Quantum Well Infrared Photodetectors Array Research Articles

Quantum well earth science testbed

Abstract A thermal hyperspectral imager is underdevelopment which utilizes the compact Dyson optical configuration and the broadband (8–12 μm) quantum well infrared photodetector (QWIP) focal plane array technology. The Dyson configuration uses a single monolithic prism-like grating design which allows for a high throughput instrument (F/1.6) with minimal ghosting, stray light and large swath width. The configuration has the potential to be the optimal high resolution imaging spectroscopy solution for aerial and space remote sensing applications due to its small form factor and relatively low power requirements. The planned instrument specifications are discussed as well as thermal design trade-offs. The current design uses a single high power cryocooler which allows operation of the QWIP at 40 K with adequate temperature stability. Calibration testing results (noise equivalent temperature difference, spectral linearity and spectral bandwidth) and laboratory emissivity plots from samples are shown using an operational testbed unit which has similar specifications as the final airborne system. Field testing of the testbed unit was performed to acquire plots of emissivity for various known standard minerals (quartz, opal, alunite). A comparison is made using data from the ASTER spectral library. The current single band (8–9 μm) testbed utilizes the high uniformity and operability of the QWIP array and shows excellent laboratory and field spectroscopic results.

Development of a 1K × 1K, 8–12 μm QWIP array

In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a 1,024 × 1,024 (1K × 1K), 8–12 μm infrared focal plane array (FPA). This 1 megapixel detector array is a hybrid using an L3/Cincinnati Electronics silicon readout integrated circuit (ROIC) bump bonded to a GaAs QWIP array fabricated jointly by engineers at the Goddard Space Flight Center (GSFC) and the Army Research Laboratory (ARL). We have integrated the 1K × 1K array into an SE-IR based imaging camera system and performed tests over the 50–80 K temperature range achieving BLIP performance at an operating temperature of 57 K. The GaAs array is relatively easy to fabricate once the superlattice structure of the quantum wells has been defined and grown. The overall arrays costs are currently dominated by the costs associated with the silicon readout since the GaAs array fabrication is based on high yield, well-established GaAs processing capabilities. One of the advantages of GaAs QWIP technology is the ability to fabricate arrays in a fashion similar to and compatible with silicon IC technology. The designer’s ability to easily select the spectral response of the material from 3 μm to beyond 15 μm is the result of the success of band-gap engineering and the Army Research Lab is a leader in this area. In this paper we will present the first results of our 1K × 1K QWIP array development including fabrication methodology, test data and imaging capabilities.

Development of a 4–15 μm infrared GaAs hyperspectral QWIP imager

Abstract In the on-going evolution of GaAs quantum well infrared photodetectors (QWIPs) we have developed a four band, 640 × 512, 23 μm × 23 μm pixel array which we have subsequently integrated with a linear variable etalon (LVE) filter providing over 200 spectral bands across the 4–15.4 μm wavelength region. This effort was a collaboration between NASA’s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory (JPL) and the Army Research Laboratory (ARL) sponsored by the Earth Science Technology Office of NASA. The QWIP array was fabricated by graded molecular beam epitaxial (MBE) growth that was specifically tailored to yield four distinct bands (FWHM): Band 1; 4.5–5.7 μm, Band 2; 8.5–10 μm, Band 3; 10–12 μm and Band 4; 13.3–14.8 μm. Each band occupies a swath that comprises 128 × 640 elements. The addition of the LVE (which is placed directly over the array) further divides the four “broad” bands into 209 separate spectral bands ranging in width from 0.02 μm at 5 μm to 0.05 μm at 15 μm. The detector is cooled by a mechanical cryocooler to 46 K. The camera system is a fully reflective, f /4.2, 3-mirror system with a 21° × 25° field of view. The project goals were: (1) develop the 4 band GaAs QWIP array; (2) develop the LVE and; (3) implement a mechanical cryocooler. This paper will describe the efforts and results of this undertaking with emphasis on the overall system characteristics.

Astronomical imaging with quantum well infrared photodetectors

Quantum well infrared photodetectors (QWIPs) are attractive for use in astronomy because they offer good sensitivity, are very stable, are easily manufactured in large sizes, and have (comparatively) warm operating temperatures. We describe an astronomical camera based on an 8.5 μm QWIP array which was built at the Jet Propulsion Laboratory and is operated at the Palomar 5-m telescope. We highlight some science results obtained with this instrument, then discuss the prospects for QWIP arrays in future NASA astronomy missions.

Monolithic integration of a quantum-well infrared photodetector array with a read-out circuit

We demonstrate the feasibility of monolithic integration of a quantum-well infrared detector and a read-out circuit on the same GaAs/AlGaAs crystal. Charge storage capability of 2×107 electrons in a 50×50 μm2 pixel is obtained. The operation of a 5×5 test array is reported, performing all the basic functions of a practical focal plane array.

Zero-bias offsets in the low-temperature dark current of quantum-well infrared photodetectors

Quantum-well infrared photodetectors (QWIPs) have potential applicability in many remote-sensing applications, even in the space environment where low background fluxes are involved. In this environment, the detector arrays may need to be operated at temperatures lower than 77 K. At these temperatures, tunneling mechanisms such as Fowler-Nordheim and trap-assisted tunneling could dominate the dark current. The device resistance, which is bias-dependent, increases by several orders of magnitude at these temperatures and may pose a problem. We have seen offsets in the current-versus-voltage (I-V) characteristics (nonzero current at zero bias not associated with dopant migration) which could impair the compatibility of a QWIP array with a readout circuit. We propose that these offsets are due to an RC time- constant effect. We further propose that the resistance in this time constant is due to tunneling mechanisms (and not due to contact resistance), which in turn are structure- and bias-dependent. We discuss our observations and present a circuit model of a QWIP that explains these observations nearly completely.