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Quantifying the potential for abrupt non-linear changes in ecological communities is a key managerial goal, leading to a significant body of research aimed at identifying indicators of approaching regime shifts. Most of this work has built on the theory of bifurcations, with the assumption that critical transitions are a common feature of complex ecological systems. This has led to the development of a suite of often inaccurate early warning signals (EWSs), with more recent techniques seeking to overcome their limitations by analysing multivariate time series or applying machine learning. However, it remains unclear whether regime shifts and/or critical transitions are common occurrences in natural systems, and – if they are present – whether classic and second-generation EWS methods predict rapid community change. Here, using multitrophic data on nine lakes from around the world, we both identify the type of transition a lake is exhibiting, and the reliability of classic and second generation EWSs methods to predict whole ecosystem change. We find few instances of critical transitions in our lake dataset, with different trophic levels often expressing different forms of abrupt change. The ability to predict this change is highly technique dependant, with multivariate EWSs generally classifying correctly, classical rolling window univariate EWSs performing not better than chance, and recently developed machine learning techniques performing poorly. Our results suggest that predictive ecology should start to move away from the concept of critical transitions and develop methods suitable for predicting change in the absence of the strict bounds of bifurcation theory.

Early warning signals (EWSs) represent a potentially universal tool for identifying whether a system is approaching a tipping point, and have been applied in fields including ecology, epidemiology, economics, and physics. This potential universality has led to the development of a suite of computational approaches aimed at improving the reliability of these methods. Classic methods based on univariate data have a long history of use, but recent theoretical advances have expanded EWSs to multivariate datasets, particularly relevant given advancements in remote sensing. More recently, novel machine learning approaches have been developed but have not been made accessible in the R environment. Here, we present EWSmethods – an R package that provides a unified syntax and interpretation of the most popular and cutting edge EWSs methods applicable to both univariate and multivariate time series. EWSmethods provides two primary functions for univariate and multivariate systems respectively, with two forms of calculation available for each: classical rolling window time series analysis, and the more robust expanding window. It also provides an interface to the Python machine learning model EWSNet which predicts the probability of a sudden tipping point or a smooth transition, the first of its form available to R users. This note details the rationale for this open-source package and delivers an introduction to its functionality for assessing resilience. We have also provided vignettes and an external website to act as further tutorials and FAQs.

CO2 Capture, Sequestration and Production of Multi-purpose fuels – Hydrogen, Methane and Biodiesel through Algae route through Post combustion CCS on a fossil fuel fired Plant has been successfully demonstrated at the DST sponsored CO2 sequestration Pilot Plant at the State Technological University of MP, the ‘RGPV’ at Bhopal in Central India. A CO2 Capture efficiency of over 93% has been achieved using MEA solvent of 18 % concentration and the required heat for stripping captured CO2 in the regeneration tower has been worked out at the level of 3.88 MJ per kg of recovered carbon dioxide, which is provided by the low pressure steam of about 150°C and 2 bar pressure @ 1.5-1.6 tons of steam per ton of CO2. Exergy analysis was carried out to optimize this regeneration energy requirement with multiple fuels including bituminous coal, fuel oil and also bio-mass aiming towards negative emission strategy. Production of multipurpose fuels like hydrogen and bio-diesel have been demonstrated in the pilot plant through lignite gasifier via CO and Algae routes respectively. Hydrogen to the tune of 21.92% by volume has been produced, while selected species of Algae grown in open pond with CO2 and Solar flux input provided Algal oil by using Soxhlet extraction process using petroleum ether. Parallel efforts are made in a Solar Thermal Pilot Unit, a MNRE sponsored project, in collaboration with Rensselaer Polytechnic Institute (RPI), NY, USA at the State Private University, RKDF in near vicinity of RGPV’s CCS pilot plant under joint collaboration. Based on above two pilot studies a feasibility analysis is carried out for installation of Carbon Capture, Utilization & Storage plant on a 500 MW Anpara Thermal Power Plant in Northern Coal Fields of India. The Feasibility Study has revealed that it is possible to implement Carbon Utilization by growing selected species of Algae culture in the Ash pond of the plant for almost doubling the Algae growth for bio-diesel production. The feasibility analysis has further revealed that energy penalty for a 30% CO2 capture for a 500 MW Unit would be around 25% including 10% for compression and pumping in an Enhanced Coal Bed Methane (ECBM) technique in proximity to the CO2 source. In any case, the energy penalty in Indian context, when our power planners are targeting at no-storage options for CCS like Carbon sequestration for conversion to multi-purpose fuels rather than CO2 storage, the energy penalty still remains at level of 15-20%. This can be further reduced to a level of 5-10 % if solar thermal device is used for production of steam for solvent regeneration and stripping of CO2. A specially designed Alkali Halide Salt developed by RPI, USA is used as 24x7 heat storage- materials in the pilot plant and the same is integrated with the CCS pilot plant for solvent regeneration and stripping of CO2. Test results of Solar Thermal pilot plant for Steam production with thermal storage, using solid pathway is given which may pave way for development of the technology of Post Combustion Carbon Capture with least energy penalty in regeneration of solvents used in the CCS process.

