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As an emerging class of hybrid nanoporous materials, metal-organic frameworks (MOFs) have attracted significant attention as promising multifunctional building blocks for the development of highly sensitive and selective gas sensors due to their unique properties, such as large surface area, highly diversified structures, functionalizable sites and specific adsorption affinities. Here, we provide a review of recent advances in the design and fabrication of MOF nanomaterials for the low-temperature detection of different gases for air quality and environmental monitoring applications. The impact of key structural parameters including surface morphologies, metal nodes, organic linkers and functional groups on the sensing performance of state-of-the-art sensing technologies are discussed. This review is concluded by summarising achievements and current challenges, providing a future perspective for the development of the next generation of MOF-based nanostructured materials for low-temperature detection of gas molecules in real-world environments.

In the last decades, nanomaterials have emerged as multifunctional building blocks for the development of next generation sensing technologies for a wide range of industrial sectors including the food industry, environment monitoring, public security, and agricultural production. The use of advanced nanosensing technologies, particularly nanostructured metal-oxide gas sensors, is a promising technique for monitoring low concentrations of gases in complex gas mixtures. However, their poor conductivity and lack of selectivity at room temperature are key barriers to their practical implementation in real world applications. Here, we provide a review of the fundamental mechanisms that have been successfully implemented for reducing the operating temperature of nanostructured materials for low and room temperature gas sensing. The latest advances in the design of efficient architecture for the fabrication of highly performing nanostructured gas sensing technologies for environmental and health monitoring is reviewed in detail. This review is concluded by summarizing achievements and standing challenges with the aim to provide directions for future research in the design and development of low and room temperature nanostructured gas sensing technologies.

Light trapping in thin silicon solar cells demands radically different fabrication approaches to standard commercial cells. Weaker optical absorption and increased sensitivity to surface recombination requires light trapping to be achieved over a broader spectral range and, ideally, without texturing the absorber itself. Nano-scale light trapping structures allow the strongest scattering to be tuned to wavelengths, where oblique scattering into the absorber is needed most. Furthermore, applying these structures “externally,” i.e., on a well-passivated planar silicon surface, reduces the surface area and permits optimal electronic conditions to be maintained. Despite these advantages, the challenges of balancing efficiency gain, cost, and lithographic fidelity have prevented the commercial use of nano-scale light trapping schemes. Here, we demonstrate the use of nanosphere lithography for producing high-quality and cost-effective nano-scale light trapping structures suitable for incorporation in thin solar cells. We have successfully fabricated large-area and uniform metal nanospheregrating structures, with embedded dielectric nanospheres, on 30 μ m thick c-Si pseudo cells and measured their effectiveness for light trapping. Comparison between simulations and the fabricated pseudo cells’ characteristics highlighted key challenges in fabricating uniform structures, including the impact of air gaps within non-conformal coatings and minor changes in the geometry. Optical characterization via absorption spectroscopy and both spectral and spatially resolved photoluminescence showed a clear enhancement in the short-circuit current density of up to 4.33 mA/cm2 in comparison with a planar 30 μ m thick device and a 3.7 times absorptance enhancement close to the bandgap of Si.

Remotely accessed hardware laboratories enable students to work on projects from any location and anytime while providing real-world responses. This paper reports on the outcomes of augmenting existing physical electronics labs of a third-year undergraduate electronics engineering course with innovative remote access ones on student engagement and learning experience. Student activity gathered in the form of time stamped mouse click data, combined with feedback survey results reveal valuable information regarding how, where and when students approach the remote lab and their level of engagement. Data show a large discrepancy between the actual and perceived time needed to complete the lab. The sparseness of activity through the lab reveal the commonly encountered problem of starting late and diffused work due to increased likelihood for distraction. Correlating the grades with the actual amount of work undertaken reveals the shortcoming of conventional labs in assessing student engagement when working in teams. Overall, the analysis of collected data and the received feedback reveal that the introduction of the remote lab had a beneficial effect on student learning.

A slow forming carrier-induced degradation effect has previously been reported for p-type multi-crystalline silicon (mc-Si) solar cells and has been observed to be most severe in passivated emitter and rear contact (PERC) designs. The as yet undetermined defect (or defects) responsible for this degradation is typically described as being light-induced. However, in this work we confirm that a directly equivalent degradation also occurs when subjecting a cell to current injection, thus the effect can therefore be more accurately described as being carrier-induced. The defect can take years to form under normal operating conditions, but acceleration of this formation and an apparent subsequent passivation through the use of high intensity illumination at elevated temperatures has recently been demonstrated. In this work we further investigate this approach, analyzing the effects of temperature and time on degradation, regeneration, and resulting stability, as well as the variations in treatment response for mc-Si materials from two different manufacturers. Susceptibility to carrier-induced degradation after 225 h of light soaking at 70 °C is shown to be reduced by 80% using a rapid, 10 s treatment with an illumination intensity of 44.8 kw/m2 at 200 °C. Stability is shown to further improve by extending treatment time but is reduced with increasing treatment temperature.

