InAs quantum dots in AlAs/GaAs short period superlattices: structure, optical characteristics and laser diodes
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InAs quantum dots in AlAs/GaAs short period superlattices: structure, optical characteristics and laser diodes Vadim Tokranov, M. Yakimov, A. Katsnelson, K. Dovidenko, R. Todt, and S. Oktyabrsky, UAlbany Institute for Materials, University at Albany–SUNY, 251 Fuller Rd, Albany, NY 12203 ABSTRACT The influence of two monolayer - thick AlAs under- and overlayers on the formation and properties of self-assembled InAs quantum dots (QDs) has been studied using transmission electron microscopy (TEM) and photoluminescence (PL). Single sheets of InAs QDs were grown inside a 2ML/8ML AlAs/GaAs short-period superlattice with various combinations of under- and overlayers. It was found that 2.4ML InAs QDs with GaAs underlayer and 2ML AlAs overlayer exhibited the lowest QD surface density of 4.2x1010 cm-2 and the largest QD lateral size of about 19 nm as compared to the other combinations of cladding layers. This InAs QD ensemble has also shown the highest room temperature PL intensity with a peak at 1210 nm and the narrowest linewidth, 34 meV. Fabricated edge-emitting lasers using triple layers of InAs QDs with AlAs overlayer demonstrated 120 A/cm2 threshold current density and 1230 nm emission wavelength at room temperature. Excited state QD lasers have shown high thermal stability of threshold current up to 130 oC. INTRODUCTION Quantum dot (QD) layers were proposed as an active gain medium for semiconductor laser diodes in 1982 [1]. After the discovery of self-assembly of QDs in the InAs/GaAs system via Stranski–Krastanov growth mode, significant research efforts were directed to obtain QD ensembles with uniform size, high density, and high emission efficiency [2,3], and to fabricate QD lasers [4,5,6]. Currently, the performance of QD lasers is comparable or even better than that of quantum well (QW) lasers, e.g. room temperature threshold current density of 16 A/cm2 for single- and 36 A/cm2 for triple-layer QD laser [6], and long wavelength (1.3 µm) lasing on GaAs substarte [5] were obtained. However, InAs/GaAs quantum-dot lasers with the lowest threshold current density (ground level near 1.3 µm emission wavelength) have not yet achieved hightemperature stability [7]. In spite of profound studies of Stranski–Krastanov growth of In(Ga)As islands on GaAs, this self-organized formation of nanoscale islands is still not completely understood. The structural and optical properties of QD ensembles are very sensitive to the growth parameters [8,9]. This phenomenon of self-assembly is further complicated by intermixing of InAs QDs with the GaAs barrier [10], segregation of In atoms when InGaAs islands are overgrown by GaAs [11,12], complex diffusion properties of adatoms. Very recently, the top Al containing layers have been used to achieve a red shift of the InAs QDs photoluminescence band [13,14]. The primary goal of the present study is the development of the self-assembled QD active medium for laser diodes operating at elevated (>1000C) temperatures. We have investigated the influence of two monolayer - thick AlAs under- and ove
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