Color vision, which is based on our ability to see different wavelengths of light, plays a critical role in object detection and identification. While there has been tremendous progress in the realization of multicolor imagers operating in the visible, this technology is still in its infancy in the infrared spectral range. Realization of multi-color or multi spectral imaging devices operating in the short-wave (1.4-3μm), mid-wave (3-8μm) and long-wave (8-15μm) parts of the infrared spectrum will benefit numerous applications, including material analysis, chemical identification, and imaging. In particular, the development of multi-spectral detectors operating in both short–wave and mid- or long-wave infrared spectral ranges will allow the utilization of two imaging regimes. In the short wave infrared (SWIR), imaging relies on the reflection of the atmospheric night sky light by the objects, whereas in the mid-wave infrared (MWIR) and long wave infrared (LWIR), imaging is based on detection of the objects’ thermal emission.

Antimonide-based semiconductors structures have numerous advantages for infrared detection. In a GaSb/InAs superlattice, the longest detectable wavelength (the so-called detector cut-off wavelength) is set by the thicknesses of the GaSb and InAs layers and is therefore adjustable by design. This allows the implementation of GaSb/InAs superlattice-based photodetectors with different cut-off wavelengths. Moreover, photodetectors with distinct cut-off wavelengths can be vertically integrated in the same device by stacking a superlattice with different cut-off wavelengths on top of each other.

The most common detector architecture is a photodiode, consisting, for example, of a thick low doped p-type absorber and a thin highly doped n-type contact. Under reverse bias, light absorbed in the absorber excites electrons which diffuse into the contacts where they are collected as a photocurrent. In the homo-diodes both the absorber and the contact are constructed from material (superlattice in this case) with the same bandgap. In the heterodiode used in this work, the contact is based on a superlattice with a larger bandgap (smaller cut-off wavelength) to decrease parasitic dark currents, which are one of the main sources of detector noise.

For two-color detection, two photodiodes with different cut-off wavelengths are connected “back-to-back” so that the anode of the first detector is connected to the anode of the second. In this configuration, at a given biasing polarity one of the diodes operates under reverse bias and functions as a photodetector with cut-off wavelength set by its superlattice bandgap, while the second diode is under forward bias and serves as the contact. Reversal of the biasing polarity switches the roles of the two diodes and activates the other color. Using this approach, researchers from Prof. Razeghi’s group successfully demonstrated a two-color Focal Plane Array (FPA) based on antimonide superlattice homo- and hetero-diodes connected back-to-back. Realization of this FPA, which operates in the SWIR (2μm cut-off) and MWIR (4μm cut-off) spectral ranges, is an important step towards practical application of infrared imagers based on antimonide superlattice.

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