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A simple and cost-effective maskless-photolithography system is developed using a digital projector and a stereo-zoom optical microscope. The optical microscope is used to focus a highly divergent light from the projector into a smaller region of space. An eyepiece of the microscope helps to ensure the pattern placement accuracy, focusing of light and re-alignment of the substrate with respect to the incident light. The patterns which are to be exposed are designed in a PowerPoint software, where the intensity and duration of the exposure are directly controlled via a computer. The system is found to be beneficial for the fabrication of various structures with minimum feature size ∼ 20 µm, over an exposure area 6×8 mm2. A uniform development of photoresist material is realized throughout the exposure area, and the exposure time (3 s) is found to be invariant for different structures having dimension 20-8000 µm. The maskless-photolithography system is used to fabricate various structures for basic research and development of semiconductor diode lasers, detectors, etc. A Hall bar structure fabricated on a GaAs/AlGaAs quantum well sample is used to perform the quantum Hall effect measurements and thereafter the transport properties of charge carrier are investigated.

Photoluminescence (PL) and Photoluminescence Excitation (PLE) measurements are performed on MOVPE grown InAlN/AlN/InGaN/GaN/Sapphire High Electron Mobility Transistor Structures. Features associated with InAlN barrier layer, InGaN channel layer and GaN buffer layer are clearly seen in the PL spectra. A blueshift of PL features with excitation intensity is observed which is claimed to be the signature of 2-dimensional electron gas formed in InGaN layer. By comparing the integrated intensity of PL peaks recorded at 7K and room temperature, it is confirmed that the optical quality of InGaN and GaN layers is superior when compared with that of InAlN barrier layer in the HEMT structure. It is also seen that the PLE features of InGaN channel layer are considerably red shifted with respect to PL features of the same layer. It is explained by considering the screening of polarization induced electric field causing a blue shift in case of PL measurements.

The effect of surface and interface states of various GaAs based structures such as n+GaAs, n-n+GaAs and p-i-n GaAs are investigated for understanding the detector performance. These structures are grown by metal organic vapor phase epitaxy (MOVPE). Surface photovoltage spectroscopy is performed in these structures to investigate the role of surface and interface states on the absorption spectra. Decrease in surface photovoltage signal with increased chopping frequency indicate the presence of slow temporal response states. These states get passivated by growing homo epitaxial layer of GaAs on n+GaAs substrate. Subsequently, for the isolation of surface and interface states on the p-i-n detector structure, n-n+GaAs and p-i-n GaAs structures are investigated separately. Further, the detectors of these structures are developed and their spectral response are recorded at room temperature. The responsivity values are evaluated by varying the incident power of 690 nm laser. Ten times increase in responsivity is observed in p-i-n GaAs photodetector compared to n-n+GaAs photodetector. A low dark current (∼ 0.5 nA) is also obtained in p-i-n GaAs photodetector which qualifies its better device performance.

GaN metal-semiconductor-metal (MSM)ultraviolet photodetectors without and with annealed ZrO 2 insulating layers are successfully fabricated. It is found that the dark current is reduced by 3 times post annealing of the oxide layer. GaN MSM photo detectors exhibit large internal gain which can be explained by considering the presence of hole traps in oxide layer. Further, reduction in gain and increase intransient response time of photodetectors post annealing of ZrO 2 layer reveal that the density of shallow trap states is considerably reduced by the annealing step. It is noticed that though the response of all the states is affected by the annealing but the fast component of photo response is rather severely hampered. The present investigations clearly demonstrate that reduction of dark current is not the sole criteria for improving the performance of a photodetector. Rather, transient response of the device and hence the bandwidth of the device along with the leakage current and gain should be taken into consideration for qualifying the overall performance of the photodetectors.

Williamson–Hall (WH) analysis is a well established method for studying the microstructural properties of epilayers grown on foreign substrates. However, the method becomes inapplicable in specific cases where the structure factor considerations and the presence of anti-phase domains forbid the data acquisition for certain reflections in conventional high-resolution X-ray diffraction (HRXRD) measurements. Here, this limitation is overcome by exploiting the large intensity (25 µW mm−2) and high photon energy (15.5 keV) of the X-ray beam obtained from a synchrotron radiation source. The lateral coherence length, vertical coherence length, tilt and micro-strain of GaAs epilayers grown on Si substrate have been successfully measured using the conventional WH analysis. The microstructure information obtained from the conventional WH analysis based on the data acquired at the synchrotron radiation source is in reasonable agreement with the results obtained from atomic force microscope and surface profiler measurements. Such information cannot be obtained on a laboratory-based HRXRD system where modification of the WH method by involving a set of parallel asymmetric crystallographic planes is found to be essential. However, the information obtained from the modified WH method is along a different crystallographic orientation.

