About 100 fragments of Roman mosaic and millefiori glass were stylistically attributed to a Hellenistic type, a Ptolemaic and Romano-Egyptian period type and an early imperial period type. Twelve representative fragments were studied by electron microprobe analysis and Raman microspectroscopy. Eleven of them display a Napronounced recipe with low K, Mg and P contents, typical for the Roman period. Minor differences in composition are unsystematic, not reflecting the stylistic classification. Ionic colouring agents are Mn 3+ for violet, Cu 2+ for light blue, Co 2+ for deep blue and Fe 3+ for brown translucent colours. Calcium antimonates, lead antimonate and cuprite are the colourants responsible for white, yellow and red colours, respectively, and additionally serve as opacifiers. Mixing of ionic colouring agents and opacifying colourants led to a more differentiated palette of colours. Pb was used as yellow colouring agent, as a flux material and as a stabiliser for the colourant crystals. The remaining fragment consisting of a K-pronounced but still Na-bearing glass matrix was most likely produced during the Middle Ages or later.

This paper reports the growth of silicon nanocrystals (SiNCs) from SiH4-O-2 plasma chemistry. The formation of an oxynitride was avoided by using O-2 instead of the widely used N2O as precursor. X-ray photoelectron spectroscopy is used to prove the absence of nitrogen in the layers and determine the film stoichiometry. It is shown that the Si rich film growth is achieved via nonequilibrium deposition that resembles a interphase clusters mixture model. Photoluminescence and Fourier transformed infrared spectroscopy are used to monitor the formation process of the SiNCs, to reveal that the phase separation is completed at lower temperatures as for SiNCs based on oxynitrides. Additionally, transmission electron microscopy proves that the SiNC sizes are well controllable by superlattice configuration, and as a result, the optical emission band of the Si nanocrystal can be tuned over a wide range

Superlattices of Si-rich silicon nitride and Si 3 N 4 are prepared by plasma-enhanced chemical vapor deposition and, subsequently, annealed at 1150 C to form size-controlled Si nanocrystals (Si NCs) embedded in amorphous Si 3 N 4 . Despite well defined structural properties, photoluminescence spectroscopy (PL) reveals inconsistencies with the typically applied model of quantum confined excitons in nitride-embedded Si NCs. Time-resolved PL measurements demonstrate 10 5 times faster time-constants than typical for the indirect band structure of Si NCs. Furthermore, a pure Si 3 N 4 reference sample exhibits a similar PL peak as the Si NC samples. The origin of this luminescence is discussed in detail on the basis of radiative defects and Si 3 N 4 band tail states in combination with optical absorption measurements. The apparent absence of PL from the Si NCs is explained conclusively using electron spin resonance data from the Si/Si 3 N 4 interface defect literature. In addition, the role of Si 3 N 4 valence band tail states as potential hole traps is discussed. Most strikingly, the PL peak blueshift with decreasing NC size, which is often observed in literature and typically attributed to quantum confinement (QC), is identified as optical artifact by transfer matrix method simulations of the PL spectra. Finally, criteria for a critical examination of a potential QC-related origin of the PL from Si 3 N 4 -embedded Si NCs are suggested. V C 2014 AIP Publishing LLC. [http://dx.

For future 4-junction Ge based multi-junction solar cells, the current generated in the Ge sub-cell gets very important. We developed an efficient rear-side passivation stack, which results in minority carrier lifetimes (τ eff ) ≈ 200 µs and investigated its performance in an accelerated aging experiment (1 MeV electron irradiation). The aging caused a strong lifetime decrease down to τ eff = 4 µs, whereas the carrier mobility stayed constant. These experimental values provide the basis for Beginning-of-Life and End-of-Life solar cell simulations, which show that the potential of the rear-side passivation for 3-junction solar-cell performance is limited, but very promising for 4junction solar cells.

Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of Si tandem solar cells since their bandgap exceeds that of bulk silicon and can be tuned by adjusting nanocrystal size. Due to this effect, size control is required to maintain a uniform bandgap throughout a Si NC film. This can be achieved by annealing superlattices of Si-rich and stoichiometric dielectrics. This paper reviews the progress that has been made using host matrices SiO2, Si3N4, and SiC. Si NCs in SiO2 show excellent NC size and shape control and strong quantum-confinement-related photoluminescence, however electrical conductivity is poor. Ordering and size control is also obtained in Si3N4, but conclusive evidence of quantum confinement is lacking. Preparing ordered but separated Si NCs in SiC is difficult, but the narrow parameter space in which this is possible has been elucidated, good electrical conductivity was obtained, and functioning single-junction and tandem cells have been produced. Si NC formation can now be well-controlled in all three materials, and the key weaknesses for photovoltaics have been identified to be the electrical conductivity of SiO2, and defect density for Si3N4 and SiC. Addressing these is expected to lead to competitive Si tandem solar cells

