Steve Arscott

Terahertz frequency difference from vertically integrated low-temperature-grown GaAs photodetector

We report on the development of a photoconductive detector based on low-temperature-grown GaAs which is vertically integrated with terahertz spiral antennas. A non steady-state velocity overshoot effect was expected in the photoresponse with a responsivity of 0.04 A/W at a bias voltage of 8 V. Photomixing experiments using two optical 0.8 μm beating lasers show a 3 dB bandwith of 700 GHz with a radiation power at terahertz frequency of 0.5 μW under 2×30u2009mW optical pumping.

Miniaturized microDMFC using silicon microsystems techniques: performances at low fuel flow rates

This paper reports the design, fabrication and characterization of high performance miniaturized micro direct methanol fuel cells (microDMFC) functioning at room temperature under a forced low input fuel flow rate (<10 µL min−1) fabricated using silicon microsystems techniques. A room temperature maximum power output of 12.5 mW cm−2 has been measured at a fuel flow rate of 5.52 µL min−1 for a fuel cell surface area as small as 0.3 cm2 (corresponding to a fuel use efficiency of 14.1% at 300 K). At a lower flow rate of 1.38 µL min−1, the fuel use efficiency rises to 20.1% although the power density falls to 4.3 mW cm−2. The study revealed that improved room temperature cell performances in terms of power density can be achieved at low flow rates (<10 µL min−1) by (i) reducing the fuel cell area and (ii) reducing the microchannel cross-section. The study also revealed that higher fuel use efficiencies are obtained at lower fuel flow rates. Fuel (methanol) for the anode and an oxidant (air) for the cathode are supplied via a compact serpentine network of micron-size microfluidic and gas microchannels; by using silicon microsystems techniques we also render the fuel cell compatible with other silicon technologies such as microelectronics and micro- and nanoelectromechanical systems (MEMS/NEMS).

Controlled mud-crack patterning and self-organized cracking of polydimethylsiloxane elastomer surfaces

Exploiting pattern formation – such as that observed in nature – in the context of micro/nanotechnology could have great benefits if coupled with the traditional top-down lithographic approach. Here, we demonstrate an original and simple method to produce unique, localized and controllable self-organised patterns on elastomeric films. A thin, brittle silica-like crust is formed on the surface of polydimethylsiloxane (PDMS) using oxygen plasma. This crust is subsequently cracked via the deposition of a thin metal film – having residual tensile stress. The density of the mud-crack patterns depends on the plasma dose and on the metal thickness. The mud-crack patterning can be controlled depending on the thickness and shape of the metallization – ultimately leading to regularly spaced cracks and/or metal mesa structures. Such patterning of the cracks indicates a level of self-organization in the structuring and layout of the features – arrived at simply by imposing metallization boundaries in proximity to each other, separated by a distance of the order of the critical dimension of the pattern size apparent in the large surface mud-crack patterns.

Moving liquids with light: photoelectrowetting on semiconductors.

By linking semiconductor physics and wetting phenomena a brand new effect termed “photoelectrowetting-on-semiconductors” is demonstrated here for a conducting droplet resting on an insulator-semiconductor stack. Optical generation of carriers in the space-charge region of the underlying semiconductor alters the capacitance of the liquid-insulator-semiconductor stack; the result of this is a modification of the wetting contact angle of the droplet upon illumination using above band gap light. The effect is demonstrated using commercial silicon wafers, both n- and p-type having a doping range spanning four orders of magnitude (6×1014−8×1018u2005cm−3), coated with a commercial amorphous fluoropolymer insulating film (Teflon®). Impedance measurements confirm that the observations are semiconductor space-charge related effects. The impact of the work could lead to new silicon-based technologies in areas such as Laboratory-on-a-Chip, Microfluidics and Optofluidics.

Circularly polarized luminescence microscopy for the imaging of charge and spin diffusion in semiconductors

Room temperature electronic diffusion is studied in 3u2002μm thick epitaxial p(+) GaAs lift-off films using a novel circularly polarized photoluminescence microscope. The method is equivalent to using a standard optical microscope and provides a contactless means to measure both the charge (L) and spin (L(s)) diffusion lengths simultaneously. The measured values of L and L(s) are in excellent agreement with the spatially averaged polarization and a sharp reduction in these two quantities (L from 21.3 to 1.2u2002μm and L(s) from 1.3 to 0.8u2002μm) is found with increasing surface recombination velocity. Outward diffusion results in a factor of 10 increase in the polarization at the excitation spot. The range of materials to which the technique can be applied, as well as a comparison with other existing methods for the measurement of spin diffusion, is discussed.Room temperature electronic diffusion is studied in 3 mum thick epitaxial p+ GaAs lift-off films using a novel circularly polarized photoluminescence microscope. The method is equivalent to using a standard optical microscope and provides a contactless means to measure charge (L) and spin (L_s) diffusion lengths. The measured values of L and L_s are in excellent agreement with the spatially averaged polarization and a sharp reduction in these two quantities (L from 21.3 mum to 1.2 mum and L_s from 1.3 mum to 0.8 mum) is measured with increasing surface recombination. Outwards diffusion results in a factor of 10 increase in the polarization at the excitation spot.

