Temperature Dependence and Dynamic Response of Quantum Dots
for 1.3 Ķm GaAs-Based Lasers
D.G. Deppe, O. Shchekin, G. Park, T.F. Boggess,* and L. Zhang*
Microelectronics Research Center, Department of Electrical and Computer Engineering
The University of Texas at Austin, Austin, Texas 78712
*Optical Science and Technology Center, Department of Physics and Astronomy
University of Iowa, Iowa City, Iowa 52242
Carrier dynamics in self-organized quantum dots (QDs) is emerging as one of the most interesting and important topics for their application to semiconductor lasers. Because of the 3-dimensional confinement, the QD dynamic response is expected to be strongly influenced by its 0-dimensional energy levels. When these levels are spaced equal to or significantly less than an optical phonon energy the carrier relaxation is expected to be fast. When the levels are detuned from the optical phonon energy, especially with much larger energy spacings, the relaxation is expected to be much slower.
Furthermore, the QDís electronic structure is unique in that the 0-dimensional energy levels are connected to a 2-dimensional density of levels associated with the wetting layer of self-organized QDs. Entropy effects can be important when electrons and holes relax from the wetting layer into the QDsí 0-dimensional levels, because of the large disparity in the effective density of states. The result of both entropy effects and slowed relaxation between the 0-dimensional energy levels is that the QDsí dynamic response can show strikingly different temperature dependencies, becoming either faster or slower with increasing temperature, depending on which effect may dominate.
The 0Ėdimensional energy levels can also have a large influence on the temperature dependence of the QDís light emission. Carrier relaxation between the 0-dimensional energy levels, as compared to the radiative light emission, is fast enough to bring the carriers to quasi-equilibrium in their respective QD prior to light emission. Thus, for steady-state excitation, the carriers take thermal occupations among the 0-dimensional energy levels that depend on level degeneracies, temperature, and the 0-dimensional energy spacings. More widely spaced energy levels lead to a smaller temperature dependence of the QDís light emission characteristics, and have a significant impact on the temperature dependence of a QD laserís threshold current.
Finally, applications of QDs to microcavities to generate the Purcell effect will also be presented. The QDs placed in microcavities become the ultimate system to control the interactions between excitons and photons in semiconductors. Different types of microcavity systems will be discussed, along with their potential benefits for different light emitters. Special emphasis will be placed on selectively oxidized Fabry-Perot microcavity suitable for electrical injection light emitting diodes.
In this talk we will present experimental results along with models that elucidate the above physics in self-organized QDs.