Charge transport in nanocrystalline SiC with and without embedded Si nanocrystals (Si NCs) prepared by annealing of a−Si1−xCx:H precursors is studied using temperature-dependent current-voltage measurements supported by electron spin resonance and mass spectrometry data. Transport is Ohmic in all films at all temperatures and the temperature dependence of conductivity shows that the materials behave as disordered semiconductors, exhibiting extended-state transport at high temperature and variable-range hopping transport at low temperature. Grain-boundary-, surface-, and interface-dominated transport is systematically ruled out. Films are n type, and films with Si NCs exhibit up to 103 times higher conductivity (up to 0.1Scm−1) after exposure to a hydrogen plasma which passivates dangling bonds, particularly on Si NCs. A forming gas anneal does not have such an effect, indicating that atomic rather than molecular hydrogen is required. The conductivity of SiC films without Si NCs is largely unchanged by passivation and the Fermi level is not raised nearly as closely to the conduction band. This is attributed to a type I band offset between Si NCs and SiC that leads to extended-state conduction in films with Si NCs taking place in a Si network. This is confirmed by the dependence of the extended-state mobility on the volume fraction of excess Si. Variable-range hopping is relatively insensitive to the presence of excess Si and is hence considered to take place via shallow defect states throughout the volume of the films. The high conductivities are found to be a consequence of background doping by oxygen and nitrogen

Silicon nanocrystals formed in the annealed SiNx/Si3N4 superlattices are attractive for research due to the smaller band offsets of Si3N4 matrix to Si in comparison with commonly used SiOx/SiO2 superlattices. However, the annealed SiNx/Si3N4 structures contain an increased number of nanocrystal interface defects, which completely suppress nanocrystal emission spectrum. In this work, we study a novel SiOxNy/Si3N4 hetero multilayer combination, which compromises the major issues of SiOx/SiO2 and SiNx/Si3N4 superlattices. The annealed SiOxNy/Si3N4 superlattices are investigated by TEM, demonstrating a precise sublayer thicknesses control. The PL spectra of the annealed SiOxNy/Si3N4 superlattices are centered at 845–950 nm with an expected PL peak shift for silicon nanocrystals of different sizes albeit the PL intensity is drastically reduced as compared to SiO2 separation barriers. The comparison of PL spectra of annealed SiOxNy/Si3N4 superlattice with those of SiOxNy/SiO2 superlattice enables the analysis of the interface quality of silicon nanocrystals. Using the literature data, the number of the interface defects and their distribution on the nanocrystal facets are estimated. Finally, it is shown that the increase of the Si3N4 barrier thickness leads to the increased energy transfer from the Si nanocrystals into the Si3N4 matrix, which explains an additional drop of the nanocrystal PL intensity

The outstanding demonstration of quantum confinement in Si nanocrystals (Si NC) in a SiC matrix requires the fabrication of Si NC with a narrow size distribution. It is understood without controversy that this fabrication is a difficult exercise and that a multilayer (ML) structure is suitable for such fabrication only in a narrow parameter range. This parameter range is sought by varying both the stoichiometric SiC barrier thickness and the Si-rich SiC well thickness between 3 and 9 nm and comparing them to single layers (SL). The samples processed for this investigation were deposited by plasma-enhanced chemical vapor deposition (PECVD) and subsequently subjected to thermal annealing at 1000-1100 degrees C for crystal formation. Bulk information about the entire sample area and depth were obtained by structural and optical characterization methods: information about the mean Si NC size was determined from grazing incidence X-ray diffraction (GIXRD) measurements. Fourier-transform infrared spectroscopy (FTIR) was applied to gain insight into the structure of the Si-C network, and spectrophotometry measurements were performed to investigate the absorption coefficient and to estimate the bandgap E-04. All measurements showed that the influence of the ML structure on the Si NC size, on the Si-C network and on the absorption properties is subordinate to the influence of the overall Si content in the samples, which we identified as the key parameter for the structural and optical properties. We attribute this behavior to interdiffusion of the barrier and well layers. Because the produced Si NC are within the target size range of 2-4 nm for all layer thickness variations, we propose to use the Si content to adjust the Si NC size in future experiments