Optically induced charge conversion of coexistent free and bound excitonic complexes in two-beam magnetophotoluminescence of two-dimensional quantum structures

We report on extensive polarization-resolved photoluminescence (PL) studies of a variety of excitonic complexes formed in high-quality symmetric GaAs quantum wells containing a high-mobility two-dimensional (2D) hole gas in a broad range of magnetic fields from 0 to 23 T and under two-beam illumination, allowing for dynamical control of the hole concentration beyond the point of conversion from $p$- to $n$-type structures. We have demonstrated charge conversion between positive and negative complexes bound to acceptors in the well, differing from the charge conversion of free trions due to charge reflection symmetry breaking by a fixed impurity, leaving a qualitative trace (exchange splitting) in the PL spectrum. The effect of switching between the electron and hole gases (in the same well) on different emission lines has also allowed us to distinguish the (direct and cyclotron-satellite) emission lines from positive trions moving almost freely in the quantum well and bound to nearby ionized acceptors in the barrier, thus demonstrating their coexistence in high-quality structures.

Figure 1

Comparison of photoluminescence spectra recorded in the absence of a magnetic field, under excitation with different combinations of red and green lasers, in the symmetric GaAs quantum well of width $w=15$ nm and dark hole concentration $p=1.51\times {10}^{11}$ cm${}^{-2}$. (Inset) Trion emission energy $E$ as a function of green laser power density $P_{\mathrm{green}}$, relative to pure red illumination (#1). The change of assignment from positive to negative trions beyond the maximum in $E\left(P_{\mathrm{green}}\right)$ indicates dynamical conversion of the structure from $p$ to $n$ type.

Figure 2

Evolution of the photoluminescence spectrum of the symmetric GaAs quantum well of width $w=15$ nm, with a high-mobility hole gas of (dark) concentration $p=1.51\times {10}^{11}$ cm${}^{-2}$, in magnetic field $B$ varied from 0 to 23 T, collected in polarizations ${\sigma }^{+}$ and ${\sigma }^{-}$, at low temperature $T=1.8$ K, and under power density $P=25$ mW/cm${}^2$ of the red laser. Emission intensity is color coded in logarithmic scale. A great number of recombination lines have been indicated; their assignment is explained in the text (e.g., $A_b$ and $A_w$ are acceptors in the barrier and in the well, “SU-” denotes shake-up, and “CR-” denotes cyclotron resonance). A pair of frames, (a) and (b), show adjacent areas of the map, recorded separately in two different spectral windows.

Figure 3

Similar to Fig. 1 but for the spectra recorded at magnetic field $B=23$ T, resolved into the two polarization channels, ${\sigma }^{-}$ (a) and ${\sigma }^{+}$ (b). The horizontal (energy) axis is broken between $E=1543$ and 1547 meV, and the energy scale differs in the two disconnected ranges. The assignment of lines is explained in the text, notation ($X$, $A$, SU, and CR) is the same as in Fig. 2, and “2DEG” and “2DHG” denote 2D electron and hole gas.

Figure 4

Comparison of the Coulomb binding energies of the positive and negative spin-singlet trions in the same quantum well (symmetric, GaAs, $w=15$ nm), dynamically converted between the $p$ and $n$ types. For the $X_s^{+}$, a binding energy at $B⩽3$ T was obtained by extrapolating the position of the exciton line (too weak to be seen in the spectrum).