In this paper, we propose a hybrid quantum dot (QD)/solar cell configuration to improve performance of interdigitated back contact (IBC) silicon solar cells, resulting in 39.5% relative boost in the short-circuit current (JSC) through efficient utilisation of resonant energy transfer (RET) and luminescent downshifting (LDS). A uniform layer of CdSe1−xSx/ZnS quantum dots is deposited onto the AlOx surface passivation layer of the IBC solar cell. QD hybridization is found to cause a broadband improvement in the solar cell external quantum efficiency. Enhancement over the QD absorption wavelength range is shown to result from LDS. This is confirmed by significant boosts in the solar cell internal quantum efficiency (IQE) due to the presence of QDs. Enhancement over the red and near-infrared spectral range is shown to result from the anti-reflection properties of the QD layer coating. A study on the effect of QD layer thickness on solar cell performance was performed and an optimised QD layer thickness was determined. Time-resolved photoluminescence (TRPL) spectroscopy was used to investigate the photoluminescence dynamics of the QD layer as a function of AlOx spacer layer thickness. RET can be evoked between the QD and Si layers for very thin AlOx spacer layers, with RET efficiencies of up to 15%. In the conventional LDS architecture, down-converters are deposited on the surface of an optimised anti-reflection layer, providing relatively narrowband enhancement, whereas the QDs in our hybrid architecture provide optical enhancement over the broadband wavelength range, by simultaneously utilising LDS, RET-mediated carrier injection, and antireflection effects, resulting in up to 40% improvement in the power conversion efficiency (PCE). Low-cost synthesis of QDs and simple device integration provide a cost-effective solution for boosting solar cell performance.

The Telecommunications Engineering degree at Macquarie is undergoing renewal, simultaneously with a transformation in pedagogy by the School of Engineering and also a change in curriculum structure by Macquarie University. This work-in-progress paper reports a study of the effect of changes in Telecommunications Engineering education. These include updated technical content imparted through an educational approach which includes project-based learning (PBL), project ownership, replacement of traditional lectures, virtual laboratories and an emphasis on software tools and programming skills.

Recently, a new carrier-induced defect has been reported in multi-crystalline silicon (mc-Si), and has been shown to be particularly detrimental to the performance of passivated emitter and rear contact (PERC) cells. Under normal conditions, this defect can take years to fully form. This Letter reports on the accelerated formation and subsequent passivation of this carrier-induced defect through the use of high illumination intensity and elevated temperatures resulting in passivation within minutes. The process was tested on industrial mc-Si PERC solar cells, where degradation after a 100 hour stability test was suppressed to only 0.1% absolute compared to 2.1% for non-treated cells. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Mie resonances of high-refractive index nanostructures provide strong and spectrally broadband scattering. In this numerical work, we investigate the feasibility of amourphous silicon nanodisks at the planar rear side of crystalline silicon solar cells for light trapping.

This paper presents a research tool that allows engineers to optimize the design of a dynamic photovoltaic array using simulation and test the hardware within an integrated single platform. The purpose of a dynamic PV array (DPVA) is to mitigate the detrimental effects caused by partial shading. As the environmental conditions can be unpredictable over long time scales, it is generally not practical to design large arrays without the use of a simulator, however using such a simulator for the virtual realisation of a DPVA only provides ideal parameters and characteristics which are likely to translate poorly to a practical system. The system presented here integrates a simulation environment with the operation of a hardware prototype to give engineers maximum insight into the optimal system design. The proposed hardware includes a switch matrix that is capable of being configured such that it emulates four of the most sophisticated DPVA techniques known to literature (Optimized String, Irradiance Equalization, Adaptive Bank and Alternating Current) with the addition of a user controlled load allowing for the operating point to be managed throughout testing. When integrated into a single control platform, it is possible to design, test and implement a fully autonomous DPVA which has had every aspect of its operation tailored to maximise performance. By using a tool such as this, it is hoped that the remaining engineering topics regarding dynamic arrays can be quickly addressed leading to advancement of the field. We reiterate the importance of using a number of standardized tests during development phases as they can be used to evaluate different techniques and validate real world conditions.

