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A very high performance two-stack, two-color, high strain (HS- ) quantum well infrared photodetector (QWIP) has been demonstrated. The sample was grown on a semi-insulating (100) GaAs by molecular beam epitaxy (MBE). It consists of two stacks of MWIR and LWIR QWIPs as the active region with a 100 nm thick highly doped contact layer grown between the two stacks. Each stack is designed to have detection in one of the two atmospheric windows, 3 - 5 micrometer (MWIR) and 8 - 12 micrometer (LWIR), respectively. The MWIR stack consists of 20 periods of 300 angstrom Al 0.38 Ga 0.62 As barrier and 24 angstrom doped In 0.35 Ga 0.65 As well sandwiched between two 5 angstrom GaAs, and the LWIR stack is composed of 20 periods of 500 angstrom Al 0.27 Ga 0.73 As barrier and 55 angstrom GaAs well. In this work, a 35% of indium has been employed in the MWIR-stack which not only shifts the peak wavelength to 4.3 micrometer, but also enhances the responsivity greatly in this wavelength band. This is due to the fact that higher indium concentration in the InGaAs QW reduces the electron effective mass and increases the intersubband absorption. Despite of the large strain induced by the high indium concentration, the device is highly uniform with very low dark current. For the MWIR stack, a peak responsivity of R p equals 0.65 A/W and D* equals 1.9 by 10 11 cm-Hz 1/2 /W at 4.3 micrometer, 3 V bias, and 77 K were obtained, while for the LWIR stack, R p equals 0.55 A/W and D* equals 2.7 by 10 10 cm-Hz 1/2 /W at 9.4 micrometer, 2 V bias, and 77 K were obtained using 45 degree light coupling. Normal incidence without grating coupling also has high responsivity with about 50% for the MWIR stack and 40% for the LWIR stack respectively, compared with the 45 degree incidence coupling. The BLIP temperature was found to be 125 K for the MWIR stack with cutoff wavelength of lambda c equals 4.6 micrometer and 70 K for the LWIR stack with (lambda) c equals 10 micrometer. |