Background: Samarakhai (Byttneria herbacea Roxb.), family Sterculiaceae, is one of the reputed folklore medicinal herbs, found in many parts of India. Although consumed as a vegetable since long time, its root and leaves are not yet reported for its nutritive value and antioxidant activities. Aim: The aim of this study was to evaluate nutritional value and antioxidant potential of root and leaf of B. herbacea Roxb. Materials and methods: Nutritional parameters such as carbohydrate, fat, protein, energy value, calcium, iron, zinc, manganese, phosphorus and vitamin C were evaluated. Antioxidant activity was evaluated through three test methods, i.e., 1,1-diphenyl-2-picrylhydrazyl, ferric-reducing antioxidant power (FRAP) and phosphomolybdenum assay. Results: B. herbacea roots and leaves showed the presence of total carbohydrate 46.39 g/100 g and 40.12 g/100 g, total fat 0.63 g/100 g and 1.20 g/100 g, true protein 11.46 g/100 g and 10.49 g/100 g, energy content 237.07 kcal/100 g and 213.24 kcal/100 g, iron 821.10 ppm and 889.64 ppm, zinc 9.2 ppm and 47.98 ppm, manganese 329.86 ppm and 474.59 ppm, phosphorus 0.40 ppm and 0.10 ppm and calcium 4856.84 ppm and 14964.49 ppm, respectively. The half-maximal inhibitory concentration values of the methanol extract of root, leaf and ascorbic acid were found to be 217.25 μg/ml, 131.42 μg/ml and 178.88 μg/ml, respectively. In FRAP assay, antioxidant activity of methanol extract of leaf (129.15 μM) was found to be more than root (73.13 μM). Conclusion: B. herbacea root contains high amount of true protein, carbohydrate and energy value, while micronutrients such as iron, zinc, manganese and calcium are more in its leaf. Both roots and leaves exhibited potent antioxidant activity where the leaves possess more values than the roots.

BACKGROUND: Byttneria herbacea Roxb., an extrapharmacopoeial plant of Ayurveda, locally known as Samarakhai by tribal people of Odisha, belongs to the family Sterculiaceae. The roots of B. herbacea is traditionally claimed to be used in the management of wounds, fractures, swellings, gynecological disorders, diarrhea, cuts, ulcers, and others. Although the plant is used traditionally, for its pharmacognostical characteristics, no proper scientific evaluation has been reported. AIMS: To establish and standardize the root of the plant for its microscopical and physicochemical characteristics along with different qualitative tests and to assess heavy metals, aflatoxins, and insecticide and pesticide residue. MATERIALS AND METHODS: After proper authentication, the roots were exposed to macroscopy, microscopy and physicochemical, qualitative and high-performance thin-layer chromatography study as per the Ayurvedic Pharmacopoeia of India. Heavy metal analysis, aflatoxin analysis, and insecticide and pesticide residue analysis of the root were carried out following the standard methods. RESULTS: Microscopic study, through transverse section of root shows outer multilayered elongated compactly arranged cork cells, multiseriate medullary rays embedded with brown content, starch grains and prismatic crystals, centrally located vascular bundle consist of xylem and phloem. Physicochemical parameters showed that water-soluble extractive value (11.995%) was more than that of methanol-soluble extractive (5.936%). Chromatography study exhibited 8 peaks at 254 nm and 4 peaks at 366 nm. Heavy metals and aflatoxins B2, G1, and G2 were found in the prescribed limit, whereas aflatoxin B1 was above the limit of quantification. Insecticide and pesticide residues were not detected in the sample. CONCLUSION: Typical microscopical characteristics are multiseriate medullary rays, tannin contents, and prismatic crystals. Heavy metals and aflatoxins B2, G1, and G2 were found below the limit of quantification. The findings of the study may be helpful to identify, standardize, and for quality assessment of root of B. herbacea.

We have grown Ga doped ZnO (GZO) thin films at moderate temperatures with Ga concentrations in the range varying from 0.25 to 3 at.% on sapphire substrates using buffer assisted pulsed laser deposition. Room temperature resistivity measured was ∼5.1 × 10 −5 Ω cm with a electron mobility of ∼41.9 cm 2 /V s for an optimum Ga concentration of ∼0.75 at.% in the GZO films. Buffer assisted growth methodology maintains relatively good crystalline quality of the GZO thin films, thereby improving the electron mobility even at high dopant concentrations. This plays a key role in decreasing the resistivity of GZO films to the aforementioned value, which to the best of our knowledge is the lowest so far. These highly conducting GZO thin films with good mobility are potential candidates for transparent conducting oxide (TCO) applications in various optoelectronic devices.

Metal organic vapor phase epitaxy grown GaAs p-i-n photodetector devices are fabricated and tested for the assessment of practical usage of the detector after the exposure to high radiation doses of γ-ray. Increased values of saturation current, ideality factor and leakage current after 360 kGy γ-ray irradiation confirm a substantial increase in the number of generation–recombination centers. It is further observed that the leakage current density, current per unit volume (Jv), increases linearly with the radiation fluence (Φ). The slope (α=ΔJv/ΔΦ) of the leakage current density versus γ-ray radiation fluences curve is two order less (4–5×10−l9 A/cm) for GaAs compared to Si (4–6×10−l7 A/cm). The lower value of α (radiation damage constant) confirms that GaAs is radiation harder than Si. Subsequently, it is also observed that the photo response of 360 kGy γ-ray irradiated GaAs device is reduced by ~50% due to the reduction of quantum efficiency by the radiation induced generation–recombination centers. The functionality of the irradiated sensor is verified by comparing the response of the pristine and irradiated detectors to the photoluminescence of semiconductor quantum well structures.