Light and health is a topic of significant importance and requires fundamental studies of human interactions with light. This talk will present a review of some of the on-going studies on light and health by holistically exposing the humans to the outdoor conditions, while they are confined in indoor spaces. The designs of a new lighting-display fusion system along with examples of emerging commercial products will be discussed. Full spectrum lighting is a critical component of such systems. Results of phosphor blends based on well known thiogallates and sulfide phosphors for designing full spectrum lighting that could replicate sunlight quality will be presented. The synchronous modulation of phosphor based full spectrum light sources with electronic display playing real time outdoor videos creates a new tool for systematically studying the cognitive psycho-physiological health of human beings under different environmental conditions.

This paper presents the development of a double spiral micro-heater and its reliability testing for gas sensing applications. The design and simulation of the micro-hotplate was carried out using MEMS-CAD Tool COVENTORWARE. The micro-hotplate structure consists of a 1.0 µm-thick thermally grown SiO2 membrane of area 600 µm × 600 µm over which a double spiral platinum resistor has been fabricated. A platinum resistor of 117 Ω is fabricated on SiO2 layer using lift-off technique. The platinum deposition was carried out using DC sputtering technique. The hotplate membrane release was accomplished by using both wet and dry etching of silicon. The temperature coefficient of resistance (TCR) of platinum as measured was found to be 2.19 × 10−3/°C. This value has been used to estimate the micro-hotplate temperature. The micro-hotplate consumes only 50 mW power when heated up to 500 °C. The results of reliability testing of fabricated device using pulse mode of operation, maximum current capability and thermal stability have been presented. The hotplate has been shown to continuously operate at 500 °C for more than 4 h and sustain maximum current of 23 mA and 130 cycles of pulse mode operation without any damage to the structure.

Red phosphors with narrow emission around 615 nm (with FWHM~5–10 nm) having chemical compositions of A0.6Ca2.16Mo0.2W0.8O6: Eu0.123+/Na0.12+ (A=Mg, Sr) have been found to exhibit the highest luminescence amongst the molybdate–tungstate family when excited by sources in the 380–420 nm wavelength range. Thus they are most suitable for enhancing color rendering index and lowering color temperature in phosphor converted white LEDs (pc-WLEDs) with near-UV/blue LED excitation sources. The excitation band edge in the near UV/blue wavelength in the reported phosphor has been attributed to the coordination environment of the transition metal ion (Mo6+, W6+) and host crystal structure. Furthermore the quantum efficiency of the phosphors has been enhanced by adjusting activator concentration, suitable compositional alloying using substitutional alkaline earth metal cations and charge compensation mechanisms.

Solid state light sources with dynamically tunable color temperature in the range of 3000–6000 K with chromaticity coordinates lying on the Planckian black body curve has been designed using mixtures of narrow emissions at 615 nm, 575 nm and 550 nm. These respective emissions lines were generated by individual phosphor crystals of trivalent rare earth (RE3+) species, europium (Eu3+), dysprosium (Dy3+) and terbium (Tb3+) activated calcium sodium molybdenum oxide (Ca1−2xNaxMoO4:RE3+x), when excited by near-ultra-violet (NUV) light emitting diode (LED) with emission wavelength of 380 nm. Highly luminescent crystals of these compounds have been grown from molten solutions (flux) of molybdenum (VI) oxide. The flux grown crystals exhibit emission intensity 2–4 times more than phosphor powders of the same compounds synthesized by traditional solid-state reactions. An optimum flux to solute ratio of 2.5 and solute dissolution temperature of 1100 °C resulted in the largest size crystals.

