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Faecal specimens of diarrhoea cases (n=2495, collected between November 2007 and October 2009) from Infectious Diseases and Beliaghata General (ID&BG) Hospital, Kolkata, India, were screened by RT–PCR using specific primers targeting region C of the capsid gene of noroviruses (NoVs) to determine the seasonal distribution and clinical characteristics of NoVs associated with diarrhoea. NoV infection was detected in 78 cases, mostly in children aged <2 years. In 22/78 positive cases, the virus was detected as the sole agent; others were as mixed infections with other enteric pathogens. Sequencing of NVGII strains showed clustering with GII.4 NoVs followed by GII.13 and GII.6 NoVs. Clinical characteristics of the diarrhoeic children and adults in Kolkata indicated that NoV infections were detected throughout the year and were associated with a mild degree of dehydration.
The effect of actuator damage on a helicopter rotor with an IBC based primary control system is studied. Such a system eliminates the swashplate and can be accomplished by trailing-edge flaps, active twist or full authority IBC, especially with smart material actuators. Damage to the collective, longitudinal and lateral cyclic are simulated for one blade, both individually and in combinations ranging from partial damage to complete failure. Numerical results are obtained using a dissimilar blade aeroelastic analysis based on finite elements in space and time for hover and forward speed conditions. It is found that the helicopter can be trimmed for all cases with all three controls having failed on the blade with actuator damage thereby showing that the IBC actuated rotor can survive an actuator failure and can be reconfigured by the pilot using the controls on the other blades. However, in case the collective fails and the longitudinal cyclic is present, there are problems in achieving trim at high damage levels at high forward speeds. Physical explanations of this phenomenon are given. The response (especially flap) for the damaged rotor blades can become high and 1/rev and 2/rev are transmitted by the reconfigured rotor to the hub. Results show that IBC based primary controls provide redundancy which can improve the survivability of a helicopter in case of actuator failure in one blade.
We report on direct measurements of surface potentials on cross sections of a-Si:H and a-SiGe:H n-i-p solar cells using scanning Kelvin probe microscopy. External bias voltage (Vb)induced changes in the electric field distributions in the i layer were further deduced by taking the derivative of the Vb-induced potential changes. This procedure avoids the effect of surface charges or surface Fermi-level pinning on the potential measurement. We found that the electric field does not distribute uniformly through the i layer of a-Si:H cells, but it is stronger in the regions near the n and p layers than in the middle of the i layer. The non-uniformity is reduced by incorporating buffer layers at the n/i and i/p interfaces in the a-Si:H solar cells. For a-SiGe:H solar cells, the electric field at the p side of the i layer is much stronger than at the n side and the middle. The non-uniformity becomes more severe when a profiled Ge content is incorporated with a high Ge content on the p side. We speculate that the increase in defect density with increasing of Ge content causes charge accumulation at the i/p interface.
The temperature dependent H evolution from a-Si:H provides unique information on the H-bonding and microstructure. Traditional undiluted a-Si:H films show a high temperature H-evolution peak near 600°C. However device-quality compact nanocrystalline silicon films grown near the phase boundary of amorphous and microcrystalline growth show a new low temperature H- evolution peak near 400°C in addition to a second high temperature peak near 600°C. The origin of this peak cannot be attributed to microvoids or a substantial density of dihydride species typical of porous low-temperature films. We have simulated the H evolution using a molecular dynamics generated model of nanocrystalline silicon, where nano-crystallites reside in a background amorphous matrix. An excess density of H occurs at the crystallite surface. We find a low temperature evolution peak at 250-400°C, where the H-evolution starts from the surface of the nano-crystallite. In addition there is a higher temperature peak at 700-800°C providing good agreement with H-evolution measurements. The mobile H is found to exist in both the bond-centered type of species and H2 molecules – which has implications for H-diffusion models.
