RBS spectra obtained in “channeling” along 〈001〉 direction (a) and in “random” (b) analysis configurations for the sample grown at 550 °C. The features of the two spectra indicate that the deposited Ge is confined at the surface, with a very limited interdiffusion.
AFM micrographs of the samples grown at different temperatures. 3D surface plots of the islands, respectively, formed at 550, 600, 650, and 700 °C. The vertical scale in images (a) and (b) extends to 20 nm, whereas to 30 nm for (c) and (d). The scanned area is 1 × 1 μm2 in all the images. The short black arrows in (c) and (d) indicate the presence of a pyramid in the respective images.
Numerical island density per unit area for the samples of series I (a). Average island separation for the same island ensembles formed at different temperatures (b). The unit area considered for all the specimens corresponds to 1 μm2 AFM scan. The data points have been fitted by a sigmoidal Boltzmann function. Contact angles are highlighted in the various TEM insets in (a) and lines serve as guides for the eyes.
Average vertical and lateral sizes of the islands of series I (a), whose error bars represent for each point the respective standard deviation from the mean value. Arrhenius plot of the average diameter for the island ensembles grown at different temperatures (b). Total integrated volumes for the island ensembles grown at different temperatures (c). The integrated volume refers to all the islands present in the AFM micrographs (i.e., 1 μm2).
Thickness of the WLs for islands grown at different temperatures (a). Examples of HR-TEM micrographs employed for the measurement of the WL thickness at the indicated temperatures (b, c, and d). Note that all the islands are coherent with the substrate and no crystalline defects are detected. The insets in the micrographs are the corresponding intensity profiles generated by the diffracted electrons.
BF TEM image of the cross-section of a small dome cluster grown at 550 °C (a). EF-TEM images in cross-section: Ge (b), Si (c), and O (d) elemental maps. Ge and Si are separately evident within the island (b) and the underlying substrate (c). The signal of Si within the island and along its surface in (c) is not discernible from the surrounding dark background. Note the thin O layer in (d), which indicates a clear surface oxidation of the WL.
BF TEM micrograph of the cross-section of a dome cluster grown at 600 °C (a). Elemental maps of: Ge (b), Si (c) and O (d). The bright signal related to Si within the island is weak in (c), though quite distinct from the surrounding dark background. Note the thin, bright wake due to O in (d), which is present on both the WL and the entire surface contour of the island.
BF TEM image of a typical dome cluster grown at 650 °C (a). Note the “feet” (white arrows) that the island has developed around its base and the presence of a thick WL. Elemental maps of: Ge (b), Si (c), and O (d). In map (c), the signal related to Si is clearly observed within the cluster and its intensity in correspondence of the island surface is enhanced. The intense wake related to O in map (d) displays the oxidation of the island surface, its WL and the base “feet.”
BF TEM image of a characteristic cluster formed at 700 °C (a). Note the flattened profile of the island that resembles a truncated pyramid. Elemental maps of: Ge (b), Si (c), and O (d). In map (c), the signal related to Si is visible within the island, its surface and the WL. The intense wake in map (d) corresponds to the oxidation of the WL and the flattened cluster. Note that the inset (e) in map (c) is obtained in correspondence of the characteristic Si plasmon loss.
AFM micrographs of the samples grown at 650 °C with different Ge coverages. 3D surface plots of the islands having equivalent θGe of approximately 5.5, 6.4, 9.1, and 12.2 MLs. The vertical scale bar in (a) is 1 nm, where only the WL has been formed. The image in (b) has a vertical scale bar of 30 nm, whereas for (c) and (d) is 40 nm and 70 nm, respectively.
Numerical island density per unit area (i.e., black circles) for the samples grown at 650 °C with various θGe and corresponding average island spacing (i.e., red squares) reported in the scale on the right (a). Average vertical and lateral sizes of the islands of series II, grown at 650 °C with different θGe (b), whose error bars for each point represent the respective standard deviation from the mean value. Note that also the sample containing 8.2MLs (i.e., belonging to series I) has been included in both plots. A sharp rise in the numerical density of the clusters is observed in (a) at 9.1 MLs, yielding an average island separation of approximately 110 nm. The peculiar features in the evolution from pure WL to super-domes are easily discerned. The large error bars in correspondence of the sample with the highest θGe in (b) are due to the fact that the introduction of super-domes results in a polydisperse ensemble of clusters, where a few tiny islands are also present.
