D. Van Vechten

Near infrared rugate filter fabrication by ion beam assisted deposition of Si (1− X ) NX films

The rugate filter employs a sinusoidal refractive index depth profile to produce high reflection in a narrow band of wavelengths. Fabrication relies on a continuously variable index of refraction in the wavelength regime of interest. The near IR refractive index of amorphous silicon-nitrogen films decreases continuously as the composition varies from pure silicon to stoichiometric silicon nitride (Si(3)N(4)). Ion implantation was found unsuitable as a fabrication method for rugate filters. Homogeneous and inhomogeneous films up to 5 microm in thickness have been produced by simultaneous deposition of electron beam evaporated silicon and of energetic nitrogen particles arising from an ion beam. The relative fluxes of beam and evaporant are found to determine the ratio of nitrogen to silicon in the films and therefore to determine the index. Single-band reflection filters of the rugate design of high peak optical density were fabricated under computer control using a quartz crystal oscillator shielded from the beam to monitor the silicon evaporation and three suppressed Faraday cups to monitor the ion beam current.

Characterization of a 3 cm Kaufman ion source with nitrogen feed gas

Abstract A commercial high current dual grid optics 3 cm ion source (normally used in argon etching systems) was purchased for use in a nitrogen ion beam assisted, electron beam evaporation (IAD) system. Control of the concentration of the reactive ions in the growing film required characterization of the source for nitrogen. The inter-relationships of beam current, beam spatial profile measurements, chamber pressure, accelerator voltage, beam voltage (500, 1000 and 1500 V), discharge voltage, and filament current are reported. The ratio of atomic to molecular ion content ( N + N + 2 ) was also investigated under some of these conditions.

Ion beam assisted deposition of substoichiometric silicon nitride

Thin films of substoichiometric silicon nitride of uniform composition have been produced by ion beam assisted deposition (IAD) of electron beam evaporated silicon on amorphized Si substrates. IAD films are found to have improved adhesion, increased density, and markedly lower oxygen contamination than non-IAD films produced simultaneously. The Reststrahlen features of the IR spectrum (due to local vibrational modes) are largely unaffected by crystallization at 900°C, indicating stable Si-N bond formation during IAD. The nitrogen content of the films was measured by RBS and in several cases by AES. Infrared reflection spectra were taken and analyzed to derive the index of refraction. The index decreases monotonically with increasing nitrogen fraction in agreement with the Lorentz-Lorentz relations for both the as deposited (amorphous) and annealed (crystalline) samples. The relationship between the incorporated nitrogen fraction and the ratio of the arriving nitrogen atom flux to the silicon atom flux is reported and modelled in a simple manner.

Suppression of extrinsic resolution limiting factors in epitaxial superconducting tunnel junction detectors

Superconducting tunnel junctions have been shown to have potential as high resolution X-ray detectors. Recent reductions in the noise environment of our experiment have yielded a substantial increase in the resolution of our Nb/Ta junctions. The junctions used in the experiment are epitaxial base layer devices including a thick Ta absorber and have survived more than 30 cycles to low temperatures. Current experiments with a /sup 55/Fe source have yielded an energy resolution of 68 eV at 5.89 keV with a quantum efficiency of 13%. Additional measurements using a higher energy /sup 109/Cd source are also reported. We discuss how a reduction of extrinsic and quasi-extrinsic noise sources affects the performance of the detectors and why we believe we are still not observing the intrinsic limitations of these junctions.<<ETX>>

Nonequilibrium dynamic conductivity of superconductors: An exploitable basis for high‐energy resolution x‐ray detectors

A new design for high‐energy radiation/particle detectors is presented. The nonequilibrium response of a superconductor to the absorption of the incident quanta is sensed by electromagnetic measurements of the altered dynamic conductivity. Microwave absorption may be used to amplify the signal. Such a detector will provide better energy resolution than semiconducting charge‐collection devices once the statistical resolution limit is reached.

Depth profiling high Tc materials with 6.2 MeV He ion elastic backscattering

Elastic backscattering of helium ions accelerated to 6.2 MeV has been used to determine the compositional depth profile of thin films of high Tc superconductors. The higher than normal bombarding energy allows separation of the contributions from Cu, Y and Ba in the measured spectra for film thicknesses up to about 5500 A, rather than about 1000 A by standard RBS at 2 MeV. An added benefit of the higher energy is greatly enhanced yield from nuclear elastic scattering by the light elements C and O at backward angles. This non‐Coulomb scattering process increases the scattering cross section by a factor of about 10 for oxygen and 50 for carbon, thereby increasing by a comparable amount the sensitivity for profiling C and O in thin films on useful substrate materials. Quantitative analysis for C and O requires the use of standards to ascertain the enhancement factor for the scattering cross section when compared to the well‐known Rutherford value.

Cascade modeling in superconductors: production, registration and implications of branch imbalance potential

Abstract The mechanism for creation of electron-hole branch imbalance in superconductors subjected to high-energy particle/radiation bombardment is considered. The physical basis for the production of imbalance lies in the geometrical difference between the densities of states in phase space for electron- and hole-like excitations in metals. The requirement of charge neutrality fails to prevent branch imbalance only in superconductors. Using appropriately modified kinetic equations, numerical solutions for the distribution functions of electron- and hole-excitation branches are obtained under steady state, non-equilibrium conditions. Recent experimental results on the response of Nb to pulsed laser energy deposition are also discussed. An order of magnitude estimate for the case of a 10 keV X-ray quantum detection is made.

Design Considerations for STJ X-Ray Detectors

The geometrical shape and layer thicknesses of superconducting tunnel junctions (STJ) influence their suitability as x-ray detectors. Examples relating to field biasing are discussed. Two related energy loss mechanisms are also modeled.

Escape of photoelectrons: a major energy resolution degrading mechanism in thin superconducting tunnel junction X-ray detectors

The authors consider the consequences of fabricating integral X-ray detectors with thin electrodes, 0.17 mu m for Nb and 0.25 mu m for Sn, with 6-keV photons incident. A simple geometric argument demonstrates that energetic electron loss during the first 1 ps of each event will cause less than the full energy of the incident photon to be captured in a large fraction of the events. This reduces the ultimate energy resolution the devices can achieve. Use of thicker absorbers (electrodes) is thus highly desirable, both from this point of view and to increase the quantum efficiencies of the devices. Nb electrodes should be about 0.8- mu m thick to reduce the escape probability of photoelectrons of energy >or=5 eV during the first 1 ps of an event.<<ETX>>

Development of a new superconducting detector for the ultra-violet and soft X-ray regimes

UV and X-ray astronomy need hyperspectral imaging devices, i.e. large format (1000/spl times/1000) arrays of pixels in which each single-photon event is tagged with spectral information. Such devices permit multicolor imagery and surveys to be obtained with a single camera. Here we discuss the first-stage of development of a new generation of UV/X-ray detectors that will provide 0.1-0.01% energy resolution in an array format.