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GaAs-based Detectors Extend To The Far Infrared

A team of French researchers claims that it has fabricated the first GaAs/AlGaAs quantum cascade detector (QCD) capable of operating at very long infrared wavelengths.

Development of this 15 μm detector could provide a stepping stone towards the manufacture of focal plane arrays operating in this spectral range that could be used for meteorology, atmospheric chemistry studies, and Earth observation missions.

Corresponding author Amandine Buffaz from the University of Paris, Diderot-Paris 7, says that the performance of the team’s detectors are comparable to those of the incumbent technology, quantum well infrared photodetectors. However, the cascading detectors have one distinct advantage – very low dark currents that enable long integration times. The team, which also includes researchers from the Alcatel-Thales 3-5 lab, produced their detectors via MBE growth on a semi-insulating GaAs (001) substrate. The detector’s epitaxial layers consist of 30 identical periods of 4 coupled quantum wells that feature AlGaAs barriers with a 232 meV conduction band offset.

Square shaped mesas with 50 μm and 100 μm sides were created with dry-etching techniques, and Au/Ge/Ni ohmic contacts were deposited onto these pixels.

The detector has a responsitivity peak of 14.3 μm, and its detectivity at 25 K and an applied bias of –0.6V is 1 x 1012 Jones.

The detector’s performance can be taken to a new level by cutting the tunneling current.

“To reach that aim we will use two theoretical models of electronic transport in QCDs: a ‘thermalized subbands’ approach that models transport based on diffusion mechanisms; and a resonant tunnelling model."

Comparing the results of each of these calculations should uncover a structure that has carrier transport dominated by diffusion rather than tunneling. Another of the team’s goals is to develop detectors operating in other regions of the infrared spectrum. “The first QCD detecting in the terahertz is under study, and in the immediate future the first thermal imager based on QCD detectors should be fabricated."

A. Buffaz et al. Appl. Phys. Lett 96 172101 (2010)

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