Atomic-scale structure and photoluminescence of InAs quantum dots in GaAs and AlAs

We have determined the size, shape, and composition of $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dots (QDs) and InAs QDs embedded in an AlAs barrier by cross-sectional scanning tunneling microscopy. The outward relaxation and lattice constant of the cleaved surface of the QDs and their wetting layers were calculated using continuum elasticity theory and compared with experimental data in order to determine the indium concentration of the dots. Based on the structural results we have calculated the electronic ground states of the dots using a single band, effective mass approach. We find that the calculated ground state photoluminescence energy of the $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ dots is in excellent agreement with the measured energy. The observed large width of the PL spectrum of $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ dots can be attributed to $\Gamma \text{−}\Gamma$ electron-hole recombination within an ensemble of dots with sizes varying between $2.4–4.2\mathrm{nm}$ in height and $10–20\mathrm{nm}$ along the base diagonal. We find that the electron-hole wave function overlap of small $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ QDs is 7.6 times larger than that of $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ QDs grown under the same conditions. This supports the explanation of the long decay times in $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ dots by an enhanced exciton exchange splitting.

Figure 1

(Color online) Filled states topography X-STM image of three QD layers embedded in $40\text{−}\mathrm{nm}$-thick AlAs barriers and two QD layers grown in GaAs, $V_{\text{sample}}=-3\mathrm{V}$. The arrow indicates the growth direction.

Figure 2

(Color online) Filled states topography X-STM images of (a) $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ QD, (b) $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ QD. (c) and (d) show the calculated outward relaxation corresponding to (a) and (b). The height scale for (a) and (c) is 0 (dark) to 600 pm (bright). The height scale for (b) and (d) is 0 (dark) to 450 pm (bright). $V_{\text{sample}}=-3\mathrm{V}$. The arrows indicate the growth direction.

Figure 3

(Color online) Calculated and measured outward relaxation profiles through the center of the QD in the growth direction for an $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ QD (a) and an $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ QD (b). (c) and (d) are the corresponding calculated and measured lattice constant profiles. (e) and (f) are measured and calculated outward relaxation profiles of the segregated wetting layers in GaAs (e) and AlAs (f). The growth direction is from left to right.

Figure 4

(Color online) Filled states topography X-STM image of the $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ segregated wetting layer (a). By using an inverse fourier filter, the background contrast is removed (b). The arrow indicates the growth direction.

Figure 5

(Color online) Photoluminescence (PL) of $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ (a) and $\mathrm{In}\mathrm{As}∕\mathrm{Al}\mathrm{As}$ (b) dots measured at $7\mathrm{K}$. The double peak from the $\mathrm{In}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ dots (a) indicates a bimodal size distribution which was confirmed by atomic force microscopy measurements of the uncapped dot layer at the sample surface. The calculated ground state PL energy is indicated by the arrow. The broad PL peak of the AlAs dots (b) can be attributed to an ensemble of dots with varying sizes. The large increase in PL intensity at the low energy end of the spectrum is due to bandgap recombination of the GaAs substrate. The arrows labeled L and S indicate the calculated ground state PL energy for large and small dots as discussed in the text.

Figure 6

Band scheme for differently sized InAs QDs embedded in an AlAs barrier, showing the shift of the lowest $\Gamma$ bound electron state above the AlAs $X$ conduction band edge with decreasing dot size. For the smallest dot the $\Gamma$ electron is no longer confined to the dot and may scatter to the surrounding GaAs matrix or to larger QDs (Ref. 13).