Approach to wetting-layer-assisted lateral coupling of $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ quantum dots

We present new results on the simulation of two-dimensional (2D) quantum dot(s) (QDs) $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{P}$ superlattices, emitting at $1.55\mu \mathrm{m}$, the optical telecommunication wavelength. QDs and wetting layer (WL) electronic energy states are close in such a system. Using the Fourier-transformed Schrödinger equation developed on a mixed basis, we describe the wetting layer-assisted inter-QDs lateral (WLaiQD) coupling by studying the influence of WL states on QDs states and vice versa. The results show that WL and QDs have to be considered as a unique system, in strong coupling conditions. The increase of QDs density on the WL leads to enhanced splitting and miniband effects on QDs states. It induces a fragmentation of WL density of states interpreted as a 0D-like confinement of WL states. A comparison is made with a real high QDs density sample. It is expected to have an impact on carrier-capture phenomena in optoelectronic devices using high-density QDs in the active region.

Figure 1

Electronic density of states of a QD superlattice, a WL, and a QD superlattice on a WL $(C=51\%)$. The difference between $\left(\mathrm{QD}\right)+\left(\mathrm{WL}\right)$ and $(\mathrm{QD}+\mathrm{WL})$ systems is a coupling effect evidence. The figure inclusion represents the QD and WL with their dimensions.

Figure 2

A comparison between $(\mathrm{QD}+\mathrm{WL})$ and (QD) energies, for 1S, 1P, and 1D levels depending on the compacity factor. Effects of weak QD/WL coupling (a), WL-assisted inter-QDs weak coupling (b), WlaiQD coupling, and direct inter-QD strong coupling (c).

Figure 3

$(\mathrm{QD}+\mathrm{WL})$ and (QD) densities of states for a sample with a 75% compacity factor. The figure highlights the WL influence on splitting and miniband effects.

Figure 4

Probability density of low-energy WL states: wave function spatial variations for a 2D-hexagonal superlattice of cylindrical QDs on a WL. A WL state is represented for an uncoupled (a) and a coupled (b) configuration. WL states lose their 2D character with coupling. Density is in arbitrary units, with a maximum at 0.84.

Figure 5

(Color online) Atomic force microscopy picture of $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{Ga}\mathrm{As}\mathrm{P}$ QDs with optimized ${\mathrm{As}}_2$ flux on a $(0.5\times 0.5)\mu {\mathrm{m}}^2$ InP (311)B surface (a) reveals a high density and a squarelike lattice organization of the QDs. Strong and localized spots along a particular axis on the 2D-Fast Fourier Transformation (b) of this picture is a evidence of QD self-organization.

Figure 6

A comparison between $(\mathrm{QD}+\mathrm{WL})$ and (QD) energies, for 1S, 1P, and 1D levels depending on the compacity factor for $\mathrm{In}\mathrm{As}∕\mathrm{In}\mathrm{Ga}\mathrm{As}\mathrm{P}$ QDs on a square lattice. Effects of weak QD/WL coupling (a), WL-assisted inter-QD weak coupling (b), WlaiQD coupling and direct inter-QDs strong coupling (c). Calculations predict a WlaiQD coupling for the sample of Fig. 5.