Heterogeneous confinement in laterally coupled InGaAs/GaAs quantum dot molecules under lateral electric fields

We study the electronic and optical properties of laterally coupled InGaAs/GaAs quantum dot molecules under lateral electric fields. We find that the electrons perceive the double-dot structure as a compound single object and tunnel through a basin connecting the dots from underneath. The holes discern two well-separated dots and are unable to tunnel. Through a combination of predictive atomistic modeling, detailed morphology studies, and single-object microphotoluminescence measurements, we show that this peculiar confinement results in an unusual heterogeneous behavior of electrons and holes with profound consequences on optical properties. We find a qualitatively different signal in optical-absorption, emission under resonant, and emission under nonresonant excitations. We explain this behavior by invoking the carriers’ dynamics following light absorption.

Figure 1

(Color online) (a) Top view [(001) plane] and (b) cross-sectional view [(110) plane] of the two lens-shaped dots and the basin. (c) Square of the single-particle electron (hole) wave functions $e_{0,1}$ $\left(h_{0,1}\right)$ as a function of the applied in-plane electric field. The shape of the dots is given in light gray and the isosurfaces contains 75% of the state densities. The applied field is within the range of $-800$ to 800 V/cm and the positive direction is shown in the figure.

Figure 2

(Color online) (a) Electron and (b) hole single-particle energies. Energies (c) of the uncorrelated electron-hole pair and (d) of the correlated exciton.

Figure 3

(Color online) Bonding-antibonding splitting for the structure define in Figs. 2a, 2b for (a) varying interdot separation $\delta$ and (b) varying second dot diameter $d$. The triangles show energy separation of the two excitons for ten QDMs with the same structural parameters [Figs. 1a, 1b] but constructed from different random-alloy realizations.

Figure 4

(Color online) Theoretical results for (a) the absorption and (b) the PL. (c) Experimental PL results obtained under nonresonant excitation.

Figure 5

(Color online) Energy separation of the two excitons ${\Delta }_{LR}$ for the structure define in Figs. 2a, 2b for (a) varying interdot separation $\delta$ and (b) varying second dot diameter $d$. The triangles show separations for ten QDMs with the same structural parameters [Figs. 1a, 1b] but constructed from different random realizations. The filled circles represent a structure similar to Figs. 1a, 1b but with two dots with a height difference of one monolayer.

Figure 6

(Color online) Confinement potential for the electrons (a) and (c) and the holes (b) and (d) in the (110) plane (a) and (b) and along the $\left[1\overline{1}0\right]$ direction (c) and (d) for different heights (positions along [001]) labeled as 1, 2, 3, 4. The green dashed lines in (a) and (c) follows the maximum (minimum) potential energy in the plane. The dashed green lines in (c) and (d) correspond to the energy values of the path given by the green lines in (a) and (b).

Figure 7

(Color online) Schematic representation of the emission process. The hole states are well confined in either of the dots while the electron states form delocalized bonding and antibonding state. Following our dynamical model, where the upper electron state is effectively emptied, only the lower state is occupied. In the lower part of the figure, we show the exciton state ensuing from the single-particle states. The exciton is localized inside the left dot for $F<F_e$, inside the right dot for $F>F_h$, and can be localized on either of the dots in the intermediate range of applied fields. The words “and/or” mean that in the case of a single exciton recombination the photon will be emitted from one or the other dot. In the case of PL, there will be a statistical average of emission from the left and the right dot.

Figure 8

(Color online) (a) Calculation of the correlated exciton states in the dot molecule. The exciton ladder has been split according to the dominant position of the hole. (b) Experimental PL results for nonresonant excitation at energy $E=1436.8\text{meV}$. (c) and (d) Experimental resonant excitations at energy $\text{ER}1=1361.9\text{meV}$ and $\text{ER}2=1359.2\text{meV}$.