Excitonic Mott transition in type-II quantum dots

Photoluminescence spectra measured on a type-II $\mathrm{Ga}\mathrm{Sb}∕\mathrm{Ga}\mathrm{As}$ quantum dot ensemble at high excitation power indicate a Mott transition from the low density state comprising of spatially indirect excitons to a high density electron-plasma state. Under the influence of a very high magnetic field, the electron-plasma that is formed at high excitation powers is transformed into magneto-excitons.

Figure 1

(Color online) Simplified band diagram showing spatially indirect excitons with electrons in the GaAs matrix and holes in the GaSb valence band of the WL and QD. The $z$ axis denotes the growth direction and the QDs are distributed in the $xy$ plane. The band diagram from an eight-band $\mathbf{k}\cdot \mathbf{p}$ calculation can be found in Ref. 7

Figure 2

(Color online) The zero-field PL spectra at different excitation powers measured at $15\mathrm{K}$. The emission around $1.1\mathrm{eV}$ is from QDs and the higher energy peak is due to the WL. The QD spectra considerably broaden at higher excitation powers, indicating the loss of excitonic resonance. The asymmetry and shoulder in the WL peak is due to the sharp cutoff in the detector response at $\sim 1.38\mathrm{eV}$. (Inset) A comparison of line shapes for the $0.2\mathrm{W}{\mathrm{cm}}^{-2}$ spectra (linear scale, offset on both $x$ and $y$ axes and normalized), measured at $15\mathrm{K}$ $(B=0\mathrm{T})$ and $185\mathrm{K}$ ($B=0\mathrm{T}$, $45\mathrm{T}$), show that at this low power the emission linewidth and the (Gaussian) spectral shape are completely determined by the inhomogeneous broadening and are independent of magnetic field and temperature.

Figure 3

(Color online) (a) PL peak transition energy as a function of magnetic field at different laser powers measured at $4.2\mathrm{K}$. Solid lines are the fits to the equation $E_0+\Delta E$, where $\Delta E$ is given by Eqs. (1) and $E_0$ is the zero-field transition energy that monotonically increases with excitation power. (b) A depiction of the corresponding electron trajectories (represented by solid lines for bound states and dotted lines for free particles) as the sample goes through a density-dependent Mott transition. (c) The values of the exciton radius (squares) and the exciton binding energy (circles) as a function of excitation power, obtained from the fits in (a). The smooth curves are just to guide the eye. The binding energy and the radius at $0.2\mathrm{W}{\mathrm{cm}}^{-2}$ excitation are obtained from fits to the data in Fig. 4c since a field of $15\mathrm{T}$ is not enough for excitons with radii $\lesssim 10\mathrm{nm}$.

Figure 4

(Color online) PL peak shift $(B⩽48\mathrm{T})$ at different excitation powers. Cryostat temperature $\sim 15\mathrm{K}$. At high powers, (a) and (b), the slope of the PL peak shift shows an unexpected change at intermediate fields. At low power (c), the conventional excitonic behavior is observed and the PL peak shift is well described by Eq. (1) (solid line). (d) Magnetic field-dependent PL linewidths at different excitation powers. Solid lines are to guide the eye.