Polarization Anisotropies in Strain-Free, Asymmetric, and Symmetric Quantum Dots Grown by Droplet Epitaxy

Abstract

We provide an extensive and systematic investigation of exciton dynamics in droplet epitaxial quantum dots comparing the cases of (311)A, (001), and (111)A surfaces. Despite a similar s-shell exciton structure common to the three cases, the absence of a wetting layer for (311)A and (111)A samples leads to a larger carrier confinement compared to (001), where a wetting layer is present. This leads to a more pronounced dependence of the binding energies of s-shell excitons on the quantum dot size and to the strong anti-binding character of the positive-charged exciton for smaller quantum dots. In-plane geometrical anisotropies of (311)A and (001) quantum dots lead to a large electron-hole fine interaction (fine structure splitting (FSS) ∼100 μeV), whereas for the three-fold symmetric (111)A counterpart, this figure of merit is reduced by about one order of magnitude. In all these cases, we do not observe any size dependence of the fine structure splitting. Heavy-hole/light-hole mixing is present in all the studied cases, leading to a broad spread of linear polarization anisotropy (from 0 up to about 50%) irrespective of surface orientation (symmetry of the confinement), fine structure splitting, and nanostructure size. These results are important for the further development of ideal single and entangled photon sources based on semiconductor quantum dots.

Symmetric Excitons in an (001)-Based InAs/GaAs Quantum Dot Near Si Dopant for Photon-Pair Entanglement

“…Due to a deviation of X1 and X2 from the main crystal axis [110] and [1][2][3][4][5][6][7][8][9][10], the erasure of FSS requires a biaxial stress (in-plane) tuning or a combination of stress (in-plane) and electric field (vertical) tuning. Besides, QD geometrical anisotropy (e.g., shape, strain or charged environment) induces light hole (lh)-heavy hole (hh) mixing (in degree β) and photon polarization mixing as the emission intensity polarization (EIP) reflects [22][23][24], which is quite correlated to the growth parameters such as growth temperature, indium deposition amount and surface migration time. For instance, in our experiments (shown elsewhere), for single InAs QDs at λ = 870 nm formed among dense small QDs at λ = 860 nm (a broad PL spectral profile) by indium migration, a little increase of indium deposition will induce a great EIP transition from a pure dipole to an ellipse and finally a circle (i.e., β ≈ 0) with QD emission intensity increasing monotonously.…”