| This dissertation describes the design, fabrication, and characterization of voltage tunable two-color quantum-well infrared photodetectors (QWIPs) that are based on the transfer of electrons between coupled QWs under an applied bias. The two QWs in each period of these multiple QW (MQW) structures are designed to detect different wavelengths. The peak detection wavelength of these two-color detectors should shift from one wavelength to the other as the polarity of the applied bias is reversed. Unfortunately, the shorter wavelength peak was present at all voltages for the detector design that existed when this thesis work was started. We investigate the origin of this problem using AlGaAs/GaAs-based detectors with 50 A barriers between the coupled QWs. We find that insufficient electron transfer between the coupled QWs under bias and low photoconductive gain of the longer wavelength photoelectrons lead to the presence of the shorter wavelength peak at all voltages. We solve the first problem by using thicker (200 A) barriers, which leads to a large potential drop and, hence, enhanced electron transfer between the QW pair. We show that graded barriers between each period of the MQW structure can be used to overcome the low photoconductive gain of the longer wavelength electrons. We demonstrate a two-color QWIP with voltage tunable peak detection and cutoff wavelengths for temperatures up to 40 K. At higher temperatures, the short wavelength peak is present for both bias polarities and becomes the main detection peak beyond 60 K. We extract absorption coefficient and photoconductive gain using corrugated-QWIPs with different corrugation periods and used these deduced spectra to provide possible explanations for the temperature dependence of the detector responsivity. |
