Semiconductor lasers based on quantum dots
Institut für Festkörperphysik, TU Berlin, Hardenbergstr. 36, D-10623 Berlin
Institut für Experimentelle Physik II, Universität Leipzig, Linnéstr. 5, D-04103 Leipzig
We review recent progress in diode lasers based on self-organized, epitaxial InGaAs/GaAs quantum dots [1,2,3]. Devices are discussed with emphasis on high output power operation and emission at 1140 nm and 1300 nm, two wavelengths with commercial interest.
Through continuous efforts of material optimization now close-to-ideal properties like low loss wave-guides (~2 cm-1) for edge emitters and high internal quantum efficiency (>90 %) at room temperature have been realized . The key technological step is annealing and growth interruption after the QD deposition in order to reduced defects in the GaAs matrix, that are generated due to the low growth temperatures necessary for the self-organized QD formation. Simultaneously a flatting of the growth front is achieved. The near-infrared lasing is accompanied by emission of mid-infrared (MIR) radiation due to inter-sublevel transitions in QDs . The possibility of MIR lasers is discussed.
Vertical cavity surface emitting lasers at 1300 nm have been realized using QDs overgrown with an InGaAs quantum well . This leads to an increase of the emission wavelength due to a modification of the strain and the confinement potential. A threshold of 1.8 mA has been found for a 8´8 µm2 oxide aperture.
A long-standing promise of QDs, the increased tolerance to defects , has been demonstrated for QD lasers recently . Upon irradiation of 2.4 MeV protons the threshold of QD laser devices degrades only 50% as much as that of comparable quantum well lasers, confirming the expectations from photoluminescence studies on irradiated QD layers.
This work has been carried out in collaboration with D. Bimberg, C. Ribbat, R. Sellin, and A. Weber of Technische Universität Berlin, A.R. Kovsh, N.N. Ledentsov, D.A. Livshits, Y.M. Shernyakov, V.M. Ustinov and A.E. Zhukov of the Ioffe Institute in St. Petersburg, Russia, J.A. Lott of the Air Force Institute of Technology, Wright Patterson AFB, Ohio, USA and N.A. Sobolev of University of Aveiro, Portugal. The work has been supported by bmb+f (13N7231/7 and NanOp), DFG in the framework of Sfb 296 and Gr1011/7, and INTAS (97-751, 96-0467).
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