Intrinsic Point Defects in a-SiO2 with Embedded Si Nanoparticles Probed by ESR.

Using the electron spin resonance (ESR) technique as basic experimentaltool, this PhD thesis is concerned with an elaborate study of, mostly native, point defects in different kinds of structures consisting of nanocrystalline-Si particles embedded in an a-SiO2 matrix. This includes superstructures manufactured through thermal treatment of the as deposited SiO/SiO2 multilayers on Si substrates, and Si+ implanted thermal SiO2 assisted by in situ ultrasound treatment. Additionally, the efforts given to trace the true atomic nature of the illusive E δ defect in a-SiO2 can be seen along these lines. We monitored and characterized the different varieties of point defects in those structures both in the as-produced state as well as after different treatments such as annealing in different ambient gases (N­2, Ar, room ambient, H2) and pre- or post-anneal irradiations (UV, VUV, and γ-rays) to alter the paramagnetic point defect densities, that is, lowering them in the case of nc-Si's but increasing them in the case of E δ defect investigation.As tonc-Si/SiO2 superstructures, an important result attained by combining extensive ESR and photolumenscence (PL) spectroscopies in combination with passivation and depassivation treatments of paramagnetic defects, is the demonstration of the possibility of switching from defect related PL to quantum confinement originating PL and vice versa in the same sample.The superb microscopic Si/SiO2 properties are retained down to the nanoscale . The interfacial Pb(0) type center is traced as main PL quenching center.In a next part, the ESR technique has first been applied to asses the effects of in situ ultrasound treatment (UST) during Si+ ionimplantation of SiO2 on true atomic level, viz., probing of point defects and their behavior over various treatments. This reveals an impressive healing effect of in situ UST, now quantified in detail on atomic level. The effect is retained even after high-T anneal, resulting in the removal of virtually all ESR active centers .A final part intends to asses the atomic structure of the irradiation induced (VUV or γ-ray) E δ point defect in a-SiO2, mainly targeted through focusing on the correct determination of the relative hf structure intensityover various microwave frequencies and a broad temperature range. The results are conclusive in that they exclude the Si dimer (Si­2) atomic model advocated by theory, while definitively establishing the unpaired electron to be in interaction with 4 (5) Si atom sites a conclusion not supported by theory. It reveals a hiat in the current understanding of this illusive point defect, though assumedly classified within the single O-vacancy type point defects in SiO2 (E family).status: publishe

A 28-nm Coarse Grain 2D-Reconfigurable Array with Data Forwarding

To answer the ever-increasing demand for computational power, coarse-grain reconfigurable architectures (CGRAs) experience a revival. As they provide efficient, dedicated datapaths to match the dataflow graphs of a wide range of applications, CGRAs successfully manage high performance tasks. Yet, modern CGRAs access data through classical memory hierarchies or global scratchpad memories, failing to take advantage of data locality. This reveals memory bandwidth as a key bottleneck for these reconfigurable architectures. This letter introduces a heterogeneous 2-D CGRA which distributes data buffers across its computational grid. Together with a single-cycle data forwarding path, this allows the CGRA to better take advantage of data locality, in order to minimize data transfers. A 28-nm CMOS instantiation of this concept realizes the mapping and execution of a variety of compute intensive kernels, such as a real-time 512-point FFT or $5\times 5$ convolutional filter at high power efficiency. The CGRA demonstrates a peak energy efficiency of 584.9 GOPS/W during a 103-tap FIR filter, marking a $2.9\times$ improvement over state-of-the-art 2-D CGRAs

Influence of the Supramolecular Organization on the Magnetic Properties of Poly(3-alkylthiophene)s in Their Neutral State

The influence of the supramolecular behavior on the magnetic properties of head-to-tail coupled poly(3-alkylthiophene)s (P3AT) is reported. Samples with a different molar mass were prepared, and differences in their behavior were illustrated by UV-vis and CD spectroscopy. In addition, tuning of the supramolecular organization was also accomplished by applying thermal annealing. ESR spectroscopy was performed on powder samples. From the line features, information on the nature of the unpaired spins, the mobility, and the supramolecular order of the sample was derived. These spins are paramagnetic. SQUID magnetometry, in contrast, shows for all samples a ferromagnetic behavior at 5 K and superparamagnetism at 300 K. The coercivity does not depend on the degree of supramolecular order but is inherent to the molecular structure and, related to this, the a-interactions. The saturation magnetization, on the contrary, seems to be mainly influenced by the fraction of planarized polymer chains.status: publishe

Magnetic Properties of Substituted Poly(thiophene)s in Their Neutral State

The magnetic behavior of undoped (neutral), substituted poly(thiophene)s is reported. In particular, the influence of the nature of the substituent (alkyl, alkoxy, thioalkyl), the substitution pattern (head-to-tail (HT) versus head-to-head-tail-to-tail (HH-TT)), and the regioregularity oil the magnetic properties has been investigated. ESR spectroscopy reveals that the nature of the substituent determines the spin density, while the line width and asymmetry of the ESR signals are mainly governed by the substitution pattern and regioregularity. The spins give rise to a paramagnetic behavior. SQUID magnetometry reveals the presence of superparamagnetic order at room temperature, while ferromagnetism is observed at 5 K. The magnetic behavior observed by SQUID magnetometry does not (solely) originate front the ESR-active spin system. Its strength does therefore not depend on the ESR spin density, but seems to be governed by the supramolecular structure.status: publishe

Implantation and activation of phosphorus in amorphous and crystalline germanium layers

We have investigated phosphorus implantation and activation in amorphous and crystalline Ge layers, deposited on Si substrates. The structure of the Ge layer has only limited influence on the dopant profile and diffusion after annealing. Surprisingly, crystalline Ge layers show better electrical results after implantation and dopant activation. For the amorphous layer, the solid phase epitaxy process is influenced in the neighborhood of P, leading to point defects, which inhibit electrical activation. This result implies that when a crystalline Ge layer is amorphized during implantation of high doses, the dopant activation can be significantly reduced. Reduced temperature ramping improves activation of P in amorphous Ge layers.status: publishe

Synthesis and electron spin resonance of La2/3Ca1/3MnO3 nanowires

La2/3Ca1/3MnO3 (LCMO) nanowires were prepared using a sol-gel process and an anodically oxidized aluminum template. Scanning electron microscopy (SEM) shows that the nanowires are almost parallel and their diameter is around 40 nm. The temperature dependence of the electron spin resonance (ESR) linewidth is similar to that observed for the case of nanostructured LCMO. From this dependence, the determined magnetic transition temperature, T-C, of the LCMO nanowires, is of the order of 120-130 K.status: publishe

Effect of heat-treatment on luminescence and structure of Ag nanoclusters doped oxyfluoride glasses and implication for fiber drawing

The effect of heat treatment on the structure and luminescence of Ag nanoclusters doped oxyfluoride glasses was studied and the implication for drawing the corresponding fibers doped with luminescent Ag nanoclusters has been proposed. The heat treatment results, first, in condensation of the Ag nanoclusters into larger Ag nanoparticles and loss of Ag luminescence, and further heat treatment results in precipitation of a luminescent-loss nano- and microcrystalline Ag phases onto the surface of the glass. Thus, the oxyfluoride fiber doped with luminescent Ag nanoclusters was pulled from the viscous glass melt and its attenuation loss was 0.19 dB/cm in the red part of the spectrum; i.e. near to the maximum of Ag nanoclusters luminescence band. The nucleation centers for the Ag nanoclusters in oxyfluoride glasses have been suggested to be the fluorine vacancies and their nanoclusters.status: publishe