Alkaline earth metal tungstate compounds activated by trivalent europium (Eu3+) are promising candidates for narrow spectral emission red phosphors suitable for high efficacy white light emitting diodes (LEDs). By growing crystals of these compounds from high temperature flux containing Na+ as the charge compensation element, enhancement in the emission intensity of A1 − xWO4: (A = Mg, Ca, Sr) has been demonstrated compared to phosphor powders synthesized using traditional solid state reaction. An optimum flux to solute ratio of 1.5:1 resulted in the largest size crystals using the sodium tungstate flux at growth temperature around 1300 °C. Amongst the three compounds studied, calcium tungstate exhibited the highest luminescence and magnesium tungstate the lowest. These results have been explained by the differences in the ionic radii of the cations compared to the europium activator and the energy bandgap of the host lattice. CaWO4:Eu3+/Na+ flux grown phosphor crystals exhibit almost five times higher emission intensity as compared to phosphor powders synthesized by solid-state reaction.

Alkaline earth metal molybdates are promising candidates as a host material for high efficiency narrow spectral emission phosphors. These phosphors could potentially be used for the fabrication of phosphor-converted light emitting diodes (pc-LEDs). Phosphor crystals of calcium molybdate doped with rare earth dopant Ln 3+ (Ln=Eu, Dy, Tb) grown using flux growth method have been shown to exhibit higher excitation efficiency than the powders synthesized by solid-state reaction process. Molybdenum (VI) oxide has been found to be a suitable flux for growing large size optically transparent high quality crystals at a temperature around 1100 °C. Using the excitation wavelengths of 465 nm, 454 nm and 489 nm for CaMoO 4 : Eu 3+ , CaMoO 4 : Dy 3+ and CaMoO 4 : Tb 3+ , respectively, intense emission lines at wavelengths of 615 nm, 575 nm and 550 nm were observed. The optimized doping concentrations of 12%, 2% and 5% for Eu 3+ , Dy 3+ and Tb 3+ , respectively, provided the highest luminescence intensity.

Optical concentrators that could be used as a flat panel type stationary photovoltaic (PV) system will enable significant reduction in the cost of PV power generation. This paper presents the analysis of a concentrator based on an optically transparent planar waveguide with a diffused reflector on the rear surface and PV devices placed at the edge of the waveguide. Optical collection efficiency of the stationary concentrator for geometric concentration ratios in the range 2–1000 has been analyzed. Despite reduction in optical collection efficiency of this concentrator design compared to traditional flat PV panel or tracking type concentrator panels, for practical operation in the geometric concentration ratio up to 40, a factor of 2–10 times reduction in cost of PV modules per unit power generated can be envisioned due to ease in manufacturing (including flexible roll-to-roll production) and significant savings in materials usage. Experimental data shows a strong dependence of the optical collection efficiency on surface roughness of the diffuser.

In this paper we study automated construction of an agent's behavior for a protocol-based multiagent system, where the exact behaviors of the other agents are unknown. This uncertainty makes it challenging to develop an optimal agent behavior that complies with the system protocol, while ensuring achievement of the agent's temporal goals. To solve this problem, we develop an automated method that uses graph search and model checking techniques to construct the optimal agent behavior with respect to a labeled transition systems model of a protocol and linear temporal logic specification of the agent's goals. We show that our algorithm is sound, complete and optimal. We also conducted computational experiments to evaluate the scalability of our method.

Zinc oxide nanoparticles were created by a top-down wet-chemical etching process and then coated with polyvinyl-alcohol (PVA), exhibiting sizes ranging from 10 to 120 nm with an average size approximately 80 nm. The PVA layer provides surface passivation of zinc oxide nanoparticles. As a result of PVA coating, enhancement in ultraviolet emission and suppression of parasitic green emission is observed. Photoconductors fabricated using the PVA coated zinc oxide nanoparticles exhibited a ratio of ultraviolet photo-generated current to dark current as high as 4.5 × 104, 5 times better than that of the devices fabricated using uncoated ZnO nanoparticles.

