6.W zyx - Y.UU ThB 1-1 Invited Paper zy GROWTH, CHARACTERIZATION, THEORY AND LASING OF VERTICALLY STACKED QUANTUM DOTS M. Grundmann, N.N. Ledentsov, R. Heitz, D. Bimberg Institut fur Festkorperphysik, Technische Universitat Berlin Hardenbergstr. 36, D - 10623 Berlin, Germany Phone: +49.30.3 14-22072, Fax: +49.30.314-22569, e-mail: mariusgr @ zyxwvuts w422zrz.physik.t~-berlin.de, WWW: http://sol.physik.tu-berlin.de V.M. Ustinov, A.Yu. Egorov, M.V. Maximov, P.S. Kop'ev, 2h.I. Alferov A.F. Ioffe Physical-Technical Institute Politekhnicheskaya 26, 19402 1 St. Petersburg, Russia A.O. Kosogov, P. Werner, J. Heydenreich, U. Gosele Max-Planck-Institut fur Mikrostrukturphysik Weinberg 2, D-06 120 Halle/Saale, Germany zyxw Abstract We investigate multiple sheets of self-organized InAs quantum dots (QD's) with zyxwvutsrq thin GaAs spacer layers. The QD's spontaneously form on top of each other and couple electronically. In comparison to single sheets of QD's (SQD's), the coupling leads to lower energy levels and an expansion of the wavefunction. Thus carrier localization and gain increase. Three- dimensional strain distribution and electronic structure are theoretically modeled. Laser operation of vertically coupled quantum dots (VECOD's) improves largely as compared to SQD lasers and room temperature operation with a threshold of 680Akm2 is obtained for a triple stack. I. Introduction The fabrication of self-organized quantum dots (QD's) from strained materials [1,2] using the strain driven coherent island formation [3] proved to result in high quality pyramidal struc- tures with ideal zero-dimensional confinement [4, 51 on a thin wetting layer (WL), suitable for lasers with QD's as active medium, first demonstrated by us [6]. Creation of population inversion for a QD laser is comparably easy, since the transparency current is only several A/cm2. The more strin- gent requirement is to obtain enough modal gain to overcome the losses (typical internal loss is qnt=2.5 cm-I plus mirror losses). The first QD lasers [6] were based on rather small QD's showing strong confinement, such that the electron starts to delocalize into the WL and Fig. 1: (1 00) cross section HREM of triple InAs/GaAs stacked dots, resulting from zy 3x (0.5nm InAs-1.5nm GaAs) deposition. Markers denote position of InAs parts. zyx 738