W. Cunningham

Large-area microelectrode arrays for recording of neural signals

To understand the neural code, that the retina uses to communicate the visual scene to the brain, large-area microelectrode arrays are needed to record retinal signals simultaneously from many recording sites. This will give a valuable insight into how large biological neural networks (such as the brain) process information, and may also be important in the development of a retinal prosthesis as a potential cure for some forms of blindness. We have used the transparent conductor indium tin oxide to fabricated electrode arrays with approximately 500 electrodes spaced at 60 /spl mu/m. The fabrication procedures include photolithography, electron-beam lithography, chemical etching and reactive-ion etching. These arrays have been tested electrically using impedance measurements over the range of frequencies important when recording extracellular action potentials (0.1-100kHz). The data has been compared to a circuit model of the electrode/electrolyte interface. One type of array (512 electrodes) behaves as theory would dictate and exhibits an impedance of 200 k/spl Omega/ at 1kHz. The other array (519 electrodes) has an impedance of 350 k/spl Omega/ at this frequency, which is higher than predicted by the models. This can perhaps be attributed to the difference in fabrication techniques. The 512-electrode array has been coupled to low-noise amplification circuitry and has recorded signals from a variety of retinal tissues. Example in vitro recordings are shown here.

Semi-insulating GaN and its evaluation for α particle detection

Abstract The application of semi-insulating GaN for detection of ionising particles, specifically α-particles, is presented. The electrical properties of GaN epitaxial layers have been investigated and the space charge limited and Ohmic currents were observed. A microwave method was used for temporal measurements of the photocurrent. Transient behaviour of the injection current and photo-response was observed, with a wide range of instantaneous time constants. The charge collection efficiency was measured to be approximately 100% and the role of inhomogeneities in the charge collection peculiarities was analysed.

Performance of irradiated bulk SiC detectors

Abstract Silicon carbide (SiC) is a wide bandgap material with many excellent properties for future use as a detector medium. We present here the performance of irradiated planar detector diodes made from 100-μm-thick semi-insulating SiC from Cree. Ohmic/Schottky diodes were produced and characterised using Schottky barrier measurements and charge collection efficiency (CCE) measurements were made for 5.48 MeV 241 Am alpha particles. Charge collection of ∼60% was measured initially. Measurements were taken of trap lifetimes in an effort to understand this. Three distinct traps with lifetimes of 4, 16, and 130 s were found. Irradiation took place at Paul Scherrer Institute (PSI) to fluences of 10 12 , 10 13 , and 5×10 14 cm −2 , with 300 MeV /c pions. Changes in the Schottky barrier height, leakage current, trap lifetimes, and CCE measurements give an indication of the possible degradation in performance of detectors of this type over their projected lifetime.

Photocurrent in epitaxial GaN

A simple kinetic model concerning photocurrent in epitaxially grown GaN is presented. Utilizing a minimal set of rate equations and kinetic parameters, it is shown that in the presence of hole centers with small probabilities of electron-hole recombinations, the time dependence of photocurrent is ruled by competition between capture of conduction bandelectrons by deep electron traps and electron-hole recombinations. If the probability of electron capture exceeds that of recombination, the decay of current after excitation is turned off shows the usual persistent photocurrent trend. If, on the contrary, the probability of recombination is larger than that of electron capture, a slow photocurrent quenching, past a maximum, can be observed. In some circumstances, after excitation is turned off, the current drops below the steady dark current, at which point the negative persistent photoconductivity effect comes into play.

Finite element modelling of the mechanical loss of silica suspension fibres for advanced gravitational wave detectors

Detection of gravitational waves remains one of the most challenging problems faced by experimental physicists. One of the most significant limits to the sensitivity of current, and future, long-baseline interferometric gravitational wave detectors is thermal displacement noise of the test masses and their suspensions. Detector suspension thermal noise will be an important noise source at operating frequencies between approximately 10 and 30 Hz, and results from a combination of thermoelastic damping, surface and bulk losses associated with the suspension fibres. However its effects can be reduced by minimizing the thermoelastic loss and optimization of pendulum dilution factor via appropriate choice of suspension fibre and attachment geometry. This paper will discuss finite element modelling and associated analysis of the loss in quasi-monolithic silica fibre suspensions for future advanced gravitational wave detectors.

