Fine structure of excitons in InAs quantum dots on GaAs(110) planar layers and nanowire facets

We investigate the optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs(110) using Bi as a surfactant. The quantum dots are synthesized on planar GaAs(110) substrates as well as on the {110} sidewall facets of GaAs nanowires. At 10 K, neutral excitons confined in these quantum dots give rise to photoluminescence lines between 1.1 and 1.4 eV. Magneto-photoluminescence spectroscopy reveals that, for small quantum dots emitting between 1.3 and 1.4 eV, the spatial extent of the exciton wave function in and perpendicular to the (110) plane is about 5 and 2 nm, respectively. The quantum dot photoluminescence is linearly polarized, and biexcitons with positive as well as negative binding energies are observed, two findings that we associate with the strain in the (110) plane. This strain leads to piezoelectric fields and to a strong mixing between heavy- and light-hole states, and offers the possibility to tune the degree of linear polarization of the exciton photoluminescence as well as the sign of the binding energy of biexcitons.

Figure 1

(a) PL spectra acquired at 10 K from the planar InAs/GaAs(110) sample grown with (triangles) and without (squares) the Bi surfactant. The spectra have been normalized and shifted vertically for clarity. The transitions related to the GaAs substrate, the WL, and the InAs QDs are indicated in the figure. (b) PL spectrum at 4.2 K of the sample grown with a Bi surfactant. (c) Enlarged view of the transition highlighted by a red rectangle in (b). The solid line is a Gaussian fit yielding a full width at half maximum of 140 $\mu \mathrm{eV}$.

Figure 2

PL spectrum obtained at 10 K from a few core-multishell nanowires grown with Bi surfactant. The inset shows an expanded view of two individual transitions. The full widths at half maxima indicated in the inset have been obtained by Gaussian fits (blue lines).

Figure 3

(a) PL spectrum of a few InAs QDs on planar GaAs(110) taken at 4.2 K with an excitation power of 290 nW. (b) PL spectra from the QD emitting at 1.3187 eV in (a) for different excitation powers in $\mu \mathrm{W}$ as indicated in the figure. The spectra have been normalized and shifted vertically for clarity. (c) Intensity of the transitions at 1.3187 and 1.3210 eV (squares and triangles, respectively) as a function of excitation power. The blue and red lines show fits yielding the slopes indicated in the figure. (d) PL spectra of a QD in a nanowire for different excitation powers. The excitation power in $\mu \mathrm{W}$ is specified on the left of each spectrum. For excitation powers up to 2.7 $\mu \mathrm{W}$, the increase in intensity of the lines labeled X and XX with increasing excitation power is nearly linear and quadratic, respectively.

Figure 4

(a) and (b) Evolution of the exciton energy $E_{\text{X}}$ in a magnetic field for a single InAs QD on (a) planar GaAs(110) and (b) a GaAs $\left\{110\right\}$ nanowire sidewall at 4.2 K. Both measurements were done in Faraday geometry. The intensities are color-coded according to the scale bars on the right. The solid lines are quadratic fits to the emission energies of the bright exciton states, yielding values for $g$ and ${\gamma }_2$ as indicated in the figures. The respective insets show PL spectra at 8 T (symbols) together with Gaussian fits (line) to determine the transition energies. In (b), the “jump” occurring at about 4.5 T is tentatively attributed to a magnetic field-induced change in the electrostatic environment of the QD. (c) and (d) Spatial extent of the exciton wave function $L_{\text{eh}}$ for (c) 14 QDs grown on planar GaAs(110) and (d) 15 QDs on nanowire sidewalls. The mean values are indicated in the figure. The insets display a schematic representation of an InAs(110) QD in GaAs.

Figure 5

(a) Polarization dependence of the transitions from neutral excitons confined in InAs QDs on planar GaAs(110) at 9 K. The intensities are color-coded according to the scale bar on the right. Angles of 0 and ${90}^{\circ }$ correspond to light polarized along the $\left[001\right]$ and $\left[1\overline{1}0\right]$ directions, respectively. (b) Polar plot of the total intensity of the transitions between 1.28 and 1.30 eV (squares) as a function of the polarization angle. The solid line is a fit, indicating that the QD emission is polarized linearly along the $\left[1\overline{1}0\right]$ direction with a polarization ratio $\rho =-0.68$. The triangles represent the PL intensity from the GaAs substrate reflecting the polarization of the setup.