Progress on MBE grown type-II superlattice photodiodes

The closely lattice-matched material system of InAs, GaSb, and AlSb, commonly referred to as the 6.1Å material system, has emerged as a fertile ground for the development of new solid-state devices. The flexibility of the system in simultaneously permitting type-I, type-II staggered, and type-II broken-gap band alignments has been the basis for many novel, high-performance heterostructure devices in recent years, including the GalnSb/InAs type-II strained layer superlattice infrared detectors proposed by Smith and Mailhiot [¹] in 1987. The type-II superlattice design promises optical properties comparable to HgCdTe, better uniformity, reduced tunneling currents, suppressed Auger recombination, and normal incidence operation [ ²,³]. In 1990, Chow and co-workers first reported Ga _1-xIn_xSb/InAs superlattice materials with high structural quality, LWIR photoresponse, and LWIR photoluminescence [⁴]. Later, researchers demonstrated excellent detectivity (approaching HgCdTe, 8-μm cutoff, 77K) on individual superlattice devices [5]. Currently, superlattice detector technology is undergoing the transition from single element detectors into high-performance focal plane imaging arrays [⁶]. Here we report on the status of superlattice diodes grown and fabricated at the Jet Propulsion Laboratory designed for infrared absorption in the mid (2-5 μm) and long wavelength (8-12 μm) infrared ranges. Our mid-wavelength infrared devices display a zero bias differential resistance-area product as high as 1e6 Ohmcm² at 80K with a 5μm cutoff, with a corresponding detectivity of nearly 1e13 Jones. These detectors continue to function at room temperature with a detectivity of nearly 1e9 Jones. In the long wavelength region, we have produced devices with detectivities as high as 8×10¹⁰ Jones with a differential resistance-area product greater than 6 Ohmcm² at 80K with a long wavelength cutoff of approximately 12μm. A typical IV curve a 12μm cutoff device is shown in Figure 1. Responsivity, detectivity and quantum efficiency for a typical 12μm cutoff device is shown in Figure 2. In addition to detector performance results, recent progress in epitaxial regrowth as a passivation method and progress on long-wavelength imaging array fabrication will be presented.