Optically induced charging effects in self-assembled $\mathrm{Ga}\mathrm{Sb}∕\mathrm{Ga}\mathrm{As}$ quantum dots

We report photoluminescence (PL) measurements on self-assembled $\mathrm{Ga}\mathrm{Sb}∕\mathrm{Ga}\mathrm{As}$ quantum dots. As the laser excitation is increased from very low levels, the PL shows a strong red shift, and then a blue shift, such that it presents a U-shaped curve. Raising the temperature causes a large $(<100\mathrm{meV})$ blue shift of the PL, and shifts the minimum of the PL energy versus laser excitation curve to higher laser powers. Applying a magnetic field at lasers powers $⪡1\mathrm{W}{\mathrm{cm}}^{-2}$ red shifts the PL energy. We explain these effects by population or depopulation of dots that are filled in the dark with holes supplied by carbon acceptors.

Figure 1

Dependence of the QD PL of sample A on incident laser power density $P$ at $4.2\mathrm{K}$. With increasing $P$ the QD PL red shifts and then blue shifts. The peak position at $P=50\mathrm{W}{\mathrm{cm}}^{-2}$, indicated by the arrow, is the same as with $P=3.6\times {10}^{-4}\mathrm{W}{\mathrm{cm}}^{-2}$, while for $P=0.045$ and $0.49\mathrm{W}{\mathrm{cm}}^{-2}$ the PL energy reaches a minimum. The inset shows a complete PL spectrum of sample A at $10\mathrm{K}$. The spectral features going from low to high energy are assigned to the QDs, the wetting layer (WL), and the GaAs matrix. The peak labeled ${\mathrm{C}}^{-}$ at $1.49\mathrm{ev}$ is attributed to recombination between free electrons and holes bound to carbon acceptors.

Figure 2

QD PL peak energy of sample A as a function of laser power at $4.2\mathrm{K}$. The upper inset shows the equivalent data for samples B (circles, $10\mathrm{K}$) and C (squares, $4.2\mathrm{K}$). The lines are guides to the eye.

Figure 3

QD PL peak energy for sample A for different laser power densities as a function of (a) temperature and (b) magnetic field. In (b) the 0.1 and ${10}^{-4}\mathrm{W}{\mathrm{cm}}^{-2}$ data have been shifted vertically for clarity. The lines are a guide to the eye. (c) and (d) schematically show the mechanisms for depopulation and repopulation of the dots with holes. At low power (c) photoexcited holes are captured by unoccupied carbon acceptors, ${\mathrm{C}}^{-}$, while photoexcited electrons recombine with the holes in the dots, reducing the hole population and causing a red shift in the PL. At high laser powers (d) all the carbon acceptors are occupied, so photoexcited holes are rapidly captured by the QD, causing a blue shift in the PL.