremely large ratios for permalloy|Cu for current perpendicular to the layers. Mechanisms such as spin-orbit coupling which mix spin channels are expected to greatly reduce the GMR effect for permalloy|Cu.
The role of the interlayer Fermi surface in driving oscillatory exchange coupling of ferromagnetic layers has been critically tested in a series of MBE-grown coherent epitaxial sandwiches prepared on single-crystal substrates. The wedge-shaped Cu (or Cu-based) interlayers have been modified in a number of different ways, achieving orientational, compositional and structural alteration of the Cu Fermi surface. Observed phenomena include multiperiodic coupling oscillations in both the ferromagnetically and antiferromagnetically coupled regions of (lOO) -oriented Co/Cu/Co samples, extended oscillatory periods across CuNi alloy interlayers, and extremely short-period oscillations in a bcc (100) Fe/Cu/Fe system. These phenomena are straightforwardly explained in terms of calculated Fermi surface variations.
Elastic neutron scattering measurements performed at the NIST reactor have been used to measure the staggered magnetization near the transition temperature in a thin antiferromagnetic epitaxial film of FeF2 of thickness 0.8μm and diameter 1cm grown on a diamagnetic (001) ZnF2substrate by MBE. The use of a thin film permits extinction-free Bragg intensities, something which has proven impossible in bulk crystals. The growth techniques yield sufficient crystal quality to observed resolution limited Magnetic Bragg scattering peaks and to approach the transition within a reduced temperature of |t| = 0.003. The structure quality of this sample has been characterized using X-ray double crystal diffraction with a measured rocking curve lin ewidth of less than 30 arc sec. The sample thickness, while small enough to eliminate extinction, is sufficiently large to assure three-dimensional Ising Model critical behavior. We indeed observe critical behavior consistent with theoretical predictions. The success of the thin film experiments demonstrates the possibilities of extinction-free Bragg scattering measurements in a variety of antiferromagnetic Materials, including multilayered systems.
Giant Magnetoresistance (GMR) is reported in as-deposited Ag1−xCox (x = 0.26–0.53) films co-sputtered on Si from separate Ag and Co targets. GMR ratios (10 K Oe Maximum field) exceeding 0.50 and 0.19 at 5 and 295 K, respectively, are observed for the Ag067Co033 films deposited at ≃28 to 175 °C. The Maximum ratios of 0.55 and 0.24 occur at a substrate temperature of ≃125 °C for these films. The ratios decrease rapidly for the films deposited at temperatures > 175 °C and reduce to ≃0.15 and 0.04 at deposition temperatures >300 °C. This deposition temperature dependence of GMR ratios is interpreted in terms of the change in the spin-dependent interfacial electron scatterings due to the change in the size and number of ferromagnetic Co particles within the electron mean free path. The initial increase and the subsequent decrease in GMR ratios with increasing deposition temperature are attributed to the increase in the mean free path, and the Co and Ag particle size, respectively. Changes in mean free path are obtained from the resistivities of these films while changes in Ag and Co particles are deduced mainly from the X-ray diffraction patterns, transmission electron Micrographs, and the coercivities of these films.
The role of interfacial exchange coupling in the magnetic behavior of metal oxide materials has been investigated through the study of Fe3O4/NiO superlattices. We report results on a series of superlattices grown where one bilayer constituent was held to a fixed thickness while varying the other from single unit cell dimensions upward. High crystalline quality was confirmed by XRD, RHEED and neutron diffraction. Magnetization profiles show substantial deviations from bulklike iron oxide results, with an increase in domain rotation energies observed in the superlattices over that of bulk iron oxide (increasing with NiO layer thickness) indicating the strong nature of Fe3O4/NiO interfacial linkage. Neutron scattering at elevated temperatures shows that the NiO remains ordered above the 523 K bulk Néel temperature. This suggests that at least a portion of the NiO within a layer remains ordered well above the Néel temperature, with an increase in effective Néel transition temperature that approaches the Fe3O4 Curie temperature in the limit of very thin NiO layers. Although the exchange coupling dominates these effects, strain also plays an important role.
The nonlinear Magneto-optical Kerr-effect (NMOKE) has been proven to be an ultrafast spectroscopie probe of the magnetic and electronic properties of ferromagnetic surfaces. Extending our previous theory we calculate the linear and nonlinear Magneto-optical Kerr-spectra of Fe. The results for the nonlinear Kerr spectra are compared with recent experiments on a Fe (110) surface. We derive and discuss general features of the nonlinear Kerr-effect, especially the details of its dependence on exchange interaction and Fermi-level crossings in the Stoner picture and beyond, frequency, and spin-orbit interaction. Furthermore we demonstrate how various electronic material properties, such as d-band width, s-d hybridization or band narrowing due to electronic correlations or caused by geometry (reduced coordination number in thin films), can be extracted from the spectra. As an example, we compare the linear and nonlinear Kerr-spectra of Fe to those of Ni. The extension of our theory to the Kerr spectra of thin magnetic films films as well as of hidden magnetic interfaces occurring in sandwiches and multilayers is discussed.
