Quantum well infrared detectors enhanced by faceted plasmonic cavities

Plasmonic cavities can prominently enhance the absorptance of quantum well infrared photodetectors (QWIPs). However, the total thickness of the semiconductor layers is usually in the deep subwavelength scale to ensure an intensified cavity mode, so the devices for the long wavelength infrared range usually contain a single quantum well. Although the relative enhancement is prominent, the absolute absorptance is not larger than common QWIPs and the ohmic loss is serious. Moreover, the ultra-thin semiconductor layer challenges the uniformity and the fabrication. Simply increasing the number of the QW stacks in a plasmonic cavity would probably ruin the critical coupling condition and result in an even worse situation. This work exhibits that creating real facets for the plasmonic cavities by etching away part of the semiconductors can enhance the light confinement capability and make the system with multiple quantum wells still close to a critical coupling status. Then, the quantum well absorptance is enhanced and the ohmic loss is suppressed. As experimentally demonstrated, the responsivity of a faceted plasmonic cavity integrated 4-stack QWIP at the wavelength around 15.5 mu m is enhanced by 2.2 times with respect to an unfaceted one, and it is 12 times higher than that of a 45 degrees edge facet coupled QWIP. In addition, the dark current is reduced by 32% due to the reduction of the photosensitive area. With the help of the enhanced light coupling, the specific detectivity D* initially increases with the number of QW stacks, reaches a peak around 3 stacks, and then drops back around 7 stacks to the similar level of the single stack structure.