Experimental investigation and modeling of the fine structure splitting of neutral excitons in strain-free $\text{GaAs}/{\text{Al}}_x{\text{Ga}}_{1-x}\text{As}$ quantum dots

We measure, by photoluminescence spectroscopy, the fine structure splitting (FSS) of bright excitons confined in a large number of elongated and unstrained ${\text{Al}}_{0.35}{\text{Ga}}_{0.65}\text{As}/\text{GaAs}/{\text{Al}}_{0.45}{\text{Ga}}_{0.55}\text{As}$ quantum dots (QDs) and quantum-well potential fluctuations (QWPFs). While the FSS values are comparable, the light polarization directions for well-oriented QDs and randomly oriented QWPFs on the same sample are substantially different. We compare the results with model calculations using as input scanning probe microscopy data of QD structures nominally identical to those investigated by photoluminescence. The model reproduces well the polarization orientation for all studied samples and also the magnitude of the FSS, at least for relatively tall dots. We discuss the results and elucidate the role of in-plane elongation on the FSS.

Figure 1

(Color online) (a) Color-coded PL intensity recorded for varying polarization angle from the sample with $D=5\text{nm}$. The white lines indicate the angles corresponding to emission polarization along the [110] crystal direction. (b) and (c) Results of Lorentzian fits of two split excitonic lines with relatively (b) small and (c) large FSS. The FSS is obtained from the amplitude of a sine fit in (b) or from the energy separation between the top and bottom flat regions in (c). See text for details.

Figure 2

(Color online) Experimental (black circles) and calculated (dots and stars) FSS as a function of exciton energy for samples with (a) $D=5$, (b) 7, and (c) 10 nm. The filled black circles in (a) correspond to QDs created from DIs. The black stars are the FSS calculated for the averaged depth profiles (insets in the upper left corners of each panel). The inset in the lower right corner in (a) shows the depth profile of a NH created by etching of a DI. The scale of all insets is specified in (a).

Figure 3

(Color online) (a) Histogram of the FSS values for QDs and (b) QWPFs measured on the sample with $D=7\text{nm}$. (c) Polarization direction of the low-energy component of the exciton for the QDs and (d) for the QWPFs. A polarization angle of $0°$ corresponds to the [110] crystal direction. The inset in (b) shows an STM image of the lower barrier of the QW. The inset in (c) shows the calculated polarization distribution for NHs with $D=5$, 7, and 10 nm; the $x$-axis scale is the same as for the main axis panel. The inset in (d) shows the polarization behavior of two representative QWPF emission lines with relative polarization angle $\Delta \varphi$.