1,721,012 research outputs found
Double-polysilicon self-aligned lateral bipolar transistors
No full textA new lateral bipolar junction transistor that utilises a double-polysilicon self-aligned structure to maximise high-frequency performance is introduced. Silicon-on-oxide (SOI) wafers are used to isolate devices from the substrate and to minimise parasitic substrate capacitances(CJCS0) around 1.3–2.6 fF (substrate is ground). A SOI thickness of 0.2–0.5 μm combined with 0.13–0.25 μm lithography could allow a reduction of transistor dimensions down to (0.2–0.5) · (0.13–0.25) lm2 and give an estimated minimum emitter/base junction capacitance(CJE0) of 0.54–1.36 fF. Simple device isolation is predicted to produce a small collector/base junction capacitance (CJC0) of 0.42–2.00 fF. Furthermore, use of a double base contact can help reduce base resistance (RB) to 0.43–1.17 kW and a wide collector window directly contacted to the collector is estimated to result in around 0.66–1.58 kW collector resistance (RC). By taking all parameters into account a cut-off frequency (fT) of 69–116 GHz and maximum oscillation frequency (fmax) of 61–128 GHz is predicted for this design, in addition a gain of 47–101(using minimum gain enhancement) and roughly 10.6–21.0 ps ECL propagation delay time, at a current of 0.4–1.0 mA could be achieved. Our simulations indicate that this new doubled-polysilicon self-aligned structure could outperform all other silicon bipolar transistors that have been reported
Nanostructured biomimetic moth-eye arrays in silicon by nanoimprint lithography
No full textThe eyes and wings of some species of moth are covered in arrays of subwavelength pillars that have been tuned over millions of years of evolution to reflect as little sunlight as possible. We are investigating ways of exploiting this to reduce reflection from the surfaces of silicon solar cells. Here, we report on the experimental realization of biomimetic antireflective moth-eye arrays in silicon using a technique based on nanoimprint lithography and dry etching. Areas of 1cm x 1cm have been patterned and analysis of reflectance measurements predicts a loss in the performance of a solar cell of only 6.5% compared to an ideal antireflective coating. This compares well with an optimized single layer Si3N4 antireflective coating, for which an 8% loss is predicted
Bio-mimetic subwavelength surface for near-zero reflection sunrise to sunset
No full textWe present a study of antireflective schemes and their operation over a full day. We compare simulation results for single and double layer antireflective coatings with biomimetic moth-eye structures, taking into account the full range of wavelengths and incident angles experienced by fixed solar cells from sunrise to sunset. We show that solar cells incorporating antireflective moth-eye arrays could produce up to 12% more energy than those employing single layer antireflective coatings
Current and future photovoltaics
No full textPhotovoltaics, now a billion-dollar industry, is experiencing staggering growth as increased concerns over fuel supply and carbon emissions have encouraged governments and environmentalists to become increasingly prepared to offset the extra cost of solar energy. Three 'generations' of photovoltaics have been envisaged that will take solar power into the mainstream. Photovoltaic production is currently 90% 'first-generation' or '1G' solar cells that rely on expensive bulk multi-crystalline or single-crystal semiconductors. Dominated by silicon wafers, they are reliable and durable but expensive. Half of the cost of 1G devices is the silicon wafer and efficiencies are limited to around 20%. Instead of using wafers, cheaper 'second-generation' (2G) solar cells would use cheap semiconductor thin-films deposited on low-cost substrates to produce devices of similar efficiencies. A number of thin-film device technologies account for around 5–6% of the market. As 2G technology reduces the active material cost, eventually the substrate will be the cost limit, and higher efficiency will be needed to maintain the $/W cost-reduction trend. 'Third-generation' devices (3G) will utilise new technologies to produce high-efficiency devices. Recently, tremendous advances outside the photovoltaic industry in nanotechnologies, photonics, optical metamaterials, plasmonics and semiconducting polymer sciences offer the prospect of cost-competitive photovoltaics based on new science and 3G concepts. Within the next 20 years, it is reasonable to expect that cost reductions, a move to 2G technologies and the implementation of some new technologies and 3G concepts can lead to fully cost-competitive solar energy
Si/SiGe near-infrared photodetectors grown using low pressure chemical vapour deposition
No full textNear-infrared photodetectors have been fabricated using standard CMOS processes in conjunction with the multilayer growth of Si/SiGe0.06 using low-pressure chemical vapor deposition (LPCVD). Cross-section scanning electron microscopy (SEM) indicates the existence of quantum dot like corrugations in devices with particularly thick SiGe0.06 quantum wells. With an accumulation of germanium atoms at the crest of such features and commensurate high germanium concentration we see a considerable enhancement of the long wavelength detection sensitivity of photodetectors in the range 1100–1300 nm. By fitting experimental data the minimum energy gap of the structure is found to be 0.88 eV corresponding to a germanium concentration of around 15%
Plasmonic and Biomimetic light-trapping for photovoltaics
