Energy relaxation of electrons in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dot molecules

The photoluminescence emission intensities of the exciton states in $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ coupled quantum dots are studied with regard to the tunneling-induced dependence on splitting energy. Even at very low excitation, emission from bonding and antibonding states is observed, as long as the splitting ranges between a few and $\sim 30\mathrm{meV}$. As the splitting is dominated by the conduction band states, this demonstrates an acoustic-phonon relaxation bottleneck for electrons. For larger splittings, only bonding state emission is observed at low excitation, indicating fast carrier relaxation, which is most likely caused by a combination of electron-hole scattering and polaron formation.

Figure 1

Left panel: splitting of the QDM $s$-shell exciton energy levels vs barrier width. Symbols give experimental data, bars give variations of splitting. The inset shows the energy splitting vs the median of bonding and antibonding state energies for the $4\mathrm{nm}$ barrier QDMs. Right panel: PL spectra recorded at different excitation powers on a single QDM with a $7\mathrm{nm}$ wide barrier.

Figure 2

PL spectra of mesa structures with varying lateral sizes prepared on the $d=5\mathrm{nm}$ molecule sample. The left panel shows low excitation spectra for mesa structures with tunnel splittings between $20\text{to}25\mathrm{meV}$. The excitation power was $1\mu \mathrm{W}$, resulting in power densities of less than $1\mathrm{W}{\mathrm{cm}}^{-2}$. In the right panel spectra recorded at varying excitation powers are shown for a $600\mathrm{nm}$ wide mesa structure with a tunnel splitting of $\sim 32\mathrm{meV}$. The high energy part of the spectra has been multiplied by a factor 10.

Figure 3

PL spectra recorded at varying excitation powers on mesa structures with nominal sizes of $300\mathrm{nm}$ on the $d=4\mathrm{nm}$ QDM sample. The energy scales have been shifted, so that the median of the bonding and antibonding states is located at zero. The excitation density was about $1\mathrm{W}{\mathrm{cm}}^{-2}$ for $1\mu \mathrm{W}$ laser power, from which densities for denoted powers can be calculated.

Figure 4

Ratio of emission intensities $I_{\mathit{AB}}∕I_B$ from antibonding $\left(\mathit{AB}\right)$ and bonding $\left(B\right)$ exciton states versus energy splitting $\Delta E_{\mathit{AB}}^s$. An intensity the sum of the peak heights of all lines within an emission band is taken.