Spectroscopic signatures of many-body interactions and delocalized states in self-assembled lateral quantum dot molecules

Lateral quantum dot molecules consist of at least two closely spaced InGaAs quantum dots arranged such that the axis connecting the quantum dots is perpendicular to the growth direction. These quantum dot complexes are called molecules because the small spacing between the quantum dots is expected to lead to the formation of molecular-like delocalized states. We present optical spectroscopy of ensembles and individual lateral quantum dot molecules as a function of electric fields applied along the growth direction. The results allow us to characterize the energy level structure of lateral quantum dot molecules and the spectral signatures of both charging and many-body interactions. We present experimental evidence for the existence of molecular-like delocalized states for electrons in the first excited energy shell.

Figure 1

(a) Cross-sectional profile of a LQDM reveals the molecular geometry and height. The cross section is along the axis shown in the inset. (b) Sample and three-terminal electrode design. The Schottky diode structure permits application of electric fields along the growth direction. The interdigitated top contacts allow application of electric fields perpendicular to the growth direction and simultaneously isolates individual LQDMs for optical spectroscopy. (c) AFM image of LQDMs grown under identical conditions to those embedded in the sample studied here. The large AFM image is a $1\times 1$ $\mu$m area and the inset is the 3D AFM image in $200\times 200$ nm area. (d) Potential profile of the three-terminal electrode structure. The top two electrodes apply a 1-V voltage while the bottom electrode applies 0 V. The magnitudes and directions of the electric fields along the two QDMs are marked with arrows respectively.

Figure 2

(a) PL spectra of an ensemble of LQDMs taken in a 5-$\mu$m gap with laser power 30 mW and no applied electric field. (b) Dependence of PL peak intensity on laser power between 0.1 and 200 mW. The inset is the enlarged graph of the area framed with the dashed line rectangle. (c) Schematic model of energy level shells as described in text.

Figure 3

(a) PL spectra of the $E_1$-$H_1$ transition as a function of applied voltage. [(b) and (c)] Schematic models of confining potential along the growth direction with no (b) and significant (c) applied voltage, indicating how the applied voltage impacts the filling of excited energy levels.

Figure 4

PL spectra of the $E_0$-$H_0$ (b) and $E_1$ to $H_1$ (a) transitions in a single LQDM as a function of applied voltage. Vertical lines indicate the applied voltage at which the $E_0$ and $E_1$ energy shells cross the Fermi level. Panel (a) is measured with about twice the exciting laser power used in panel (b) in order to populate the $E_1$ energy shell. Scale bars in the top right corner indicate the colors associated with increasing PL intensity.

Figure 5

(a) PL spectrum of the $E_1$-$H_1$ transitions in a single LQDM as a function of applied voltage. The scale bar at the bottom left corner indicates the color mapping of increasing PL intensity. (b) Intensity of PL lines A, B, and C as a function of applied voltage. (c) Depiction of the charge configuration assigned to each labeled transition as described in the text. (d) Close up of panel (a) a showing possible spin fine structure correlations.