Substrate removal of dual-colour InAs/GaSb superlattice focal plane arrays

A removal, as far as possible, of the GaSb substrate is needed to optimize the quantum efficiency and the longevity, i.e., the maximum number of cool down cycles, of dual-colour InAs/GaSb superlattice focal plane array detectors for the mid-wavelength infrared atmospheric transmission window at 3-5 µm. For this purpose, three different vertical detector structures have been investigated and are compared in the present paper. A basic structure, comprising only the essential layers is evaluated against two more advanced structures, which include a lattice-matched InAsSb etch stop layer. For the basic structure the remaining substrate thickness amounts to roughly 20 µm. With a second structure, which employs an etch stop separated from the active diode structure by a sufficiently thick GaSb spacer layer, the substrate and the etch stop layer can be completely removed by combining mechanical lapping and polishing as well as different wet chemical etchants. Here, only epitaxially grown layers remain, but the spacer is still common to all pixels. A third structure combined with further optimized front- and backside processes is suited for the complete mechanical separation of dual-colour detector pixels in the focal plane array arrangement.