A graphite catalyzer has been used to grow μc-Si:H films using the thermo-catalytic (HW) chemical vapor deposition (CVD) technique. The films grown in the amorphous-microcrystalline ‘transition regime’ have been found to exhibit a high photosensitivity of the order of 102-103 at a crystalline volume fraction of 0.2-0.4. The effects of deposition parameters such as silane concentration, pressure and substrate temperature on the microstructure and electrical properties of the films have been studied. It has been found that the graphite catalyzer offers a wider window of the deposition parameters for the growth of the ‘transition regime’ films as compared to the conventional W and Ta catalyzers. In addition, the graphite wires also exhibit significantly greater chemical as well as mechanical stability than the W and Ta wires, which results in improvement of the reproducibility of the technique. However, at the same filament temperature and other conditions, the deposition rates are about 10 times lower than for W or Ta filament. Increasing of the filament temperature, on the contrary, lead to radiative heating and carbon contamination of the growing film.
A comparison of the threshold voltage shift after gate-bias stress in hydrogenated and fully deuterated amorphous silicon thin film transistors (TFTs) is presented. A series of fully deuterated bottom gate TFTs consisting of a deuterated n+ contact layer, deuterated intrinsic amorphous silicon (deposited at a range of pressures) and deuterated silicon nitride gate insulator have been produced. A similar series of fully hydrogenated bottom gate TFTs have also been produced, and the stability of the two sets of devices compared. Deuterated and hydrogenated amorphous silicon deposited under the same process conditions do not have the same material properties due to the difference in the ion energy of H and D in the plasma. However, deuterated and hydrogenated material deposited at the same growth rate have almost identical structural properties. Hydrogenated and deuterated TFTs are found to exhibit the same variation in stability as a function of growth rate. In particular, there is no evidence for increased stability in deuterated TFTs. Previous reports of more stable deuterated TFTs, by other groups, can be explained by a change in the Si network properties due to the higher ion energy of deuterium in comparison with hydrogen, when using similar deposition conditions. The implication of our experimental results is that, for the same amorphous network and hydrogen/deuterium concentration, the stability is identical for hydrogenated and deuterated TFTs. Therefore, there is no giant isotopic effect in amorphous silicon TFTs. The study also further supports the idea that Si-Si bond breaking is the rate-limiting step for Si dangling bond defect creation, rather than Si-H bond breaking.
A systematic study has been made of the influence of the deposition conditions on the properties of SiO2 grown by liquid phase deposition (LPD), and a-Si:H manufactured by plasma enhanced chemical vapour deposition (PECVD) with the novel facility of source-gas heating. It is demonstrated that LPD-SiO2 can be grown at 50°C with good dielectric properties. Material has been produced with a resistivity of 1015 &cm and a dielectric strength of 9 MVcm-1.The oxide was found to have a negative fixed oxide charge of 4 × 1011 cm−2, with a dielectric constant of 3.08 and a refractive index of 1.44. In the case of a-Si:H, pre-heating the source gases has enabled material to be grown at 125°C with a hydrogen content of ∼ 10 at%, with a predominance of monohydride bonding and a photosensitivity of ∼ 104. Inverted-staggered thin film transistors have been fabricated incorporating these films with an On/Off ratio of five orders of magnitude, a sub-threshold slope of 1.3 Vdecade−1 and a field effect mobility of 0.20 cm2V−1s−1
Electrical switching due to metallic filament formation in hydrogenated amorphous silicon (a-Si:H) is studied in metal/a-Si:H/metal structures. We examine the effects of a-Si:H switch layer thickness, applied voltage and polarity, metal contact material, and contact interface properties. For switching, the voltage applied to the contacts must be large enough to establish: 1) a minimum threshold voltage of about 2V at the contacts and 2) a bias field of about 1 MV/cm in the bulk. Changing contact material and polarity strongly affects the switching behavior.
