Through computer simulations and surface plasmon resonance (SPR) measurements, we establish optimum parameters for the design and fabrication of SPR sensors of high sensitivity, resolution, stability, and long decay-length evanescent fields. We present simulations and experimental SPR data for variety of sensors fabricated by using bimetal (Ag/Au) and multilayer waveguide-coupled Ag/Si3N4/Au structures. The simulations were carried out by using the transfer matrix method in MATLAB environment. Results are presented as functions of the thickness of the metal (Ag or Au) and the waveguide dielectric used in Ag/Si3N4/Au structures. Excellent agreement is observed between the simulations and experiments. For optimized thickness of the Si3N4 waveguide (150 nm), the sensor exhibits very high sensitivity to changes in the refractive index of analytes, Sn≈52°/RIU, extremely high resolution (FWHM≤0.28°), and long penetration depth of evanescent fields (δ≥305nm).

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Direct measurements of doubly differential (angle and energy) distributions of backscattered positrons are reported using an electrostatically guided slow-positron beam. Backscattering yields for 35-keV positrons incident normally on the target were measured as a function of both scattering angle 8 and target atomic number Z (4~ZS 82). Absolute backscattering coefficients for positrons are com-

Positron beam lifetime spectroscopy has been utilized to study the depth distribution of vacancy-type defects in molecular beam epitaxy GaAs grown at low temperature. Lifetime spectra were measured as a function of positron energy. From the analysis of the positron lifetime in as-grown and annealed low temperature grown GaAs, the concentrations of Ga monovacancies and voids are estimated. Our results show that in an as-grown sample the Ga monovacancy concentration is >3×1018 cm−3. It is also known that vacancy-cluster concentration in an annealed sample exceeds 1018 cm−3 with a nonuniform spatial distribution.

An electrostatically guided positron beam which is variable in energy from 0 to 60 keV is described. A frozen inert gas (Ar, Kr, and Xe) is used as the positron moderator. The efficiency, ε, defined as the ratio of the number of slow positrons at the target to the number of positrons emitted by the source has been measured; the values obtained for Ar, Kr, and Xe were ε∼ 2.5×10−4, 1.6×10−4, and 1.1×10−4, respectively. We estimate a source/moderator efficiency of εm∼7.6×10−4, 4.7×10−4, and 3.0×10−4 for Ar, Kr, and Xe, respectively, which includes source effects, grid transmission, geometrical losses, and beam transport. These values are comparable to the best reported values for thin metal foil moderators. The measured εm is specific to the electrostatic system; significantly higher values for the same moderator should be attainable in magnetically guided beams, where higher transmission for moderators with large energy spreads is usually obtained.

We have utilized an infrared absorption technique to study deep level defects in molecular beam epitaxy GaAs grown at 250 °C. From an observed broad absorption band below the conduction edge, the concentration of defects is estimated to be ≊5×1019 cm−3 in an as-grown sample. The concentration of defects decreases exponentially by one order of magnitude due to annealing of the sample at temperatures of 400–500 °C. From the temperature dependence of the defect concentration, the migration energy of defects is calculated to be 0.52±0.02 eV. The measured migration energy shows that part of the defects are arsenic interstitials.

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We have utilized x-ray photoelectron and variable energy positron beam spectroscopies for deplh profiling excess arsenic. arsenic precipitates. and vacancy-type defects in GaAs grown by molecular beam epitaxy at low temperatures (LT-G~As). XPS results show about 1.3% excess arsenic in as-grown LT-G~As and a non-uniform depth profile of arsenic Concentration in annealed LT-G~As. Doppler broadening of the positron-electron annihilation radiation (S parameter) reveals a non-uniform depth profile of defects in annealed LT-G~As. We observe a clear comlation between the depth profile of the S parameter and As in annealed LT-GAS.Molecular beam epitaxy (MBE) GaAs grown at low temperatures (LT-GaAs) has been the