The influence of La and Nd co-substitution on structure, electric and magnetic properties of epitaxial thin films of BiFeO3 (BFO) was examined. We demonstrate structural phase transition in co-doped La and Nd BFO thin films using Raman spectroscopy. Based on group theoretical analysis of the number and symmetry of Raman lines, we provide strong experimental evidence that the structure has been changed from rhombohedral to monoclinic due to co-doping in BFO. The change in structure was also reflected in morphology of these films. Room temperature magnetic hysteresis curves showed that doped films exhibit enhanced ferromagnetic properties with remnant magnetization of ~10 emu/cm3 and coercive field of 1.2 kOe. The enhanced magnetic properties highlight the potential applications of doped BLNFO thin film for smart devices. Copyright © 2015 John Wiley & Sons, Ltd.

Effect of charge carrier confinement and ultra-low disorder acquainted in AlGaAs/GaAs multi-quantum well system is investigated via Magneto-photoluminescence spectroscopy. Significant increase of effective mass is observed for the confined exciton in narrow QWs. The foremost reason behind such an observation is due to the induced non-parabolicity in bands. Moreover, as the thickness of the QW are reduced, confined excitons in QW experience atomic irregularities at the hetero-junctions and their effects are prominent in the photoluminescence linewidth. Amount of photoluminescence line-broadening caused by the atomic irregularities at the hetero-junctions is correlated with average fluctuation (δ1) in QW thickness. The estimated δ1 for Al0.3Ga0.7As/GaAs QWs are found to be ±(0.14 − 1.6)× ‘one monolayer thickness of GaAs layer’. Further, the strong perturbations due to magnetic field in a system helps in realizing optical properties of exciton in QWs, where magnetic field is used as a probe to detect ultralow defects in the QW. Additionally, the influence of magnetic field on the free and bound exciton luminescence is explained by a simple model. The proposed approach for measuring the interface and volume defects in an ultra-low disordered system by Magneto-PL spectroscopy technique will be highly beneficial in high mobility devices for advanced applications.

We show that in a GaAs0.86P0.14/Al0.7Ga0.3As near-surface quantum well, there is coherent oscillation of holes observed in time-resolved reflectivity signal when the top barrier of the quantum well is sufficiently thin. The quantum well states interact with the surface states under the influence of the surface electric field. The time period of the observed oscillation is 120±10 fs.

The band alignment and the quantum states of InAsxP1−x/InP surface quantum well (SQW) are investigated using ultraviolet photoelectron spectroscopy (UPS) and photoluminescence techniques. The analysis of UPS spectra of core levels In 4d, As 3d, P 3p and P 2p confirms that the top surface of InAsxP1−x/InP SQW contains an admixture of oxides, which convert into a more stable form of oxide, In(PO3)3, after optimum Ar+ ion sputtering. The values of valence band offsets between In(PO3)3/InP and InAsxP1−x/InP (x=0.57) SQW are 2.1 eV and 0.3 eV respectively, as obtained from the onsets of UPS spectra. Further, the band structure of InAsxP1−x/InP SQW is also estimated from the UPS spectra of core levels and the valence band onsets after optimum Ar+ ion sputtering. The emission of the quantum states (e1–hh1) transition blue-shifts by about 60 meV for InAs0.38P0.62/InP SQW when the top InP barrier layer thickness is decreased from 22 A to 11 A. The experimental values of e1–hh1 electronic transition in SQW match well with those of the theoretically calculated ones using a 3.6 eV potential barrier at the InP surface.

Background : Balanites aegyptiaca (Linn.) Delile. & Moringa oleifera Lam. are well known drugs in Ayurveda for their wound healing property as they possess krimighna action or ability to kill foreign pathogens. Ingudi & Shigru leaves are used for wound healing purpose. Therefore leaves of both the plants were selected for antimicrobial study against surgical wound infection causative organisms. Methods: Both aqueous and alcoholic extracts of leaves of both plants at 10, 20, 30% concentrations were tested for antimicrobial activity by agar well diffusion method against a range of gram-positive and gram-negative bacteria. Zone of inhibition of extracts were determined, then activity index & IC50 value was calculated. Results: Both ethanol and aqueous extracts of Balanites aegyptiaca (Linn.) Delile. & Moringa oleifera Lam. inhibited the growth of all the tested strains of bacteria. Aqueous extract of Balanites aegyptiaca (Linn.) Delile has more potent action against microorganism Staphylococcus aureus, Escherichia coli, Klebsiella aerogenes, ZOI (15mm,11mm,19mm respectively), AI(0.6, 0.61, 0.70 resp.), IC50 (1.778, 1.586, 1.196 resp.). Conclusion: Leaves of Balanites aegyptiaca (Linn.) Delile possess good antimicrobial effects against microorganism Staphylococcus aureus, Escherichia coli, Klebsiella aerogenes compared to Moringa oleifera Lam. Efficacy is very much significant against Klebsiella aerogenes.Â