HR-TEM micrograph of the specimen grown at 650 °C with θGe ≈5.5 MLs. The inset shows the profile of the diffracted electrons in correspondence of the dotted line orthogonal to the sample surface. Note that the WL is coherent with the underlying substrate and that the measured thickness (0.982 nm) is higher than that associated to 5.5 MLs of pure Ge (i.e., 0.775 nm).
Total integrated volumes of the different islands grown at 650 °C as a function of their respective θGe. Each value refers to the overall volume of the clusters present in 1 μm2 having a definite Ge coverage. Volumes by AFM and doses by RBS have been expressed in terms of equivalent MLs for a direct comparison. The integrated volumes were first converted in cm3 and then multiplied by the atomic density of Ge, nGe = 4.41 × 1022 at./cm3. These values were further divided by 10−8 cm2 (i.e., the scanned area) to give equivalent doses and ultimately divided by 6.78 × 1014 at./cm2 to calculate the equivalent MLs. Note that the intercept of the linear fit with the x-axis provides an approximate evaluation of the critical dose (e.g., thickness), D3D. The value of θGe for the specimen containing only the WL (see open circle) has been reported for further comparison.
BF TEM image of a typical super-dome formed by depositing 12.2MLs at 650 °C (a). Note that the island has fully developed facets with definite angles and slight depressions at its base (see black arrows). Crystalline defects are highlighted by the white arrows, which point at dislocations and stacking faults. Elemental maps of Ge (b) and Si (c). Note the rather uniform distribution of both elements in the super-dome, where no Si-enriched spots are detected.
Schematic drawings representing possible diffusion pathways for Si intruding the base of coherent dome clusters (a) and defective super-domes (b). Note that the system requires an additional energy expense to have Si at the very surface of the coherent island in (a), while this circumstance could be compensated by an effective elastic relaxation of the overall volume strain energy of the island, so as to preserve the coherence of the entire island-WL system.
AFM micrographs of the uncapped island layer grown at 650 °C in a multi-layered structure. 3D surface view of the islands present in the topmost layer (a). Plane view of the same 1 μm2-area (b). Crystallographic orientations 〈110〉 and 〈100〉 are indicated by the arrows in (b). The selected black and white pattern for the vertical scale helps highlighting the particular shape of the formed clusters. Note that the vertical scale bar is 50 nm in both images and that the islands enclosed by the dotted circles in (b) are in a “transition state” between a pyramid and a dome cluster.
BF TEM image at low magnification of vertically aligned islands in a multi-layered structure grown at 650 °C (a). BF TEM close-up image (b) showing the progressive enlargement of the islands in successive layers and the thick WL and “feet” developed by the dome in the topmost layer. Elemental maps of Ge (c) and Si (d) related to image (b). In map (c), the signal related to Ge is immediately noticed for each stacked island and its corresponding WL. The presence of Si within the stacked islands is evident in (d) and the white arrow points to a Si-rich surface layer. The line labeled with numbers in (d), probes the local composition of the islands in a direction perpendicular to the surface of the sample. The intensity of theelectrons generating the Si map in correspondence of this line is shown in (e), where the various depths are indicated by their respective numbers. Asterisks (*) indicate the lowest concentrations of Si, which alternatively correspond to the highest Ge contents in the core of the islands. The hash symbol (#) refers to thebackground signal.
Synoptic table of the deposited samples. Series I refers to the specimens that have about the same Ge coverage deposited at different temperatures, whereas series II includes the samples grown with distinct doses at a fixed temperature of 650 °C. The Ge doses, σ, have been determined via RBS analysis and the associated equivalent Ge monolayer thicknesses, θGe, have been calculated by dividing the RBS dose by 6.78 × 1014 at./cm2, which corresponds to 1 ML of pure Ge. The deviation for both the dose andthe equivalent monolayer thickness is reported as: Δσ = ± 0.1 × 1014 at./cm2, ΔθGe = ± 0.02 MLs.
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