D. Brent Mott

Direct observation of transferred‐electron effect in GaN

We report the direct observation of transferred‐electron effect in unintentionally doped GaN epilayers grown by metalorganic chemical vapor deposition. The negative differential resistivity (NDR) was observed from the current‐electric field characteristics in GaN using a metal‐semiconductor‐metal (M‐S‐M) system. The threshold field for the onset of NDR was independent of the spacing of M‐S‐M fingers, and was measured to be 1.91×105 V/cm for GaN with an n‐type carrier concentration of 1014 cm−3. This value is very close to the value obtained from theoretical simulation. This observation is an experimental evidence of transferred‐electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn‐effect devices using GaN materials.

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.

Characterization of the Detector Subsystem for the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope

We present interim results from the characterization test development for the Detector Subsystem of the Near-Infrared Spectrograph (NIRSpec). NIRSpec will be the primary near-infrared spectrograph on the James Webb Space Telescope (JWST). The Detector Subsystem consists of a Focal Plane Assembly containing two Teledyne HAWAII-2RG arrays, two Teledyne SIDECAR cryogenic application specific integrated circuits, and a warm Focal Plane Electronics box. The Detector Characterization Laboratory at NASAs Goddard Space Flight Center will perform the Detector Subsystem characterization tests. In this paper, we update the initial test results obtained with engineering grade components.

Principal Components Analysis of a JWST NIRSpec Detector Subsystem

We present principal component analysis (PCA) of a flight-representative James Webb Space Telescope Near Infrared Spectrograph (NIRSpec) Detector Subsystem. Although our results are specific to NIRSpec and its T ~ 40 K SIDECAR ASICs and 5 μm cutoff H2RG detector arrays, the underlying technical approach is more general. We describe how we measured the system’s response to small environmental perturbations by modulating a set of bias voltages and temperature. We used this information to compute the system’s principal noise components. Together with information from the astronomical scene, we show how the zeroth principal component can be used to calibrate out the effects of small thermal and electrical instabilities to produce cosmetically cleaner images with significantly less correlated noise. Alternatively, if one were designing a new instrument, one could use PCA to determine a set of environmental requirements (temperature stability, electrical stability, etc.) that enabled the planned instrument to meet performance requirements.

ACCESS: design and preliminary performance

ACCESS, Absolute Color Calibration Experiment for Standard Stars, is a series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from the National Institute of Standards and Technology (NIST) to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of 500 across the 0.35.1.7μm bandpass. Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant to many astrophysical problems. Systematic errors associated with problems such as dark energy now compete with the statistical errors and thus limit our ability to answer fundamental questions in astrophysics. The ACCESS design, calibration strategy, and an updated preliminary performance estimate are discussed.

Simulations of Sample-Up-The-Ramp for Space-Based Observations of Faint Sources

We have conducted simulations of a memory-efficient up-the-ramp sampling algorithm for infrared detector arrays. Our simulations use realistic sky models of galaxy brightness, shapes, and distributions, and include the contributions of zodiacal light and cosmic rays. A simulated readout is based on the HAWAII-2RG arrays, and includes read noise, dark current, KTC noise, reset anomaly, persistence, and random telegraph noise. The up-the-ramp algorithm rejects cosmic rays, RTN, and KTC noise. The reset anomaly and persistence are also correctable. It produces a best estimate of the source flux under the assumption of very low signal-to-noise, while the overall dynamic range is increased. We present an analysis of the fidelity of image brightness recovery with this algorithm. This work is motivated by the need for sensitive, precise, accurate photometry for Destiny, a mission concept under study for the Joint Dark Energy Mission (JDEM).

ACCESS: design and sub-system performance

Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant to many astrophysical problems. ACCESS, “Absolute Color Calibration Experiment for Standard Stars”, is a series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from the National Institute of Standards and Technology (NIST) to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of 500 across the 0.35-1.7µm bandpass.

ACCESS: thermal mechanical design and performance

Establishing improved spectrophotometric standards is important for a broad range of missions and is relevant to many astrophysical problems. ACCESS, “Absolute Color Calibration Experiment for Standard Stars”, is a series of rocket-borne sub-orbital missions and ground-based experiments designed to enable improvements in the precision of the astrophysical flux scale through the transfer of absolute laboratory detector standards from the National Institute of Standards and Technology (NIST) to a network of stellar standards with a calibration accuracy of 1% and a spectral resolving power of 500 across the 0.35−1.7μm bandpass. Achieving a calibration accuracy of 1% not only requires an accurate calibration transfer from the detector standards to the instrument, but it also requires characterization and stability of the detector as well as a thermal background that contributes less than 1% to the flux per resolution element in the near-infrared (1.7μm) spectral region of the ACCESS bandpass. This paper describes the thermal mechanical design for achieving a low thermal background across the ACCESS spectral bandpass.

Stray light reduction in testing of NIRSpec subsystems: the focal plane array and micro-shutter assembly

The James Webb Space Telescope (JWST) is an infrared, space-based telescope scheduled for launch in 2013. JWST will hold four scientific instruments, including the Near Infrared Spectrograph (NIRSpec). NIRSpec operates in the wavelength range from 0.6 to 5 microns, and will be assembled by the European Space Agency. NASA/Goddard Space Flight Center (GSFC) is responsible for two NIRSpec subsystems: the detector subsystem, with the focal plane array (FPA), and the micro-shutter subsystem, with the micro-shutter assembly (MSA). The FPA consists of two side-by-side Rockwell Scientific HgCdTe 2Kx2K detectors, with the detectors and readout electronics optimized for low noise. The MSA is a GSFC developed micro-electro-mechanical system (MEMS) that serves as a programmable slit mask, allowing NIRSpec to obtain simultaneous spectra of >100 objects in a single field of view. We present the optical characterization test plan of the FPA. The test plan is driven by many requirements: cryogenic operating temperature, a flight-like beam shape, and multi-wavelength flux from 1 to 10,000 photons per second, thus low stray light is critical. We use commercial optical modeling software to predict stray light effects at the FPA. We also present the optical contrast test plan of the MSA. Each individual shutter element operates in an on/off state, and the most important optical metric is contrast. The MSA is designed to minimize stray and scattered light, and the test setup reduces stray light such that the optical contrast is measurable.

SIMULATION OF AN ELE CTROSTATICALLY ACTUA TED MICRO -MACHINED FABRY -PEROT ETALON

The development of a micro -scale tunable Fabry - Perot interferometer with narrow band pass (0.3 ∝m) and application to wide -field spectroscopy is presented. In this design, a silicon wafer is micro -machined using deep reactive ion etching (DRIE) to fabricate two etalon plates. The first etalon is a monolithic structure that includes the etalon fram e, its spring suspension, and support frame. The second is a fixed reference etalon plate. A silicon nitride membrane is suspended in tension across a thick support ring on both the moveable and fixed etalon plates as a result of back etching. Due to the processes of thin film deposition, high stresses are transmitted throughout the structure. Because of stringent flatness and parallelism requirements on the etalon plates, simulations are used to aid in the design of a suitable optical coating, which is deposited onto the membrane. Tuning the optical gap is achieved via electrostatic actuation. Simulating force - deflection behavior of the tunable etalon plate is also performed to characterize mechanical stiffness of the spring suspension system, as it det ermines electrostatic stiffness. Theoretical coupled field analysis is compared with an electrostatic simulation to predict voltage required to precisely tune the optical gap (17.5 ∝m maximum) in accordance with well -defined Fabry - Perot figures of merit. Test data gathered from three different experimental setups - nanoindentation, wave front sensing interferometry, and electrostatic actuation - correlate well with predictions based on finite element analysis.