Intersubband Transitions in In x Ga 1-x As/AlGaAs Multiple Quantum Wells for Long Wavelength Infrared Detection
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Intersubband Transitions in InxGa1-xAs/AlGaAs Multiple Quantum Wells for Long Wavelength Infrared Detection Clayton L. Workman, Zhiming Wang, Wenquan Ma, Christi E. George, R. Panneer Selvam, Gregory J. Salamo, Qiaoying Zhou1, and M. Omar Manasreh1 Center for Semiconductor Physics in Nanostructures (C-SPIN) and the MicroelectronicsPhotonics Program, University of Arkansas, Fayetteville, AR 72701, USA 1 Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM 87131, USA ABSTRACT We report on intersubband transitions in InxGa1-xAs/AlGaAs multiple quantum wells (MQWs) grown by molecular beam epitaxy. The conduction band offset for this material system is larger than that of the well known GaAs/AlGaAs system, thus making it possible to design, grow, and fabricate quantum well infrared photodetectors operational beyond the 14 µm spectral region with minimized dark current. We have grown InxGa1-xAs/AlGaAs MQWs with indium compositions ranging from x = 0.08 to 0.20 verified by in situ RHEED oscillations, band offset measurements, and high-resolution X-ray diffraction. Band-to-band transitions were verified by photoluminescence measurements, and intersubband transitions were measured using Fourier transform infrared (FTIR) spectroscopy. Due to the high strain and introduction of dislocations associated with the high indium content, wells with indium compositions above ~ 0.12 did not result in intersubband transitions at silicon doping levels of 2×1018 cm-3. A thick linear graded InxGa1-xAs buffer was grown below the MQW structures to reduce the strain and resulting dislocations. Intersubband transitions were measured in InxGa1-xAs wells with indium compositions of x = 0.20 and greater when grown on top of the linear graded buffer. In addition to these results, FTIR measurements on InGaAs/AlGaAs MQW multi-color, long-wavelength infrared detector structures are reported. INTRODUCTION Intersubband transitions in semiconductor MQWs for infrared detection have been a subject of research for the last two decades. As an alternative to bulk semiconductor interband infrared detection, devices based on intersubband transitions show promise as low background, high detectivity detectors [1-5]. Additionally, intersubband infrared detectors can be easily tuned to particular wavelengths by varying the spacing between energy levels. Energy levels spacings are effortlessly controlled because of mature epitaxial growth techniques of III-V materials that allow control over the quantum well width and depth. GaAs/AlGaAs MQWs are the most mature intersubband detectors to-date since these materials are lattice matched and are easily grown together to form devices. However, the conduction band offset of GaAs/AlGaAs quantum wells limits their intersubband absorption to the long-wavelength (8-14 µm) spectral range. Both the long-wavelength (8-14 µm) and midwavelength (3-5 µm) ranges are of interest for terrestrial imaging applications since the earth’s atmosphere is transparent in these regions. Thus, the
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