Continuum and discrete excitation spectrum of single quantum rings

Photoluminescence and excitation of the photoluminescence spectroscopy has been performed in single InGaAs self-assembled quantum rings embedded in a field effect structure device. To determine their electronic structure, bias-dependent optical transitions have been analyzed both, for individual quantum rings, and for the averaged ensemble. Our results are compared with a theoretical model, and also with results reported by other authors studying similar nanostructures.

Figure 1

(a) Semilogartithmic $I∕V$ characteristic of the device in darkness and under illumination above the GaAs edge as obtained at $10\mathrm{K}$. (b) The band diagram in the vicinity of the active layer is shown for an externally applied reverse bias $V_G$. A typical AFM image of a QR is also shown in the inset. (c) Ensemble-averaged PL spectra as a function of $V_G$ obtained at $10\mathrm{K}$ and low excitation powers.

Figure 2

(a) Voltage-dependent $\mu$-PL spectra obtained at $4.2\mathrm{K}$ are shown. (b) The ground state emission of a negatively charged single QR is redshifted due to the Coulomb interactions. Only the triplet $X^{2-}$ resonance could be detected with our signal to noise ratio. (c) A schematic energy level diagram for a typical QR with addition energies deduced assuming a parabolic in-plane potential.

Figure 3

Excitation spectrum images of a single QR at $V_G=0.2\mathrm{V}$ are depicted in the upper panels (left and right spectra were recorded with different resolutions, $0.1\mathrm{nm}$/step and $1\mathrm{nm}$/step, respectively). Averaging over $\Delta E_{\mathit{DET}}$, we obtain the $\mu$-PLE spectra shown in the lower panels.

Figure 4

Several excitation spectra of single QRs obtained in the spectral region below the WL edge are shown. Together with the sharp resonances coming from discrete optical transitions, an increasing continuous background is observed in some cases for excess energies $\Delta E>40–70\mathrm{meV}$.

Figure 5

The ensemble-averaged emission bands are compared with statistics performed in several $\mu$-PLE spectra (gray bars). The corresponding optical transitions are labeled according to the calculations discussed in the text.