Sensitivity Improvement of Quantum Well Infrared Photodetector
Details
We investigated the influence of the presence of GaAs substrates on the sensitivity of QWIP-FPA(Quantum Well Infrared Photodetectors Focal Plane Array)with the pseudo-random grating optical coupler. When the thick GaAs substrate was removed from the QWIP-FPA back surface, its sensitivity increased about ten times over that of QWIP-FPA whose substrate remained. This sensitivity enhancement was observed when selected pixels were illuminated by the IR incidence from outside QWIP, using the bar patterned high temperature targets. For uniform IR incidence for all pixels, the sensitivity was independent on the presence of the thick GaAs substrate. These results clearly indicate that the internal total reflection is confined into one pixel for thin film QWIP-FPA and it runs away from the originally incident pixel for thick substrate QWIP-FPA. We experimentally confirmed that the pseudo-random grating coupler for QWIP has really enhanced the IR absorption by utilizing multi internal total reflections. A method for simulation of optical coupling in QWIP with two-dimensional random gratings has been developed. It provides information on practical aspects of design and fabrication. The absorption efficiency varies with unit cell size and has a maximum value when cell width to wavelength ratio is 1.2. A novel optical coupling diffraction pattern with elliptical curve for QWIPs is developed using the simulation. It provides high coupling efficiency and fabrication ease. Improved performance has been experimentally verified.
