Broadband quantum well infrared photodetectors

Typical quantum well infrared photodetectors (QWIPs) exhibit rather narrow spectral bandwidth of 1 to 2 mu m. For certain applications, such as spectroscopy, sensing of a broader range of infrared radiation is highly desirable. In this work, we report the design of five broadband (BB-) QWIPs sensitive over the 7 to 14 mu m spectral range. Three n-type BB-QWIPs consisting of three, four, and five quantum wells of different thickness and/or composition in a unit cell, which are then repeated 20 times for the three and four quantum wells (QW) devices and 3 times for the five QWs device to create the BB-QWIP structures, are demonstrated. The three-well n-type InxGa1-xAs/AlyGa1-yAs BB-QWIP is designed to have a response peak at 10 mu m, with a 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 very large responsivity of 2.31 A/W at 10.3 mu m and V-b = +4.5 V, but also achieves a broader bandwidth of Delta lambda/lambda(p) = 29 % than the three-well device. The five-well n-type InxGa1-xAs/GaAs BB-QWIP has achieved a FWHM bandwidth of Delta lambda/lambda(p) = 28 % at V-b = 1.75 V. In addition, two p-type InxGa1-xAs/GaAs BB-QWIPs 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 very large FWHM bandwidth of Delta lambda/lambda(p) = 48 % at V-b = -1.5 V and T = 40 K. The variable thickness p-type BB-QWIP was found to have an even broader FWHM bandwidth of Delta lambda/lambda(p) = 63 % at V-b = 1.1 V and T = 40 K, with a corresponding peak responsivity of 25 mA/W at 10.2 mu m. The results reveal that p-type BB-QWIPs have a broader and flatter spectral bandwidth but lower responsivity than that of n-type BB-QWIPs under similar operating conditions.