Shape-dependent electronic properties of strained quantum dots

Robert Heitz

Institut für Festkörperphysik

Technische Universität Berlin

Hardenbergstrasse 36

10623 Berlin

Tel.: 030 31422184

Fax.: 030 3122569

Email: heitz@sol.physik.tu-berlin.de

A challenging aspect of self-organized quantum dots (QDs) is the experimental and theoretical investigation of the correlation of the electronic and the structural properties. Understanding the structure-dependence of the electronic states as well as the dynamical properties would provide a powerful tool to exploit the flexibility of the self-organized growth to generate different QD shapes and compositions for, e.g., device optimization.

The excited state spectrum of self-organized QDs is investigated in size-selective photoluminescence excitation experiments. For approximately pyramidal InAs/GaAs QDs a variety of excited state transitions are observed and a pronounced size-dependence of the excited state splittings1 is demonstrated. The discussion is based on the predictions of 8 band k× p calculations for exciton transitions in ideal pyramidal InAs/GaAs QDs.2 A good agreement allows for a detailed identification of the excited state transitions and gives insight into the effects of the symmetry-lowering caused by the piezoelectric potential.

The structural properties have a pronounced effect on the dynamical properties of excitons in self-organized QDs. Recombination- and relaxation-limited exciton dynamics is demonstrated, respectively, for pyramidal InAs and disk-like InGaAs QDs in time-resolved photoluminescence investigations under resonant excitation. The shape-dependent asymmetry of the electron and hole wavefunctions in such inhomogeneously strained QDs favors phonon-assisted relaxation or recombination, respectively. A pyramidal shape provides for local charge separation, reducing the electron-hole overlap and enhancing the polar exciton-LO-phonon coupling.3 On the contrary, a disk-like shape favors local charge neutrality, leading to a large oscillator strength and quenched polar exciton-LO-phonon coupling.

1 R. Heitz, O. Stier, I. Mukhametzhanov, A. Madhukar, D. Bimberg, Phys. Rev. B 62, 11017 (2000)

2 O. Stier, M. Grundmann, D. Bimberg, Phys. Rev. B 59, 5688 (1999)

3 R. Heitz, I. Mukhametzhanov, O. Stier, A. Madhukar, D. Bimberg, Phys. Rev. Lett. 83, 4654 (1999)