Photoluminescence Studies of Mid-infrared InAs/AlAs Quantum Dot Cascade Laser Structures
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Photoluminescence Studies of Mid-infrared InAs/AlAs Quantum Dot Cascade Laser Structures Elif Demirbas1 and Xifeng Qian1 1
Physics and Applied Physics, University of Massachusetts, Lowell, MA, 01854, U.S.A.
ABSTRACT We present the optical characterization of quantum dot cascade laser (QDCL) structures using photoluminescence. In this work, two InAs/AlAs quantum dot structures are investigated with different GaAs QW thicknesses. Low temperature photoluminescence measurements show peaks at 1.03 eV, 1.28 eV, and 1.51 eV for QDCL1, and 1.07 eV, 1.27 eV, and 1.47 eV for sample QDCL2, corresponding to ee-hh transition in both QD and QW. A three dimensional QD-QW model in Nextnano is developed to simulate the energy states, wave functions, and transition rates between conduction and valence bands using a typical QD size obtained from atomic force microscope measurements. The simulation results agree well with the experimental data. This allows us not only to understand the optical characteristics of the QD-QW structure, but also to optimize the QDCL structure design. INTRODUCTION Since their first invention at Bell Laboratories in 1994 [1], quantum cascade lasers (QCLs) have been extensively studied for their intersubband electron transition inside a quantum well structure that can be designed to emit light at different wavelengths simply by changing the thickness of the component layers. QCLs are based on a mature technology that yields continuous-wave, high output power, room temperature operation, and high wall-plug efficiency [2]. The demonstrated QCL emission wavelength covers a wide range from MIR (3-25 µm) [2, 3] to THz (1.2-4.9 THz) [4, 5]. In QCL devices, the non-radiative transitions between QW subbands compete with stimulated emission, therefore reducing the output luminescence and power. This also limits the wall-plug efficiency of QCLs. For this reason, it was proposed that using three dimensional confined systems such as quantum-dots (QDs) make it far less likely that a phonon can induce non-radiative transition, as compared to the continuum of in-plane states in the QW [6]. This results in higher quantum efficiency and higher operating temperature than conventional QW based QCLs. In addition, it is theoretically shown that quantum dot cascade lasers (QDCLs) operate at lower threshold current density [7]. Recently, an InAs/AlAs quantum dot cascade emitter has been experimentally demonstrated [8]. In such devices, carriers are injected into the higher electronic state in QDs. Transition of the carriers from the higher state to lower state in quantum dots results in emission, and carriers are then extracted through the ground states in QW to achieve population inversion. The realization of this QD-QW hybrid structures benefits from the mature technique of epigrowth of both QD and QW. Although an InAs/AlAs quantum dot cascade emitter has been demonstrated, its electronic states and transport mechanism were not extensively studied. In this paper, a theoretical model is first developed using Nextnano software to
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