A new phosphor alloy Ca1+xSr1-xGa2S3F2: Ce3+, Eu2+ with x = 0.04 exhibits a full spectrum emission in the entire visible wavelength range from 420 to 700 nm when excited by a LED chip with 400 nm emission. By simply depositing varying thicknesses (amount) of this phosphor powder on a 400 nm excitation LED source, white LEDs with correlated color temperatures (CCT) between 1700 K and 20,000 K and (x,y) chromaticity coordinates lying on the Planckian locus of the CIE chromaticity diagram has been demonstrated. To the best of our knowledge, this is the first report on the demonstration of a single full spectrum phosphor based white LEDs exhibiting the entire gamut of white light color temperature with CRI exceeding 90. Specifically CRI greater than 95 is reported for the CCT range of 2200 K – 8500 K.

Bulk growth of phosphorus and arsenic based ternary III–V semiconductor crystals using pseudo-binary melts such as GaP–InP, GaP–GaAs, AlAs–GaAs, etc. is significantly challenging due to the high vapor pressures of group V species in conjunction with slow growth rates and the need for melt replenishment and mixing during growth. Lowering the growth temperature is desirable such that the vapor pressures of P and As can be easily handled. Low growth temperatures could be achieved by using Ga or In rich solutions. However, this approach is less attractive for growing bulk crystals due to several experimental difficulties including sticking of the growth solution to the crucible wall and to the grown crystal, making it challenging for crystal extraction. Growth of ternary crystals from low temperature quaternary melts has been found to be attractive. In this paper, we will present a new method for the growth of Ga1−xInxP from InSb rich Ga1−xInxPySb1−y melts at low growth temperatures in the range of 800–1050 °C. Thermodynamic phase diagrams calculated at various temperatures using a Gibbs free energy minimization software and materials databases commercially available from Thermo-Calc software will be presented along with experimental validation for Ga1−xInxP crystals grown at 1000 °C.

We have developed and established a unique technique for growing highly lattice mismatched ternary and quaternary compounds on binary substrates using a quaternary melt thermo-chemistry. In this technique, growth is initiated from a pseudo-quaternary melt on compatible substrates. Growth of GaInAs, InAsSb, GaInAsSb and InSb epilayers on GaAs has been achieved using In–Ga–As–Sb melts. The grown epilayers have a uniform composition and are very thick ( > 100 μ m ) . Between the GaAs substrate and the uniform composition epilayer, there exists a graded composition quaternary which is found to be extremely beneficial in relieving misfit. No specific efforts were made to change growth conditions (during epi-growth) to compositionally grade the buffer layers. Hence, this growth scheme is extremely simple to implement and can be used for the growth of a variety of alloy semiconductors by appropriately selecting the growth temperature and melt composition. One of the key highlights of this work is the growth of In x Ga 1 - x As y Sb 1 - y epilayers with cut-off wavelength of 10 μ m on GaAs substrates. This paper will discuss the melt thermo-chemistries and the process for this new epilayer growth method.

Indium arsenide (InAs) is a direct bandgap semiconductor ( E g =0.36 eV) and is well suited for non-linear optic (NLO) applications. The NLO applications require high linear optical transmission for below-bandgap energy photons in the material. Due to this stringent criterion, there is a need for understanding the fundamental optical absorption mechanisms in the material as well as correlating them with the presence of native defects and impurities introduced during the crystal growth. In this paper, we will present the experimentally measured and theoretically analyzed infrared absorption in an undoped and Te-doped n-type InAs in the wavelength region of 4–20 μm for horizontal Bridgman grown crystals. Transmission measurements at 300 K were performed on samples of undoped and Te-doped n-type InAs with carrier concentration in the range of 2.6×10 16 –1.7×10 17 cm −3 . The dependences of various absorption mechanisms as a function of wavelength have been presented. Free carrier absorption by electrons is found to be the dominant absorption mechanism at longer wavelengths ( λ >9 μm) for all the samples. At shorter wavelengths ( λ n i =1×10 15 cm −3 ), the inter-valence band transitions by free holes contributes significantly to the total absorption. Unique fitting parameters have been deduced to theoretically explain the infrared absorption mechanism in InAs for the above-mentioned carrier concentration range.

