The unipolar barrier device architecture introduced by the nBn [1] and XBn [2] has led to significantly improved performance in III-V semiconductor infrared detectors. In particular,the combination of the unipolar barrier device architecture and antimonide absorbers, including the InAsSb and the GaInAsSb bulk alloys, the InAs/GaSb type-II superlattice (T2SL), and the InAs/InAsSbtype-II strained-layer superlattice (T2SLS), has enabled a new generation of high-performance infrared detectors that can provide continuous cutoff wavelengths coverage in the short-, mid-, and long-wavelength range. Notably, focal plane arrays (FPAs) based on the mid-wavelength Ga-free InAs/InAsSb T2SLS unipolar barrier infrared detector have demonstrated a 40 – 50 K higher operating temperature than the InSb FPA, while retaining the same III-V semiconductor manufacturability and affordability benefits [3]. We will provide an overview of the progress and challenges [4] in the development of antimonide unipolar barrier infrared detectors, as well as some of their applications for NASA infrared spectral imaging needs.

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

[1] S. Maimon and G. W. Wicks, Appl. Phys. Lett. 89(15), 151109 (2006).

[2] P. C. Klipstein, Proc. SPIE6940, 6940–2U (2008).

[3] D. Z. Ting, A. Soibel, A. Khoshakhlagh, S. B. Rafol, S. A. Keo, L. Höglund, A. M. Fisher, E. M. Luong, and S. D. Gunapala, Appl. Phys. Lett. 113, 021101 (2018).

[4] D. Z. Ting, A. Khoshakhlagh, A. Soibel, and S. D. Gunapala, J. Elec Materi49, 6936 (2020).

NRL recently demonstrated the first MWIR resonant cavity IR detectors (RCIDs) to exhibit high performance [1]. At resonance wavelength λ_res = 4.0 μm, devices with a grown GaSb/AlAsSb bottom mirror, dielectric top mirror, and absorber thickness of only 50 nm attained external quantum efficiency EQE = 34%, with linewidth 46 nm. U. Lancaster subsequently reported RCIDs with InAs absorber displaying λ_res» 3.3 mm and 52% EQE [2].

Here we report devices for which an nBn detector chip with 20 InAs/InAsSb active quantum wells and total absorber thickness 103 nm was bonded to a GaAs/AlGaAs mirror with reflectivity > 99%. The GaSb substrate was then removed, and mesas processed from the backside of the as-grown detector epitaxy. The RCID’s very thin absorber allows rapid extraction of the photoexcited carriers for high frequency response. For a device with small diameter (d = 21 µm) for reduced capacitance, the optical heterodyne data illustrated in Fig. 1 (taken before the top dielectric mirror was deposited) confirm a 3dB response of 5.9 GHz. The EQE spectrum for a larger RCID (d =105 μm) was measured with an FTIR by fitting the ratio of photocurrent to measured blackbody flux through a calibrated narrowband filter (blue curve in Fig. 2). This yielded peak EQE =57% at λ_res= 4.618 μm,with FWHM = 31 nm. EQEs for the same device were also characterized from the photocurrent induced by excitation from a quantum cascade laser (QCL) with calibrated incident power, in independent measurements at NRL (blue points) and Intraband, LLC (red points).

Several strain-balanced 5.3 µm midwave and 12.0 µm longwave InAs/InAsSb superlattices are grown on (100) GaSb substrates by molecular beam epitaxy and examined using X-ray diffraction and temperature-dependent photoluminescence. A significant amount of surface Sb incorporates into the tensile InAs layer that subsequently affects the design and performance. For a given wavelength design the presence of unintentional Sb in the InAs layer i) increases the tensile electron well thickness, thereby increasing electron confinement and decreasing electron wavefunction intensity, ii) decreases hole well depth, thereby decreasing hole well width and hole confinement, and as a result iii) decreases the absorption coefficient by 8% for midwave and 11% for longwave. This is presented in Figure 1 in terms of total wavefunction overlap and that within the tensile and compressive layers for structures with and without unintentional Sb as a function of the compressive layer Sb mole fraction. The solid curves show the case with unintentional Sb and the dashed curves show the case without unintentional Sb, with the compressive layer in red, the tensile layer in black, and the total (sum of red and black) in blue. The values for the grown superlattices are shown as open circles. The measurements and calculations are for an operating temperature of 77 K. The grown superlattice tetragonal distortion ranges from -0.019% to 0.020% with a -0.001% average for midwave and from 0.021% to 0.039% with a 0.027% average for longwave. A combination of X-ray diffraction and photoluminescence is utilized to determine that the unintentional Sb mole fraction in the tensile layer is 1.9% for midwave and 1.2% for longwave.