ABSTRACTP-type InGaAs/InP quantum-well infrared photodetectors operated at 4.55 μm require the growth of ultra-thin (10 Å) quantum wells. We report a study of interfaces in QWIPs grown by gas-source molecular beam epitaxy in which we optimized the group V source supply sequence so that a 6 K photoluminescence linewidth as narrow as 8.4 meV was observed from a structure with 10 Å wells. Analysis of the PL suggests that interface roughness was minimized. Cross-sectional scanning tunneling microscopy, double crystal x-ray diffraction, and cross-sectional tunneling electron microscopy confirmed that high-quality interfaces and uniform layers were obtained. Using the derived structural parameters, photocurrent spectral response was theoretically predicted for these QWIPs and then experimentally verified.