Spin disorder scattering in a ferromagnetic insulator-on-graphene structure

We theoretically study the transport properties of a single graphene layer between two insulating materials, i.e., a ferromagnetic EuO thin film and a nonmagnetic SiC substrate. An exchange interaction between the charge carrier spins in graphene and the localized magnetic moments in the ferromagnetic insulator is assumed. This proximity effect and the large spin fluctuations at temperatures close to the ferromagnetic transition temperature TC lead to spin disorder scattering, which is calculated using a Green's function technique. Numerical results indicate that at temperatures close to TC the contribution of the spin disorder scattering to the total electron mobility is clearly observable even in the case of a weak exchange interaction and a low background mobility of the graphene layer. This enables the experimental determination of the exchange interaction parameter using the present model and a simple resistivity measurement.Peer reviewe

Efficient photon capture on germanium surfaces using industrially feasible nanostructure formation

Nanostructured surfaces are known to provide excellent optical properties for various photonics devices. Fabrication of such nanoscale structures to germanium (Ge) surfaces by metal assisted chemical etching (MACE) is, however, challenging as Ge surface is highly reactive resulting often in micron-level rather than nanoscale structures. Here we show that by properly controlling the process, it is possible to confine the chemical reaction only to the vicinity of the metal nanoparticles and obtain nanostructures also in Ge. Furthermore, it is shown that controlling the density of the nanoparticles, concentration of oxidizing and dissolving agents as well as the etching time plays a crucial role in successful nanostructure formation. We also discuss the impact of high mobility of charge carriers on the chemical reactions taking place on Ge surfaces. As a result we propose a simple one-step MACE process that results in nanoscale structures with less than 10% surface reflectance in the wavelength region between 400 nm and 1600 nm. The method consumes only a small amount of Ge and is thus industrially viable and also applicable to thin Ge layers.Comment: 8 pages, 4 figures. Full citation details and link to manuscript published in Nanotechnology were adde

Al-neal Degrades Al2O3 Passivation of Silicon Surface

Atomic layer deposited (ALD) aluminum oxide (Al2O3) has emerged as a useful material for silicon devices due to its capability for effective surface passivation and ability to generate p(+) region underneath the oxide as active or passive component in semiconductor devices. However, it is uncertain how Al2O3 films tolerate the so-called Al-neal treatment that is a necessary process step in devices that also contain silicon dioxide (SiO2) passivation layers. Herein, it is reported that the Al-neal process is harmful for the passivation performance of Al2O3 causing over eightfold increase in surface recombination velocity (SRV) (from 0.9 to 7.3 cm s(-1)). Interestingly, it is also observed that the stage at which the so-called activation of Al2O3 passivation is performed impacts the final degradation strength. The best result is obtained when the activation step is done at the end of the process together with the Al-neal thermal treatment, which results in SRV of 1.7 cm s(-1). The results correlate well with the measured interface defect density, indicating that the Al-neal affects defects at the Si/SiO x /Al2O3 interface. The root causes for the defect reactions are discussed and possible reasons for the observed phenomena are suggested.Peer reviewe

Passivation of Detector-Grade Float Zone Silicon with Atomic Layer Deposited Aluminum Oxide

Silicon radiation and particle detectors are traditionally passivated with thermal silicon dioxide. It has been shown that aluminum oxide (Al2O3) films provide better surface passivation due to their high negative charge, but studies on Al2O3 surface passivation are usually performed on low-resistivity substrates. Herein, the passivation of high-resistivity, detector-grade float zone silicon (FZ-Si) with Al2O3 is studied, with a specific emphasis on the effect of post-annealing temperature on carrier lifetimes and film properties. It is confirmed that Al2O3 provides excellent surface passivation also on high-resistivity FZ-Si substrates, with a low interface defect density of around (2-4) x 10(11) cm(-2) eV(-1) and a high negative oxide charge of 1 x 10(12) to 3 x 10(12) q cm(-2), when post-annealed at temperatures of up to 450-500 degrees C. In addition, high-resistivity samples are studied for the phenomenon of bulk lifetime degradation occurring at typical post-annealing or metal sintering temperatures, which has been reported for low-resistivity FZ-Si. At post-annealing temperatures of >500 degrees C, reduced bulk lifetimes are observed if the substrates did not receive high-temperature treatment prior to surface passivation. Furthermore, it is noticed that n-type samples exhibit lower bulk lifetimes even when a high-temperature treatment is performed, which indicates a connection between FZ-Si bulk lifetime degradation and doping type.Peer reviewe

Should thermostatted ring polymer molecular dynamics be used to calculate thermal reaction rates?