We have studied the influence of substrate temperature and hydrogen dilution ratio on the properties of silicon thin films deposited on single-crystal silicon and glass substrates. We varied the initial substrate temperature from 200° to 400°C and the dilution ratio from 10 to 100. We also studied the effectiveness of the use of a seed layer to increase the crystallinity of the films. The films were analyzed by atomic force microscopy, X-ray diffraction, Raman spectroscopy, and transmission and scanning electron microscopy. We found that as the dilution ratio is increased, the films go from amorphous, to a mixture of amorphous and crystalline, to nanocrystalline. The effect of substrate temperature is to increase the amount of crystallinity in the film for a given dilution ratio. We found that the use of a seed layer has limited effects and is important only for low values of dilution ratio and substrate temperature, when the films have large amounts of the amorphous phase.
We present femtosecond time-resolved studies of the photoexcited carrier response in the far-infrared spectral range in PECVD a-Si:H and a-SiGe:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties as a function of time delay following the excitation. These measurements are sensitive to carrier processes at low energy, corresponding to a range of approximately 1 - 10 meV, a key energy scale in these materials. We find that the observed photoexcited carrier dynamics are consistent with trapping of carriers into band tail states on a picosecond time scale.
Switching in a-Si:H and a-Si:HNx layers is investigated by pulse current transient and Auger scanning microspectroscopy measurements. Switching in a-Si:H with Ag and Cr contacts exhibits 2 different regimes depending on the voltage pulse polarity. With a positive top Ag contact, switching occurs in nanoseconds after a certain latency time, which depends on voltage exponentially. For a negative Ag contact, there is no latency time provided the voltage exceeds a certain critical value. This might be related to interface effects on contact properties or field-assisted metal diffusion. Scanning Auger element micromaps reveal metallic filaments in the switched films. They contain both Ag and Cr throughout the film thickness. Two phases of the filament formation are suggested – a precursor phase and a post-switching phase characterized by local heating and atomic diffusion. Soft and hard switching are observed in a-Si:HNx films simultaneously and their rates depend strongly on the contact material and applied voltage. Soft switching might be related to the charge trapping in this wide bandgap material.
Photoconductive Semiconductor Switches (PCSS) were fabricated in planar structures on high resistivity 4H-SiC and conductive 6H-SiC and tested at DC Bias voltages up to 1000 V. The gap spacing between the electrodes is 1 mm. The average on-state resistance and the ratio of on-state to off-state currents were about 20 Ω and 3×1011 for 4H-SiC, and 60 Ω and 6.6×103 for 6H-SiC, respectively. The typical maximum switch current at 1000 V is about 49 A for 4H-SiC. Photoconductivity pulse widths for all applied voltages were 8-10 ns. The observed performance is due in part to the removal of the surface damage by high temperature H2 etching and surface preparation. Atomic Force Microscopy (AFM) images revealed that very good surface morphology, atomic layer flatness and large step widths were achieved with this surface treatment and these atomically smooth surfaces likely contributed to the excellent switching performance of these devices.
A set of 8 rf deposited a-Si:H thin films of various thickness (4-1031nm) have been used to explore the applicability of two optical techniques, thin film cavity ringdown spectroscopy (tfCRDS) and second harmonic generation (SHG), for the measurement of small defect-related absorptions. In this paper we will give a first overview of the different aspects of these techniques, which are novel in the field of amorphous silicon materials. It is shown that tf-CRDS is capable of measuring defect-related absorptions (associated with dangling bonds) as small as 10-7 for a single measurement, without the need for elaborate calibration procedures. The results are compared with photothermal deflection spectroscopy (PDS) for a broad spectral range (0.7 – 1.7 eV) and show good agreement. Furthermore the existence of a defect-rich surface layer with a defect density of 1.1×1012 cm-2 has been proven. The absorption spectrum of a 4 nm thin film has revealed a different spectral signature than a bulk dominated (1031 nm) film. The SHG experiments on a-Si:H films have shown that the second harmonic signal arises from the surface states and polarization dependent studies have revealed that the surface states probed have an ∞m-symmetry. From this it can be deduced that the absorbing surface states are isotropically distributed. A spectral scan suggests that the second harmonic signal, whose origin has not been unrevealed yet, has a resonance at an incident photon energy of 1.22 eV.
