Normal Incidence Intersubband Transitions in InGaAs/GaAs Quantum Dots With Non-monotonic Shift
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Normal Incidence Intersubband Transitions in InGaAs/GaAs Quantum Dots With Non-monotonic Shift M. L. Hussein, W. Q. Ma, and G.J. Salamo Microelectronics-Photonics, University of Arkansas, Fayetteville, Arkansas 72701 Abstract Multiple layers of self assembled In0.3Ga0.7As quantum dots of different size were grown on GaAs (100) using molecular beam epitaxy. Fourier-transform infrared spectroscopy shows absorption in the long-wavelength infrared region (8-10 µm) under normal incidence. The absorbance peak shift with dot size was investigated and revealed non-monotonic behavior of intersubband transitions. The optical absorption coefficient was calculated to be in order of 3.8×103 cm-2. Introduction Quantum dots (QDs) are currently attracting much attention for their unique underlying physical properties. One exciting application is in the area of long-wavelength infrared detectors. Due to their three-dimensional confinement, optical transition selection rules for QDs allow normal incident light to couple with electrons that undergoes intersubband transitions. This is in sharp contrast to quantum wells (QWs), where optical transitions for normal incident light are forbidden. This restriction has forced the development of optical detectors to add a grating layer to the detector in order to redirect normal incident light to propagate along the well and optically couple to the QW. Of course, this solution increases fabrication cost. If QDs are to find application as infrared detectors the optical absorption of QD structures must be well characterized to calculate and predict the quantum efficiency. The results of long wavelength infrared detectors are usually expressed by photocurrent or photoresponse [1,2]. In addition to the absorption coefficient, another important parameter is the peak absorption wavelength that is a sensitive function of dot size. For example, the infrared absorbance peakshift in quantum dots [3,4] and in quantum wires [5] has been demonstrated as a function of nanostructure size with a predicted monotonic behavior of the shift of the absorption peak. While there are several theoretical calculations of the absorption coefficient of QDs [6,7] structures, there are few corresponding experimental measurements. In this paper, we report and explain a non-monotonic absorbance peak-shift in Si doped In0.3Ga0.7As /GaAs multiple QDs structures. In addition, we also report on measurements of the optical absorption coefficient in the investigated samples. Experiment The InGaAs/GaAs quantum dot structures were grown by a solid-source molecular beam epitaxy system on semi-insulating GaAs (100) substrate. A typical structure consists of a 0.5 µm thick Si doped (1018 cm-3) GaAs buffer layer followed by 20 periods of Si doped In0.3Ga0.7As dots and 30 nm GaAs barrier. The structure is then capped by a 0.5 µm thick Si doped (1018 cm3 ) GaAs layer. The quantum dot layers are doped with [Si]~ 1.5×1018 cm-3. The growth temperature of InGaAs/GaAs multi-layers was 510 ºC with As4 to Ga beam equivalent pressure (B
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