Search

Silicon nanowires (SiNWs) hold potential applications in optoelectronics and SiNW-based optical sensors. Here, a photoluminescence study of SiNW arrays fabricated with a simple two-step silver (Ag) catalyzed etching of silicon wafers is presented. The morphology and photoluminescence properties were investigated for SiNWs of different lengths obtained by varying Ag concentration (as silver nitrate, AgNO 3 ) and etching time. The samples consist of vertically aligned SiNWs with length in the range ~10-40 µm. Our foremost result is that the photoluminescence intensity from the SiNW arrays is an order of magnitude higher than that from bulk Si. This is accompanied by a red-shift in the peak position of approximately 0.09 eV, which may be attributed either to the variation in size of the Ag-nanoparticles created during the etching process or different lengths of the SiNWs. The results indicate that SiNWs obtained by simple and cost-effective metal catalyzed etching, are potentially promising for optoelectronic applications.

We investigate the formation and evolution of Ga droplets over GaAs (001) substrate with and without Ge top layer (8 nm). Various morphological transformation from holes to droplets formation were observed as a function of annealing parameters. It was observed that the application of Ge layer plays a vital role in incorporating oxygen into the system. Moreover, a potential application of these structures as a photocathode is discussed, demonstrating increased photoemission.

We investigate the photoluminescence properties of structures comprising of Si1-xGex nanoparticles (NPs) within SiO2, GeO2, TiO2 and Ta2O5 oxide matrices. Of the investigated structures, it was observed that the structures with GeO2 and TiO2 matrices provide increased spectral response (at ~907 and 844 nm respectively) and increased PL intensity. The improved PL characteristic have been attributed to increased diffusion barrier against oxygen which otherwise would result in formation of unwanted oxide at the film-oxide interface, thereby deteriorating the optical properties., This work is partially funded by the Icelandic Research Fund Grants nos. 218029 and 196141., Pre-print (óritrýnt handrit)

The effect of room temperature hydrogen plasma treatment on the photoconductive properties of the SiO 2 matrix containing SiGe nanoparticles is investigated. A considerable increase in photocurrent intensity is observed after plasma treatment. The increase is partly attributed to neutralization of dangling bonds around the nanoparticles and partly to passivation of non-radiative centers and defects in the matrix and at the nanoparticles-matrix interfaces.

We studied the effect of short term furnace annealing over the photoconductive properties of tristacked layer i.e. TiO2/(SiGe/TiO2)3. The structure was prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A transmission electron microscopy and grazing incidence spectroscopy was used to analyze the morphology of the structure. Photoconductive properties were studied by measuring photocurrent spectra at different applied voltages and temperatures. Tristack layers were obtained with 5–10 nm SiGe nanocrystals (NCs) by annealing at 600 °C for 5 min. No sign of SiO2 formation was found inside stacked layers. A maximum in the photocurrent spectra was observed at 994 nm at 300 K but it red-shifted gradually to 1045 nm with decrease in temperature to 100 K. This transition in peak maxima is attributed to SiGe NCs, due to lattice vibration and to contribution of non-radiative recombination at low temperatures.

A molecular dynamics based code has been developed for high-fidelity simulations of nano- and microscale vacuum electronics. The force acting upon any electron in the system is calculated directly from the applied fields and Coulomb interaction with every other electron in the system and all of the image charge. This code captures scattering and other important discrete particle effects in proximity to the cathode better than Particle-in-Cell and fluid codes that are commonly used for simulation of electron beams. Electron emission is treated stochastically in a self-consistent manner with regard to spacecharge effects.

We describe a molecular dynamics based simulation code for high-fidelity models of electron emission and propagation in micro-diodes and near cathode regions. Some practical uses of the code are described.

The term “shielding” or “screening” is used to describe the effect on the local field enhancement factor of a high aspect ratio emitter, due to neighboring emitters in an ungated array1–4, as might be the case with an array of nanowires or carbon nanotubes. In this work, a related issue is examined; namely the effect on the local electric field at an emitter site, and subsequent influence on field emission from it, by space-charge due to current from neighboring emitters. Molecular dynamics based simulations of field emitted electrons in a vacuum microdiode5, 6 are used to examine how the beamlet coming from an emitter is affected by current from its neighbors. Continuous models are used to simulate larger emitter arrays where the MD based approach is not practical.

