Nanoengineered Quantum Dot Active Medium for Thermally-Stable Laser Diodes
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Nanoengineered Quantum Dot Active Medium for Thermally-Stable Laser Diodes V. Tokranov, M. Yakimov, A. Katsnelson, M. Lamberti, G. Agnello and S. Oktyabrsky, School of NanoSciences and NanoEngineering, University at Albany–SUNY, Albany, NY 12203, U.S.A. ABSTRACT The influence of a nanoengineering procedure on the properties of single- and multi-layer self-assembled InAs quantum dot (QDs) has been studied using photoluminescence, transmission electron microscopy (TEM), and electroluminescence. Optical properties of QDs were optimized by shape engineering via adjustment of a GaAs overlayer thickness prior to a heating (truncation) step. TEM micrographs have confirmed that the employed growth procedure results in truncated pyramidal QDs covered entirely by AlAs with smooth top interfaces. Truncated QDs with twomonolayer-thick AlAs capping have demonstrated a strong blue shift of ground state (GS) energies and up to 10 meV larger separation between GS and first excited state energy levels as compared to non-capped QDs. We believe that AlAs capping, in combination with truncation procedure, results in significant suppression of carrier transport between QDs within the same layer as well as between QD layers. A record high characteristic temperature for GS lasing threshold, T0 = 380 K up to 55 0C, as well as a maximum saturated gain of 16 (5.3 per layer) cm1 , were measured for a 1.22 µm edge-emitting lasers with this truncated triple layer QD gain medium. A maximum saturated gain, 27 (3.9 per layer) cm-1, and T0 = 110 K have been demonstrated in a 1.19 µm lasers with truncated seven layer QD medium. INTRODUCTION InAs quantum dot (QD) laser medium is still attracting the attention of researchers due to superior performance parameters of laser diodes (both predicted [1] and partially achieved [2, 3]) and long wavelength extension of GaAs-based lasers. However, the lowest threshold current density edge-emitting lasers employing ground state (GS) emission from multi-layer QDs (MQDs) have exhibited a significantly lower saturated gain in comparison with multi-layer quantum well (QW) lasers [4]. As a result, all-epitaxial vertical cavity surface emitting lasers (VCSELs) with GS emission from MQD are not reachable. Recent theoretical studies [5] estimated that e-h coupling and maximum saturated gain will be increased significantly in QDs with a more symmetrical shape. In addition, a correlation between QD shape, lateral size and electronic structure, such as GS and excited state (ES) energies has been studied theoretically [6]. Over the last few years, some experimental investigations have focused on the effects of QD capping layer [7-9] and overgrowth procedures [10] with respect to QD shape management, efficiency, and size distribution. Improved temperature stability was demonstrated experimentally in QD laser medium with larger distance between GS and first ES [11]. All these facts stimulate further research on QD nanoengineering methods to improve the shape and size distribution of MQD media. In the previous
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