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Detailed knowledge of composition and its distribution within and around QDs is important for a thorough understanding of the QD growth mechanism and of the structure-property relationship of QDs. However, the determination of quantitative composition profiles of QDs is a very challenging task because of the small size of the QDs. As a result, the composition of QDs has been less investigated compared to their shape and size.

Most investigations of QD composition suggest that the average composition of a QD is not necessarily the composition of the deposited materials, with the evolution of the composition being a complicated process involving alloying with the substrate material, elemental enrichment [193], and segregation.

Tersoff [194] argued that because an enriched larger-misfit component (e.g., In in a InGaAs/GaAs system and Ge in a GeSi/Si system) will significantly reduce the energy barrier of nucleation, and because the subsequent island growth and final growth stage may consume an indium-(or germanium-) depleted film, the resulting island will have a core that is rich in the larger misfit component (In-rich core in InGaAs/GaAs systems, Ge in GeSi/Si (larger atoms at compressive regions)). This results in a so-called "self-capping" process. The predicted self-capping process is illustrated schematically in Figure 10.

Liao et al. [195] presented the first experimental evidence of the elemental enrichment and segregation in QDs. They studied plan-view TEM diffraction contrast images and compared the details of the contrast with that predicted for

Figure 10. Schematic diagrams showing (a) an indium-enriched island nucleated on the wetting film and (b) the island further growing by consuming progressively In-depleted wetting film. The indium compositions are represented by grey scale with darker areas having higher indium composition. (After Tersoff [194])

Figure 10. Schematic diagrams showing (a) an indium-enriched island nucleated on the wetting film and (b) the island further growing by consuming progressively In-depleted wetting film. The indium compositions are represented by grey scale with darker areas having higher indium composition. (After Tersoff [194])

QDs modelled using finite element analysis. They found that under the [001] zone-axis imaging condition, the image detail (in the form of Maltese crosses, see Fig. 11) could be used to investigate the elemental distribution with the QD. In this way, they demonstrated In enrichment and segregation in Ina6Gaa4As/GaAs QDs.

Walther et al. [196] also reported elemental enrichment and segregation in MBE grown In0 25Ga075As/GaAs (the lowest In concentration at which islanding occurs), using energy-filtering imaging (EFI) in TEM. They observed strong inhomogeneous In enrichment within the islands (up to X(In) ~ 0.6 at the apex), resulting in a simultaneous In depletion in the remaining flat layer. Their results also demonstrated strong vertical and lateral In segregation with the highest In content at the island apex.

Krost et al. [197] use high-resolution X-ray diffraction and pole figure analysis to study a 15-fold InAs/InGaAs/GaAs QD stack grown by MOCVD. Through the analysis of the diffuse scattering intensity in the vicinity of the (113) reflection in the symmetric scattering geometry, they found that the In concentration in QDs is more than double the mean In concentration (43%) in the wetting layer, indicating a strong In enrichment during the S-K formation process. Crozier et al. [198] also found strong In enrichment in buried In05Ga05As/GaAs QDs grown at 550 °C, with the highest In/As ratio up to 0.8, by measuring quantitatively the composition profiles using electron energy-loss spectroscopy in a scanning transmission electron microscope.

Experimental observation of enrichment and segregation of the larger-misfit component has also been found in the Ge(Si)/Si system by Liao et al. [199]. They deposited pure Ge on Si(001) by gas-source MBE at 575 °C and then investigated cross-section samples using the EFI technique. They found that both the wetting layer and the islands are GeSi alloys, implying that alloying occurred during the deposition process. However, as illustrated in Figure 12, there is a much higher Ge content in the islands than in the wetting layer and the highest Ge content is located at the island center. The observed composition distribution and interfacial morphology (island/substrate interface slightly below wetting layer and the position of the highest Ge content slightly above the wetting layer) in Figure 12 can

20 nm

Figure 11. A comparison of an experimental (a) and a simulated (b) image of an Inx Ga1-xAs/GaAs QDs. The simulated image was obtained using a model QD with composition segregation. The conclusion of In enrichment and segregation was reached through the comparison. Reprinted with permission from [124], J. Zou et al., Phys. Rev. B 59, 12279 (1999). © 1999, American Physical Society.

Figure 12. A pseudo-color (spectrum) image showing the Ge/Si distribution within a Ge(Si)/Si(001) island, where the area with the highest Ge content is presented in red color while the lowest is purple. A horizontal black line is drawn along the wetting layer/substrate interface.

be explained by following Tersoff's theoretical analysis [194], while also assuming that alloying occurred during deposition and that interdiffusion between islands and substrate at the island/substrate interface occurred to further release strain energy.

