Germanium-based quantum emitters towards a time-reordering entanglement scheme with degenerate exciton and biexciton states

Abstract

We address the radiative emission of individual germanium extrinsic centers in ${\mathrm{Al}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ epilayers grown on germanium substrates. Microphotoluminescence experiments demonstrate the capability of high temperature emission (70 K) and complex exciton configurations (neutral exciton $X$ and biexciton $XX$, positive ${X}^{+}$ and negative ${X}^{\ensuremath{-}}$ charged excitons) of these quantum emitters. Finally, we investigate the renormalization of each energy level showing a large and systematic change of the binding energy of $XX$ and ${X}^{+}$ from positive to negative values (from $\ensuremath{\sim}+5$ meV up to $\ensuremath{\sim}\ensuremath{-}7$ meV covering $\ensuremath{\sim}70$ meV of the emission energy) with increasing quantum confinement. These light emitters, grown on a silicon substrate, may exhibit energy-degenerate $X$ and $XX$ energy levels. Furthermore, they emit at the highest detection efficiency window of Si-based single-photon detectors. These features render them a promising device platform for the generation of entangled photons in the time-reordering scheme.