Silicon-germanium is an important material that is used for the fabrication of SiGe heterojunction bipolar transistors and strained Si metal-oxide-semiconductor (MOS) transistors for advanced complementary metal-oxide-semiconductor (CMOS) and BiCMOS (bipolar CMOS) technologies. It also has interesting optical properties that are increasingly being applied in silicon-based photonic devices. The key benefit of silicon-germanium is its use in combination with silicon to produce a heterojunction. Strain is incorporated into the silicon-germanium or the silicon during growth, which also gives improved physical properties such as higher values of mobility. This chapter reviews the properties of silicon-germanium, beginning with the electronic properties and then progressing to the optical properties. The growth of silicon-germanium is considered, with particular emphasis on the chemical vapour deposition technique and selective epitaxy. Finally, the properties of polycrystalline silicon-germanium are discussed in the context of its use as a gate material for MOS transistors.

This paper presents a novel system for producing the optimum power output from photovoltaic (PV) arrays using dynamic cell reconfiguration. The proposed approach is the first in the literature that creates strings using individual substrings that have been characterized and categorized ensuring maximum power extraction for a given irradiance profile. This optimized and decentralized PV architecture can produce significantly more power than a static equivalent (by an average of 22.6%) and also outperforms the sophisticated alternative known as an irradiance equalized dynamic photovoltaic array (IEq-DPVA) by an average of 13.7% for the relevant tests carried out. This paper identifies the hardware requirements to produce such a system and it describes an algorithm that performs the optimized-string reconfiguration strategy. Finally, a simulator programmed in MATLAB is used to compare the performance of the optimized-string DPVA against an IEq-DPVA in a series of flexibility tests.

This paper reports the effects of varied deposition conditions on the resultant energy conversion efficiencies of thin-film silicon solar cells. Cells were deposited using an Oxford Instruments PlasmaLab System 100 on to Pilkington TEC-8 TCO glass, and thoroughly investigated using electrical methods. Thin film devices were fabricated using the decomposition of silane gas within a reaction chamber of 13.56 MHz plasma discharge. The deposition conditions, including substrate temperature, gas flow rates, RF power, chamber pressure, and film thickness are all explored to determine the optimum cell performance. A view toward high-efficiency solar structures, including heterojunction with intrinsic thin layer (HIT) and micro-morph cell designs using the same conditions is presented.

The growth of (0002) orientated polycrystalline undoped and Ga-doped ZnO films by DC sputtering under Ar is described. The (0002) peak intensity decreases with increasing substrate temperature in both doped and undoped samples. The average grain sizes are very small. This indicates that ZnO films with low crystallinity are obtained at high substrate temperatures. It is deduced that surface damage can be increased by high energy plasmas of neutral Ar particles at high substrate temperatures. The average surface roughness for both undoped and Ga-doped ZnO films decreases with increasing substrate temperatures. It is deduced that energies of sputter particles decrease with increasing substrate temperatures due to collisions with Ar particles. The surface roughness corresponds well to the structure model.

The current limitations of commercially available thermoelectric (TE) generators include their incompatibility with human body applications due to the toxicity of commonly used alloys and possible future shortage of raw materials (Bi-Sb-Te and Se). In this respect, exploiting silicon as an environmentally friendly candidate for thermoelectric applications is a promising alternative since it is an abundant, ecofriendly semiconductor for which there already exists an infrastructure for low-cost and high-yield processing. Contrary to the existing approaches, where n/p-legs were either heavily doped to an optimal carrier concentration of 1019 cm−3 or morphologically modified by increasing their roughness, in this work improved thermoelectric performance was achieved in smooth silicon nanostructures with low doping concentration (1.5 × 1015 cm−3). Scalable, highly reproducible e-beam lithographies, which are compatible with nanoimprint and followed by deep reactive-ion etching (DRIE), were employed to produce arrays of regularly spaced nanopillars of 400 nm height with diameters varying from 140 nm to 300 nm. A potential Seebeck microprobe (PSM) was used to measure the Seebeck coefficients of such nanostructures. This resulted in values ranging from −75 μV/K to −120 μV/K for n-type and 100 μV/K to 140 μV/K for p-type, which are significant improvements over previously reported data.

We 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.

In this paper, we present morphological and electrical characteristics of a junction formed of Si p-type films deposited on an n-type silicon wafer using a hot wire chemical vapor deposition (HWCVD) tool. We describe the fabrication process and study the influence of diborane flow and postprocess annealing in improving junction characteristics. Our morphological studies undertaken using atomic force microscopy show that the initial deposition suffered from voids rather than being a uniform film; however, this improves significantly under our annealing treatment. The improvement in morphology was observed in the electrical characteristics, with estimated $V_{\text{oc}}$ doubling and rectification of the junction improving by several orders of magnitude. Fitting of the current–voltage curves to a two-diode model showed that increasing the diborane flow in the process helps reduce the saturation current and ideality factors, while increasing the shunt resistance. Electrochemical capacitance–voltage (ECV) and quasi-steady-state photoconductance measurements are used to characterize the deposited films further. A solar cell device with a silicon epitaxy emitter is modeled using industry-standard 3-D modeling tools and input parameters from experimental data, and the impact of defects is studied. A potential efficiency approaching 25% is shown to be feasible for an optimized device.