Cryogenic measurements of mechanical loss of high-reflectivity coating and estimation of thermal noise

We report on low-frequency measurements of the mechanical loss of a high-quality (transmissivity T<5 ppm at λ(0)=1064 nm, absorption loss <0.5 ppm) multilayer dielectric coating of ion-beam-sputtered fused silica and titanium-doped tantala in the 10-300 K temperature range. A useful parameter for the computation of coating thermal noise on different substrates is derived as a function of temperature and frequency.

Role of Potential Barriers in Epitaxial Layers of Semi-Insulating GaN Layers

The electrical properties of semi-insulating GaN were investigated by dc and microwave techniques. The different epitaxial GaN layers were grown by Metal Organic Chemical Vapour Deposition on a Al2O3 (0001) substrate. Space charge effects along and perpendicular to the direction of layer growth were proposed to explain the observed peculiarities of the dark current and photocurrent. From the temporal and temperature dependences of characteristic time constants in the photoconductivity decay, the details of non-equilibrium states were analysed in the semi-insulating GaN. The well-resolved alpha-particle spectra obtained demonstrates the promise of semi-insulating GaN for ionising radiation detection.

Fabricating high-density microarrays for retinal recording

Understanding how the retina encodes the visual scene is a problem, which requires large area, high-density microelectrode arrays to solve. The correlated signals that emerge from the output (ganglion) cells of the retina form a code, which is not well understood. We use a combination of electron beam lithography, photolithography and dry-etch pattern transfer to realise a 519-electrode array in the transparent conductor indium tin oxide (ITO). The electrodes are spaced at 60 µm in a hexagonal close-packed geometry. A mix and match lithography procedure is utilised, whereby the high-density inner region is fabricated using electron beam lithography whilst the outer sections are realised by photolithography. Reactive ion etching (RIE), using CH4/H2, of the ITO forms the array structure and SF6 RIE allows resist removal and patterning of vias through a plasma deposited Si3N4 protective layer. The electrical properties of the ITO layer are unaffected by the etching procedures, A reliable method for achieving low-impedance electroplated platinum electrodes has been employed to yield electrode impedances of ∼ 20 kΩ. An array fabricated using these dry-etch techniques is shown to record action potentials from live retinal tissue in neurophysiological experiments.

Comparison of bulk and epitaxial 4H-SiC detectors for radiation hard particle tracking

Measurements and simulations have been carried out using bulk and epitaxial SiC detectors. Samples were irradiated to fluences of around 10/sup 14/ hardrons/cm/sup 2/. Material of thickness 40/spl mu/m gave a charge collection efficiency of 100% dropping to around 60% at 100 /spl mu/m thickness. Detailed MEDICI simulations incorporated the main defect levels in SiC, the vanadium center, Z-center and a mid-gap level as measured by deep level transient spectroscopy and other techniques. Calculated recombination currents and charge collection efficiencies at varying fluences were comparable to experimental data. The study suggests that SiC detectors will operate up to fluences around 10/sup 16//cm/sup 2/ as required by future particle physics experiments.

Detection of retinal signals using position sensitive microelectrode arrays

The development of high-density, multi-channel circuitry for the detection of low-level signals in particle physics has potential application in other areas of science. Here we use this technology to study biological neural networks. For example understanding the code that the retina uses to process visual images is of great interest. To this end we have fabricated a 61-channel microelectrode array on the transparent conductor indium tin oxide. We analyse the effect of contact size on electrode characteristics and demonstrate the performance of these arrays in neurophysiological experiments using retinal tissue. We present some preliminary results in the development of 519 channel arrays to allow an increased area of the retina to be studied.