Shape Engineered InAs Quantum Dots with Stabilized Electronic Properties
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Shape Engineered InAs Quantum Dots with Stabilized Electronic Properties V. Tokranov, M. Yakimov, A. Katsnelson, K. Dovidenko, M. Lamberti, and S. Oktyabrsky, School of NanoSciences and NanoEngineering, University at Albany–SUNY, Albany, NY 12203, U.S.A. ABSTRACT We have studied the influence of overgrowth procedure and a few monolayer-thick AlAs overlayer on the properties of self-assembled InAs quantum dots (QDs) using scanning electron microscopy (SEM) and photoluminescence (PL). PL spectroscopy was used to optimize optical properties of the QDs by shape engineering (QD truncation) through adjustment of the thickness of overlayers and temperature of the subsequent heating. QDs with 6 nm - thick overlayer with subsequent heating up to 560oC was found to have the highest PL intensity at room temperature and the lowest FWHM, 29 meV. Ground state energy of the truncated QDs is very stable against variations of growth parameters. 1.23 µm edge-emitting laser of triple-layer QD structure demonstrated room temperature threshold current density, 74 A/cm2. INTRODUCTION Recently, the fabrication of quantum dots (QDs) has been intensively studied due to superior performance parameters of laser diodes [1, 2]. However, the lowest threshold current density InAs/GaAs QD lasers have not yet achieved high-temperature stability [3], and vertical cavity surface emitting laser (VCSEL) employing ground state QD emission with epitaxial AlGaAs/GaAs DBR-mirrors is not available yet. Extensive studies have been carried out to characterize the growth of InAs self-organized dots [4, 5]. There are relatively few recent studies which have focused on the effect of the capping layer [6-10] and on the development of QD overgrowth procedure [11, 12] which are required to fabricate InAs QDs for device applications. On the other hand, theoretical studies reporting on correlation between QD size, shape and electronic structure, such as ground and excited state energies are available [13, 14]. It was proven experimentally that QD laser with large distance between the ground and excited state demonstrated better temperature stability [15]. Also it was theoretically estimated that QD gain should increase significantly with the shape modification to more symmetrical than typical pyramidal shape [16]. All these facts stimulate the search of effective methods of QD shape management. The primary goal of the present study is the optimization of capping and overgrowth procedure of self-assembled InAs QDs to achieve higher internal quantum efficiency of the active medium for laser diodes operating at elevated (>100oC) temperatures. We have investigated the influence of 2 monolayers (ML) AlAs capping layer and GaAs overlayer with different thicknesses (8 - 25 ML) on optical properties of the QDs embedded into short period superlattice (SPSL). We implemented an InAs QD overgrowth procedure similar to that in Ref. 11 with additional capping layer and modified truncation. EXPERIMENTAL DELAILS The InAs QDs were grown in EPI MOD GEN II Molecular Beam Epit
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