Growth of InAs/AlAs1-ySby Quantum Cascade Detectors

Abstract

Quantum cascade detectors (QCDs) are mid-infrared (MIR) room-temperature, high-speed, narrow-band, low-noise, photovoltaic detectors based on intersubband (ISB) transitions [1]. They have a variety of applications, including thermal imaging, remote detection, chemical sensing, and astronomy [2]. The main figures of merit of QCDs are responsivity and detectivity, which are inversely proportional to the effective electron mass (m e*) in the quantum well and are the result of the design and choice of the material system. The lower limit for the wavelength is imposed by the conduction band offset (CBO). Commonly, QCDs are based on lattice-matched InGaAs/InAlAs superlattices (SLs) on InP substrates [2]. In this case, the CBO between the barrier and the well materials limits the lowest detectable wavelength to 4 μm. The InAs/Al(As)Sb material system offers the benefits of a wide CBO (2.28 eV) and a low m e* (0.023m 0) in InAs quantum wells. As a result, the optical absorption strength is higher and this enables thicker wells, which are easier to grow. Two different substrates can be chosen for QCDs: InAs [3] and GaSb [4], both offering advantages and disadvantages. On the one hand, InAs/AlAs0.16Sb 0.84 QCDs can be grown lattice-matched to InAs substrates, maximizing design freedom, but the bandgap of InAs (E g=0.36 eV) makes the substrate absorbing at wavelengths lower than 3.5 μm, cutting off part of the MIR spectrum and limiting some devices to top-side illumination. Additionally, the use of mixed group Vs complicates growth conditions. On the other hand, GaSb has a larger bandgap (E g=0.73 eV), which is transparent down to 1.7 μm. Therefore, the QCDs can be fabricated in the standard double-pass 45° facet method, where the illumination occurs through the substrate. The epitaxy of InAs/AlSb QCDs on GaSb poses several challenges. The SLs need to be grown strain-balanced, due to the mismatches between GaSb, and the intermixing between the group-V elements in the barriers and wells must be prevented. As and Sb are adjusted to obtain sharp interfaces.