Long-wavelength infrared detectors based on strained InAs–Ga1−xInxSb type-II superlattices

We demonstrate that infrared detectors made from strained type-II superlattices consisting of III–V semiconductors can have favorable properties for long-wavelength (λc 10 μm) detection applications. We specifically consider InAs–Ga1−x Inx Sb strained-layer superlattices with x≊0.4 . This is a type-II superlattice where the conduction-band minimum of InAs is lower in energy than the valence-band maximum of Ga1−x Inx Sb. Electrons are localized in the InAs layers while holes are localized in the Ga1−x Inx Sb layers. Generally, in a type-II superlattice such as InAs–GaSb, large absorption coefficients cannot be achieved at the small values of band gap necessary for long-wavelength detection because the localization of electrons and holes in adjacent layers leads to small optical matrix elements. Therefore, small band gaps and large optical matrix elements appear to be two mutually exclusive requirements in the InAs–GaSb superlattice. Here, we show that it is possible to obtain large optical absorption coefficients at the values of band gap required for long-wavelength detection by taking advantage of internal lattice-mismatch induced strains present in InAs–Ga1−x Inx Sb superlattices. This feature eliminates the principal difficulty associated with type-II superlattices as candidate materials for long-wavelength detection applications. In fact, the near-threshold optical absorption coefficient of the InAs–Ga0.6 In0.4 Sb strained-layer superlattice is found to be as large as that of conventional Hg1−x Cdx Te alloys in the λc ≊ 10–12 μm region. Moreover, the electron effective mass of the InAs–Ga0.6 In0.4 Sb strained-layer superlattice is found to be nearly isotropic and much larger (m*super/m ≊0.05 at λc ≊10 μm) than that of the Hg1−x Cdx Te alloy. This value of the electron effective mass is a favorable feature of the InAs–Ga0.6 In0.4 Sb strained-layer superlattice since it is large enough to lead to small diode tunneling currents, yet it is small enough to lead to large mobilities and diffusivities. As a result, leakage currents are expected to be much less of a problem in infrared detector diodes based on InAs–Ga0.6 In0.4 Sb strained-layer superlattices than they are in conventional Hg1−x Cdx Te-based infrared detectors. Overall, InAs–Ga1−x Inx Sb strained-layer superlattices are expected to have superior materials properties for long-wavelength detection than the Hg1−x Cdx Te alloy.