Tensile-strained GaAs quantum wells and quantum dots in a $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ matrix

We report on the fabrication and analysis of tensily strained thin GaAs layers (“insertion layers”) in a $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ matrix $(x<0.05)$. The GaAs insertion layers vary in nominal thickness in the range of 0.6–2 monolayers. They were grown by molecular beam epitaxy on GaSb substrates. Transmission electron microscopy studies indicated either growth of ultra-thin quantum wells (QWs) or self-organized formation of quantum dots (QDs), depending on the nominal thickness of the insertion layers. Two photoluminescence (PL) peaks are observed at photon energies below the $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ band edge. One of them, detected near $0.7\mathrm{eV}$ for any insertion layer thickness, is attributed to the emission of a type II QW. The second peak shifted toward smaller energies in the thicker insertion layers is assigned to the PL of type II QDs. This assignment is supported by microscopic tight-binding calculations of the QW electron states.

Figure 1

(004) reflection XRD rocking curves measured in a type B sample without any insertion in the $\mathrm{Ga}{\mathrm{As}}_{0.05}{\mathrm{Sb}}_{0.95}$ matrix (a), a type B sample with a graded thickness GaAs insertion in the $\mathrm{Ga}{\mathrm{As}}_{0.02}{\mathrm{Sb}}_{0.98}$ matrix (b), and a type A sample with a graded thickness GaAs insertion in the $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ matrix with $0.035<x<0.055$ (c).

Figure 2

Cross-section TEM images of a GaAs insertion with different nominal thicknesses: $0.6\mathrm{ML}$ (a), $0.8\mathrm{ML}$ (b), $1.2\mathrm{ML}$ (c), and $2.0\mathrm{ML}$ (d).

Figure 3

Plan-view TEM image of islands formed within the $1.2\mathrm{ML}$ thick GaAs insertions. The inset shows the lateral size distribution of the islands.

Figure 4

PL spectra measured at successive points along the gradient direction in a type A sample with graded thickness. The respective GaAs nominal thicknesses are marked at each curve. The spectra are multiplied by the factors given at the curves and are arbitrarily offset.

Figure 5

Temperature dependent PL spectra measured at an excitation power density of $10\mathrm{W}∕{\mathrm{cm}}^2$ ($532\mathrm{nm}$ laser line) in a graded thickness type A sample at three points with different nominal GaAs thicknesses: $0.8\mathrm{ML}$ (a), $1.05\mathrm{ML}$ (b), and $1.2\mathrm{ML}$ (c). For clarity the spectra are arbitrarily offset and normalized. Dashed lines are eye guides.

Figure 6

PL spectra measured at $5\mathrm{K}$ versus excitation power density ($532\mathrm{nm}$ laser line). Dashed curves are plotted only to guide the eye. The spectra are arbitrarily offset and normalized.

Figure 7

PL peak energy (a) and full width at half maximum (FWHM) (b) for $I_1$ (open circles) and $I_2$ (solid squares) peaks versus excitation power density. Dashed curves are plotted only to guide an eye.

Figure 8

Relative positions of the conduction band bottom (el) and heavy hole (hh) and light hole (lh) valence band tops for a $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ solid alloy. Dashed curves correspond to the unstrained solid alloy, whereas solid curves are relevant to the tensily strained layer grown pseudomorphically on a GaSb substrate. The inset illustrates band line-ups and optical transitions for a type II QW of $\mathrm{Ga}{\mathrm{As}}_y{\mathrm{Sb}}_{1-y}∕\mathrm{Ga}{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ (see text).

Figure 9

Strain-induced splitting of heavy- and light-hole bands in a $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$ solid alloy grown pseudomorphically on a GaSb substrate, versus the As content $x$. The solid line represents the results of the calculation and circles are from the PL data. Solid circles correspond to type B samples and the open circle represents a type A sample.

Figure 10

Electron localization energy for a GaAs QW in a GaSb matrix. The solid curve represents the data obtained within the Kane model. Open squares show the results of the tight-binding calculation, where for low widths each square corresponds to a monolayer step.