Effect of charge carrier confinement and ultra-low disorder acquainted in AlGaAs/GaAs multi-quantum well system is investigated via Magneto-photoluminescence spectroscopy. Significant increase of effective mass is observed for the confined exciton in narrow QWs. The foremost reason behind such an observation is due to the induced non-parabolicity in bands. Moreover, as the thickness of the QW are reduced, confined excitons in QW experience atomic irregularities at the hetero-junctions and their effects are prominent in the photoluminescence linewidth. Amount of photoluminescence line-broadening caused by the atomic irregularities at the hetero-junctions is correlated with average fluctuation (δ 1) in QW thickness. The estimated δ 1 for Al0.3Ga0.7As/GaAs QWs are found to be ±(0.14 − 1.6)× ‘one monolayer thickness of GaAs layer’. Further, the strong perturbations due to magnetic field in a system helps in realizing optical properties of exciton in QWs, where magnetic field is used as a probe to detect ultralow defects in the QW. Additionally, the influence of magnetic field on the free and bound exciton luminescence is explained by a simple model. The proposed approach for measuring the interface and volume defects in an ultra-low disordered system by Magneto-PL spectroscopy technique will be highly beneficial in high mobility devices for advanced applications.

InAsxP1-x/InP quantum wells grown using metal organic vapor phase epitaxy are investigated for infrared detector applications. The structural parameters of the QWs are evaluated from high resolution x-ray diffraction. The electronic transition energies measured from surface photo voltage and photoconductivity confirms that these QWs can be used for fabricating IR detectors in the wide wavelength range, i.e. 0.9–1.46 µm by inter-band transitions and 7–18 µm by inter-sub-band transitions. Subsequently the functionality of one such fabricated InAsxP1-x/InPQW detector is verified by measuring the photoluminescence of suitable semiconductor quantum well structure.At the request of all authors of the paper, and with the agreement of the Proceedings Editor, an updated version of this article was published on 24 June 2016. The original version supplied to AIP Publishing contained an error in the Figures 1 and 2 where the right side of the images were cutoff. The error has been corrected in the updated and re-publ...

Coal gasification has been internationally accepted as one of the most viable and effective clean coal technology for power generation. Presently, the coal being produced in India is having high ash content and it is a major constraint for most of the commercial applications in process industries. The present paper deals with the variation of higher heating value (HHV) of the product gas and carbon conversion with different inherent properties under different operating conditions in fluidized bed gasification. It has been observed that HHV of product gas increases with volatile mater, fixed carbon and temperature, whereas, mineral matter, air and steam show decreasing effect on HHV. On the other hand, carbon conversion increases with volatile matter, air, steam and temperature. It has also been observed that mineral matter provides catalytic effect to a certain level for carbon conversion, whilst, decreasing trend has been observed with the fixed carbon.

Coal gasification is one of the most viable and practical clean coal technologies for power generation. In this technology, the low calorific value fuel (LCVF) gas containing mainly carbon monoxide (CO), hydrogen (H2) and carbon dioxide (CO2) are obtained by blowing a mixture of air and steam through a bed of coal at atmospheric or elevated pressures. In this paper, coal gasification data from 18 fluidized bed gasifiers (FBGs) operating at a steady-state and located in India and other countries have been used to develop two types of exclusively data-driven steady-state FBG models. Specifically, multivariate regression (MVR) and multilayer Artificial Neural Network (ANN) strategies have been employed for the prediction of gas produced rate and heating value of the product gas. Both types of models use six inputs describing coal properties and gasification process parameters namely fixed carbon, volatile matter, mineral matter, air feed per kg of coal, steam feed per kg of coal and temperature. The output prediction accuracy results of the developed models indicate that the ANN-based models outperform the corresponding MVR models. The models presented here can be gainfully employed for evaluating steady-state performance of an FBG as well as the thermal potential of a coal prior to its usage for the generation of the LCVF gas. These models would also assist in the design of a gasifier for coals with varying ash content.

