Simulation, modeling, and crystal growth of Cd0.9Zn0.1Te for nuclear spectrometers

High-quality, large (10 cm long and 2.5 cm diameter), nuclear spectrometer grade Cd_0.9Zn_0.1Te (CZT) single crystals have been grown by a controlled vertical Bridgman technique using in-house zone refined precursor materials (Cd, Zn, and Te). A state-of-the-art computer model, multizone adaptive scheme for transport and phase-change processes (MASTRAP), is used to model heat and mass transfer in the Bridgman growth system and to predict the stress distribution in the as-grown CZT crystal and optimize the thermal profile. The model accounts for heat transfer in the multiphase system, convection in the melt, and interface dynamics. The grown semi-insulating (SI) CZT crystals have demonstrated promising results for high-resolution room-temperature radiation detectors due to their high dark resistivity (ρ≈2.8 × 10¹¹ Θ cm), good charge-transport properties [electron and hole mobility-life-time product, μτ_e≈(2–5)×10⁻³ and μτ_h≈(3–5)×10⁻⁵ respectively, and low cost of production. Spectroscopic ellipsometry and optical transmission measurements were carried out on the grown CZT crystals using two-modulator generalized ellipsometry (2-MGE). The refractive index n and extinction coefficient k were determined by mathematically eliminating the ∼3-nm surface roughness layer. Nuclear detection measurements on the single-element CZT detectors with ²⁴¹Am and ¹³⁷Cs clearly detected 59.6 and 662 keV energies with energy resolution (FWHM) of 2.4 keV (4.0%) and 9.2 keV (1.4%), respectively.     

Characterization and optimization of absorbing plasma-enhanced chemical vapor deposited antireflection coatings for silicon photovoltaics

We have optimized plasma-enhanced chemical vapor deposition (PECVD) of SiN-based antireflection (AR) coatings with special consideration for the short-wavelength (<600 nm) parasitic absorption in SiN. Spectroscopic ellipsometry was used to measure the dispersion relation for both the refractive index n and the extinction coefficient k, allowing a precise analysis of the trade-off between reflection and absorption in SiN-based AR coatings. Although we focus on photovoltaic applications, this study may be useful for photodetectors, IR optics, and any device for which it is essential to maximize the transmission of light into silicon. We designed and optimized various AR coatings for minimal average (spectrally) weighted reflectance (? R(w) ?) and average weighted absorptance (? A (w) ?), using the air mass 1.5 global solar spectrum. In most situations ? R (w) ? decreased with higher n, but ? A (w) ? increased because k increased with n. For the practical case of a single-layer AR coating for silicon under glass, an optimum refractive index of ~2.23 (at 632.8 nm) was determined. Further simulations revealed that a double-layer SiN stack with an n = 2.42 film underneath an n = 2.03 film gives the minimum total photocurrent loss. Similar optimization of double-layer SiN/SiO(2) coatings for silicon in air revealed an optimum of n = 2.28 for SiN. To determine the allowable tolerance in index and film thickness, we generated isotransmittance plots, which revealed more leeway for n values below the optimum than above because absorption begins to reduce photocurrent for high n values.     

Normal-incidence generalized ellipsometry using the two-modulator generalized ellipsometry microscope

A new microscope is described that is capable of measuring the polarization characteristics of materials in normal-incidence reflection with a demonstrated lateral resolution of 4 microm. The instrument measures eight parameters of the sample Mueller matrix, which can be related to the diattenuation, retardation, circular diattenuation, direction of the principal axis, and the polarization factor. With proper calibration, the eight elements of the sample Mueller matrix can be determined to better than 0.001-0.002 for small values. Examples are given for aluminum, rutile (TiO2), and calcite (CaCO3).     

