We investigate and compare the performance of 30 layers strain-coupled quantum dot (SCQD) infrared photodetectors capped with one of two different layers: a quaternary (In0.21Al0.21Ga0.58As) or ternary (In0.15Ga0.85As) alloy of 30 Å and a GaAs layer with a thickness of 120–150 Å. Measurements of optical properties, spectral responsivity, and cross-sectional transmission electron microscopy were conducted. Results showed that quaternary capping yielded more superior multilayer QD infrared photodetectors than ternary capping. Quaternary capping resulted in enhanced dot size, order, and uniformity of the QD array. The presence of Al in the capped layer helped in the reduction in dark current density and spectral linewidth as well as led to higher electron confinement of the QDs and enhanced device detectivity. The vertically ordered SCQD system with quaternary capping exhibited higher peak detectivity (~1010 cm Hz1/2/W) than that with ternary capping (~107 cm Hz1/2/W). In addition, a very low noise current density of ~10−16 A/cm2 Hz1/2 at 77 K was achieved with quaternary-capped QDs. © 2014, Springer-Verlag Berlin Heidelberg.