A novel Zn diffusion technique in n-GaSb substrate from a low temperature chemical bath deposited ZnS layer has been developed to obtain high breakdown voltages. Junctions formed by this technique have breakdown voltages of ∼ 18.5 V , low reverse leakage current (0.01– 0.03 A / cm 2 at - 3 V ), excellent reverse current saturation and ideality factor of ∼ 1.3 . The high breakdown voltages obtained are due to the co-doping of zinc and sulfur from the ZnS film. Sulfur forms shallow and deep levels that compensate the p-doping of zinc. The non-linear relation of the inverse of the zero-bias resistance area product ( 1 / R 0 A ) versus perimeter to area ratio ( P / A ) in these diodes indicates surface leakage is the dominant leakage mechanism. CdS has been used to passivate the mesa photodiodes. After passivation, the 1 / R 0 A product reduces from 0.3 to 0.02 Ω - 1 cm - 2 for a 150 μ m diameter device. The 1 / R 0 A product is also independent of the diode dimension confirming effective passivation. ZnS surface passivation on the mesa walls is not effective and is found to increase the leakage current.

This paper reports the fabrication and characterization of GaInAsSb photodetectors operating at 2 µm. At room temperature, the performance of these photodiodes under reverse bias conditions is limited by the surface leakage. A model has been developed to separate the bulk (diffusion and generation-recombination (g-r)) and the surface leakage contributions toward the total leakage current. By fitting this model to the experimental data, values of material parameters such as minority carrier diffusion length and lifetime have been estimated. The highest R0A of 55 Ω-cm2 has been obtained with a responsivity of 0.44 A/W at 2 µm.

Using ZnO nano-colloids, thin films of ZnO have been deposited and characterized. The nano-colloid has been synthesized using a top-down wet chemistry method starting from submicron ZnO particles. The steric stabilization technique is implemented using stearic acid as a capping agent to prevent agglomeration of the ZnO nanoparticles. A stable suspension is achieved, exhibiting strong optical absorbance and a band edge at 376 nm. The simple chemistry enables a uniform film on a variety of substrates using spin coating. Current-voltage (I–V) measurements of ZnO films deposited on quartz after annealing treatment show a resistivity of 1.89×105 Ω · cm.

Bulk crystal growth of aluminum antimonide (AlSb) (50 mm diameter) in silica crucibles without any sticking problem has been demonstrated for the first time. The adhesion of AlSb melt to silica crucible could be eliminated by employing a LiCl–KCl encapsulation in conjunction with excess antimony in the melt. This paper also presents a new surface preparation recipe based on KOH treatment that has been found to result in superior AlSb surfaces necessary for making reliable ohmic contacts for electrical property measurements.

The mode of redistributing the excess solute species at the melt–solid interface accumulated during the growth of bulk ternary semiconductor crystals is a key factor dictating the crystalline quality and spatial alloy composition. Since the solidification temperatures vary with alloy composition, a control over both heat and mass transport during crystal growth is necessary for achieving spatial compositional homogeneity in the grown crystals. The effects of forced and natural convections, as well as diffusion (in the liquid) on the radial alloy distribution in vertical Bridgman growth Ga 1− x In x Sb bulk polycrystals (20–50 mm diameter) have been investigated. In the absence of forced convection, the shape of the solid–liquid interface has been found to be highly curved under growth conditions necessary for avoiding constitutional supercooling (high axial temperature gradients and low growth rates). Lowering the axial temperature gradient does flatten the interface shape, but extremely slow growth rate (below 0.1 mm/h) is necessary for avoiding constitutional supercooling. Forced convection helps in accelerating the removal and re-distributing the excess solute from the growth interface to the entire melt volume thus leading to a 4–5 fold increase in the growth rate without constitutional supercooling and flattening the solid–liquid interface necessary for uniform radial composition in the crystal. However, the mechanism of forced convection coupled with axial temperature gradient in the melt leads to thermal fluctuation at the growth interface. As a consequence, fluctuation in radial alloy composition is observed. A comparative study of various melt stirring schemes with different axial temperature gradients and the resulting alloy distribution in the crystals has been presented in this paper.