A study has been made of the magnetic properties of epitaxial 6 Monolayer (ML) fcc-Fe films on Cu (100) with various thicknesses of epitaxial Cu deposited on top of the Fe. It was found that the magnetic properties undergo striking changes as a function of the Cu thickness. The easy axis of magnetization goes from being in-plane for the bare Fe to perpendicular upon the deposition of 1 ML Cu. Concurrently there is a dramatic decrease in the Kerr signal intensity at saturation. Upon depositing a second ML of Cu the Kerr signal intensity more than doubles, and the easy axis remains perpendicular. For Cu overlayers of 3 ML to 10 ML the Kerr signal intensity at saturation gradually diminishes to below the level of detectability, as if the Fe were nonmagnetic. A superlattice consisting of 60 ML Cu/ (6 ML 57Fe/10 ML Cu)×5/Cu (100) was fabricated and studied at room temperature by conversion electron Mòssbauer spectroscopy. The results confirmed that the Fe is indeed nonmagnetic. The four inner Fe layers of the 6 ML film have the same isomer shift as bulk fcc-Fe in precipitates in Cu, and the two boundary Fe layers exhibit an asymmetric quadrupole doublet.
An experimental study of conventional x-ray absorption and of magnetic circular x-ray dichroism has been performed on Ce/Fe Multilayers at the Ce L2,3 and the Fe-K edge, to characterize the impact of the interaction of Ce and Fe on the local electronic and magnetic structure of the interface. The spectra reveal that the interaction has two effects. First, on a depth scale of up to 15 A near the interface, the Ce atoms adopt the electronic structure of the oc-phase, with itinerant 4f states. Second, a large portion (=10 Å) of these cc-like Ce atoms is magnetically polarized and carries an ordered Magnetic 5d Moment. The Fe-3d and Ce-5d Moments are antiferromagnetically coupled. Interface Mixing on a length scale of the magnetic polarization can be excluded.
The paper discusses the electronic properties of Cu(In,Ga)Se2–based heterojunction solar cells with a special focus on questions which at present are not satisfactorily understood. First, we discuss an apparent quantitative contradiction between measured concentrations of recombination centers in the Cu(In,Ga)Se2 absorber material and the actual recombination rate in the solar cells. We propose, as a possible explanation for that observation, that the defect concentration in Cu(In,Ga)Se2 is spatially inhomogeneous with a systematic increase towards the heterojunction interface. Second, we address the issue of electronic metastabilities in ZnO/CdS/Cu(In,Ga)Se2 heterojunctions and, especially, in devices that use alternative buffer materials instead of CdS. Starting from a brief review of the experimentally observed types of metastabilities, we demonstrate by thermally stimulated capacitance measurements that a specific type of metastability that severely limits the performance of solar cells with non-CdS buffers is present also in high-efficiency standard devices though it has virtually no influence on the output parameters in the latter case. A possible explanation of this type of metastability points to a metastable defect reaction localized in the close to surface region of Cu(In,Ga)Se2. At the moment we cannot propose conclusive models for both open questions. However, we can localize the answers to both problems in the close-to-surface region of the Cu(In,Ga)Se2 absorber.
We investigate radiation-induced defects in high-efficiency Cu(In,Ga)Se2/CdS/ZnO heterojunction solar cells after 1-MeV electron and 4-MeV proton irradiation. We use electron and proton fluences of more than 1018 cm−2 and up to 1014 cm−2, respectively. The irradiation experiments performed at three independent electron irradiation facilities consistently prove the superior radiation resistance of these Cu(In,Ga)Se2 devices compared to other types of solar cells. The reduction of the solar cell efficiency in all experiments is predominantly caused by a loss ΔVOC of the open circuit voltage VOC. An analytical model describes ΔVOC in terms of radiation-induced defects enhancing recombination in the Cu(In,Ga)Se2 absorber material. From our model we extract the defect introduction rates for recombination centers in Cu(In,Ga)Se2 for the respective particles and energies. Isochronal annealing steps fully recover VOC of the irradiated Cu(In,Ga)Se2 solar cells. Exposure to temperatures of approx. 400 K are sufficient to restore the initial VOC within less than 5 %, even after excessive irradiation. The annealing process displays an activation energy of EA = 1.1 eV. Admittance spectroscopy directly reveals the generation and the annealing of radiation-induced defects.