Disorders acquainted in semiconductor quantum structures can change the crystal potential and thus play an important role in the performance of optoelectronic devices. Such disorders are critical in the performance of InAsP/InP quantum well devices, which are useful for telecommunication applications. These disorders originate due to the difference in adsorption and the sticking coefficient of arsenic and phosphorus. In our previous paper [J. Appl. Phys.119, 095708 (2016)JAPIAU0021-897910.1063/1.4943031], we reported the impact of carrier localizations on the performance of InAsP/InP quantum well detectors. In this work, the magnetic field is used to investigate exciton properties in InAsP/InP quantum wells. Line shape analysis of magneto-photoluminescence (magneto-PL) distinguishes the role of disorders on the excitonic properties that are useful for the identification of sample quality. Furthermore, the blue shift in PL peak energy with an applied magnetic field is used to estimate the effective mass of excitons in InAsP/InP quantum wells. The estimated effective mass of excitons confined in the quantum well is significantly higher than that of the corresponding bulk value, which is explained by considering the non-parabolicity of the conduction band. The non-parabolicity parameter is also estimated and used for obtaining the electron dispersion E-k curve for InAsP/InP quantum wells.

Temperature dependence of photo-luminescence for Nd3+ doped PLZT(9/65/35) was measured across its structural phase transition temperature. The absorption corresponding to 4 I 9/2 → 4 F 5/2, 2 H 9/2 energy level near 808 nm was used for exciting the sample. The major emission bands at 0.9, 1.06, and 1.34 μm were observed. No shift in the peak position of these emission bands was observed across the diffused structural transition with increase in temperature. However, a 10% increase in FWHM was observed. The absence of any discernible peak shift is attributed to the shielding of the intra-configurational 4f-4f transitions, in trivalent Nd3+ ions, by external 5 s and 5d orbitals. The red shifted phonon side bands appeared with increase in temperature.

A novel application of the surface photovoltage spectroscopy (SPS) technique, using a Fourier transform infrared (FTIR) interferometer, has been developed for the investigation of narrow-gap mid-infrared semiconductor quantum well (QW) laser structures. We demonstrate the FTIR-SPS technique at room temperature for a GaInSb/AlGaInSb type I QW laser structure, grown on GaAs, emitting in the mid-infrared at ∼4 μm, revealing an unambiguous detection of the ground and excited state transitions of the GaInSb QW, as well as the AlGaInSb barrier bandgap energy. As a further corroboration of the new technique, the FTIR-SPS spectrum of a ∼1.5 μm communication laser structure has also been measured and shown to be in good agreement with conventional grating-based electro-modulated reflectance and SPS spectroscopic measurements on this sample. A detailed numerical analysis of the room temperature FTIR-SPS results for the mid-infrared semiconductor laser structure is also presented, including a comparison with theoretical predictions of the QW transition energies.

In this study, two tri-metal quaternary InGaAlAs quantum well (QW) laser structures grown on InP are compared to a conventional InGaAsP QW structure, also grown on InP, all designed to lase at 1.55 μm. Several spectroscopic methods are used to study the samples including surface photo-voltage (SPV) and electro-modulated reflectance (ER). In the tri-metal samples the ground- and two excited-state QW transitions are detectable in both types of spectroscopy. The SPV and ER give results in agreement to within a few meV of each other. By comparing the position of the measured QW transitions with those predicted theoretically, the conduction band offsets of the tri-metal samples can be determined and compared to that in the InGaAsP sample. It is found that the trimetal samples have a much larger conduction band offset of ∼66% compared to the ∼40% in the InGaAsP sample.

Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300 K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons ( B X ) and their phonon replicas. With increasing temperature, free exciton ( F X ) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B X peak with line width of ∼6 meV at 10 K exhibited thermal activation energy of ∼17 meV, consistent with the exciton-defect binding energy. The separation between the F X and B X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films.

Laser diodes are by far the most varied, flexible, cheapest and the most abundant lasers. For long-term, reliable operation and for maximum optical power utilization, laser diode is coated with the anti-reflection (AR) and high-reflection (HR) coatings by means of dielectric layers on the front and the back facet, respectively. For the optimum performance, accurate monitoring and control over the reflectivity of the dielectric thin film layers at specific lasing wavelength is necessary. We have demonstrated a simple and inexpensive in-situ reflectivity measurement system for the facets-coating of laser diodes. The system relies on the measurement of dynamic optical reflectance of the growing thin film by means of intensity of a laser-beam reflected from the gallium arsenide test-substrate kept in the close vicinity of the laser diode facet being coated. The in-situ reflectivity monitoring of single layer AR film and multilayer HR films have been demonstrated and verified with ex-situ reflectivity measurements.

