High performance dual-band long-wave infrared focal plane array based on type-II InAs/GaSb superlattices

Abstract

As requirements for infrared (IR) sensing become more stringent, demanding identification of the object rather than mere detection, imagers sensitive to a single waveband are no longer adequate in some applications. In these scenarios, the ability to "see" in multiple wavebands through a single infrared camera is indispensable. For terrestrial-based IR imaging, long-wave (LWIR) detectors are particularly suitable since the emission peaks of room temperature objects are positioned in the 8 to 12µm atmospheric window according to Planck's law. The state-of-the-art dual-band detector systems in the LWIR spectra are based on mercury cadmium telluride, though control of its spatial bandgap uniformity towards this wavelength regime can be challenging [1]. Type-II superlattices (T2SLs) enjoy a unique advantage because of the way its band-structure is determined. The electronic structure of the superlattice is controlled by the layer thicknesses, which is solely determined by the impinging rate of the group III element and does not vary much with the substrate temperature nor the flux ratios. Because of this, a wide range of cutoff wavelengths can be realized with great spatial homogeneity, which is of great benefit for imager operability and manufacturing yield, especially as imager resolutions increase.High performance dual-band T2SL imagers have only been demonstrated in the mid-wave IR [2]. Initial efforts to extend multispectral detection into longer cutoff wavelengths have been limited by the inversely decreasing energy gap that encourages coherent tunneling and surface leakage. However, in our studies on single color diodes, these issues were addressed first by blocking majority carrier tunneling across the p-n junction with a M-barrier [3] while leaving photo-generated minority carriers unhindered. The dark current characteristics improved by an order of magnitude and were observed to be limited by conductive channels along the mesa sidewalls. Subsequent modifications to the manufacturing process [4] decreased the dark current by another order of magnitude. These changes incur benefits for eventual imager performance by reducing electrical shot noise and increasing the dynamic range of the read-out integrated circuit (ROIC), which is a multiplexer with charge storage capacitors that the sensors are hybridized to for image read-out.Following these observations, we targeted for a dual-band T2SL LWIR imager [5] employing the design modifications applied to single color detectors. Unlike single color imagers, however, each spectrally sensitive layer is stacked one on top of another in an n-p-p-n fashion forming two back-to-back diodes with the shorter wavelength channel placed closer to the illumination source. The shorter "blue" channel had its 100% cutoff wavelength at 9.5µm (~130meV) while the "red" channel's cutoff was at 13µm (~95meV), approaching the VLWIR atmospheric window. Although a mere 2µm in active region thickness was grown for each channel, the quantum efficiencies (QE) were enhanced by means of Fabry-...