We apply Thermostatted Ring Polymer Molecular Dynamics (TRPMD), a recently proposed approximate quantum dynamics method, to the computation of thermal reaction rates. Its short-time transition-state theory limit is identical to rigorous quantum transition-state theory, and we find that its long-time limit is independent of the location of the dividing surface. TRPMD rate theory is then applied to one-dimensional model systems, the atom-diatom bimolecular reactions H + H2, D + MuH, and F + H2, and the prototypical polyatomic reaction H + CH4. Above the crossover temperature, the TRPMD rate is virtually invariant to the strength of the friction applied to the internal ring-polymer normal modes, and beneath the crossover temperature the TRPMD rate generally decreases with increasing friction, in agreement with the predictions of Kramers theory. We therefore find that TRPMD is approximately equal to, or less accurate than, ring polymer molecular dynamics for symmetric reactions, and for certain asymmetric systems and friction parameters closer to the quantum result, providing a basis for further assessment of the accuracy of this method.TJHH acknowledges a Research Fellowship from Jesus College, Cambridge, and helpful comments on the manuscript from Stuart Althorpe. YVS acknowledges support via the Newton International Alumni Scheme from the Royal Society. YVS also thanks the European Regional Development Fund and the Republic of Cyprus for support through the Research Promotion Foundation (Project Cy-Tera ΝΕΑ ΓΠΟΔΟΜΗ/ΣΤΡΑΤΗ/0308/31).This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.492859

Impact of doping and silicon substrate resistivity on the blistering of atomic-layer-deposited aluminium oxide

Aluminium oxide (Al2O3) thin films grown at low temperatures using atomic layer deposition (ALD) are known to often suffer from local delamination sites, referred to as "blisters", after post-deposition annealing during device processing. In this work, we report our observation that doping of the silicon substrate has an effect on blister formation. The introduction of a highly doped layer by diffusion or implantation is found to significantly reduce blistering, compared to the non-doped regions in the immediate vicinity. Similar behavior is observed for both phosphorus and boron doping. Further investigation of this phenomenon using substrates with different resistivities reveals that even when introduced already during silicon crystal growth, doping affects the blistering of aluminium oxide films. Changes in several properties of silicon affected by doping, most importantly surface terminating groups, native oxide growth, and passivation of defects with hydrogen, are discussed as potential reasons behind the observed effect on blistering.Peer reviewe

Cavitation-induced force transition in confined viscous liquids under traction

We perform traction experiments on simple liquids highly confined between parallel plates. At small separation rates, we observe a simple response corresponding to a convergent Poiseuille flow. Dramatic changes in the force response occur at high separation rates, with the appearance of a force plateau followed by an abrupt drop. By direct observation in the course of the experiment, we show that cavitation accounts for these features which are reminiscent of the utmost complex behavior of adhesive films under traction. Surprisingly enough, this is observed here in purely viscous fluids.Comment: Submitted to Physical Review Letters on May 31, 2002. Related informations on http://www.crpp.u-bordeaux.fr/tack.htm

Black ultra-thin crystalline silicon wafers reach the 4n2 absorption limit–application to IBC solar cells

Cutting costs by progressively decreasing substrate thickness is a common theme in the crystalline silicon photovoltaic industry for the last decades, since drastically thinner wafers would significantly reduce the substrate-related costs. In addition to the technological challenges concerning wafering and handling of razor-thin flexible wafers, a major bottleneck is to maintain high absorption in those thin wafers. For the latter, advanced light-trapping techniques become of paramount importance. Here we demonstrate that by applying state-of-the-art black-Si nanotexture produced by DRIE on thin uncommitted wafers, the maximum theoretical absorption (Yablonovitch's 4n2 absorption limit), that is, ideal light trapping, is reached with wafer thicknesses as low as 40, 20, and 10 µm when paired with a back reflector. Due to the achieved promising optical properties the results are implemented into an actual thin interdigitated back contacted solar cell. The proof-of-concept cell, encapsulated in glass, achieved a 16.4% efficiency with an JSC = 35 mA cm-2, representing a 43% improvement in output power with respect to the reference polished cell. These results demonstrate the vast potential of black silicon nanotexture in future extremely-thin silicon photovoltaics.Peer ReviewedPostprint (published version

Breakdown of smoothness for the Muskat problem

In this paper we show that there exist analytic initial data in the stable regime for the Muskat problem such that the solution turns to the unstable regime and later breaks down i.e. no longer belongs to $C^4$.Comment: 93 pages, 10 figures (6 added

Generic critical points of normal matrix ensembles

The evolution of the degenerate complex curve associated with the ensemble at a generic critical point is related to the finite time singularities of Laplacian Growth. It is shown that the scaling behavior at a critical point of singular geometry $x^3 \sim y^2$ is described by the first Painlev\'e transcendent. The regularization of the curve resulting from discretization is discussed.Comment: Based on a talk given at the conference on Random Matrices, Random Processes and Integrable Systems, CRM Montreal, June 200