High-quality InGaAs/InP quantum wells with ultra-narrow well widths (∼10Å) and peak response at 4.55 μm were grown by gas source molecular beam epitaxy. These structures were characterized by cross-sectional tunneling microscopy (XSTM), double-crystal x-ray diffraction (DCXRD), and cross-sectional transmission electron microscopy (XTEM). Based on the structural parameters determined by XTEM, XSTM, and DCXRD, the field dependent photocurrent spectra were simulated using a six-band effective bond-orbital model. The theoretical calculations are in excellent agreement with experimental data. When used to fabricate p-type InGaAs/InP quantum-well infrared photodetectors (QWIPs), and combined with the high responsivity of 8.93 μm n-type InGaAs/InP QWIPs, these structures offer the possibility of dual band monolithically integrated QWIPs.