Carl Michael Stahle

A concept for a submillimeter-wave single-photon counter

We discuss the design for a submillimeter-wave photometer, using a combination of superconducting and single-electron devices, which would have high quantum efficiency, very low noise-equivalent powers, and eventually even submicrosecond timing resolution. The absorption of above-gap photons occurs in a small strip of superconducting Al, whose normal-state resistance can be matched efficiently to an antenna of a higher gap (Nb) superconductor. The quasiparticles produced by photon absorption are then confined via Andreev reflection, and forced to tunnel through a small SIS tunnel junction. The tunneling time is much shorter than the known (>10 /spl mu/s) quasiparticle recombination time, so collection efficiency will be high. The device sensitivity would be limited by the small subgap current in the high-quality Al/AlO/sub x//Al tunnel junction at temperatures (100 mK) well below T/sub c/. Scaling based on the larger junctions used in X-ray detector applications suggests that the total dark current can be <0.1 pA, or of order 10/sup 5/ electrons/second, corresponding to an NEP of less than 10/sup -19/ W//spl radic/Hz at 500 microns (600 GHz). The photocurrent will be measured using a fast single-electron transistor (RF-SET), which allows a shot-noise-limited performance even for the very small currents delivered from this low capacitance and high impedance SIS junction. Results of initial fabrication and dc characterization of an integrated photodetector are also given.

Low‐temperature photoluminescence of detector grade Cd1−xZnxTe crystal treated by different chemical etchants

Low‐temperature photoluminescence (PL) spectra of detector grade Cd1−xZnxTe (x=0.1) have been measured to obtain information about shallow level defect concentration introduced during mechanical polishing and chemical etching processes. We present here a comparative PL study of Cd0.9Zn0.1Te crystals treated by different chemical solutions used for nuclear detector surface treatment. The results show that the 5% Br–MeOH+2%Br–20% lactic acid in ethylene glycol treatment combines the advantages of bromine and lactic acid for chemical etching and results in the best surface condition, as evidenced by the largest I(D0,X)/Idef intensity ratio and the narrowest full width at half‐maximum of the main peak (D0,X). Changes in the surface morphology were also analyzed by atomic force microscopy and correlated with the PL results. Current–voltage (I–V) curves and the room‐temperature 55Fe spectral response of the sample etched by the best treatment are also presented and discussed.

Fabrication of CdZnTe strip detectors for large area arrays

A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy has been built as a prototype for a space flight gamma ray burst instrument. Two dimensional orthogonal strip detectors with 100 micrometer pitch have been fabricated and tested for a large area array (approximately 60 cm2 with 36 detectors). Details for the fabrication and evaluation of the detectors are presented. Critical issues to be addressed include fabricating metal contacts with low leakage current and with excellent wire bonding yield, achieving high yield for good strips, and surface cleaning and passivation.

Multiplexing of Radio-Frequency Single Electron Transistors

We present results on wavelength division multiplexing of radio-frequency single-electron transistors. We use a network of resonant impedance matching circuits to direct applied rf carrier waves to different transistors depending on carrier frequency. A two-channel demonstration of this concept using discrete components successfully reconstructed input signals with small levels of crosscoupling. A lithographic version of the rf circuits had measured parameters in agreement with electromagnetic modeling, with reduced crosscapacitance and inductance, and should allow 20–50 channels to be multiplexed.

Design and fabrication of superconducting transition edge X-ray calorimeters

Abstract We report on progress made so far at NASA Goddard Space Flight Center towards the development of arrays of X-ray microcalorimeters as candidates for the high-resolution X-ray spectrometer on the Constellation-X mission. In the design concept presently under consideration, the microcalorimeter consists of (i) a Bi/Cu multilayer absorber for stopping and thermalizing the incident X-rays, (ii) an e-beam evaporated Mo/Au proximity bilayer with sputtered Nb leads for sensing the resultant temperature rise, and (iii) a silicon nitride membrane to provide a weak thermal link to the sink temperature so that the calorimeter can return to its equilibrium temperature. Fabrication details and preliminary results are reported.

Fabrication of superconducting bilayer transition edge thermometers and their application for spaceborne X-ray microcalorimetry

The transition between normal conduction and superconductivity in superconducting materials can be exploited as a highly sensitive thermometer. Transition temperatures can be tailored through the selection of materials, their component cases of more than one material. Two bilayer configurations, Ag/Al and Au/Mo, are examined, including details of preparation, testing, and encountered difficulties. Proposed designs for spaceflight detector applications are discussed.

Commentary: JWST near-infrared detector degradation— finding the problem, fixing the problem, and moving forward

The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.

The Effect of Cathode Bias (Field Effect) on the Surface Leakage Current of CdZnTe Detectors

Surface resistivity is an important parameter of multi-electrode CZT detectors such as coplanar-grid, strip, or pixel detectors. Low surface resistivity results in a high leakage current and affects the charge collection efficiency in the areas near contacts. Thus, it is always desirable to have the surface resistivity of the detector as high as possible. In the past the most significant efforts were concentrated to develop passivation techniques for CZT detectors. However, as we found, the field-effect caused by a bias applied on the cathode can significantly reduce the surface resistivity even though the detector surface was carefully passivated. In this paper we illustrate that the field-effect is a common feature of the CZT multi-electrode detectors, and discuss how to take advantage of this effect to improve the surface resistivity of CZT detectors.

1/f Noise and Hot Electron Effects in Variable Range Hopping Conduction

In the course of developing microcalorimeters as detectors for astronomical X-ray spectroscopy, we have undertaken an empirical characterization of non-ideal effects in the doped semiconductor thermometers used with these detectors, which operate at temperatures near 50 mK. We have found three apparently independent categories of such behavior that are apparently intrinsic properties of the variable-range hopping conduction mechanism in these devices: 1/f fluctuations in the resistance, which seems to be a 2D effect; a departure from the ideal coulomb-gap temperature dependence of the resistance at temperatures below T 0 /24; and an electrical nonlinearity that has the time dependence and extra noise that are quantitatively predicted by a simple hot electron model. This work has been done largely with ion-implanted Si:P:B, but similar behaviors have been observed in transmutation doped germanium.

CdZnTe background measurements at balloon altitudes

Because of its high atomic number and convenient room temperature operation, CdZnTe has great potential for use in both balloon and space borne hard x-ray (5 - 200 keV) astrophysics experiments. Here we present preliminary results from the first CdZnTe background measurements made by a balloon instrument. Measurements of the CdZnTe internal background are essential to determine which physical processes make the most important background contributions and are critical in the design of future scientific instruments. The PoRTIA CdZnTe balloon instrument was flown three times in three different shielding configurations. PoRTIA was passively shielded during its first flight from Palestine, Texas and actively shielded as a piggyback instrument on the GRIS balloon experiment during flights 2 and 3 from Alice Springs, Australia. PoRTIA flew twice during the Fall 1995 Alice Springs, Australia campaign using the thick GRIS NaI anticoincidence shield. A significant CdZnTe background reduction was achieved during the third flight with PoRTIA placed completely inside the GRIS shield and blocking crystal, and thus completely surrounded by 15 cm of NaI. These background results are presented and contributions from different background processes are discussed.