Luminescence imaging is a widely used characterization technique for silicon photovoltaics. However, the tools used to acquire images typically utilize a silicon CCD array for detection, which is a poor absorber at silicon luminescence wavelengths. This leads to a smearing effect in the measured image which can be characterized by a point spread function (PSF). If the true PSF is known then the measured image can be restored through deconvolution. Several methods exist for determining a PSF for a particular imaging system and different extraction techniques can lead to variations in the PSF result, yet no studies have provided comprehensive analysis of PSF deconvolution accuracy when applied to luminescence imaging. In this work, several new techniques have been designed and investigated in order to test PSF deconvolution results, with a view to quantifying improvement or errors generated and potentially leading towards improved image restoration.

Over the last decade, plasmonic nanoparticle arrays have been extensively studied for their light trapping potential in thin film solar cells. However, the commercial use of such arrays has been limited by complex and expensive fabrication techniques such as e-beam lithography. Nanosphere lithography (NSL) is a promising low-cost alternative for forming regular arrays of nanoscale features. Here, we use finite-difference time-domain (FDTD) simulations to determine the optical enhancement due to nanosphere arrays embedded at the rear of a complete thin film device. Array parameters including the nanosphere pitch and diameter are explored, with the FDTD model itself first validated by comparing simulations of Ag nanodisc arrays with optical measurements of pre-existing e-beam fabricated test structures. These results are used to guide the development of a nanosphere back-reflector for 20 μm thin crystalline silicon cells. The deposition of polystyrene nanosphere monolayers is optimized to provide uniform arrays, which are subsequently incorporated into preliminary, proof of concept device structures. Absorption and photoluminescence measurements clearly demonstrate the potential of nanosphere arrays for improving the optical response of a solar cell using economical and scalable methods.

We use helium ion microscopy (HeIM) to image the nanostructure of poly(3-hexylthiophene)/[6,6]-phenyl-C(61)-butric acid methyl ester (P3HT/PCBM) blend thin-films. Specifically, we study a blend thin-film subject to a thermal anneal at 140 °C and use a plasma-etching technique to gain access to the bulk of the blend thin-films. We observe a domain structure within the bulk of the film that is not apparent at the film-surface and tentatively identify a network of slightly elongated PCBM domains having a spatial periodicity of (20 ± 4) nm a length of (12 ± 8) nm.

The use of a helium ion microscope with an integrated gas injection system for nanofabrication is explored by demonstrating the milling of fine features into single layered graphene and the controlled deposition of tungsten and platinum wires from gaseous precursors. Using pattern generator software to control the path of the beam, nanoelectronic device designs are transferred directly into graphene. Four point contact designs are also defined on SiO"2/Si surfaces with atomic force microscopy used to characterize the resulting depositions. Although further optimization of the processes is required and questions of beam-induced damage to the delicate graphene lattice are yet to be answered, the helium ion microscope shows potential to go beyond what is possible with Ga^+ focused ion beam technologies in nanoscale device fabrication.

In this report, helium ion microscopy (HIM) is used to study the micro and nano-structures responsible for structural color in the wings of two species of Lepidotera from the Papilionidae family: Papilio ulysses (Blue Mountain Butterfly) and Parides sesostris (Emerald-patched Cattleheart). Electronic charging under the beam of uncoated scales from the wings of these butterflies is successfully neutralized, leading to images displaying a large depth-of-field and a high level of surface detail, which would normally be obscured by traditional coating methods used for scanning electron microscopy (SEM). The images are compared to those from variable pressure SEM, demonstrating the superiority of HIM at high magnifications. In addition, the large depth-of-field capabilities of HIM are exploited through the creation of stereo pairs which allows the exploration of the third dimension. Furthermore, the extraction of quantitative height information which matches well with cross-sectional transmission electron microscopy (TEM) measurements from the literature is demonstrated.

This paper reports the production of high quality copper indium diselenide thin films using pulsed DC magnetron sputtering from a powder target. As–grown thin films consisted of pin-hole free, densely packed grains. X–ray diffraction showed that films were highly orientated in the (112) and/or (204)/(220) direction with no secondary phases present. The most surprising and exciting outcome of this study was that the as-grown films were of near stoichiometric composition, almost independent of the composition of the starting material. No additional steps or substrate heating were necessary to incorporate selenium and create single phase CuInSe2. Electrical properties obtained by hot point probe and four point probe gave values of low resistivity and showed that the films were all p–type. The physical and structural properties of these films were analyzed using x-ray diffraction, scanning electron microscopy and atomic force microscopy. Resistivity measurements were carried out using the four point probe and hot probe methods. The single step deposition process can cut down the cost of the complex multi step processes involved in the traditional vacuum based deposition techniques.

