The way conduction electrons respond to ultrafast external perturbations in low dimensional materials is at the core of the design of future devices for (opto)electronics, photodetection and spintronics. Highly charged ions provide a tool for probing the electronic response of solids to extremely strong electric fields localized down to nanometre-sized areas. With ion transmission times in the order of femtoseconds, we can directly probe the local electronic dynamics of an ultrathin foil on this timescale. Here we report on the ability of freestanding single layer graphene to provide tens of electrons for charge neutralization of a slow highly charged ion within a few femtoseconds. With values higher than 1012 A cm−2, the resulting local current density in graphene exceeds previously measured breakdown currents by three orders of magnitude. Surprisingly, the passing ion does not tear nanometre-sized holes into the single layer graphene. We use time-dependent density functional theory to gain insight into the multielectron dynamics.
Despite the improvement of the quality of CVD grown single-layer graphene on copper substrates, transferring the two-dimensional layer without introducing any unintentional defects still poses a challenge. While many approaches focus on optimizing the transfer itself or on necessary post-transfer cleaning steps, we have focused on developing a pre-treatment of the monolayer graphene on copper to improve the quality and reproducibility of the transfer process. By pressing an ethylene-vinyl acetate copolymer foil onto the monolayer graphene on copper using a commercially available vacuum bag sealer graphene is stabilized by the attachment of functional carbon groups. As a result, we are able to transfer graphene without the need of any supporting layer in an all-H2O wet-chemical transfer step. Despite the general belief that the crumbling of graphene without a support layer in a H2O environment is caused due to differences in surface energy, we will show that this assumption is false and that this behavior is caused rather by the polar interactions between graphene and water. Suppressing these interactions protects graphene from ripping and results in extremely clean, highly crystalline graphene with a coverage close to 100%.
We report on a study of ESR and conductivity on a series of hot-wire CVD microcrystalline silicon samples prepared with different hydrogen dilution of silane. We observe two different types of dangling bond defects in ESR in different microscopic environments. One type of defect is located at outer surfaces accessible to oxygen and/or chemicals, the other is located at inner boundaries presumably at columnar structures. We correlate changes of the defect density induced by either annealing, exposure to air or wet-chemical treatment with the morphology and electronic properties of the films. We find that annealing at 200 °C induces irreversible changes in donor concentration as monitored by an ESR signal at g = 1.9981±3.
We compare the electronic properties of nanocrystalline silicon from hot-wire chemical vapor deposition in a high-vacuum and an ultra-high-vacuum deposition system, employing W and Ta as filament material. From the constant photocurrent method we identify a band gap around 1.15 eV while, in contrast, a Tauc plot from optical transmission data guides to a wide band gap above 1.9 eV. The sudden change-over from nanocrystalline to amorphous structure in a hydrogen dilution series is also find in the dark and photoconductivity measurements. The samples show a metastability effect in the dark conductivity upon annealing in vacuum with an increase in the dark conductivity, with the large dark conductivity decreasing slowly after the annealing cycle when the cryostat is flushed with air. We identify larger values for the mobility-lifetime products, which corresponds to the smaller defect density shoulder in constant photocur- rent spectra, for the ultra-high-vacuum deposited material compared to the high-vacuun counterpart.
An ambitious neurotechnology program has been started in Germany in 1995, in the long run aiming at the realization of visual prostheses for blind people. A broad technological approach has been chosen which besides crystalline silicon microelectronics also involves amorphous silicon photodiodes for subretinal implantation, because thin film technology may be able to offer better solutions to some of the complex problems. Special topics we report on here include the development of low temperature deposition techniques for enabling the use of flexible plastic or bio-degradable substrate foils, the study of protective and bio-compatible coatings, as well as novel contacting and energy supply schemes. The key issue for stimulating retina cells by the use of technically generated photocurrents is an optimum capacitive coupling to these cells. For this purpose we study several contact layers (p-doped a-Si:H, microcrystalline Si, metal-induced crystallization) which provide high perpendicular but at the same time low lateral conductivity, thereby greatly reducing parasitic losses to the surrounding tissue. The photovoltaic mode of operation of the implanted photodiodes may be limited due to shortcomings in the charge transfer to the nerve cells, in which case additional infrared energy has to be coupled into the devices. Local light-induced stimulation can then be realised by using an a-Si:H i-layer as a photoresistor on top of the IR-sensitive solar cell.
Combining real-time ellipsometry and atomic force microscopy (AFM) the growth of hydrogenated amorphous silicon (a-Si:H), deposited on crystalline silicon wafers with a native oxide layer on top and on fused silica from a dc glow discharge, has been studied from initial nucleation to the final morphology. By in-situ ellipsometry we detected the evolution of morphology changes. The surface structure has been determined by AFM in the nucleation phase and in the subsequent growth stage. During nucleation on crystalline silicon only few (about 40 per 1μm) flat islands of a-Si:H (up to 4 nm high and up to 50 nm in diameter) grow with a strongly enhanced rate compared to bulk deposition. Once the crystalline surface has completely been covered by a-Si:H, the fast deposition of these islands stops and further surface structures, comparable with the initial ones, start to grow gradually until a homogeneous final roughness up to 5nm high is formed. Nucleation of a-Si:H on fused silica yields densely distributed nuclei (up to 1.5 nm high and up to 25 nm in diameter), indicating a shorter surface diffusion length on this substrate compared to the growth on silicon wafers. The ongoing film deposition, however, finally results in a morphology comparable to the one of a-Si:H grown on crystalline silicon. Using hydrogen dilution we found that the final roughness is affected by the dilution ratio; furthermore infrared spectroscopy reveals the surface structure to be correlated with the hydrogen content of the a-Si:H films.
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