The below bandgap infrared transmission (up to 25 µm) in undoped Ga1−xInxSb bulk crystals has been studied for the first time and found to be limited by native defects such as antisites and vacancies found in antimonide-based III–V compounds. For the gallium-rich alloy compositions (x 0.5 (the indium-rich alloy compositions), the crystals exhibit n-type conductivity when the net donor concentration and indium content in the crystals increase. A correlation between the optical transmission and the residual carrier concentration arising from the native acceptors and donors has been observed. Due to donor-acceptor compensation, crystals with alloy compositions in the range of x=0.5−0.7 exhibit high optical transmission for a wide wavelength range (up to 22 µm). The light hole to heavy hole interband transition in the valence band and the free electron absorption in the conduction band have been found to be the two dominant absorption processes.

Enhancement in below bandgap room temperature infrared transmission has been observed in tellurium (Te)-doped GaSb bulk crystals. The effect of Te concentration on the transmission characteristics of GaSb has been experimentally and theoretically analysed. Undoped GaSb is known to exhibit p-type conductivity with residual hole concentration of the order of (1–2) × 1017 cm−3 at room temperature due to the formation of native defects. For such samples, inter-valence band absorption has been found to be the dominant absorption mechanism. The residual holes could be compensated by n-type dopants such as Te. With increasing Te concentration, free carrier absorption due to electrons and inter-valley transitions in the conduction subband become significant. The dependences of various absorption mechanisms as a function of wavelength have been discussed in this paper.

The optical and electrical properties of tellurium- (Te) compensated Ga1−xInxSb bulk crystals with alloy compositions ranging from x=0.37 to x=0.98 have been investigated. It has been observed that the Burstein-Moss shift plays an important role in the optical properties of Ga1−xInxSb crystals for the alloy compositions greater than x=0.5 and net donor concentrations in the 2.9×1017 to 2.6×1018cm−3 range at 300K. A good agreement has been obtained between the theory and experimental observations. Furthermore, electrical characterizations at 300 and 77K show that Te compensates the native defects in Ga1−xInxSb, irrespective of the alloy composition.

Heavily compensated GaSb (2-in.-diameter substrates) with resistivity as high as 7 × 103 Ω-cm, corresponding to a net donor concentration of 3.5×1013 cm−3 at 77 K, and 16.4 Ω-cm, corresponding to net donor concentration of 1.16×1016 cm−3 at 300 K, have been obtained by tellurium (Te) compensation in vertical-Bridgman-grown bulk crystals. Very interesting p- to n-type as well as n- to p-type changes have been observed as a function of temperature in these samples.

A novel process has been developed which allows the growth of device grade ultra-smooth epitaxial layers of antimonide based III-V compounds with a controlled thickness using liquid phase epitaxy (LPE). GaSb epilayers (with thickness in the range of 20-50 /spl mu/m) on GaSb single crystalline substrates have been grown with a root mean square surface roughness of less than 1 nm over an area of 5/spl conint/ /spl larr/ 5 /spl mu/m.

Radio-frequency (RF) photoreflectance measurements and one-dimensional device simulations have been used to evaluate bulk and surface recombination parameters in doubly capped, 0.50–0.59-eV, p-type InGaAsSb epitaxial materials. The InGaAsSb lifetime structures with variable active-layer thicknesses are used to extract the surface recombination velocity (SRV), while samples with different active-layer doping concentrations have been used to determine the Auger and radiative recombination coefficients. The RF photoreflectance measurements and analysis are compatible with a radiative recombination coefficient (B) of approximately 3×10−11 cm3/s, Auger coefficient (C) of 1×10−28 cm6/s, and SRV of ∼103 cm/s or lower for 0.50–0.59 eV, doubly capped, p-type InGaAsSb epitaxial layers.

A single-step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high-efficiency GaSb thermophotovoltaic (TPV) cells. The junction depth was controlled through monitoring of light current-voltage (I–V) curves (photovoltaic response) during the post-diffusion emitter-etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum efficiencies (QEs) and the open-circuit voltages in the fabricated devices. An optimum junction depth for obtaining the highest QE and open-circuit voltage is presented based on diffusion lengths (or minority carrier lifetimes), carrier mobility, and the typical diffused impurity profile in GaSb.

