Nature Allows High Sensitivity Thermal Imaging With Type-I Quantum Wells Without Optical Couplers: A Grating-Free Quantum Well Infrared Photodetector With High Conversion Efficiency

Quantum well infrared photodetectors (QWIPs) have facilitated thermal imagers with excellent pixel operability, uniformity and stability. The main disadvantage of the standard QWIP technology is the low conversion efficiency (CE) as a result of weak quantum efficiency (QE) and photoconductive gain inhibiting the utilization of the sensor for low background and/or high frame rate applications. The other problem is the requirement of an optical grating which loses its efficiency with decreasing pixel pitch, as well as limiting the performance of dual-band focal plane array (FPA) due to the wavelength dependence of the diffraction-grating coupling efficiency. The author reports a grating-free 15 $\mu \text{m}$ pixel pitch $640\times 512$ mid-wavelength infrared (MWIR) QWIP FPA constructed with the InP/GaInP/In0.83Ga0.17 As material system with normal incident radiation absorption ability. The pixels yielded peak QE, CE and specific detectivity of 23%, ~40% and $1\times 10 ^{11}$ cmHz1/2/W (at ~80 K with f/2 optics) in spite of the absence of diffraction grating, substantially high cut-off wavelength (5.8 $\mu \text{m}$ ) and broad spectral response ( $\Delta \lambda /\lambda _{\mathbf {p}}=31$ %). Together with excellent (uncorrected) responsivity and noise equivalent temperature difference nonuniformities of 5.9% and 17%, the results illustrate tremendous improvement over the performance of the conventional MWIR QWIP FPA exhibiting great potential to revive the QWIP technology especially in dual-band imaging.