Quantum-well infrared photodetectors (QWIPs) using inter-subband transition in GaAs/AlGaAs multi-quantum wells require optical coupling to absorb normally incident radiation. Using a simple method developed for simulating optical coupling, we have designed a novel, optical coupling diffraction pattern formed with elliptical-curves in a single etching step. The design minimizes the radiation escaping from the pixels while maximizing the random diffraction. Simulation results also indicated that fabrication errors in the lithographic process had less effect of reducing coupling efficiency for the elliptical-curves diffraction pattern than for a conventional pseudo-random grating formed with square unit cells. The performance of a QWIP with the elliptical pattern was experimentally compared with that of a QWIP with the pseudo-random grating by fabricating photodetector arrays and measuring the photocurrent output from individual pixels. The elliptical-curve pattern produced about 30% higher quantum efficiency than the pseudo-random grating. The superior performance of the elliptical-curve pattern is due primarily to its diffraction characteristics and to a lesser extent to its smaller dependence on fabrication error. © 2001 Elsevier Science B.V.
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