We report on a transparent gate silicon MOS photo-impedance sensor, where a gated light sensitive semiconductor layer connects fixed capacitances. The resistance of the semiconductor and the capacitance of the MOS structure change with illumination. The frequency dispersion makes the coupling of these capacitances sensitive to light intensity extending the sensor dynamic range and tuning the sensitivity of the sensor. Our modeling results demonstrate advantages of this novel sensor in terms of sensitivity and dynamic range. The design and concept of this device could be extended to many other semiconductor materials, where frequency dispersion is related either to traps, or embedded nanoparticles or carrier generation processes.

We report on the effects of the frequency dispersion in light sensitive materials used in photoimpedance wireless sensors. An example of such a sensor is a gated semiconductor connecting two or more fixed capacitances. The impedance of the device under illumination is changed by the change in the photoresistance of the semiconductor layer and the change in the gate-semiconductor capacitance. We report on the design and simulation of the frequency dispersion of the impedance of this device in silicon and discuss the physics and device performance. We also evaluate the dynamic range and sensitivity of the wireless photoimpedance sensors and show their advantages for wireless sensing applications compared to more conventional light sensors.

The energy band gap and lattice constant of the quaternary alloy GaxIn1−xAsySb1−y were determined over the entire composition space (x, y) using a correlated function expansion (CFE) technique to interpolate from observed ternary compound data. Considerable anomalous band gap behavior (i.e., deep bowing) was found, which produced band gap minima as a function of composition. This deep bowing effect was experimentally observed in a quasibinary alloy (GaSb)1−z(InAs)z at low values of z. In addition, the CFE lattice matching relations of the alloy grown on GaSb and InAs were obtained as a function of composition, and the corresponding band gaps were estimated. The CFE estimates were in good agreement with existing experimental data.

Axial and radial segregation of (i) Ga in Ge and (ii) InSb in GaSb has been evaluated in crystals grown by the submerged heater method. The values of di!usion coe$cients obtained by "tting the Tiller’s equation to the initial transients in composition are signi"cantly lower than the values in the literature, obtained by using shear cells with capillaries. ( 2000 Elsevier Science B.V. All rights reserved.

Based on intrinsic alloy phase formation chemistry and thermodynamics, a novel and unique way of producing compositionally homogeneous multi-component (binary, ternary, quaternary) semiconductor materials is presented. A free energy minimization computer program licensed from AEA Technology Engineering Software, Inc., has been employed to study the composition of the solidifying phases from Ga-In-As-Sb melts at different temperatures and with various liquid compositions. The solid phases have been identified (theoretically and experimentally) to be either ternary compounds of Ga1−xInxAsySb1−y depending on the melt temperature and composition. By engineering the thermochemistry of preferential phase formation in the Ga-In-As-Sb melt, compositionally uniform, single phase, crack free, large polycrystalline Ga1−xInxSb and Ga1−xInxAs have been grown.

Exceptionally uniform, steady-state tellurium segregation was observed in a GaSb single crystal grown by the submerged heater method. The single crystal was grown at 5 mm/h in a 20 mm diameter silica crucible using a 20 mm diameter 〈1 1 1〉 B seed and with melt encapsulation by NaCl–KCl salt. The axial dopant profile consists of three regions: an initial transient, a steady state uniform region and a last to freeze inhomogeneous portion. In the steady state region, the dopant concentration is equal to the initial tellurium content ( C o ) in the melt. The axial tellurium profile was best fitted by the Tiller's equation for diffusion controlled segregation with k =0.37 and D =1.0×10 −5 cm 2 /s. The inhomogeneous composition in the last to freeze portion is caused by Marangoni convection which takes place after the baffle separates from the melt. The electrical properties of the crystal were evaluated by Hall measurements and four-point probe technique.

Large crystals of a quasi-binary semiconductor alloy (GaSb) 1~x (InAs) x with x "0.02—0.05 have been grown from melt for the first time. The family of quasi-binary crystals (GaSb) 1~x (InAs) x grown and reported here are di⁄erent from the conventional Ga 1~x In x As y Sb 1~y quaternaries due to growth behavior and physical properties. Significant narrowing of the band gap was observed in these crystals compared to the conventional quaternary Ga 1~x In x As y Sb 1~y (with x"y). With an InAs content of about 2—5 at%, band gaps in the range of 0.6—0.65 eV have been demonstrated. The possible origins of the band gap narrowing (i.e., high bowing parameter) include chemical and structural alterations in the grown crystals, resulting from the association of Ga—Sb and In—As in the melt. ( 1999 Elsevier Science B.V. All rights reserved.