A nanofluidic emitter tip obtained by focused ion beam nanofabrication.

We report here the design, fabrication and testing of a novel nanofluidic device which we term a nano-nib due to its resemblance to a nano-fountain pen. The nanofluidic device is an emitter tip which incorporates a nanofluidic capillary slot coupled to a microfluidic capillary slot. The microfluidic capillary slot is fabricated using reactive ion etching (RIE) whilst the nanofluidic capillary slot is fabricated using focused ion beam (FIB) etching. The microfluidic capillary slot has a length of 2 mm and capillary slot dimensions (w x h) of 1 microm x 4 microm, i.e. a volume of a few picolitres (pl). The smallest nanofluidic capillary slot has a length of 3 microm and capillary slot dimensions as small as 21 nm x 300 nm, i.e. a volume of a few attolitres (al). Current-voltage characterization in electrospray mode revealed a current of 1 nA at an applied voltage as low as 40 V. The applications for this nanofluidic device lie in high sensitivity electrospray mass spectrometry, direct nanowriting, ultralow volume sample extraction/spotting and printing.

Fluidic assembly of hybrid MEMS: a GaAs-based microcantilever spin injector

A proof-of-concept fluidic assembly of a hybrid MEMS GaAs microcantilever spin injector is presented here. Instead of monolithically forming MEMS from pre-deposited layers, we fabricate a hybrid MEMS by assembling pre-fabricated parts. Sub-millimetre sized patches of GaAs having a thickness of 3 µm are pre-fabricated, as is a metalized fused silica support layer. The GaAs patches are manipulated and assembled onto the silica support using capillary forces; the resultant hybrid MEMS comprises a GaAs microcantilever on a robust fused silica support. A novel ohmic contact is demonstrated by bonding a GaAs patch (p-type carbon doped to 1 × 1018 cm−3) onto the pre-metalized silica support layer prior to annealing; measurements revealed ohmic behaviour and a specific contact resistivity of ~10−5 Ω cm. Preliminary investigations show that, when contacting the cantilever against a metallic or magnetic surface, injected photocurrents as large as several tens of nA can be obtained, for which the spin polarization is equal to 16%.

Tunable contact angle hysteresis by micropatterning surfaces

Lithography is used to form circular micropatterns which govern the evaporation of a water droplet. The surfaces are composed of concentric circular defects having a smooth indentation profile. When a droplet encounters a micropattern, evaporation occurs with distinct discontinuities in the droplet wetting contact angle and base radius. The addition of gaps into the patterns enables modification of the contact angle hysteresis; the receding angle of fluorocarbon coated SU-8 can be tuned between 34.6° and 89.1° and that of SU-8 from 5.6° to 43.3° depending on the gap length. A model is developed which accurately predicts the observed behavior.

Electrowetting and semiconductors

Electrowetting is coming to fruition for many applications such as display technologies, droplet transport, smart optics, flexible systems, remote switching, electronic paper, miniaturized chemistry and energy harvesting. Surprisingly, although most of the workings of this technology have been achieved using semiconductor fabrication techniques – electrowetting has yet to fully take advantage of the wealth of physical properties (electrical, optical, mechanical, thermal, magnetic…) offered by semiconducting materials. This short review paper is intended to bridge the gap a little between electrowetting and the possible use of semiconductor properties for novel applications involving liquids and technology.

Surface recombination in doped semiconductors: Effect of light excitation power and of surface passivation

A self-consistent expression for the surface recombination velocity S and the surface Fermi level unpinning energy as a function of light excitation power (P ) is presented for nand p-type semiconductors doped above the 10 cm range. Measurements of S on p-type GaAs films using a novel polarized microluminescence technique are used to illustrate two limiting cases of the model. For a naturally oxidized surface S is described by a power law in P whereas for a passivated surface S varies logarithmically with P . Furthermore, the variation in S with surface state density and bulk doping level is found to be the result of Fermi level unpinning rather than a change in the intrinsic surface recombination velocity. It is concluded that S depends on P throughout the experimentally accessible range of excitation powers and therefore that no instrinsic value can be determined. Previously reported values of S on a range of semiconducting materials are thus only valid for a specific excitation power.For n- and p-type semiconductors doped above the 1016u2009cm−3 range, simple analytical expressions for the surface recombination velocity S have been obtained as a function of excitation power P and surface state density NT. These predictions are in excellent agreement with measurements on p-type GaAs films, using a novel polarized microluminescence technique. The effect on S of surface passivation is a combination of the changes of three factors, each of which depends on NT: (i) a power-independent factor which is inversely proportional to NT and (ii) two factors which reveal the effect of photovoltage and the shift of the electron surface quasi Fermi level, respectively. In the whole range of accessible excitation powers, these two factors play a significant role so that S always depends on power. Three physical regimes are outlined. In the first regime, illustrated experimentally by the oxidized GaAs surface, S depends on P as a power law of exponent determined by NT. A decrease of S such as the one induc...