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

Typical quantum-well infrared photodetectors (QWIPs) exhibit a rather narrow spectral bandwidth of 1-2 μm. 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 QWIPs (BB-QWIPs) sensitive over the 8-14-μm spectral range. Two n-type BB-QWIPs, 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 In_xGa_1-xAs-Al_yGa_1-yAs BB-QWIP was designed to have a response peak at 10 μm, with a full-width at half-maximum (FWHM) bandwidth that varies with the applied bias. A maximum bandwidth of Δλ/λ_p=21% was obtained for this device at V_b=-2 V. The four-well n-type In_xGa_1-xAs-GaAs BB-QWIP not only exhibits a large responsivity of 2.31 A/W at 10.3 μm and V_b=+4.5 V, but also achieves a bandwidth of Δλ/λ_p=29% that is broader than the three-well device. In addition, two p-type In _xGa_1-xAs-GaAs BB-QWIPs with variable well thickness and composition, sensitive in the 7-14-μm spectral range, are also demonstrated. The variable composition p-type BB-QWIP has a large FWHM bandwidth of Δλ/λ_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 Δλ/λ _p=63% at T=40 K and V_b=1.1 V, with a corresponding peak responsivity of 25 mA/W at 10.2 μm. The results show that a broader and flatter spectral bandwidth was obtained in both p-type BB-QWIP´s than in the n-type BS-QWIP´s under similar operating conditions