In fact, alloying (a reverse process to elemental enrichment) during the deposition process is a commonly seen phenomenon [200-204]. One of the earliest reports was by Joyce et al. [205] in which they employed STM to study the growth by MBE of InAs QDs on GaAs(001). They measured the volume of the QDs at different temperatures as a function of InAs deposition. They found that a high substrate temperature (>420 °C) resulted in QDs with a much greater volume than the deposited volume, which implies significant mass transport from both the wetting layer and the substrate to the QDs. Furthermore, the wetting layers in the InAs/GaAs system and in the Ge/Si system have been reported to be an (In, Ga)As alloy [149] and a GeSi alloy [206], respectively, with temperature-dependant composition. Chaparro et al. [207] reported that strain-driven alloying in Ge/Si(001) coherent islands results in an increased mean Ge/Si(001) dome size and a delayed onset of island dislocation formation as the growth temperature increases.

Although elemental segregation appears to be common in QD islands, the segregation mode depends on the growth conditions. For example, Liao et al. [208, 209] showed that for Ge(Si)/Si(001) grown by solid-source MBE at 700 0C, the highest Ge content is at the island top, while in another study [199] they showed that Ge(Si)/Si(001) grown by gas-source MBE at 575 0C had the highest Ge content at the island center. The reason for the different segregation modes is discussed later.

There are many other investigations that report elemental segregation with the highest content of a larger-misfit component located either at the island center [210, 211] or at the island top [187, 212-217]. Using high-resolution TEM combined with finite element calculations, Kret et al. [210] investigated In035Ga0 65As/GaAs(001) QDs grown by MBE at 510 0C. They concluded that the In concentration in the island center is significantly high (up to ~50%), while the concentration at the island edges is low (down to ~20%). Litvinov et al. [211] studied CdSe QDs in a ZnSe matrix grown by MBE on GaAs(001) and used lattice fringe analysis in TEM to evaluate the distribution of Cd (the larger-misfit component in the CdSe/ZnSe system). They showed that the highest Cd content was in the QD center. Rosenauer et al. [215] reported that the lattice mismatch in MBE grown InxGa1-xAs/GaAs(001)

QDs at 560 °C increases from the bottom towards the top of the QDs. Stangl et al. [216] investigated the composition distribution of GeSi islands grown by MBE on Si(001) at 600 °C using grazing incidence, noncoplanar, scattering X-ray diffraction. They compared the experimental diffraction pattern with patterns calculated, based on the QD strain fields obtained from finite element analysis. Using surface-sensitive X-ray diffraction, Kegel et al. [217] investigated the tomographic nanometer-scale images of 530 °C MBE grown InAs/GaAs(001) QDs. Based on the three-dimensional intensity mapping of selected regions in reciprocal space, they extracted information about the interdiffusion profile along the QD growth direction on a subnanometer scale and found that QD composition varies continuously from GaAs at the island/substrate interface to InAs at the top. From this comparison, they concluded that although pure Ge was deposited, the Ge content varies from 50% at the island base to 100% at the island top. Using electron energy-loss spectroscopy (EELS), Walther et al. [218] showed vertical and lateral Ge segregation in Si08Ge02/Si (001), where the Ge atoms tend to occupy tensile positions.

Although Tersoff's theoretical investigation [194] predicts islands forming with enriched a larger-misfit component in the center to lower the nucleation barrier, it is energetically favorable to have larger atoms in the regions that are (relatively) more tensile and smaller atoms in more compressive regions [219]. Different atomic mobilities on the surface can also lead to substantial segregation, even when the atomic sizes are identical [219]. Therefore, in the case where kinetic conditions (higher growth temperature, slower deposition speed) allow a system to reach an equilibrium state, QDs will have the segregation mode with the larger-misfit component located at the island top. Otherwise, the enriched larger-misfit component will remain in the center of the QDs. This explains the different observations discussed above.

Because of the dynamic, often nonequilibrium, nature of QD growth, the real situation may be more complicated than is covered by the above discussion. A surprising compositional uniformity in Si08Ge02/Si (001) islands was reported by Floro et al. [220]. Bimodal composition distribution in InGaAs/GaAs QDs grown using low-pressure MOCVD has been reported by Saint-Girons et al. [221] They demonstrated that the presence of a bimodal inhomogeneous broadening of the photoluminescence was correlated with a bimodal QD contrast distribution in the bright-field [001], plan-view TEM images, which they believe are from In-poor and In-rich QDs.

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