The ongoing development of QWIP focal plane arrays at IRnova (formerly Acreo) has resulted in the launch of several
new formats up to 640 by 512 pixels and the introduction of major improvements to all products. The achieved
performance and imagery will be evaluated. In the light of the development of new formats, the results of hybridization a
640 by 512 detector with 20 &mgr;m pitch will be discussed. The driving forces behind these improvements have been the
demands from both industrial applications where the requirements for the operating temperature are high due to the life
time issues, and from space applications where the requirements for the quantum efficiency and dark current are
extreme. For the latter type of applications a number of QWIPs covering the 4 to 20 &mgr;m wavelength band have been
grown and evaluated. The demands for better performance are met by ongoing increases in light coupling, improvements
of the quantum well structures, as well as fine tuning of the epitaxial growth parameters. This has led to FPAs that can
operate at 75 K and operation close to 80 K is within reach. IRnova is also looking at other material systems to fulfill the
requirements of next generation photon detectors.
The ultimate performance of QWIP implies hard requirements on the response-to-dark-current ratio for both high operating temperature and low background, e.g. space, applications. A way to improve this ratio by finding the optimal combination of band structure and material parameters is suggested. Experiments have been conducted on GaAs/AlGaAs structures optimised for 8.5 to 16 μm with similar types of band profile.
The doping concentration in the quantum well (QW) is the principal parameter in such optimisation because it affects linearly the photocurrent and exponentially the dark current. As a result of the first experiment series we found an optimal QW doping concentration corresponding to the maximum response-to-dark-current-ratio, thus verifying the validity of the widely used hydrodynamic model.
Experiments with a varying number of quantum wells for a constant total thickness were also carried out and analyzed. The resulting variation in barrier thickness changes the balance between the quantum efficiency and photoconductive gain. A critical thickness was found, where the temperature-independent component of the dark current increases drastically.
For low background applications, especially in combination with long wavelength detection, it is not enough to only reduce the thermally-assisted and sequential tunnelling components of the dark current. Other sources of the dark current usually neglected at high temperature start to play a role. Interface shape and background doping in the barriers are examples of increasingly important factors. We discuss the contribution of these factors to the dark current.
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.