It was demonstrated that forced convection or mixing in the melt during directional solidification of bulk InxGa1−xSb (0

The effect of aluminium on the defect structure of GaSb crystals, grown by the vertical Bridgman technique, has been investigated by cathodoluminescence (CL) in the scanning electron microscope. Crystals have been found to be highly homogeneous along the growth axis with the exception of the top end which showed Al accumulation. The CL results show decoration of extended defects by Al but contrary to the case of other dopants Al has not been found to cause a significative reduction of native accepters. A CL band at about 850 meV appears to be related to the presence of aluminium.

〈1 1 1〉B and 〈1 0 0〉 oriented tellurium-doped GaSb single crystals have been grown by the vertical Bridgman technique, with and without a baffle in the melt. The axial and radial segregation has been investigated. Nearly diffusion controlled steady-state segregation was reached with a baffle submerged in the melt above the melt–solid interface. The experimental data has been compared with the theoretically calculated curves using a segregation coefficient of 0.37 and distribution coefficient of 3×10 −5 cm 2 /s for Te in GaSb.

In0.2Ga0.8Sb has been synthesized and polycrystals have been grown by the vertical Bridgman technique. Sticking of ingot to the quartz ampoule and ampoule cracking have been found to be major problems. The presence of native oxides on the surface of the starting material impedes the formation of homogeneous compounds due to which the structural quality of the ingots is poor. Worst sticking is observed with improper homogeneization and when excess of pure indium is present on the surface of the crystal. Several procedures have been adopted to avoid sticking and cracking. The most efficient way of solving this problem is to employ a high-temperature baking in dynamic vacuum prior to synthesis by placing the ampoule in a region of the furnace with a temperature gradient. The post-baking cooling rate is also extremely critical to avoid cracking. The polycrystalline alloy thus formed by adopting the pre-synthesis baking cycle shows better homogeneity and structural quality.

We present the homogeneity and luminescence properties of bulk GaSb obtained by the cathodoluminescence (CL) technique in the scanning electron microscope. The samples used in this study are as-grown undoped and impurity diffused (tellurium) and doped (chromium) material. CL investigations have revealed a non uniform distribution of native defects in GaSb wafers. Post growth annealing in vacuum, gallium or antimony atmospheres causes an increase in homogeneity in CL images. Te diffusion and Cr doping provides new information about defects in GaSb. CL images and CL spectra recorded in these samples support that the type of defects formed is a function of diffusion time and impurity concentration.

Spatial compositional analysis has been carried out on single and polycrystal wafers of GaSb grown from stoichiometric and non-stoichiometric melts. In crystals grown from stoichiometric melt, the ratio of Ga to Sb is slightly more and remains uniform throughout. At the grain boundaries in polycrystals, the Sb content is more than in the other regions of the crystal. Crystals grown from either Ga- or Sb-rich melts exhibit inclusions of the excess component. Post-growth annealing treatments in vacuum and Ga-rich atmospheres have been performed. Heat treatments in vacuum atmosphere produce very little effect on the local composition of the crystal. On the other hand, localized crystallization at grain boundaries and inclusions takes place in the presence of excess gallium. It has been shown that annealing treatments in Ga ambient can produce defect-free wafers with extremely homogeneous composition. It is concluded that the excess Sb which is liberated from the crystal during growth resides at the grain boundaries and other extended defect centers. The vacant Sb sites are then responsible for the formation of the native acceptor centers like $V_{Ga}$ and $Ga_{sb}$.

High quality layers of Al 0.82 Ga 0.18 As y Sb 1 − y have been grown on GaSb substrates by the liquid phase epitaxial (LPE) technique. The effect of arsenic incorporation at various growth temperatures on the surface morphology, lattice mismatch and photoluminescence has been studied. The quality of the epilayers has been found to depend strongly on the pre-growth melt dissolution sequence and improves with increase in growth temperature. Perfectly lattice matched epilayers with excellent optical properties could be grown at 650°C.