ABSTRACT
We report a detailed study of the electronic properties of self-organized InAs/GaAs quantum dots (QDs) by photoluminescence (PL), time-resolved PL, and PL excitation (PLE) experiments. High-quality InAs/GaAs QDs of tunable size were obtained using the variable deposition amount approach in MBEgrowth, yielding ultimately room-temperature emission at 1.3 µm for island area densities of ∼400 µm−2. The experiments emphasize the role of a slowed down carrier relaxation in the QDs, being important, for example, for energy transfer processes between QDs and the temperature dependence of the carrier capture processes. The quantum size effect of the excited-state spectrum is revealed in PLE experiments and shown to be in good agreement to numerical results for pyramidal QDs based on eight band k · p theory. Finally, phonon-assisted recombination processes are identified demonstrating an enhanced excitonLO-phonon coupling. Excellent agreement with estimations in the adiabatic approximation suggests that this enhancement is the consequence of the particular quantum confinement and the piezoelectricity in the strained low-symmetry QDs.
The formation of nanoscale coherent islands in highly strained semiconductor epitaxy has been extensively studied as a means to generate optically active
quantum dots (QDs) [1]. In spite of the large size inhomogeneity [2-5] of ∼10% such Stranski-Krastanow QDs have been successfully employed in devices [6-10], demonstrating partly the predicted advantages of QD-based devices [11, 12]. Though self-organized QDs are easily incorporated in conventional device structures, the interdependent nature of the QD density and size makes the adaptation of the QDs to the device needs difficult. Additionally, the inhomogeneous broadening of the discrete density of states for self-organized QD ensembles hampers detailed investigations of the excited-state spectrum and of energy relaxation (and recombination) processes, which are both of basic physical interest and critical for design and performance of devices. In fact, it is often necessary to distinguish between extrinsic ensemble effects and intrinsic properties of single QDs, for example in describing the carrier dynamics [13, 14]. In recent years, extensive work has been devoted to the study of the excited states, as well as the temperature dependence and dynamical behaviour of the optical properties of self-organized QDs [4, 6, 15-27]. Furthermore, the prediction [28] of slowed-down carrier relaxation due to restricted inelastic phonon scattering in QDs was evidenced by the observation of multi-phonon resonances in photoluminescence excitation (PLE) spectra [24,29-31]. However, the electronic properties of such self-organized QDs are still controversially discussed owing to the inhomogeneous broadening and the wide spread of reported structural properties of the individual QDs. Further progress in the understanding of the electronic properties of the self-organized QDs might be stimulated by improved samples.
