Intersubband Transitions in In 0.3 Ga 0.7 As/GaAs Multiple Quantum Dots of Varying Dot-Sizes
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Intersubband Transitions in In0.3Ga0.7As/GaAs Multiple Quantum Dots of Varying Dot-Sizes Y.C. Chua a), Jie Liang a), B.S. Passmore a), E.A. DeCuir a), M.O. Manasreh a), Zhiming Wang b), and G.J. Salamo b) a) Department of Electrical Engineering and Microelectronics-Photonics Program, University of Arkansas, Fayetteville, AR 72701, USA b) Department of Physics, University of Arkansas, Fayetteville, AR 72701, USA Abstract The optical absorption spectra of intersubband transitions in In0.3Ga0.7As/GaAs multiple quantum dots (MQDs) grown by molecular beam epitaxy were investigated. By varying the number of In0.3Ga0.7As monolayers deposited, a series of samples with varying dot sizes ranging from 10 – 50 monolayers were obtained. The quantum dots grown with size less than 15 monolayers or more than 50 monolayers did not yield any observable measurements of intersubband transition. This suggests that there exist a critical upper and lower limit of In0.3Ga0.7As quantum dots for infrared detectors. A wavelength range of 8.60 – 13.70 µm is achieved for structures grown with the above monolayers range. The theoretical line-shape of the intersubband transition absorption was compared to the experimental measurements. From the lineshape, it was deduced that bound-to-continuum transtition is present in thick quantum dots and bound-to-bound transition is present in thinly grown quantum dots.
I. Introduction The most extensively used material systems for infrared imaging is the II-VI semiconductor, HgCdTe. Although this material possesses a very high quantum efficiency and detectivity [1,2], it suffers from manufacturing related problems. Apart from being difficult to grow and process, the material itself is very costly compared to the III-V semiconductors. The technology for producing HgCdTe detectors is not yet mature enough to achieve uniformity over large areas and allow for reliable reproduction [2,3]. These obstacles lead to low yields and high detector costs. These disadvantages lead many researchers to seek out an alternative solution which eventually lead to the birth of the quantum well infrared photodetector (QWIP) [2-6]. Quantum well infrared photodetectors are designed from wide bandgap (III-V) semiconductor materials in such a way where quantum confinement is created. Unlike HgCdTe which utilizes electronic transitions across the fundamental bandgap, QWIPs relies on transitions between two or more bound energy levels within quantum wells. This extends the range of infrared sensing to the far infrared. The material growth and fabrication technology of III-V based devices are more attainable as compared to these based on II-VI group. However, the QWIP has several limitations such as the
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insensitivity to normal incidence radiation. Photons incident normal to the structure are not absorbed due to selection rules associated with intersubband transitions. Special grating layers must be etched onto the top contact layer of the structure to scatter incident photons at an angle into the structure. In recent
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