InAs/InGaSb superlattice (SL) materials are an excellent candidate for infrared photodiodes with cutoff wavelengths beyond 15 μm, i.e., in the very long-wavelength infrared (VLWIR) range. There are relatively few options for high-performance infrared detectors to cover wavelengths longer than 15 μm, especially for operating temperatures above 15 K. A variety of possible SL designs cover the VLWIR range, including designs with and without indium alloying of the GaSb layers. For homogeneous InGaSb alloys, transport modeling found that alloy scattering should be negligible for electrons. In addition, there can be benefits for incorporating InGaSb into the VLWIR SL design; these benefits include a higher molecular beam epitaxy (MBE) growth temperature, which should reduce point defects in the InGaSb, simpler interfaces with the continuous indium flux, suppressed Auger recombination rates, and larger absorption coefficients due to thinner periods in these designs.
Our focus is on designs with 25% indium in the gallium antimonide to achieve energy bandgaps less than 50 meV with a SL period on the order of 68 Å. Similar to the work reported on InAs/GaSb LWIR and VLWIR SLs, our designs employ InGaSb layers less than seven monolayers in width. While the SL designs are strain balanced to the GaSb substrate, care was also taken to minimize strain spikes in the interfacial regions. High-resolution transmission electron microscope images were analyzed to create strain mapping profiles of the SL layers and interfaces. By focusing on a narrow set of VLWIR SL designs, the deposition parameters for the MBE SL growth could be carefully optimized.
The electrical and optical properties of the VLWIR SLs were characterized by variable-temperature Hall effect measurements and by infrared photoresponse spectra. The photoresponse spectra consistently showed a bandgap energy of 47 ±3 meV for the samples studied and a 50% cutoff wavelength at ~19 mm. The repeatability of these very narrow-bandgap SLs over multiple sample depositions even while some growth parameters were being adjusted shows the tight control obtainable with MBE. Narrow-bandgap designs are very sensitive to small changes in layer widths. Variable-temperature Hall effect measurements found that the mobility of electrons in the SL was ~10,000 cm2/V•s below 80 K and was relatively constant in magnitude.
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