Structural and optical changes induced by incorporation of antimony into InAs/GaAs(001) quantum dots

We present experimental evidence of Sb incorporation inside InAs/GaAs(001) quantum dots exposed to an antimony flux immediately before capping with GaAs. The Sb composition profile inside the nanostructures as measured by cross-sectional scanning tunneling and electron transmission microscopies show two differentiated regions within the quantum dots, with an Sb rich alloy at the tip of the quantum dots. Atomic force microscopy and transmission electron microscopy micrographs show increased quantum-dot height with Sb flux exposure. The evolution of the reflection high-energy electron-diffraction pattern suggests that the increased height is due to changes in the quantum-dot capping process related to the presence of segregated Sb atoms. These structural and compositional changes result in a shift of the room-temperature photoluminescence emission from 1.26 to $1.36\mu \text{m}$ accompanied by an order of magnitude increase in the room-temperature quantum-dot luminescence intensity.

Figure 1

(Color online) $2\times 2\mu {\text{m}}^2$ AFM images of InAs QD ensembles (a) without Sb exposure and (b) after a 10 s exposure to an Sb pressure of $P_{\text{Sb}}=8.0\times {10}^{-7}\text{mbar}$.

Figure 2

[(a)–(f)] AFM statistics of QD surface density vs height with a 1 nm height binning as a function of the increasing Sb pressure at a constant exposure time of 10 s and [(g)–(l)] corresponding room-temperature PL spectra.

Figure 3

QD three-dimensional (3D) RHEED pattern intensity evolution for InAs QD exposed to As and Sb followed by GaAs capping. When QD are exposed to Sb, the 3D pattern remains visible longer time during capping. The inset shows the RHEED pattern immediately before Sb.

Figure 4

Dark-field 002 TEM images of: (a) the QD four-layer stack. The bottom layer is an InAs QD reference layer, and the upper layers have increasing Sb exposures of $P_{\text{Sb}}=2.0\times {10}^{-7}\text{mbar}$, $P_{\text{Sb}}=8.0\times {10}^{-7}\text{mbar}$, and $P_{\text{Sb}}=3.1\times {10}^{-6}\text{mbar}$. (b) Detail of the layers exposed to Sb. (c) Detail of the top layer QDs exposed to $P_{\text{Sb}}=3.1\times {10}^{-6}\text{mbar}$. (d) Reference InAs QD exposed to As.

Figure 5

InAs(Sb) QD heights (as measured by TEM and AFM) and corresponding emission wavelengths plotted against the pressure of the Sb beam during the exposure step.

Figure 6

Calculated diffracted 002 intensities as a function of composition in the ${\text{In}}_x{\text{Ga}}_{1-x}{\text{Sb}}_y{\text{As}}_{1-y}$ system.

Figure 7

(Color online) Cross-sectional scanning tunneling micrographs of: (a) InAs QD exposed to $P_{{\text{As}}_4}=2.0\times {10}^{-6}\text{mbar}$. (b) InAs QD exposed to an Sb pressure $P_{\text{Sb}}=8.0\times {10}^{-7}\text{mbar}$ during 10 s before capping. This growth conditions optimize optical properties. The images have been flattened by removing low frequency components with a Fourier filter.

Figure 8

Atomic plane spacing profiles taken in the growth direction for QD with and without Sb exposure at $P_{\text{Sb}}=3.1\times {10}^{-6}\text{mbar}$. The origin (0 nm) is at the QD base. Error bars represent the standard error of the mean.

Figure 9

Outward strain relaxation profiles of InAs reference WL and InAs WL exposed to Sb $\left(P_{\text{Sb}}=3.1\times {10}^{-6}\text{mbar}\right)$. The latter is compared with its Sb distribution along the growth direction [001]. Sb concentration has been estimated by atom counting. The origin (0 nm) is set in the first WL monolayer. Error bars represent the binomial proportion confidence interval at a 90% confidence level.

Figure 10

Accumulated stress evolution during InAs QD formation, Sb exposure, and subsequent GaAs capping in different experiments: (a) InAs QD exposed to $P_{\text{Sb}}=3.6\times {10}^{-7}\text{mbar}$ and $P_{\text{Sb}}=3.4\times {10}^{-6}\text{mbar}$, and a control experiment exposing InAs QD to $P_{\text{As}}=2.0\times {10}^{-6}\text{mbar}$. The inset shows the detail of the $\Sigma \sigma$ evolution during Sb exposure growth step for the different pressures. (b) Samples with QD and without QD, instead, with 1 ML InAs quantum well. The Sb exposure step took place at $P_{\text{Sb}}=3.6\times {10}^{-7}\text{mbar}$. The signal for the QD samples is shown for cantilevers cut in such a way that the long side is parallel either to the [110] or to the $\left[1\overline{1}0\right]$ crystallographic directions. The inset shows the detail of the $\Sigma \sigma$ evolution during the 10 s Sb exposure step in both crystallographic directions and in the quantum well.

Figure 11

Scheme of the structural changes induced by Sb exposure immediately before QD capping. (a) Initial uncapped InAs QDs. (b) Structural changes induced by GaAs capping. (c) InAs QDs exposed to Sb before capping, with segregated Sb layer, and Sb incorporation at the apex of the QDs. (d) Effects of GaAs capping after Sb exposure. The QD height is significantly higher than in case (b).