Phonons in $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ single quantum dots observed by optical emission

Photoluminescence emission spectra of self-assembled single $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ quantum dots with variable size reveal typical sharp lines below $1.8\mathrm{eV}$, corresponding to transitions between fully confined states. Above this energy the sharp peaks disappear due to the $\Gamma \text{−}X$ scattering of conduction electrons. Excitation spectra of the biexciton line in single quantum dots show marked maxima at the optical-phonon frequency of the quantum dot and at its first overtone. The quantum dot phonon energy observed is $31\mathrm{meV}$. Our results indicate an efficient polaron coupling in these quantum dots.

Figure 1

PL with and without mask. PL from QDs is observed only below $1.8\mathrm{eV}$ as a consequence of the $\Gamma \text{−}X$ crossover with the AlAs barrier. Inset: Band sketch of the active region illustrating transitions between fully confined QD states and recombination of electrons scattered into the AlAs $X$ band.

Figure 2

Power dependence of two peaks in the PL of a single quantum dot, normalized to the $X$ peak intensity. For excitation power below $1\mathrm{mW}$ the $X$ and $XX$ peaks grow as $P^{\alpha }$ with $\alpha =0.5$ and 1.5, and are attributed to the exciton and biexciton, respectively (see text).

Figure 3

Waterfall plots of the $X$ and $XX$ emission (see Fig. 2) for varying excitation energy $E_{\mathit{exc}}$ which is indicated in eV. In (a) the 1LO polaron state is excited, in (b) the 2LO polaron state.

Figure 4

PL $(E_{\mathit{exc}}=1.71\mathrm{eV})$ and PLE spectra of the $XX$ peak (see Fig. 2). PLE reveals polaron states, separated by the phonon energy of the dot $\left(31\mathrm{meV}\right)$, which is close to the bulk InAs LO phonon frequency. Squares and circles correspond to two different experimental runs. At 1LO the polaron state is also observed in PL.