A1. High-resolution X-ray diffraction A1. Characterization A1. Defects A1. Doping A3. Metal organic vapor-phase epitaxy B2. Semiconducting III-V materials abstract A two-step growth process of gallium phosphide (GaP) epilayer on silicon substrate is carried out using metal organic vapor-phase epitaxy (MOVPE). This process includes the growth of a low-temperature GaP nucleating layer and a high-temperature GaP epilayer. In the first step, a GaP nucleating layer of thickness � 80 nm with high V/III ratio � 1725 is grown at 4251C. This is followed by the growth of a GaP layer of thickness � 760 nm with V/III ratio � 100 at 770 1C. The total thickness of the epilayer is � 845 nm as measured on a cross-section by a scanning electron microscope (SEM). A GaP layer grown by the two- step growth process shows significant improvement in morphology compared with that grown by the single-step process, as confirmed by Raman and SEM studies. High-resolution X-ray diffraction studies show that the epilayer is of single crystalline nature and structurally coherent with the silicon substrate. Epilayers grown using the two-step process show reduced dislocation density and micro-strains, with significant improvements in their structural properties. These layers also show n-type behavior with an electron density of � 8.5 � 10 17 cm � 3 , while the single-step-grown layers show p-type behavior. This change in type is explained by a reduced/increased incorporation of silicon into phosphorus/gallium sites, respectively, i.e. a predominant donor nature of silicon in GaP grown by the two-step growth process. Photoluminescence (PL), surface photovoltage spectroscopy (SPS) and transport measurements confirm a significant reduction in sub-band gap states and carrier density.

The optical output power of a laser diode can be enhanced by anti-reflection (AR) and high-reflection (HR) facet coatings, respectively, at the front and back facet. AR and HR coatings also serve the purpose of protection and passivation of laser diode facets. In this work, we have designed and optimized a single layer λ /4 thick Al 2 O 3 film for the AR coating and a stack of λ /4 thick Al 2 O 3 / λ /4 thick Si bi-layers for the HR coating for highly strained InGaAs quantum-well edge emitting broad area (BA) laser diodes. Effect of the front and back facet reflectivities on output power of the laser diodes has been studied. The light output versus injected current ( L–I characteristics) measurements were carried out on selected devices before and after the facet coatings. We have also carried out the numerical simulation and analysis of L–I characteristics for this particular diode structure. The experimental results have been compared and verified with the numerical simulation.

Surface photovoltage (SPV) and photoelectron spectra (PES) of crystalline n-GaP wafers have been studied at 300 K. The magnitude of the surface potential (Vs) decreases in the presence of photons with energy more than the band gap, however the magnitude of Vs increases in the presence of photons with sub band gap energy. The SPV spectrum is helpful in understanding the rigid shift of PES spectra of n-GaP wafers towards higher kinetic energy in the presence of secondary white light from a tungsten lamp.

Metal organic vapour phase epitaxy (MOVPE)-grown InGaAs quantum wells (QWs) with low and high built-in strain are investigated by using complementary spectroscopic techniques like photoluminescence (PL) and surface photovoltage spectroscopy (SPS). The built-in strain is varied by increasing the indium content while keeping the QW thickness constant. An extreme case of high built-in strain of ultrathin InAs QW is also discussed and useful information has been obtained from SPS technique at room temperature itself where no PL signal was seen due to either presence of high defect/dislocation density or thermal escape of charge carriers in case of a shallow QW.

Growth of gallium phosphide layer on silicon substrate has been carried out using metal-organic vapor phase epitaxy. Epitaxial layers were grown at 845 °C with a V/III ratio of 100 and a growth rate of 1.7 A s −1 at a reactor pressure of 30 mbar. The nominal thickness of the gallium phosphide epitaxial layer is ∼600 nm as measured by cross-sectional scanning electron microscopy. Growth of gallium phosphide epilayer is confirmed by Raman spectra studies. High-resolution X-ray diffraction studies show that the epilayer is of single crystalline nature and structurally coherent with silicon substrate. It is also inferred from these measurements that the in- and out of plane strain arising from small mismatch confirms a relaxed epilayer. As-grown epilayer shows p-type behavior with a hole carrier density of ∼1.2×10 18 cm −3 and hole mobility 114 cm −2 V s −1 at room temperature. Annealing at 550 °C for 10 min shows significant improvements in crystalline quality of the epilayer. The annealed layer shows a reduced hole density (∼6.7×10 17 cm −3 ) and increased hole mobility (155 cm −2 V s −1 ).

Cataract surgery in a patient with previous trabeculectomy is considered to have an adverse effect on the long-term survival of the filtering bleb. 5-Fluorouracil (5-FU) has been used in patients with a pre-existing bleb, undergoing cataract surgery, but there are no published reports for or against its use in such cases. This study was carried out to evaluate the protective role of subconjunctival 5-FU on the pre-existing bleb from trabeculectomy in patients undergoing phacocoemulsification. This retrospective study was carried out on patients with pre-existing filtering bleb from trabeculectomy for primary open-angle glaucoma who then underwent phacoemulsification at least 12 months after trabeculectomy. Data were collected for two groups of patients. Group 1 (22 patients) received 5-FU at the end of successful phacoemulsification, whereas group 2 (25 patients) did not receive it. The two groups were comparable with respect to age, gender, intraocular pressure (IOP), type of glaucoma, number of adverse risk factors present, and duration of follow-up. Any worsening of IOP control was analysed using mean IOP and mean change in the treatment for glaucoma. Mean IOP was comparable in the two groups, but there was a significant difference in mean change in glaucoma treatment between the two groups at 12 months postoperatively. Worsening of IOP control was seen in 13.6% of the patients in group 1 and in 36.4% in group 2 (P-value=0.03). Our study suggests that 5-FU has a protective effect on the functioning bleb and may be used routinely at the end of phacoemulsification in such cases.