We describe the code used for high resolution molecular dynamics (MD) simulations of vacuum nanodiodes. The code includes full Coulomb interaction of electrons and offers several types of emission mechanisms with different geometries. It has been successfully used to study field emission and Child-Langmuir limited emissions.

The photosensing properties related to the structure of GeSi/TiO2 multilayers prepared under different conditions are studied. TiO 2 cap/(GeSi/TiO 2 ) 2 multilayers (ML) were deposited by magnetron sputtering (MS) and annealed by rapid thermal annealing. Trilayers of TiO 2 cap/GeSi/TiO 2 (TL) were also deposited using reactive high power impulse MS (HiPIMS) for TiO 2 layers and dc MS for the GeSi layer. For TL samples a two-step annealing was employed, one before and the second after depositing TiO 2 cap. Structure and morphology characterization (X-ray diffraction, scanning and transmission electron microscopy) was carried out and photocurrent measurements (voltage dependences, spectral curves) were performed. The annealed ML samples are formed of GeSi NCs with 5–10 nm sizes, while in the annealed TL samples, the GeSi NCs are larger (20–30 nm). These morphologies determine the multilayers photosensing properties in VIS-NIR of ML structures and in UV in TL ones, respectively.

We analyse theoretically the possibilities of contactless control of in-gap states formed by a pair of electrons confined in a triangular quantum ring. The in-gap states are corner-localized states associated with two electrons occupying the same corner area, and thus shifted to much higher energies than other corner states, but still they are below the energies of corner-side-localized states. We show how the energies, degeneracy and splittings between consecutive levels change with the orientation of an external electric field relatively to the polygonal cross section. We also show how absorption changes in the presence of external electric and magnetic fields., 4 pages, 2 figures

Beam emittance 1 has several contributing factors that have been studied in considerable detail. These include effects of temperature, surface structure, and space-charge 2. Taking into consideration typical values for spacing between neighboring electrons coming off the cathode, it is reasonable to expect that discrete particle effects, such as Rutherford scattering, Coulomb blockade, and discontinuous emission may be a contributing factor to emittance near the cathode surface.

High resolution molecular dynamics simulations with full Coulomb effect are used to investigate field emission in planar nanodiodes and from a prolate-spheroidal tip. The effects of space charge and emitter radius in a planar nanodiode are examined and compared to previous results concerning transition from Fowler-Nordheim to Child-Langmuir current1,2. Similarly, different geometries for the prolate-spheroidal tips are investigated to determine the influence of space-charge and Coulomb effects on emission current and beam quality from the tip. The Fowler-Nordheim law is used to determine the current density injected into the system and the Metropolis-Hastings algorithm to find a favourable point of emission on the emitter surface.

Summary form only given. Vacuum diodes with gap spacing of the order 1μm and gap voltages of the order 1V exhibit space-charge induced current modulation in the THz regime. The frequency can be tuned by simply altering the applied DC vacuum electric field, and the frequency range corresponding to the range of the applied field can be set by selection of the size of the emitter area on the cathode. The current in a microdiode of this type will typically range from tens to hundreds of microamperes, thus limiting the THz power that can be drawn from a single microdiode. Coupling between microdiodes is a problem of interest as it could offer an avenue to generate higher power levels by using an array of emitters. We present an overview of work done on vacuum microdiode oscillators, with some new results concerning beam quality in a single microdiode and interaction between beams from neighboring emitters.

Summary form only given. Numerical simulations with full representation of Coulomb effects in space-charge dominated photoemission in vacuum microdiodes indicate the presence of a mechanism leading to the formation of well-defined electron bunches corresponding to THz frequency1. We present the results of survey that examines the effects of such parameters as gap-spacing, diode voltage and electron emission velocity on the structure of the beam, e.g. modulation frequency, debunching and current density.