Hypochlorite Solution Expiration and Stability in Household Water Treatment in Developing Countries

Household water treatment with dilute hypochlorite solution has been shown to improve microbiological water quality and reduce diarrheal disease in developing countries. One concern raised by governmental agencies, implementers, and nongovernmental organizations is whether the hypochlorite solution remains stable during distribution and after the bottle is opened and usage begins. In this study, laboratory and field research was conducted to determine the expiration date and stability in the home of hypochlorite solution. Expiration-date testing found that pH-stabilized ～ 1.25% hypochlorite solution made using various production techniques in Ethiopia, Guinea, and Nigeria was stable for a minimum of 12 months in temperatures less than 35°C. Stability testing during laboratory simulated normal usage showed no degradation, and 77–91% of bottles collected from households tested met standards. Hypochlorite solution concentration drops rapidly when pH is not stabilized, when pH levels fall, and when solution is exposed to direct sunlight.     

Use of fluorescent-antibody staining of cytocentrifuge-prepared smears in combination with cell culture for direct detection of respiratory viruses

Over a 3-year period, 1,003 respiratory samples were collected and examined for selected respiratory viruses with cytocentrifuged prepared smears stained with fluorescently labeled antibodies (IFA) in conjunction with cell culture. IFA results were compared with results obtained by cell culture. Viruses were isolated or detected by direct means in 401 samples. Agreement between culture and IFA was 90%.     

Development of Spark Chambers for Use in Measurements of the Charge Spectrum of High Energy Cosmic Rays

The High Energy Cosmic Ray Experiment, flown on balloons by a group at Goddard Space Flight Center, is designed to measure the charge, the energy, and the arrival direction of high-energy cosmic rays. A wiregrid spark chamber is employed to determine the trajectory of each incident primary particle which satisfies the experiment trigger criteria. Optimum trajectory-determination efficiency depends on parameters, the values of which vary with the charge of the incident particle. The functional dependence on the voltage applied to the wire grids and on the delay in application of the high voltage trigger for several different values of the DC clearing field was measured for fully ionized nitrogen nuclei and for singly charged particles. The voltage was found to be the most suitable parameter to vary in order to maximize the spark formation efficiencies and to minimize spurious spark formation. Because of the low fluxes of high-Z, high-energy cosmic rays, long experiment exposure times such as those obtained in satellites are desirable. To adapt the spark chambers for use with experiments with a duration of more than one year and total accumulated events in excess of 3 × 108, the useful life times of both the high-voltage triggering circuit and the spark-chamber gas needed to be improved. Krytron high-voltage triggering devices have been developed which will perform satisfactorily for more than 3 × 108 simulated events at a rate of 10 events/ sec, an improvement of approximately a factor of 10.     

Pseudodielectric functions of uniaxial materials in certain symmetry directions

The pseudodielectric function is often used to represent ellipsometric data and corresponds to the actual dielectric functions of materials when there is no surface overlayer and the material is isotropic. If a uniaxial material is oriented such that the optic axis is in the plane of incidence or is perpendicular to the plane of incidence, then the cross-polarization terms are zero and appropriate pseudodielectric functions can be determined from the ellipsometry data. We calculate the pseudodielectric functions for uniaxial crystals in three primary symmetry directions: (1) the optic axis is perpendicular to the plane of incidence, (2) the optic axis is in the plane of the sample surface and parallel to the plane of incidence, and (3) the optic axis is in the plane of the sample surface and perpendicular to the plane of incidence. These results are expanded in terms of the difference in the ordinary and extraordinary dielectric functions and compared with the approximation of Aspnes [J. Opt. Soc. Am. 70, 1275 (1980)]. Comparisons are made with experimental results on oriented crystals of rutile (TiO2), and a simple procedure is presented to determine the complex dielectric function from standard ellipsometry techniques.     

Transmission two-modulator generalized ellipsometry measurements

The two-modulator generalized ellipsometer has been used to measure samples in transmission. In this configuration, the instrument can completely characterize a linear diattenuator and retarder, measuring birefringence, diattenuation, the angle of the principal axis, and the sample depolarization simultaneously and accurately. This instrument can be operated in two modes: (1) spectroscopic, in which measurements are made through the entire sample aperture as a function of wavelength, and (2) spatially resolved, in which measurements are made at a single wavelength and a birefringence picture is made of the sample. Current spatially resolved measurements have been made at a resolution of approximately 40 microm. Four samples have been examined with this instrument: (1) a mica plate, (2) a Polaroid polarizer, and (3) two quartz plates.