We present a systematic study on the polycrystalline Cu(In,Ga)(S,Se)2 alloys with a gallium to indium ratio of Ga/(Ga+In)<0.3 and a sulfur to selenium ratio varying in the range between 0<S/(S+Se)<1. All samples were grown by coevaporation of the elements at constant rates under high vacuum conditions. The formation of island-like (Cu,S,Se) segregations correlate with the sulfur to selenium ratio in the layer and are found in the growth region near the copper rich phase boundary. These segregations are related to a preferred incorporation of sulfur in the copper rich growth mode. We obtain solar cell grade material from an indium rich growth mode up to a sulfur to selenium ratio of S/(S+Se)=0.9. A detailed analysis of the electronic and optical properties of Mo/CIGSSe/CdS/ZnO:Al heterojunctions allows us to determine the energetic position of the bands within the Cu(In,Ga)(S,Se)2 alloy system. We find that contrary to the Cu(In,Ga)(Se)2 alloy system the valence band position is significantly lowered with increasing bandgap.
A new technique for improving the diffusion barrier properties of thin, thermallyevaporated nickel, chromium and nichrome films on silicon is described. In this technique, known as “Rapid Thermal Annealing” (RTA), profound differences in the diffusion barrier properties of the films annealed in ammonia ambient at 550-750°C, in comparison to films annealed only in vacuum, were observed. The films annealed in ammonia retained their integrity while the films annealed in vacuum showed diffusion of the silicon into the metal overlayer throughout the entire thickness of the metal in some cases. The film sheet resistance increase for the latter was consistent with the formation of the metal silicide. The possibility of extending this technique to electroplated films used in integrated and hybrid device fabrication is being studied.
A simple statistical model describes measured absorption and photoluminescence data of Cu(In1-x, Gax)Se2 thin films. The broadening of the transition peak in the absorption spectra stems from band gap fluctuations. The extent of the spatial inhomogeneities as expressed in the standard deviation Eg μ reaches a maximum of Eg μ = 90 meV for films with equal amounts of indium and gallium, indicating alloy disorder as one possible source of the band gap fluctuations. The fluctuations observed lead to a decrease δVOC of the maximum possible open-circuit voltage VOC of almost 150 mV. However, the experimentally measured, low VOC of solar cells with high gallium content cannot be explained by band gap fluctuations alone. Consequently, our analysis suggests that the dominant recombination process in Cu(In1-x, Gax)Se2 thin film solar cells with high gallium content is not governed by the band gap energy, but is more likely due to deep levels within the forbidden gap.
The epitaxial thickening of a thin polycrystalline Si (poly-Si) film (seed layer) is a promising approach to realize an absorber layer of a poly-Si thin-film solar cell on glass. Such cell concept combines the benefits of crystalline Si and the high potential for cost reduction of a thin-film technology. Here, we discuss the influence of post-deposition treatments on the properties of absorber layers grown by electron-cyclotron resonance chemical vapor deposition (ECRCVD) and the solar cell performance, respectively. After the absorber growth and prior to the emitter deposition, defect annealing was used to improve the structural quality of the absorber layers and to increase the doping efficiency. For this, we used rapid thermal annealing (RTA) processes as well as thermal annealing in a conventional quartz furnace. In order to avoid damaging of the glass, only short annealing times (up to 400 s) were applied at temperatures of up to 950 °C. Defect passivation treatments were carried out at temperatures of about 350 °C to passivate the remaining defects in the films by hydrogen. The impact of both treatments on the solar cell parameter will be discussed. Excellent VOC's of up to 361 mV were achieved without hydrogenation showing the high potential of ECRCVD-grown absorbers. Applying both treatments resulted so far in an increase of VOC of about 400 mV. Because of the fact, that both post-treatments (particularly hydrogenation) are still not yet optimized, further improvements can be expected.
This article investigates chemical modifications of Cu(In, Ga)Se2 thin film by treatment in cadmium solutions with varying [NH3] concentration. Secondary ions mass spectroscopy and Xray photoelectron-spectroscopy analysis show that Cd-diffusion into the Cu(In, Ga)Se2 film is intimately tied to the reduction-oxidation of the Cu(In, Ga)Se2 surface, which is controlled by the [NH3] concentration. Reaction products, like Se and metal hydroxides of the redox process as wellas thediffusedCdinCu(In, Ga)Se2 film, result in degradation of solar cell performance.