Hierarchical Self-Assembly of $\mathrm{GaAs}/\mathrm{AlGaAs}$ Quantum Dots

A novel structure containing self-assembled, unstrained GaAs quantum dots is obtained by combining solid-source molecular beam epitaxy and atomic-layer precise in situ etching. Photoluminescence (PL) spectroscopy reveals light emission with very narrow inhomogeneous broadening and clearly resolved excited states at high excitation intensity. The dot morphology is determined by scanning probe microscopy and, combined with single band and eight-band $k·p$ theory calculations, is used to interpret PL and single-dot spectra with no adjustable structural parameter.

Figure 1

AFM images of GaAs nanoholes prior to (a) and after (b) overgrowth with $D=10\mathrm{nm}$ of ${\mathrm{Al}}_{0.45}{\mathrm{Ga}}_{0.55}\mathrm{As}$. The inset of (b) shows a $150\times 150{\mathrm{nm}}^2$ magnification of a nanohole with widths $W_x$ and $W_y$ in the $[1\overline{1}0]$ and [110] directions, respectively. (c) Average depth $⟨h⟩$ and (d) widths of holes as a function of $D$. Error bars in (c) represent standard deviations.

Figure 2

(a) Schematic sample structure and magnification of the active layer. (b) Low excitation PL spectra of samples with different AlGaAs-barrier thickness $D$. Measurement temperature $T$ and laser excitation energy $E_{\mathrm{exc}}$ are indicated. (c) Positions of the peaks associated with GaAs QDs ($E^0$) and with the GaAs QW ($E_{\mathrm{QW}}$) and (d) FWHM of the QD peak as a function of $D$. Open symbols refer to the result of a 1D simulation (see the text).

Figure 3

PL spectra for a sample with $D=7\mathrm{nm}$ for various excitation intensities $I_{\mathrm{exc}}$. When $I_{\mathrm{exc}}$ is increased, well defined excited states $E^1$, $E^2$, $E^3$, and $E^4$ appear while the peak $E^0$ slightly redshifts. The laser spot diameter ${\varphi }_{\mathrm{spot}}$ is also indicated.

Figure 4

(a) PL ($E_{\mathrm{exc}}=1.96\mathrm{eV}$) and PLE spectra of a single QD with $D=7\mathrm{nm}$ for different $I_{\mathrm{exc}}$. The bottom graph is a calculated excitonic absorption spectrum and the inset shows 3D representations of ground and first excited state wave functions for electrons (E) and holes (H) in the QD. (b) STM topograph, with contrast enhanced by a first derivative filtering, taken at constant current (0.2 nA) and sample bias ($-4.2\mathrm{V}$), of the AlGaAs nanohole ($D=7\mathrm{nm}$) used for the calculation shown in (a). (c) Line scans of the STM image.