Implementation of an algorithmic spectrometer using Quantum Dot Infrared Photodetectors

A series of Quantum Dot Infrared Photodiodes (QDIPs) incorporating a dot-in-a-well structure was grown by Molecular Beam Epitaxy. These QDIPs were grown with 2.4 monolayers of InAs sandwiched between two 1 nm In0.15Ga0.85As layers which were in turn grown in a 10.85 nm GaAs quantum well with 50 nm Al0.1Ga0.9As barriers. Thirty stacks were grown with the quantum dot doping density of 0, 1 and 2 electron/dot. Very low dark current density below 10−4 A cm−2 was measured at voltage range of −1.9 V to +1.5 V at room temperature in the undoped QDIPs, this is the lowest dark current reported for QDIPs at room temperature. Incorporation of doping was found to increase the dark current density at room temperature by 2–4 orders of magnitude in the QDIPs with 1 and 2 electron/dot, respectively. The undoped QDIPs were further characterised. A high peak detectivity value of 6.7 × 1010 cm W/Hz1/2 with a peak absorption wavelength of 6.3 μm was measured at a bias of +20 V and temperature of 77 K. Our QDIPs exhibit highly tuneable spectral response across the voltage range of −21 to +22 V, making them an excellent candidate for an algorithm based infrared spectrometer. A polyethylene sheet was used to filter the black body radiation incident on the QDIPs. Responsivity measurements as a function of voltage was incorporated into a post processing algorithm which assigns appropriate weighting factors to each responsivity value. The spectra characteristics of the polyethylene sheet with an absorption feature as narrow as 0.3 μm was successfully reconstructed using our QDIPs and algorithmic spectrometer demonstrating the potential of our QDIPs for compact, cheap and highly flexible multi and hyperspectral infrared imaging.