We have studied the optical transmission properties of a number of different dielectric diffraction gratings that each exhibit two-dimensional chirality in their grating elements. We have found that the diffracted beams transmitted through these structures exhibit large anisotropic polarization changes and polarization-dependent intensity modulations even though the chiral elements are arranged in square grating arrays. The strength and functional form of both of these effects differ for each diffracted beam. They are also dependent on the chirality of the surface patterning and the thickness of the patterned dielectric film. We show how these properties could be exploited to provide a new and more versatile form of polarimeter, and how the operation of such a device is inherently dependent on the optical non-reciprocity of these chiral gratings.

An improvement to the method of creating close-packed polystyrene nanosphere arrays on silicon via liquid surface self-assembly has been attained via modification of the deposition technique. This modification facilitates consistent and large area deposition of high quality close-packed polystyrene nanosphere monolayers on silicon substrates that was achieved in this work. This technique has the potential to be used for photonics applications such as creating low cost, large area antireflective surfaces in solar cells. (c) 2009 Elsevier B.V. All rights reserved.

Silver nanoparticles have been fabricated on glass and silicon substrates, and silicon solar cells, by evaporation of 10 nm layers followed by thermal annealing. Distinct localized surface plasmon resonances are observed in the optical spectra of the annealed samples, and these strongly affect the cell reflectance and spectral response, both positively and negatively. At short wavelengths photocurrent is improved due to forward-scattering by quadrupolar modes, while at longer wavelengths photocurrent is deteriorated due to back-scattering by dipolar modes. Back-scattering is attributed to modification of the angular scattering distribution of the nanoparticles by the silicon substrate. [All rights reserved Elsevier].

We present an approach for the optimization of thin film antireflective coatings for encapsulated planar silicon solar cells in which the variations in the incident spectra and angle of incidence (AOI) over a typical day are fully considered. Both the angular and wavelength dependences of the reflectance from the surface, absorptance within the coating, and transmittance into the device are calculated for both single- and doublelayer antireflection coatings with and without thin silicon oxide passivation layers. These data are then combined with spectral data as a function of time of day and internal quantum efficiency to estimate the average short-circuit current produced by a fixed solar cell during a day. This is then used as a figure of merit for the optimization of antireflective layer thicknesses for modules placed horizontally at the equator and on a roof in the UK. Our results indicate that only modest gains in average short-circuit current could be obtained by optimizing structures for sunrise to sunset irradiance rather than AM15 at normal incidence, and fabrication tolerances and uniformities are likely to be more significant. However, we believe that this overall approach to optimization will be of increasing significance for new, potentially asymmetric, antireflection schemes such as those based on subwavelength texturing or other photonic or plasmonic technologies currently under development especially when considered in combination with modules fixed at locations and directions that result in asymmetric spectral conditions.

A 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 lm combined with 0.13–0.25 lm 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.

Near-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%.

Electron beam lithography has been applied to the fabrication of nanoscale two-dimensional chiral structures with various designs. A fabrication process for planar chiral structures in a thin silicon nitride layer using electron beam lithography and dry etching is presented. A top conductive coating is applied during electron-beam exposure to prevent the occurrence of charging effects caused by the non-conductive silica substrate. Different doses are chosen during the lithography process depending on the complexities and densities of different chiral designs. A very well defined transmission diffraction pattern from arrays of Peano-Gosper fractals is obtained. The optical activity of these dielectric metamaterials shows great potential in their applications of optoelectronic devices and communications.

The characterization of scattered light is complex and relatively nonstandardized despite being of great importance to many optical technologies. While total scatter can be efficiently measured using integrating-sphere-based techniques, a detailed determination of the full bidirectional scattering distribution function is far more challenging, often requiring complicated and expensive equipment as well as substantial measurement time. Due to this, many research groups rely on simpler, angle-resolved scattering (ARS) measurements, yet these are typically carried out using a single wavelength source, therefore providing limited information. Here, we demonstrate a custom-built broadband angle-resolved optical spectrometer, which utilizes a supercontinuum white light laser source combined with a custom automated goniometer and a Si CCD array spectrometer in order to carry out broad spectral measurements of ARS. The use of a collimated supercontinuum allows for small area measurements that are often crucial for investigation of nanophotonic samples created using expensive fabrication techniques. The system has been tested and calibrated, and accuracy and reproducibility have been verified by integrating wavelength and ARS data over the angular range and comparing to calibrated integrating sphere measurements.

