Effects of strain and confinement on the emission wavelength of InAs quantum dots due to a $\mathrm{Ga}{\mathrm{As}}_{1-x}{\mathrm{N}}_x$ capping layer

A GaAsN capping layer grown on InAs quantum dots (QDs) induces a strong redshift of the emission wavelength and extends it beyond $1.3\mu \mathrm{m}$. We investigated this effect systematically by changing the nitrogen content in the GaAsN layer, varying the thickness of this layer, and embedding a GaAs spacer layer between the GaAsN layer and the QDs. The samples were grown on GaAs(001) substrates by plasma-assisted solid-source molecular beam epitaxy (MBE). Additionally, we simulated the band structure and the electron and hole energy levels based on $6\times 6\mathbf{k}\mathbf{\bullet }\mathbf{p}$ calculations, including strain and piezoelectric effects. We found that the wavelength extension is caused by the decrease of the confining energy barrier for the electron wave function in the QDs due to the lower conduction band energy of the GaAsN layer with respect to GaAs. The strain inside the QDs is almost unaffected by the overgrowth with the tensilely strained GaAsN layer. The insertion of a GaAsN layer below the QDs yields only a very small change in wavelength compared to the effect produced by a GaAsN capping layer. This difference is attributed to a reduced QD volume due to the growth on GaAsN that is suggested in cross-sectional scanning tunneling microscopy (XSTM) measurements. The blueshift due to this structural change of the QDs compensates for the redshift that is induced by the decreased confinement.

Figure 1

Schematic of (a) the grown sample structure for the PL measurements, and (b) the simulated QD shape, size and composition (including the wetting layer). Note that the indium content inside the core of the QD in (b) increases linearly from 50% at the lower interface to 100% at the upper interface of the QD.

Figure 2

(a) Emission wavelength of the QDs at $300\mathrm{K.}$ as a function of the nitrogen content in a $10\text{−}\mathrm{nm}$ thick GaAsN layer either below or above the QDs. The solid symbols correspond to QDs capped with GaAsN, and the open symbols to QDs grown on GaAsN, whereas squares with solid lines denote experimental results, and the triangles with dashed lines correspond to the simulations. The experimental data point for the reference sample with nominally zero nitrogen in the GaAsN layer on top of the QDs is set to a nitrogen content of 0.1% in order to show that a small amount of nitrogen has been incorporated from the nitrogen plasma even though the shutter in front of the plasma cell was closed (see text). The corresponding PL spectra are presented in (b) for the QDs capped with GaAsN, and in (c) for the QDs grown on GaAsN.

Figure 3

XSTM images of QDs. The QD in (a) and (c) is embedded in GaAs, and the QD in (b) and (d) is grown on $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$ and capped with GaAs. The lines in (c) and (d) indicate the shape of the QDs. Below the images are schematic top views of truncated pyramids with (e) {137} side facets and (f) {101} facets. The dotted lines indicate possible cleavage planes through the QDs. The images are taken at (a) and (c) $V_T=-1.7\mathrm{V}$, $I_T=70\mathrm{pA}$ as well as at (b) and (d) $V_T=-3.3\mathrm{V}$ and $I_T=70\mathrm{pA}$, respectively.

Figure 4

Simulated emission wavelengths of different QD models embedded in GaAs (0%) and with an adjacent $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$ layer. The QD sizes of models A and B are described in the text. For the QD in model C, we used squares of $20\times 20{\mathrm{nm}}^2$ and $7\times 7{\mathrm{nm}}^2$ for the bottom and top square, respectively. The other parameters of the model are the same as in model A and B.

Figure 5

Band structures (conduction and valence bands) corresponding to the simulations in Fig. 2 with a nitrogen content of 1.2% in the $10\text{−}\mathrm{nm}$-thick GaAsN layer (a) above and (b) below the QD, and the reference structures without nitrogen. For the QD sizes, we used model A in (a) and model B in (b). The band structure is taken along a line in growth direction through the center of the QD.

Figure 6

Simulated transition energies between the lowest electron and highest hole energy level for QDs capped with GaAs (0%), $10\text{−}\mathrm{nm}$ $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$, or hypothetical materials (additional data points at 1.2%). The first hypothetical material has the same lattice constant as GaAs, but the band structure is the same as for $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$. In the second hypothetical material, the changes are reversed.

Figure 7

Schematic model of the distortion in a QD due to capping with GaAs. The arrows indicate the difference when capping the QDs with GaAsN: The compressive strain of the QD is stronger in the growth direction but smaller perpendicular to it with respect to a GaAs capping layer.

Figure 8

(Color) Cross section of the calculated strain tensor components in the QD region in growth direction $\left({\epsilon }_{zz}\right)$ (d)–(f) and perpendicular to it $\left({\epsilon }_{yy}\right)$ (a)–(c), and the hydrostatic strain $\left({\epsilon }_{\mathrm{hydro}}\right)$ (g)–(i) with GaAs and $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$ caps. Only a fraction of the simulated region is shown. Positive values denote tensile and negative values compressive strain. The difference images in (c), (f), and (i) are obtained by subtracting the values of the strain in the case of the GaAs cap from the strain in the case of the $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$ cap, i.e. (b) minus (a), (e) minus (d), and (h) minus (g). Note the different scale of the color coding in particular for the difference images.

Figure 9

(a) Emission wavelength of the QDs at $300\mathrm{K}$ as a function of the thickness of the $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$ capping layer. The squares correspond to the experimental data and the triangles to the simulated data. Lines are visual guides. (b) Corresponding PL spectra.

Figure 10

(a) Emission wavelength of the QDs at $300\mathrm{K}$ as a function of the thickness of a GaAs spacer layer between the QDs and the $10\text{−}\mathrm{nm}$ $\mathrm{Ga}\mathrm{As}{\mathrm{N}}_{1.2\%}$ capping layer. The squares correspond to the experimental and the triangles to the simulated data. Lines are visual guides. (b) Corresponding PL spectra.