Characterisation of anisotropic etching in KOH using network etch rate function model: influence of an applied potential in terms of microscopic properties

Using the network etch rate function model, the anisotropic etch rate of p-type single crystal silicon was characterised in terms of microscopic properties including step velocity, step and terrace roughening. The anisotropic etch rate data needed have been obtained using a combination of 2 wagon wheel patterns on different substrate and 1 offset trench pattern. Using this procedure the influence of an applied potential has been investigated in terms of microscopic properties. Model parameter trends show a good correlation with chemical/electrochemical reaction mechanism and mono- and dihydride terminated steps reactivity difference. Results also indicate a minimum in (111) terrace roughening which results in a peak in anisotropic ratio at the non-OCP applied potential of −1250 mV vs OCP

Development of mirror coatings for gravitational-wave detectors

Gravitational waves are detected by measuring length changes between mirrors in the arms of kilometrelong Michelson interferometers. Brownian thermal noise arising from thermal vibrations of the mirrors can limit the sensitivity to distance changes between the mirrors, and, therefore, the ability to measure gravitational-wave signals. Thermal noise arising from the highly reflective mirror coatings will limit the sensitivity both of current detectors (when they reach design performance) and of planned future detectors. Therefore, the development of coatings with low thermal noise, which at the same time meet strict optical requirements, is of great importance. This article gives an overview of the current status of coatings and of the different approaches for coating improvement. This article is part of a discussion meeting issue ‘The promises of gravitational-wave astronomy’

Ab Initio studies of the atomic structure and electronic density of states of pure and hydrogenated a-Si

We propose a method to simulate a-Si and a-Si:H using an ab initio approach based on the Harris functional and thermally amorphisized periodically continued cells with at least 64 atoms, and calculate their radial distribution functions. Hydrogen incorporation was achieved via diffusive random addition. The electronic density of states (DOS) is obtained using density functional theory with the aid of both the Harris-functional and Kohn-Sham-LDA approaches. Two time steps are used, 2.44 and 10 fs for the pure, and 0.46 and 2 fs for the hydrogenated, to see their effect on the topological and DOS structure of the samples. The calculated long time-step radial features of a-Si are in very good agreement with experiment whereas for a-Si:H the short time-step partial and total radial features agree well; for the long time-step simulation molecular hydrogen appears during annealing.The long time-step a-Si has a well defined gap with two dangling bonds, that clears and increases upon hydrogen addition and relaxation, as expected. The short time-step structures have more defects, both dangling and floating bonds, that are less characteristic of a good sample; however the radial structures of a-Si:H are in better agreement with experiment indicating that the experimental work was done on defective samples.Comment: 11 pages, RevTeX, 16 figures, submitted to Phys. Rev. B 16 June 200

One-Pot Synthesis of Single-Source Precursors for Nanocrystalline LED Phosphors M2Si5N8:Eu2+ (M = Sr, Ba)

Highly efficient red-emitting nitridosilicate phosphors Sr2Si5N8:Eu2+ and Ba1.5Sr0.5Si5N8:Eu2+ (doping level 1%) applicable to phosphor converted pc-LEDs were synthesized in nanocrystalline form at low temperatures employing a novel single-source precursor approach. Synthesis starts from nanocrystalline silicon and uses mixed metal amides M(NH2)2 with M = Sr, Ba, Eu as reactive intermediates. In a second approach, a single-source precursor mixture obtained from a one-pot reaction of the corresponding elements (Sr/Ba, Eu, Si) was obtained in supercritical ammonia. Thermoanalytical in situ investigations gain a deeper insight into the degradation mechanism of the mixed metal amide precursors and revealed the onset for the formation of the 2-5-8 phosphor materials at temperatures slightly above 900°C. Formation of the products is complete below 1400°C. Under these conditions, the nitridosilicate phosphors form spherically shaped particles with crystallites of 200 nm in size. Spherical particles are desirable for phosphor application because light extraction may be improved by decreased light trapping and re-absorption losses. As a major advantage of the one-pot precursor approach, the exact Sr/Ba content in the solid solution series Sr2−xBaxSi2N8:Eu2+ and the doping concentration of Eu2+ can easily be controlled in a wide range by the relative amount of the elemental starting materials (Sr, Ba, Eu, Si). Simultaneously, thorough mixing of these elements down to an atomic level (Sr, Ba, Eu) or at least at nanoscopic dimensions (silicon) is achieved by the solution approach. As a consequence, no milling and pre-reaction steps are necessary which might give rise to contamination. Advantageously, this approach can easily be extended to large-scale processes by simultaneously preserving complete mixing. Furthermore, the influence of the starting materials (single-source precursor, nanocrystalline silicon) and the reaction conditions on the crystal shape and finally on the luminescence properties of the products was investigated. The obtained nanophosphors exhibit luminescence properties comparable to coarsely crystalline nitridosilicate phosphor powders prepared by conventional high-temperature processing

Inclusion of Experimental Information in First Principles Modeling of Materials

We propose a novel approach to model amorphous materials using a first principles density functional method while simultaneously enforcing agreement with selected experimental data. We illustrate our method with applications to amorphous silicon and glassy GeSe$_2$. The structural, vibrational and electronic properties of the models are found to be in agreement with experimental results. The method is general and can be extended to other complex materials.Comment: 11 pages, 8 PostScript figures, submitted to J. Phys.: Condens. Matter in honor of Mike Thorpe's 60th birthda

Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics

We review our recent efforts in building atom-scale quantum-dot cellular automata circuits on a silicon surface. Our building block consists of silicon dangling bond on a H-Si(001) surface, which has been shown to act as a quantum dot. First the fabrication, experimental imaging, and charging character of the dangling bond are discussed. We then show how precise assemblies of such dots can be created to form artificial molecules. Such complex structures can be used as systems with custom optical properties, circuit elements for quantum-dot cellular automata, and quantum computing. Considerations on macro-to-atom connections are discussed.Comment: 28 pages, 19 figure

Optical Absorption Measurement at 1550 nm on a Highly-Reflective Si/SiO2_2 Coating Stack

Future laser-interferometric gravitational wave detectors (GWDs) will potentially employ test mass mirrors from crystalline silicon and a laser wavelength of $1550\,\rm{nm}$, which corresponds to a photon energy below the silicon bandgap. Silicon might also be an attractive high-refractive index material for the dielectric mirror coatings. Films of amorphous silicon (a-Si), however, have been found to be significantly more absorptive at $1550\,\rm{nm}$ than crystalline silicon (c-Si). Here, we investigate the optical absorption of a Si/SiO$_2$ dielectric coating produced with the ion plating technique. The ion plating technique is distinct from the standard state-of-the-art ion beam sputtering technique since it uses a higher processing temperature of about 250$^\circ$C, higher particle energies, and generally results in higher refractive indices of the deposited films. Our coating stack was fabricated for a reflectivity of $R=99.95\,\%$ for s-polarized light at $1550\,\rm{nm}$ and for an angle of incidence of 44$^\circ$. We used the photothermal self-phase modulation technique to measure the coating absorption in s-polarization and p-polarization. We obtained $\alpha^{\rm coat}_{s}=(1035 \pm 42)\,\rm{ppm}$ and $\alpha^{\rm coat}_{p}=(1428 \pm 97)\,\rm{ppm}$. These results correspond to an absorption coefficient which is lower than literature values for a-Si which vary from $100\,\rm{/cm}$ up to $2000\,\rm{/cm}$. It is, however, still orders of magnitude higher than expected for c-Si and thus still too high for GWD applications