Brazing techniques for the fabrication of biocompatible carbon-based electronic devices

Prototype electronic devices have been critical to the discovery and demonstration of the unique properties of new materials, including composites based on carbon nanotubes (CNT) and graphene. However, these devices are not typically constructed with durability or biocompatibility in mind, relying on conductive polymeric adhesives, mechanical clamps or crimps, or solders for electrical connections. In this paper, two key metallization techniques are presented that employ commercially-available brazing alloys to fabricate electronic devices based on diamond and carbonaceous wires. Investigation of the carbon - alloy interfacial interactions was utilized to guide device fabrication. The interplay of both chemical ( adhesive ) and mechanical ( cohesive ) forces at the interface of different forms of carbon was exploited to fabricate either freestanding or substrate-fixed carbonaceous electronic devices. Elemental analysis in conjunction with scanning electron microscopy of the carbon - alloy interface revealed the chemical nature of the Ag alloy bond and the mechanical nature of the Au alloy bond. Electrical characterization revealed the non-rectifying nature of the carbon - Au alloy interconnects. Finally, electronic devices were fabricated, including a Au circuit structure embedded in a polycrystalline diamond substrate

Feasibility Assessment of an Optically Powered Digital Retinal Prosthesis Architecture for Retinal Ganglion Cell Stimulation

Clinical trials previously demonstrated the notable capacity to elicit visual percepts in blind patients affected with retinal diseases by electrically stimulating the remaining neurons on the retina. However, these implants restored very limited visual acuity and required transcutaneous cables traversing the eyeball, leading to reduced reliability and complex surgery with high postoperative infection risks. To overcome the limitations imposed by cables, a retinal implant architecture in which near-infrared illumination carries both power and data through the pupil to a digital stimulation controller is presented. A high efficiency multi-junction photovoltaic cell transduces the optical power to a CMOS stimulator capable of delivering flexible interleaved sequential stimulation through a diamond microelectrode array. To demonstrate the capacity to elicit a neural response with this approach while complying with the optical irradiance limit at the pupil, fluorescence imaging with a calcium indicator is used on a degenerate rat retina. The power delivered by the laser at the permissible irradiance of 4 mW/mm2 at 850 nm is shown to be sufficient to both power the stimulator ASIC and elicit a response in retinal ganglion cells (RGCs), with the ability to generate of up to 35 000 pulses per second at the average stimulation threshold. This confirms the feasibility of generating a response in RGCs with an infrared-powered digital architecture capable of delivering complex sequential stimulation patterns at high repetition rates, albeit with some limitations.Comment: 11 pages, 13 figure

Advanced surface texturing for silicon solar cells

The multi-crystalline silicon (me-Si) solar cell is considered to be one of the most promising cells capable of achieving high efficiency at low cost and high reliability. Improving solar cells efficiency using low cost materials requires careful design considerations aiming to minimise the optical and electrical losses. In this work plasma texturing was employed to reduce optical reflections from silicon surfaces well below 1%. Plasma texturing is used to form light trapping structures suitable for silicon solar cells. Several plasma texturing methods are investigated and associated defects are analysed. Masked as well as mask-less texturing techniques are investigated. Conventional parallel plate Reactive Ion Elching (RIE), Inductively Coupled Plasma (ICP) and Electron Cyclotron Resonance (ECR) plasma system are used to compare the plasma induced defects in silicon. The influence of various plasma etch parameters on plasma induced defect is investigated. A correlation between the minority carriers lifetime and surface area increased by texturing is established. Effective lifetime measurements using Quasi Steady State Photo Conductive (QSSPC) technique is mainly used to estimate the plasma induced defect in textured silicon substrates. Sinton lifetime tester is used to measure the effective lifetime of the substrates. The implied open circuit voltage is calculated from the lifetime data for textured substrates. In this work low temperature photoluminescence spectroscopy is also used to analyse the defect caused by plasma on me-Si substrates. Photoluminescence (PL) data is obtained using the 514.5 nm line of an Ar⁺ laser as an excitation source. The luminescence is dispersed with SPEX 1700 spectrometer with a liquid nitrogen cooled Germanium detector. Reflectance measurements are performed on textured surfaces usmg a purpose built integrating sphere attachment of a high accuracy spectrophotometer. Modelling is also performed using PV-optics software to compare the experimental and theoretical results. Finally, silicon solar cells are fabricated with measured efficiency around 18% . The efficiency is estimated from the I-V characteristics data obtained using a calibrated halogen lamp and a HP semiconductor parameter analyser. Spin-on-dopant source as well as solid diffusion source is used to form the ewitter junction of the solar cells fabricated on p-type silicon wafers. Multicrystalline silicon, CZ- silicon and FZ silicon wafers are used to fabricate solar cells in this thesis. The effect of single and double layer antireflection coatings on diffused reflections is also investigated

Metallisation of single crystal diamond radiation detectors

Properties such as a large band gap, high thermal conductivity and resistance to radiation damage make diamond an extremely attractive candidate for detectors in next generation particle physics experiments. This paper presents our technique for metallisation of a single crystal diamond grown by chemical vapour deposition (CVD) for use as a radiation detector, suitable for operation in places such as the Large Hadron Collider. The front and back side of the diamond are metalised with aluminium and gold on top of titanium respectively, after which the diamond is mounted and read out via a charge sensitive preamplifier. The device is found to collect charge at an efficiency of 97%