BACKGROUND: The lack of prospective data comparing early surgery and medical management in primary open-angle glaucoma (POAG) in the developing world led us to conduct a small randomised controlled clinical trial to evaluate acceptance and effectiveness of early trabeculectomy in these patients. METHODS: A total of 60 patients with moderately advanced POAG were randomised into three groups (Group I-Conventional medical management, Group II-Option for early trabeculectomy, Group III-Received an educational package about their disease before an option for early trabeculectomy). The patients were followed up for a period of 6 months for visual acuity, intraocular pressures (IOP), and subjective satisfaction. RESULTS: The three study groups were statistically similar with respect to mean IOP, demographic, and socio-economic profile. 35% of the patients accepted early surgery when offered a choice between early surgery and medical management in one of the groups. 65% of patients in another group expressed willingness for an early surgery after receiving health education on glaucoma. The mean IOP in the operated eyes was lower than the medically treated eyes at 2 weeks (16.6 vs 23.0 mmHg), 6 months (18.5 vs 22.8 mmHg), and 1-year review (17.9 vs 22.3 mmHg) (P

Highly strained InxGa1–xAs quantum wells (QWs) with GaAs barriers emitting around 1.2 µm are grown on GaAs substrates by metal organic vapour phase epitaxy (MOVPE) at low growth temperatures using conventional precursors. The effects of growth temperature, V/III ratio and growth rate on QW composition and luminescence properties are studied. The variation of indium incorporation with V/III ratio at a growth temperature of 510 °C is found to be opposite to the results reported for 700 °C. By an appropriate choice of the growth parameters, we could extend the room temperature photoluminescence (PL) wavelength of InGaAs/GaAs QWs up to about 1.24 µm which corresponds to an average indium content of 41% in the QW. The results of the growth study were applied to broad area laser diodes emitting at 1193 nm with low threshold current densities. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

With the aim of realizing laser diodes in the wavelength range beyond 1100 nm on GaAs, we have studied the indium incorporation behaviour into pseudomorphic InGaAs-quantum wells with extremely high indium content grown by metalorganic vapour phase epitaxy. A wide growth temperature range between 490°C and 770°C as well as the dependence on V/III-ratio and strain compensation has been studied. At the maximum In-content of 41%, a photoluminescence wavelength of 1238 nm at room temperature is obtained. Laser diodes with an emission wavelength up to 1206 nm were processed. Structures with a slightly reduced In-content, emitting at 1120 nm, were processed to broad-area devices (100 μm×1000 μm) and show output powers up to 12 W, which corresponds to a record high internal power density of 23 MW/cm 2 , with a good reliability. LOC structures with a reduced far field below 30° and a higher indium content emit 8.5 W at 1170 nm.

Surface photovoltage spectroscopy studies on thick semi-insulating GaAs wafers are reported in the range 850–950 nm using the chopped light geometry. We observed some interesting sharp features in the sub-band-gap of SI-GaAs, which were reported recently [Appl. Phys. Lett. 79, 1715(2001); Rev. Sci. Instrum. 73, 1835 (2002)]. In this article, we present the dependence of these features on the chopping frequency and the source intensity. The intensity variation in the above-band-gap region and for the A peak (898 nm) in the sub-band-gap region could be fitted with single component while it is necessary to consider more than one component to fit the data for the Q peak (887 nm) in the sub-band-gap region. A model consistent with the observed features is also proposed.

Purpose To assess the aetiology and changing patterns of childhood blindness in one school for the blind in the UK and to assess the use of the World Health Organisation Prevention of Blindness (WHO/PBL) methodology and reporting form in a developed country. Methods One hundred and seven children in one school for the blind and visually impaired in Edinburgh were examined using the WHO/PBL childhood blindness assessment form. Results Of the 107 children examined, 87 (81%) were blind or severely visually impaired (corrected visual acuity of

A procedure for choosing the appropriate chopping frequency (f) for the surface photovoltage spectroscopy (SPS) measurements in order to obtain the absorption related features is presented. We could obtain the absorption edge of thick n+ GaAs wafer (thickness ≈700 μm) by performing SPS measurements at f⩾1 kHz at room temperature (300 K). The similar information for semi-insulating (SI) GaAs could not be obtained due to the carrier trapping at deep levels or surface states at 300 K. However, we could obtain the absorption edge of SI-GaAs by performing SPS measurements at 395 K at f=3 kHz. Here, we demonstrate the capability of the SPS technique to measure large absorption coefficient (α) values for thick wafers by performing SPS measurements and normalizing this with the reported α value at one wavelength in the above band gap region. For comparison, we also perform quasisimultaneous SPS and transmission spectroscopy (TS) measurements. The SPS technique could provide α values up to 104 cm−1 for 700-μm-thic...

