Investigation of broad-band quantum-well infrared photodetectors for 8-14-μm detection

Typical quantum-well infrared photodetectors (QWIP's) exhibit a rather narrow spectral bandwidth of 1-2 I'm For certain applications, such as spectroscopy, sensing a broader range of infrared radiation is highly desirable, In this paper, we report the design of four broad-band QWIP's (BB-QWIP's) sensitive over the 8-14-mu m spectral range. Two n-type BB-QWIP's, consisting of three and four quantum wells of different thickness and/or composition in a unit cell which is then repeated 20 times to create the BB-QWIP structure, are demonstrated. The three-well n-type InxGa1-xAs-AlyGa1-yAs BB-QWIP was designed to have a response peak at 10 mu m, with a full-width at half-maximum (FWHM) bandwidth that varies with the applied bias. A maximum bandwidth of Delta lambda/lambda(p) = 21% was obtained for this device at V-b = -2 V. The four-well n-type InxGa1-xAs-GaAs BB-QWIP not only exhibits a large responsivity of 2.31 A/W at 10.3 mu m and V-b = +4.5 V, but also achieves a bandwidth of Delta lambda/lambda(p) = 29% that is broader than the three-well device. In addition, two p-type InxGa1-xAs-GaAs BB-QWIP's with variable well thickness and composition, sensitive in the 7-14-mu m spectral range, are also demonstrated. The variable composition p-type BB-QWIP has a large FWHM bandwidth of Delta lambda/lambda(p) = 48% at T = 40 K and V-b = -1.5 V. The variable thickness p-type BB-QWIP was found to have an even broader FWHM bandwidth of Delta lambda/lambda(p) = 63% at T = 40 K and V-b = 1.1 V, with a corresponding peak responsivity of 25 mA/W at 10.2 mu m. The results show that a broader and flatter spectral bandwidth was obtained in both p-type BB-QWIP's than in the n-type BB-QWIP's under similar operating conditions.