No full textThe challenge when applying photonics to photovoltaics is the need to provide broadband, multiple-angle solutions to problems and both plasmonics and biomimetics offer broadband approaches to reducing reflection and enhancing light-trapping. Over millions of years nature has optimised nanostructures to create black, transparent, white and mirrored surfaces, the antireflective “moth-eye” structures are perhaps the best known of these biophotonic materials. In this paper we use simulated and experimental studies to illustrate how careful optimisation of nanoscale features is required to ensure the optimum match between reflectivity, spectral bandwidth and device quantum efficiencies. In the case of light-trapping by plasmonic scattering there is more room for design and specific spectral regions can be targeted by precise control of the size, shape and density of particular metal nanoparticles. We describe how the best opportunity for plasmonics within inorganic solar cells appears to be enhanced light-trapping of near-band edge photons
Optical characterisation of a spectrally tunable plasmonic reflector for application in thin film silicon solar cells
No full textWe have investigated the interaction between a random two-dimensional array of Ag islands near a Ag reflector, with the aim of producing a plasmonic back reflector structure with high diffuse reflectivity in the near-infrared, 600–1100 nm wavelength, region. We have demonstrated the ability to tune the power scattered and absorbed by varying the distance between the plasmonic layer and the reflector. Finite-difference-time-domain (FDTD) simulations demonstrate the tunability of the scattered and absorbed power with separation distance for a single Ag nanosphere near a planar Ag reflector. The tunability of the optical properties can be attributed to the modulation in the electric field driving the plasmonic resonance with separation distance. The simulation results indicate an intermediate distance where the scattered power peaks with minimal absorption losses. Random arrays of metal-islands were fabricated on varying thicknesses of a ZnO separation layer on a Ag reflector. Compared to a conventional textured Ag reflector, which has ?2% diffuse reflectance in the near-infrared spectral region, the fabricated plasmonic reflector with ?200 nm sized Ag metal islands at 100 nm separation distance from the Ag reflector shows a relatively higher, ?24%, integrated diffuse reflectance in the near bandgap, 600–1100 nm wavelength, region for thin film silicon solar cells
A new model of geometric chirality for two-dimensional continuous media and planar meta-materials
No full textWe have, for the first time, identified ten tenets of two-dimensional (2D) chirality that define and encapsulate the symmetry and scaling behaviour of planar objects and have used them to develop three new measures of geometric 2D chirality. All three models are based on the principle of overlap integrals and can be expressed as simple analytical functions of the two-dimensional surface density, rho(r). In this paper we will compare the predicted behaviour of these models and show that two of them are fully integrable and scalable and can therefore be applied to both discrete and continuous 2D systems of any finite size, or any degree of complexity. The only significant difference in these two models appears in their behaviour at infinite length scales. Such differences could, however, have profound implications for the analysis of chirality in new generations of planar meta-materials, such as chiral arrays, fractals, quasi-periodic 2D crystals and Penrose tiled structures
Giant optical activity in dielectric planar metamaterials with 2D chirality
No full textFor the first time, all-dielectric planar chiral metamaterials consisting of arrays of silicon nitride gammadions on fused silica substrates have been fabricated, and shown to be capable of inducing large changes to the polarization states of transmitted light in a manner that is dependent on the two-dimensional chirality of the microstructured silicon nitride film. The polarization response is found to reverse for opposite enantiomers, and also for the same enantiomer when it is illuminated from opposite sides of the structure. In addition, the polarization states of the various diffracted beams are found to be non-reversible. These structures therefore appear to display elements of non-reciprocal behaviour. The polarization responses of complementary designs, different chiral geometries and various silicon nitride film thicknesses have also been studied. As a result we conclude that multiple reflections within the patterned silicon nitride layer play an important role in defining the mechanism by which these structures are able to modify the polarization states of diffracted light
Tunable reflection minima of nanostructured antireflective surfaces
Get PDFBroadband antireflection schemes for silicon surfaces based on the moth-eye principle and comprising arrays of subwavelength-scale pillars are applicable to solar cells, photodetectors, and stealth technologies and can exhibit very low reflectances. We show that rigorous coupled wave analysis can be used to accurately model the intricate reflectance behavior of these surfaces and so can be used to explore the effects of variations in pillar height, period, and shape. Low reflectance regions are identified, the extent of which are determined by the shape of the pillars. The wavelengths over which these low reflectance regions operate can be shifted by altering the period of the array. Thus the subtle features of the reflectance spectrum of a moth-eye array can be tailored for optimum performance for the input spectrum of a specific application
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