InN thin films were grown on (0 0 0 1) sapphire substrates at 400°C by N 2 glow discharge plasma-assisted pulsed laser deposition using elemental In as a target. X-ray diffraction (XRD) showed that the InN films consisted of a predominant zincblend (ZB) structure along with a low-intensity hexagonal wurtzite (WZ) peak. The full-width at half-maximum of the (0 0 2) ZB peak was about 0.5° in high-resolution XRD pattern. The lattice constant after tilt correction was found to be 5.09±0.04 A. Scanning electron micrograph of the InN thin films showed a granular-type surface morphology. The presence of both ZB and WZ phases was confirmed by Raman spectra.

AlGaN/GaN heterostructures are investigated by performing complementary spectroscopic measurements under novel experimental configurations. Distinct features related to the band edge of AlGaN and GaN layers are clearly observed in surface photovoltage spectroscopy (SPS) spectra. A few more SPS features, which are associated with defects in GaN, are also identified by performing the pump-probe SPS measurements. SPS results are strongly corroborated by the complementary photoluminescence and photoluminescence excitation (PLE) measurements. A correlation between the defect assisted SPS features and yellow luminescence (YL) peak is established by performing pump-probe SPS and PLE measurements. It is found that CN-ON donor complex is responsible for the generation of YL peak in our sample. Further, the deep trap states are found to be present throughout the entire GaN epilayer. It is also noticed that the deep trap states lying at the GaN/Fe-GaN interface make a strong contribution to the YL feature. A phenome...

A general method based on the valence band mixing is proposed to establish a correlation between electron spin polarization and the degree of circular polarization of luminescence in a strained quantum well (QW). Key issues that are critical for establishing such a correlation are highlighted by invoking 4 band k.p calculations in a strained QW system. The proposed method takes care of the complex valence band mixing in strained QWs by simultaneously analyzing the heavy hole (e-hh) and light hole (e-lh) related luminescence transitions along the quantization direction. It is shown that the ratio of the degree of circular polarizations measured for the e-hh and e-lh features is a sensitive probe to estimate the values of band mixing. The method is reasonably supported by carefully conducted experiments on a tensile strained GaAsP/AlGaAs QW sample in the temperature range of 10 to 150 K. The proposed method shall be helpful for understanding the spin injection/transfer across the interfaces of QW, especially for the cases where heavy hole-light hole coupling is expected to be strong, and both the features are experimentally observed. It might lead to new directions in the field of semiconductor spin-optoelectronics in QWs.

A new technique is presented for an accurate determination of the bandgap of semiconductor bulk and quantum structures under the strong influence of localized states. It is based on a pump-probe configuration where an additional sub bandgap cw pump laser beam is used in a conventional chopped light geometry surface photovoltage spectroscopy setup. The main role of pump beam is limited to saturate the sub bandgap localized states whose contribution otherwise swamp the information related to the bandgap of material. The pump beam is found to be very effective in suppressing the effect of surface/interface and bulk trap states.

Photo-induced inverse spin Hall effect (ISHE) measurements on a Au/InP metal–semiconductor junction are performed. Spin polarized electrons, which are generated in InP by the circularly polarized laser light of 2.33 eV (532 nm), provide a net spin current in thin Au-layer after crossing the Schottky barrier. This generates a transverse electric current because of large spin orbit coupling in Au-layer. ISHE origin of the measured signal is confirmed by a linear variation of the spin current with the degree of circular polarization of incident light. Furthermore, angle dependence of spin current matches well with the predicted trends.

An appropriate method for an accurate determination of carrier concentration from Hall measurements for samples having large inhomogeneities is presented. Parasitic contributions in such samples generally limit the capabilities of Hall experiments where even the measurement of carrier type in some samples becomes doubtful. Here, we eliminate the major parasitic contributions from the measured Hall voltage through a systematic averaging procedure over the current and magnetic field polarities. Further, the carrier concentration values are unambiguously determined from the magnetic field dependent Hall measurements, where the slope of true Hall voltage versus magnetic field plot provides realistic values of carrier concentrations.

In GaAs/InGaP/GaAs structures grown by metal organic vapor-phase epitaxy (MOVPE), the two heterointerfaces are not identical. Normal photoluminescence (PL) features corresponding to the band gaps of GaAs and InGaP are seen for InGaP layer grown on GaAs. However, an intense long-wavelength feature is observed if we grow GaAs on InGaP (inverted structure) while the features of InGaP and GaAs are suppressed. The nature of interfacial regions is investigated by using different gas switching sequences, which can influence the interfacial region composition. Significantly, we find anomalous PL features similar to those observed in the case of inverted structure if we briefly interrupt the growth of InGaP on GaAs and introduce AsH 3 during the growth interruption. Secondary-ion mass spectrometry (SIMS) measurements and preliminary results of the compositional analysis of the interfacial layers based on high resolution X-ray diffraction (HRXRD) and PL measurements suggest that a deleterious effect arises with the exposure of InGaP surface to AsH 3 and is attributed to the formation of an interfacial InGaAsP layer.