The structure of germanium thin films prepared on glass by plasma enhanced chemical vapor deposition was characterized by Raman spectroscopy, atomic force microscopy (AFM) and field emission scanning electron microscopy (SEM). Crystallinity, surface roughness, and grain size were measured as functions of film thickness and deposition temperature. Grain nucleation was apparent for films as thin as 10 nm. Over the thickness range studied, grain size increased with film thickness, whereas average surface roughness started to increase with film thickness, but then remained fairly constant at approximately 1 nm for a film thickness greater than 25 nm.
μc-Si has traditionally been deposited by Hot Wire CVD at a low filament temperature. At these temperatures, silicides rapidly form on the filament surface, leading in the case of a tungsten filament to both film reproducibility and filament lifetime issues. By depositing films consecutively using identical deposition parameters, these issues are chronicled for a filament temperature of ∼ 1750°C. Upon increasing the filament temperature to ∼ 1825-1850°C, these reproducibility and lifetime issues disappear and, by lowering both the substrate temperature and chamber pressure, device quality μc-Si is deposited at high deposition rates in a filament regime where tungsten silicide formation is minimal. Both single junction and tandem solar cells are fabricated using this material, confirming the validity of this approach.
Materials grown close to the phase boundary of amorphous and microcrystalline growth have the best electronic properties for solar cells. Systematic molecular dynamics methods have generated such nano-crystalline silicon, consisting of a mixed phase of nano-crystallites in an amorphous matrix, using an embedding method. An excess density of H resides on the surface of the nanocrystallites. The structure of this heterogeneous phase will be characterized by atomic distribution functions and structure factors. The electronic band structure of smaller models of nanocrystalline silicon reveals no midgap states and is similar to a-Si:H. There is a highly strained region surrounding the crystallites. The presence of localized strain region may increase the stability of the material.
Nanocrystalline germanium thin-films deposited on glass by plasma-enhanced chemical vapor deposition from germane and hydrogen were doped with phosphorus and boron. We report some electrical transport and structural properties of Ge films as a function of dopant species and doping levels. The dark conductivities of the phosphorus- and boron-doped films are approximately three to four orders of magnitude higher than the intrinsic nanocrystalline germanium. In the solid phase, phosphorus comprised about 1 atomic percent in the Ge bulk over the range of source gas ratios used, and the conductivity remained fairly constant, indicating saturated conditions. Boron comprised about 10 atomic percent in Ge at the highest dark conductivity, while increased doping turned the films amorphous. To test the doped layers for device applications, an all-nanocrystalline germanium p-i-n diode was constructed and showed rectification when measured in the dark at room temperature.
μc-Si has traditionally been deposited by Hot Wire CVD at a low filament temperature. At these temperatures, silicides rapidly form on the filament surface, leading in the case of a tungsten filament to both film reproducibility and filament lifetime issues. By depositing films consecutively using identical deposition parameters, these issues are chronicled for a filament temperature of ∼ 1750°C. Upon increasing the filament temperature to ∼1825-1850°C, these reproducibility and lifetime issues disappear and, by lowering both the substrate temperature and chamber pressure, device quality μc-Si is deposited at high deposition rates in a filament regime where tungsten silicide formation is minimal. Both single junction and tandem solar cells are fabricated using this material, confirming the validity of this approach.
We have carried out measurements to try to correlate amorphous silicon film properties with companion solar cell device performance. The dc plasma deposited intrinsic films were prepared with various hydrogen dilution levels, and increasing power levels to increase growth rate. The electronic properties were determined using admittance spectroscopy and drive-level capacitance profiling (DLCP) techniques as well as transient photocapacitance and photocurrent spectroscopy. Cell and film performance were explored in both as-grown and light-soaked states. We observed that, although cell performance decreased systematiclly with increasing growth rate, it depended on factors other than the deep defect density in the matched films. On the other hand, we did observe that increases in defect density caused by the light-induced degradation led to fairly predictable decreases in the cell fill factors.