Improved Wavelength-Insensitive Optical Coupling for Quantum Well Intersubband Photodetectors

GaAs based Quantum Well Intersubband Photodetectors (QWIP) are attractive devices for large staring Focal Plane Arrays (FPA) at mid- and long- infrared wavelengths. The mature GaAs processing technology allows larger arrays with higher pixel operability and better uniformity to be manufactured at lower cost than competing technologies that can access both the MWIR (3-5 micron) and LWIR (8-12 micron) atmospheric windows. Due to their materials and operation, these detectors also benefit from inherent radiation hardness and resistance to thermal blooming effects.QWIPs suffer from two important limitations: low sensitivity to normal incidence light due to the physics of the absorption process, and relatively high dark current due to the photoconductive, rather than photovoltaic, nature of the carrier collection. QED proposes to address both these limitations by developing a reflection based coupling scheme, that relies on the natural crystalographic etching properties of certain solutions, such as H{sub 2}SO{sub 4}:H{sub 2}O{sub 2}:H{sub 2}O. Light is collected by the facets over virtually the entire pixel area; however, since the active area is reduced significantly by the material etched away, the dark current can be reduced by a factor of 2. Because the coupling relies on reflection rather than diffraction, it is much less sensitive to wavelength than a grating coupler and more applicable to use in dual band detectors. The relatively large features, several microns wise, will also ease processing constraints compared to grating couplers, especially for applications in the mid infrared.