Helium ion beam lithography (HIBL) is an emerging technique that uses a sub-nanometre focused beam of helium ions generated in the helium ion microscope to expose resist. It benefits from high resolution, high sensitivity and a low proximity effect. Here we present an investigation into HIBL on a novel, negative tone fullerene-derivative molecular resist. Analysis of large area exposures reveals a sensitivity of ~40µC/cm2 with a 30keV helium beam which is almost three orders of magnitude higher than the sensitivity of this resist to a 30keV electron beam. Sparse line features with line widths of 7.3nm are achieved on the ~10nm thick resist. The fabrication of 8.5 half-pitched lines with good feature separation and 6nm half-pitched lines with inferior but still resolvable separation is also shown in this study. Thus, sub-10nm patterning with small proximity effect is demonstrated using HIBL using standard processing conditions, establishing its potential as an alternative to EBL for rapid prototyping of beyond CMOS devices. Display Omitted A novel, negative tone fullerene molecular resist was patterned using helium ion beam lithography (HIBL).Almost 3 orders of magnitude sensitivity improvement was achieved in HIBL compared to EBL for this resist.Dense single-pixel, clearly seperated line features with a half pitch down to 8.5nm were fabricated.Sub-10nm patterning was achieved with high resolution, high sensitivity and low proximity effect.

For 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.

This paper reviews progress in SiGe Heterojunction Bipolar Transistors (HBT) on Silicon-On-Insulator (SOI) technology. SiGe HBTs on SOI are attractive for mixed signal radio frequency (RF) applications and have been of increasing research interest due to their compatibility with SOI CMOS (Complementary Metal Oxide Semiconductor) technology. In bipolar technology, the use of SOI substrate eliminates parasitic substrate transistors and associated latch-up, and has the ability to reduce crosstalk, particularly when combined with buried groundplanes (GP). Various technological SOI bipolar concepts are reviewed with special emphasis on the state-of-the-art SOI SiGe HBT devices in vertical and lateral design. More in depth results are shown from a UK consortium advanced RF platform technology, which includes SOI SiGe HBTs. Bonded wafer technology was developed to allow incorporation of buried silicide layers both above and below the buried oxide. New electrical and noise characterisation results pointed to reduced 1/f noise in these devices compared to bulk counterparts. The lower noise is purported to arise from strain relief of the device structure due to the elasticity of the buried oxide layer during the high temperature epitaxial layer growth. The novel concept of the silicide SOI (SSOI) SiGe HBT technology developed for targeting a reduction in collector resistance, as well as for suppressing the crosstalk, is outlined. The buried tungsten silicide layers were found to have negligible impact on junction leakage. Further to vertical SiGe HBTs on SOI, the challenges of fabricating a lateral SOI SiGe HBT structure are presented.

An investigation of the microscopic mechanisms of Ge self-assembling island growth is of great importance for future optoelectronic applications of quantum dot nanostructures. In this study, two sets of self-assembled germanium islands on Si (001) substrate, with and without preconditioning using a high-temperature hydrogenation step on their nucleation and subsequent temporal evolution, were grown by low-pressure chemical vapor deposition (LPCVD). The average germanium concentration, mean diameter of Ge crystalline regions and the strain inside the germanium quantum dots are characterized with high resolution micro-Raman spectroscopy (?RS). Both the intensity and peak position of the Si–Si vibration mode at about 520.07 cm?1 in the Raman spectra have been used as a reference to separate the germanium Raman signal from the overlapping localized Si–Si optical phonon at ?300 cm?1. In the absence of preconditioning, both the island size and germanium composition increase steadily as a function of deposition time. However, on the H2 preconditioned surface, the nucleation and growth rates are greatly increased during the first stages and slow down significantly after deposition for 10 s. Our results indicate that the compressive strain inside the islands acts as a barrier for Ge adatoms to diffuse from the wetting layer into the islands. For the growth times used in this study, for both sets of samples with and without H2 preconditioning, the normalized rate of increase of the Ge concentration (%? [Ge]/? t) decreases by ?0.13/s for a 1% compressive strain increase. The H2 preconditioning can initially increase the density of island nucleation sites, but cannot accelerate the Ge island growth. It tends to lower %? [Ge]/? t by 0.015/s instead. The decreased strain due to surface roughing is the principal reason why the Ge islands grow so rapidly at the beginning on the H2 preconditioned samples.

