Mino Green

Structured Silicon Anodes for Lithium Battery Applications

Pillar arrays fabricated on silicon substrates have been tested as potential anodes for lithium batteries. Electrodes of array characteristics, diameter 580 ′ 150 nm: fractional surface coverage 0.34: height 810 nm are reported here. Cyclic voltammetry (CV) and cyclic galvanostatic tests of alloying/dealloying of electrochemically produced lithium with silicon were carried out, and results correlated with SEM studies. Aerial current densities in the low and fractional mA cm - 2 , and voltage 25 mV to 2 V (vs. L/Li + ) were used. CV features correspond to various Zintl phase compounds (ZPCs). Structured electrodes of Si pillars maintained their structural integrity throughout cycling; planar Si electrodes showed cracks (2 μm features) after 50 cycles. A model is advanced in which lithium diffuses through a layer of ZPC to react with Si: growing ZPCs plastically deforms where necessary. Upon lithium dealloying vacancies coalesce to form voids at the ZPC/Si interface, Si rejoins the substrate. or precipitates out as a nanocrystalline material, and the voids appear as a fine pattern of cracks, looking like dried mud. The extra surface area that a pillar structure can confer on Si electrodes is essential and makes it practical to consider the possible eventual use of such anodes in integrated battery structure;.

Space Charge Calculations for Semiconductors

The electric field at the surface of a semiconductor is obtained as a function of the semiconductor bulk properties and the potential difference across the space charge region. The treatment is general enough to take into account degenerate free carrier distributions and partial ionization of impurities, either in the neutral bulk or the space charge region, or both. A one‐dimensional model with a spacially homogeneous impurity distribution is assumed. The density of states in the conduction and valence bands is assumed to be that appropriate for spherical and ellipsoidal energy surfaces. The common model for simple donor and acceptor states is used in the main body of the paper. Conditions under which the equation for the field can be conveniently simplified are discussed, and a particular case is treated numerically.The validity of the free carrier distribution functions in the high fields near the semiconductor surface is discussed in Appendix I. The equation for the electric field at the semiconductor...

Fabrication of buried channel waveguides on silicon substrates using spin-on glass.

A new process for the deposition of thick (≈10-µm) films of silica and titania-doped silica on silicon substrates is described. Films are built up by repetitive operation of a simple process cycle in which a layer of sol-gel material is deposited by spin coating, then densified by rapid thermal annealing. Stressfree layers are obtained through careful choice of the anneal temperature. Bilayer structures suitable for waveguide fabrication may also be constructed by performing two successive deposition runs using sol-gel precursors with different titania concentrations. These bilayers may be patterned topographically into ridges by using reactive ion etching, and the ridges may be planarized by applying additional layers of sol-gel material to form buried channel waveguides.

SERS platforms for high density DNA arrays

Surface Enhanced Raman Scattering (SERS) gives rise to analytical applications with much promise. In our approach three steps are necessary. We require a SERS platform of high enhancement. This has been achieved using the special technique of Island Lithography, combined with Ag deposition by galvanic exchange, yielding an enhancement factor of 10(8). Probe oligonucleotide molecules are attached to a specific area on the platform, at the optimized surface concentration, using thiolated single stranded (ss) DNA molecules. The optimum surface concentration has been determined and interpreted in the light of the polyelectrolyte behaviour of ssDNA. Finally the change in SERS produced by hybridisation of the probe molecules to a target DNA molecule is measured. Highly discernible changes have been obtained. No change in probe signal is seen when presented with one base mismatched target. From this work it is concluded that the prospects for label-free DNA detection in high-density arrays is now close to achievement.

Effects of H2O on structure of acid-catalysed SiO2 sol-gel films

Abstract Thin silica films were deposited on silicon wafers by the sol-gel technique, using spin coating. The sols were prepared by HCl catalysis of tetraethylorthosilicate (TEOS) diluted in ethanol, using different molar ratios, R , of H 2 O:TEOS. The films were then baked at various temperatures, and characterised using ellipsometry, profilometry, optical scattering and infrared spectroscopy. It was found that the thickness, shrinkage, porosity and pore sizes all decrease with increasing R . It was also found that high water levels yield films of higher homogeneity and finer texture, and less tensile stress.

