Quantum Dot Lasers and Amplifiers
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Quantum Dot Lasers and Amplifiers Udo W. Pohl and Dieter Bimberg Institut für Festkörperphysik, PN 5-2, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany ABSTRACT Progress in self-organized epitaxy of quantum dots lead to fabrication of edge- and surfaceemitting lasers featuring excellent device characteristics. Exceedingly low lasing threshold, an internal quantum efficiency near unity, high characteristic temperature was proved, and high power operation was demonstrated. We present recent achievements on lasers and optical amplifiers based on InGaAs quantum dots in GaAs matrix. INTRODUCTION Zero-dimensional charge carrier localization in the active region of a semiconductor laser was predicted two decades ago to lead to superior device performance, e.g. with respect to a decreased threshold current and high temperature stability [1]. Since then, self-organized formation of quantum dots (QDs) using Stranski-Krastanow growth has evolved into the decisive pathway to fabricate defect-free QDs, which display properties of artificial atoms in a dielectric matrix and present a basis for novel opto-electronic devices. The first realization of a QD injection laser by us [2] demonstrated the fundamental validity of previous predictions. We subsequently developed new concepts for strain engineering and interface control to define QD size and density and to reduce losses. Today we have actually achieved unique device performance for both, edge and surface emitting QD lasers grown using MOCVD and MBE. QUANTUM DOT EPITAXY The first QD lasers demonstrated the potential of active zero-dimensional nanostructures at cryogenic temperatures, particularly a low lasing threshold with a low temperature sensitivity, and a very high gain and differential gain [2,3]. A then insufficient carrier confinement, gain saturation and high cavity losses defined issues of subsequent work. A further demand was a design to tune the emission to the 1300 nm wavelength range for datacom applications. The regime of self-organized Stranski-Krastanow growth of QDs generally requires low deposition temperatures, for InGaAs/GaAs QDs well below step flow growth of the matrix. GaAs cap layer growth at such low temperatures favors the formation of defects, which affect the optical properties of QDs. Based on the thermal stability of covered QDs below 600°C [4], we established an overgrowth procedure which maintains a high radiative recombination efficiency [5]. Here, the dots are buried by a thin (2-3 nm) GaAs cap at QD growth temperature, and further GaAs overgrowth is accomplished during a temperature increase to 600°C. Besides an essential improvement of optical properties and ensuing material gain, a flat growth front is
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re-established on top of the corrugated surface of QDs (Fig. 1). This is a precondition for continued QD growth in a stack, made to achieve an increased confinement factor and to overcome gain saturation. Furthermore, the flattening reduces interface roughness to a subsequently deposited
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