A selective epitaxy process for raised sources and drains is investigated, with growth performed at a pressure in the 1 Torr regime, rather than the more common CVD (10s of Torr) or UHV–CVD (1–40 mTorr) regimes. It is shown that selective growth can be achieved using a mixture of silane and dichlorosilane without any requirement for Cl2 or HCl in the gas stream. The selectivity of the process can be controlled by varying the silane:dichlorosilane ratio in the gas mixture, with a ratio between 1:1 and 3:1 giving selective growth. Facet-free selective epitaxy is achieved, the process is selective to silicon nitride and a growth activation energy of 2.4 eV is obtained. Raised source/drain MOSFET devices with channel lengths down to 50 nm have been fabricated and the thickness of the selective epitaxial silicon layer has been varied to investigate the effect of this parameter on device performance. Excellent subthreshold characteristics are obtained and the sub-threshold slope, S, improves from 102 to 81.9 mV/dec as the raised source/drain thickness is increased from 50 nm to 100 nm. The raised source/drain also improves threshold voltage roll-off and drain induced barrier lowering. A decrease in both Ion and Ioff is seen with increasing RSD thickness, but the overall Ioff/Ion trade-off is unchanged. 2005 Elsevier Ltd. All rights reserved.

We have fabricated curved optical micromirrors on silicon. We expect to be able to form open optical cavities between these mirrors and plane mirrors coated on the ends of optical fibres. The curved mirror templates have been prepared with less than 10 nm surface roughness by means of isotropic chemical etching. Two different methods are used for the fabrication of the mirrors: the first method uses a silicon oxide mask, while the second uses dry etching to form an indentation in the silicon surface that is opened out by wet isotropic etching. After coating the silicon substrate with gold, good quality mirrors are obtained with R ~ 98% in the near infra-red. We find that a reflectivity of approximately 98% can be achieved by this method. Cavities formed from these mirrors will be useful for manipulating single atoms.

We 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, ρ(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.

Confined and unconfined selective epitaxial growth using SiH4 and a novel DCS/SiH4/H2 mixture by low-pressure chemical vapor deposition (LPCVD) have been studied. Results indicate that DCS/SiH4/H2 growth with a DCS/H2 prebake provides a very suitable process for confined growth, providing both good epitaxial quality and selectivity. Results also show that unconfined and confined growth using the DCS/SiH4/H2 process gives similar growth in terms of growth rate and faceting. Thickness uniformity was also found to be good in areas away from the wafer edge, and no loading effects were observed.

The time evolution of self-assembled Ge islands, during low-pressure chemical vapor deposition (LPCVD)of Ge on Si at 650 Deg C using high growth rates, has been investigated by atomic force microscopy,transmission electron microscopy, and Rutherford backscattering spectrometry. We have found three different island structures The smallest islands are "lens-shaped" and characterized by a rather narrow size distribution, ~4nm high and ~20nm wide. Next to form are a distinct population of multifaceted "dome shaped" islands, up to 25nm high and 80-150 nm wide. Finally, the largest islands that form are square-based truncated pyramids with a very narrow size distribution ~50nm high and ~250nm wide. The pyramidal islands normally seen in the intermediate size range (~150nm) are not observed. The small lens-shaped islands appear to be defect free, while some of the multifaceted islands as well as all the large truncated pyramids contain misfit dislocations. The existence of multifaceted ialands, in the size range where multifaceted "dome shaped" islands have previously been reported, is attributed to the high growth rate used. Furthermore, under the growth conditions used, the truncated-pyramid-shaped islands are characterized by a very narrow size distribution.

Measuring the spectral response of photoluminescence (SRPL) in solar cells has recently attracted attention as it can be used as a contactless relative measure of external quantum efficiency (EQE) prior to full device fabrication. However, this technique requires that the monitored PL spectrum originates mainly from a region in the solar cell with uniformly distributed majority carriers. For a stack of thin films with a similar material composition, the slightly different emission spectrum from each layer may lead to the superposition of several luminescence peaks. This letter presents the measurement of the SRPL from GaAsP tandem solar cells and outlines a method for separating the individual layer contributions. Good agreement between measured SRPL and EQE at short wavelengths has been achieved, and the deviations at longer wavelengths have been analyzed. This study also reveals unexpected bandgap narrowing resulting from a variable material composition within the active region.

A novel approach to the production of interdigitated back contact (IBC) solar cells is presented. IBC cells have all the contacts on the rear of the cell, thus preventing the ‘dead areas’ that appear on conventional solar cells with front-side contacts. Current IBC cell designs are relatively expensive, but by using laser grooved contacts it is possible to create cheaper IBC solar cells using solar grade silicon wafers. The concept is verified by experiments and detailed TCAD simulations.

