Acreo is one of the leading producers of QWIP FPAs in the world and is also intensively running R&D activities. The European Space Agency has awarded Acreo the contracts "Far-IR Linear Detector Array" in 6-18 μm infrared range within the Darwin mission's frameworks and "Quantum Well Infrared Photodetector Arrays" in 11-15 μm range for Earth observation (EO). The Darwin project imposes hard requirements on the dark current, while for the EO project the operating temperature is a stringent constraint. The goal of both contracts is to establish and demonstrate the ultimate performance of Acreo's QWIP-technology for these applications at the highest possible operating temperature. For this purpose Acreo designed, grew and characterised QWIP material sensitive to different wavelengths in the range of 6-18 μm. To investigate transport properties and verify the validity of the hydrodynamic model of the dark current, experiments with varying numbers of quantum wells per thickness unit and periods were conducted. A structure for long infrared region with an increased number of periods revealed a drastic reduction of the dark current at transient temperature. The dependence of the capture probabilities on the electron energy in the miniband resulting in different dependencies of the photoconductive gain for the photo- and dark currents on the number of periods is suggested as the reason for that. Such hypothesis shows possibilities for improvement of the balance between the photo- and dark current. Optimisation of the photoconductive gain changes the geometrical parameters of the detector and requires optimisation of the optical coupling.
Acreo in Sweden has been invovled in Quantum Well IR Photodetector (QWIP) reserach and development since 1986. During the first years a small group led by Jan Andersson was dedicated to research on QWIP structures and means of coupling radiation into the quantum-well structure. One of the resarech results is a 2D optical grating couler to create an optimal elecgtric field pattern for highest possible absorption and responsivity. Acreo holds a patent for this grating coupler. Since 1988 FLIR Systems, later FMV, Saab Dynamics, Celsius Tech and NUTEK have sponsoered an R and D project with the goal to develop QWIP detectors and start up production. Soon it became clear that an adapted ROIC deisng and the hybridization of the focal plane array are key issues in order to achieve the highest possible performance and operability of the complete detector device for voluem production. Extended measures where taken in 1996. In 1997 the industrial interest increased further, ClesiusTech and Saab Dynamics, merged in 2000, and now a division of FLIR Systems AB started to sponsor the R and D project. Because of its success during the last years it has now been expanded and scheduled until 2003 and beyond. Volume production of QWIP FPAs started in 2000.
Multiple quantum well spatial light modulators (MQW SLMs) are promising devices for future high-speed applications. We present results obtained with a single-pixel amplitude modulator. We discuss the status of our work on a 128x128-pixel ternary SLM. This SLM will run at 10 kHz and have one low-reflectance level and two high reflectance levels with a phase difference of pi. We also present a study of the relation between the coding domain and the structural design of modulators.
A camera system has been designed using a focal plane array with 320 X 240 pixels. The detector array is based on quantum wells in the GaAs/AlGaAs material system grown onto a GaAs substrate and flipchip mounted to a readout circuit. The camera system uses f-number equals 1.5 optics to create an image of the scene on the FPA. The detector is cooled to approximately 70 K by an integrated Stirling cooler. The system also includes electronics for amplification and analog to digital conversion of the detector signal. The images are either displayed on a monitor or stored in digital format on an integrated hard disk. The short-term temporal noise was measured and the noise equivalent temperature difference was calculated to 16 mK. The spatial noise was found to be comparable to the temporal noise. The properties of the infrared images were valuated with respect to short and long term stability. The stability was found to be very good, giving a high quality image even 1 hour after a calibration. The number of dead pixels was less than 0.1% for several detectors.
Pulsed laser deposition has been developed as a technique for the preparation of accurate planar models of catalysts. The procedure is exemplified by copper/copper oxide on alumina, a DENOX catalyst, ablated under dry or wet conditions. Ablation of hydrogen free oxides in moisture is a novel route for hydroxylation. The work wad done in a fully UHV compatible multichamber system and is part of a coherent research program to trace the role of different paths for NO reduction with hydrocarbons over alumina/Cu<SUB>x</SUB>O.
Pulsed laser deposition have been adapted for precursor fabrication of Hg-Ba-Ca-Cu-O thin films. The preparation involves a sequential deposition of HgO and Ba<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>x</SUB> thin layers at 200 degree(s)C on MgO and SrTiO<SUB>3</SUB> substrates. Processing parameters were optimized by studying surface morphology of the individual layers and by strict Hg content control. Ex-situ thermal postannealing and oxygen content have not yet been optimized. The obtained films show almost single phase HgBa<SUB>2</SUB>CaCu<SUB>2</SUB>O<SUB>6+(delta</SUB> ) with a c-axis of 12.65 angstrom and T<SUB>c</SUB> (onset) of 110 K as determined from magnetic measurements.