Finite Carrier Confinement and Biexcitonic Complexes in Self-Assembled Inas Quantum Dots
- PDF / 1,607,016 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 34 Downloads / 170 Views
emitted by a single QD [3,4]. A confocal scanning optical microscope with a spatial resolution of less than 1 im is used here for spectroscopic studies of single QDs resulting in new insights in their electronic properties. We also used samples with a very low QD density to avoid interference from neighboring QDs. Coulomb interactions are enhanced in QDs with respect to semiconductor structures of higher dimensionality because of carrier confinement in all directions. Recently in nonlinear photoluminescence spectroscopic studies of single QDs, a multiplicity of PL peaks has been observed with increasing excitation power [3-5]. Comparison with theoretical calculations suggested the formation of multiexcitonic complexes [5-7] and/or charged excitons [8]. In this work, we will discuss results on biexcitonic complexes (XX) in single tnAs self-assembled QDs which are formed by Coulomb coupling of two excitons (X) of opposite spins. The biexciton gives rise to a sharp PL line at an energy E 0xx below the groundstate exciton peak at E0 x. The separation between exciton and biexciton lines is commonly called biexciton binding energy (BBE). The measured BBE decreases from nearly 5.0 meV for thick QDs down to less than 2.0 meV in thin QDs. This result is compared to calculations based on QD potential barriers which are either infinite [9] or finite [10-12]. Both models predict for QDs sizes larger than the bulk exciton Bohr radius an increase of BBE with decreasing QD size. However, if the QD size becomes smaller than the bulk exciton Bohr radius this increase of BBE continues with decreasing QD size only in the case of infinite potential barriers whereas with finite 271 Mat. Res. Soc. Symp. Proc. Vol. 583 © 2000 Materials Research Society
S.
Fig. 1 Atomic force microscopy images of an uncapped InAs self-assembled quantum dot sample with a gradient in quantum dot density n. The QD density varies from (a) n=1010 cm2 to (b) n=4. 109 cm"2 and vanishes in (c). barriers a decrease of BBE with decreasing QD size is predicted. Since the QDs under investigation have heights smaller than the bulk exciton Bohr radius the present data can only be explained by a model based on finite barrier heights of the QDs. EXPERIMENT Samples The QDs samples under investigation were grown by molecular beam epitaxy (MBE) on a semi-insulating GaAs (100) substrate. They contain the following layer sequence: an AlAs/GaAs superlattice buffer (2 nm/2 nm, 40 pair), a buffer layer of 100 nm GaAs, an InAs dots layer, and a cap layer of 100 nm GaAs. Small islands connected by a very thin (1.7 ML) wetting layer form during deposition of InAs on GaAs due to the Stranski-Krastanov growth mode. High quality QDs with diameters of less than 50 nm are produced by capping these selfassembled InAs islands with an epitaxial layer of GaAs [ 13]. All QDs layers were grown without rotation of the substrate, so that a gradual variation of In flux is achieved across the wafer resulting in a gradient of both the density and the average size of the dots across the epitaxia
Data Loading...