Optical properties of dilute hydrogen tungsten bronze thin films

Transmission and reflection spectra in the range of 0.5–4.1 eV have been obtained for HxWO3 polycrystalline thin films of 5‐nm mean grain size. The composition range x=0–0.16 has been investigated. The measurements were refined to give the absorption coefficient and other optical parameters versus photon energy. The data have been interpreted in terms of the interband transition model due to Green and Travlos [Philos. Mag. B 51, 501 (1985)]. It is noted that main gap changes because of changes in crystal structure, and there is a sensitivity of the main gap value to grain size which may have its origins in quantum confinement effects.

Electrochemistry of the Semiconductor‐Electrolyte Electrode. I. The Electrical Double Layer

The thermodynamics of the semiconductor‐electrolyte electrode (S/C—E electrode) are examined. The following is concluded about the distribution of the p.d. (potential difference) due to excess chargeV in the interphase (assuming the dipole distribution to be invariant with semiconductor electron concentration): (i) For a semiconductor, with a nondegenerate free carrier distribution throughout and no surface states, in contact with a concentrated electrolyte solution (such that the Gouy layer p.d.→0), most of V occurs within the semiconductor phase and the p.d. across the compact Helmholtz layer is small (millivolts). Changes in p.d. across the S/C—E electrode caused by an externally applied p.d. will occur mainly in the S/C phase (if the electrolyte is concentrated); (ii) For a S/C—E electrode with a high concentration of ionized surface states of one kind, p.d. caused by excess charge in the Helmholtz layer, H will be large (∼1 volt) and the electrode behaves like a metalelectrode. The Helmholtz layer p.d., H, is found to be essentially independent of bulk S/C electron concentration. An applied p.d. is mainly manifest across the Helmholtz layer (when the electrolyte solution is concentrated).

Mesoscopic hemisphere arrays for use as resist in solid state structure fabrication

The kinetics of formation of disordered arrays of hemispherically shaped islands of cesium chloride on an oxidized silicon wafer, starting from a deposited thin film of CsCl, has been investigated. The essential process step is the exposure of the CsCl thin film to water vapor. Transport of dissolved material in the adsorbed water layer occurs, driven by the size dependent solubility of CsCl. Growth rate is dominated by the kinetics of dissolution and deposition at the solid/solution boundary, rather than by interisland diffusion. The equipment and procedure are arranged so that constant humidity conditions prevail at every moment of hemisphere development. The process variables were: the effective thickness of the CsCl film, the relative humidity and the exposure time. The nature of the solid surface was found to be important and it was monitored using contact angle measurements of the water/solid interface: hydrophilic surfaces were used. The resulting arrays of hemispheres are characterized by a mean d...

Structural Fabrication Using Cesium Chloride Island Arrays as a Resist in a Fluorocarbon Reactive Ion Etching Plasma

Arrays of hemispherical islands of cesium chloride are formed on an oxidized silicon wafer when a deposited thin film of CsCl is subsequently exposed to water vapor. High packing densities, P, and a wide, preselectable range of mean diameter, , can be obtained. Use of such arrays for nanoscale lithography is demonstrated. Silicon pillars and cones have been fabricated with CsCl used as resist in a fluorocarbon reactive ion etching scheme. Cones of tip angle 28° and P = 0.18 and = 85 ± 20 nm were made. The wall angle as a function of etching parameters was determined. CsCl arrays were used in a negative resist strategy, by coating the substrate and the hemispheres in aluminum and then lifting off the CsCl (with their Al coating). Wells were made in the exposed SiO2 on Si with = 920 ± 150 nm with a packing density of P = 35. Applications to field emission systems are noted.

Quantum pillar structures on n+ gallium arsenide fabricated using ‘‘natural’’ lithography

Random arrays of CsCl hemispherically shaped islands with average diameters as small as 500 A have been made on n+GaAs substrates. The CsCl behaves as a resist of high selectivity when the GaAs is reactively ion etched in a BCl3 plasma. The resulting structure is a set of pillars all the same height, but with varying diameters, typically ±15% of the average value, 〈D〉. Typical pillar packing density, S, is 20%. Photoluminescence (PL) studies were made at 10 K on n+ and semi‐insulating GaAs using 514.5 nm exciting radiation. PL spectra from n+GaAs structures of 〈D〉=520±78 A, h=800 A, S=20%; 〈D〉=610±97 A, h=450 A, S=12%; 〈D〉=1215±210 A, h=600 A, S=20%, as well as a plane surface, are reported. There is a shift in the peak value of the PL curves with respect to the planar structure of 26, 13, and 2.5 meV, respectively. It may be possible to understand these shifts in terms of quantum confinement effects if the effective pillar size is reduced by surface space charge effects.