Tailored-Potential Semiconductor Quantum Nanostructures Grown in Inverted Pyramids
We have focused on the ability to tune the aluminum (Al) content of the AlGaAs alloy along the growth direction in inverted pyramids (by MOVPE), thus tailoring the potential along a QWR nanostructure. Three parabolic-potential QDs (PQDs) of different potential gradients have thus been realized. The parabolic profiles were changed systematically by fixing the minimum and maximum Al content and varying the total length (120 nm, 240 nm, 480 nm) of the resulting parabolic QDs. The impact of the different effects of quantum confinement was investigated using polarization-resolved and time-resolved photoluminescence spectroscopy. The results indicate that such PQDs can be used as a model structure for investigating the transition in optical properties from QWR-like to QD-like structures. Two extreme cases in terms of confinement strength were also realized: a thin, lens shaped GaAs QD, and a long (480 nm) AlGaAs QWR as a model structure of infinitely long QWRs. Polarization-resolved measurements show that the QD and the QWR emit in different polarization, aligned perpendicular and mostly parallel to the growth direction, respectively. This is because the low energy states in the valence band (VB) of the QWR are dominated by light hole (LH) components whereas for the QD, the VB states are more heavy hole (HH) -like. Using the PQDs as a model structure with intermediate confinement between QWRs and thin QDs, we have investigated the impact of the quantum confinement on the optical polarization properties. Using polarization resolved spectroscopy we have analyzed and compared the degree of linear polarization (DOLP) of PQDs of different confinement strength. Whereas these PQDs emit light mostly polarized along the growth direction, like the corresponding QWRs, the characteristics of their DOLP spectra are more complex. Supplementing the experimental results with detailed numerical modelling, we show that the features of the DOLP spectra depend on valence band mixing, broadening of individual transitions and state filling effects. We have also demonstrated a method for extracting the occupation numbers, the exciton effective temperature and the Fermi levels in these structures, and observed multiexcitonic states in the emission spectra. MOVPE in inverted pyramids provides nanostructure engineering possibilities with fine control of the confinement geometry and thus the polarization of the emitted photons. We identified several geometries in which fine tuning of the geometry can yield switching from HH-like and LH-like transitions. In particular, single QDs of different aspect ratios, QD molecules of different barrier structures, and several QD superlattice systems were investigated numerically. In addition, the impact of an external electric field on the VB states of such system structurally tuned near the switching point was studied theoretically. The results demonstrate that dynamic switching of polarization can be achieved with electric fields, and first step towards experimental implementation using Schottky diodes are presented. In conclusion, in this work we show that MOVPE in inverted pyramids provides possibilities to grow a large variety of quantum nanostructures with high structural accuracy. Control of the confinement geometry was used as a basis for controlling the polarization state of the emitted light. The knowledge obtained in this work should be useful for the fabrication of future polarization-controlled single photon devices.