This paper will address the feasibility of ZnO as photonic material for the UV region. ZnO films are grown by plasma-assisted MBE. Detailed XRD studies suggest the growth of structurally different epilayers on Al2O3 (0001) and MgAl2O4 (111) substrates. PL shows dominant excitonic emission and very low deep level emission which is indicative of low density of defects or impurities. Stimulated emission based on the excitonic mechanism has been achieved up to 550 K. It is concluded that ZnO can be used for exciton-based photonic device applications owing to the high exciton binding energy. Optically pumped lasing has been demonstrated at room temperature.

Cadmium sulfide thin films, grown by pulsed laser deposition (PLD) on glass substrates, show green photoluminescence and lasing characteristics under optical excitation at room temperature. X-ray diffraction shows the polycrystalline films are of wurtzite structure and have (0 0 2) preferred orientation. Laser thresholds are around 1.8 MW cm−2 and the peak emission is at 501 nm. Fabry–Perot laser modes are spaced at 16 nm indicating cavity lengths of 2.9 μm. Results indicate that laser microcavities are consistently self-formed within the hexagonal lattice.

Photoluminescence at the fundamental transition of thin (1.5 μm) CdS films formed by laser ablation on glass is investigated in the range 4–300 K. In contrast to previous studies on thin CdS films and crystals, the emission takes the form of a single peak over the whole temperature range centered at 487 and 500 nm at 4 and 300 K, respectively. By the application of the van Roosbroeck–Shockley relation and Urbach’s rule, a direct relationship is shown between emission and absorption. It is further demonstrated that the LO phonon is found to form the Urbach tail and determines the temperature induced gap shift.

We report on the growth and characterization of beryllium–chalcogenide layers prepared on GaAs (100) by molecular beam epitaxy. Be- and Te-terminated BeTe surfaces show (4×1) and (2×1) reconstructions, respectively. The stability of each surface is investigated by reflection high energy electron diffraction as a function of substrate temperature. The dependence of growth rate of BeTe on growth temperature and Be cell temperature is investigated. The best full width at half maximum (FWHM) of a (400) x-ray rocking curve of BeTe is 78 arcsec. The dependence of the ZnBeSe energy gap on Be composition is obtained by four-crystal x-ray diffraction (XRD) and low temperature photoluminescence measurements. The energy gap of Zn1−xBexSe varies as Eg=0.0107x+2.790 (eV) for small Be composition (x

ZnO single crystal thin films were grown on c-plane sapphire using oxygen microwave plasma assisted molecular beam epitaxy. Atomically flat oxygen-terminated substrate surfaces were obtained by pre-growth cleaning procedures involving an oxygen plasma treatment. A two dimensional nucleation during the initial growth which is followed by a morphology transition to three dimensional nucleation was observed by in situ reflection high energy electron diffraction. X-ray diffraction (XRD) and photoluminescence investigations suggest that the ZnO epilayer consists of a high quality layer on top of a transition layer containing a high density of defects in the interfacial region. A full width at half maximum (FWHM) of 0.005° is obtained for the ZnO(0002) diffraction peak in an XRD rocking curve, while a broad tail extending from the peak can also be observed. The photoluminescence spectra exhibit dominant bound exciton emission with a FWHM of 3 meV at low temperatures and free exciton emission combined with a ver...

ZnO quantum disks are found to form on the top surface of a thick ZnO epilayer as a result of a morphology transition occurring during the growth of ZnO single-crystal thin films on c-plane sapphire using plasma-assisted molecular beam epitaxy. These disks manifest themselves as triangular pyramids and organize themselves to conform to the six-fold symmetry of the ZnO epilayer underneath with the base edges along 〈1 1 0 0〉 and side facets of {1 2 1 6}. Atomic-force microscopy measurements show a size distribution with base width centered at 140 and 24 nm height, the height/width ratio remaining constant due to the formation of special facets. Cathodoluminescence studies reveal a strong near-band-gap emission from the regions with a high density of pyramids.

ZnSe heteroepitaxial layers have been grown on GaAs (100), (110) on axis, and (110) 6° miscut substrates by molecular beam epitaxy. ZnSe on GaAs (110) shows smooth and featureless spectra from Rutherford backscattering channeling measurements taken along major crystalline directions, whereas ZnSe on GaAs (100) without pre-growth treatments exhibit large interface disorder in channeling spectra. ZnSe films grown on GaAs (110) on axis show facet formation over a wide range of growth conditions. The use of (110) 6° miscut substrates is shown to suppress facet formation; and under the correct growth conditions, facet-free surfaces are achieved. Etch pit density measurements give dislocation densities for ZnSe epitaxial layers grown on GaAs (100), (110) on axis, and (110) 6° miscut substrates of 107/cm2, 3 × 105/cm2 and 5 × 104/cm2, respectively. These results suggest that with further improvements to ZnSe growth on GaAs (110)-off substrates it may be possible to fabricate defect free ZnSe based laser devices.