Strain effects on individual quantum dots: Dependence of cap layer thickness

We have studied the effects of strain on individual self-assembled quantum dots (QDs) exemplified by InP dots embedded in GaInP. The quantum dot sample was etched from the top and in this way the amount of capping material was reduced. In a sequence of etch cycles, the cap layer was thinned, and the photoluminescence from several individual QDs could be followed as a function of cap layer thickness. The evolution of the emission spectra clearly depended on the quantum dot size. We interpret this as arising from differences in the aspect ratio for quantum dots of different sizes. The influence of the capping layer, for different QD geometries, was modeled using deformation potential theory with the strain calculated using a full three-dimensional linear elasticity model. The results agree well with the experimental observations.

Figure 1

Estimated etch depth (solid squares), measured average surface root-mean square (RMS) roughness (circles), and etch depth measured by TEM (open squares) as a function of etching time. The surface roughness was measured by AFM over $10\times 10\mu {\mathrm{m}}^2$ areas at several different locations on the surface for each sample.

Figure 2

Photoluminescence spectra of a single small InP QD for varying cap layer thicknesses. (a) The effect of cap layer thickness on the emission. The spectra have been normalized and offset for clarity. (b) The same data as in (a) but shifted in energy to facilitate comparison between the different cap layer thicknesses. (c) The evolution of the photon-emission for the dot as a function of excitation power $(P_0=0.1\mathrm{W}{\mathrm{cm}}^{-2})$, at a cap layer thickness of $26\mathrm{nm}$.

Figure 3

Photoluminescence spectra of fully developed InP QDs for varying cap layer thicknesses. The spectra have been normalized and offset for clarity, and shifted in energy to facilitate comparisons. (a) Quantum dot emitting around $1.66\mathrm{eV}$. (b) Larger quantum dot emitting around $1.62\mathrm{eV}$. The insets show the unshifted (measured) spectra.

Figure 4

The results of the theoretical calculations for dots with three different heights (aspect ratios). The solid lines show the experimental data and the dotted lines are the calculated energies. The insets show the geometry of the QDs and the aspect ratios used for the modeling.