Quantitative determination of Ge profiles across SiGe wetting layers on Si (001)

The commonly accepted Stranski-Krastanow model, according to which island formation occurs on top of a wetting layer ͑WL͒ of a certain thickness, predicts for the morphological evolution an increasing island aspect ratio with volume. We report on an apparent violation of this thermodynamic understanding of island growth with deposition. In order to investigate the actual onset of three-dimensional islanding and the critical WL thickness in the Ge/Si͑001͒ system, a key issue is controlling the Ge deposition with extremely high resolution ͓0.025 monolayer ͑ML͔͒. Atomic force microscopy and photoluminescence measurements on samples covering the deposition range 1.75-6.1 ML, taken along a Ge deposition gradient on 4 in. Si substrates and at different growth temperatures ͑T g ͒, surprisingly reveal that for T g Ͼ 675°C steeper multifaceted domes apparently nucleate prior to shallow ͕105͖-faceted pyramids, in a narrow commonly overlooked deposition range. The puzzling experimental findings are explained by a quantitative modeling of the total energy with deposition. We accurately matched ab initio calculations of layer and surface energies to finite-element method simulations of the elastic energy in islands, in order to compare the thermodynamic stability of different island shapes with respect to an increasing WL thickness. Close agreement between modeling and experiments is found, pointing out that the sizeable progressive lowering of the surface energy in the first few MLs of the WL reverts the common understanding of the SK growth onset. Strong similarities between islanding in SiGe and III/V systems are highlighted.

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“…21 In Fig. 3͑c͒ the shift of the no-phonon ͑NP͒ WL transition energy is plotted versus ⌰ Ge for T g = 625 and 700°C.…”

Section: Experiments: Growth Photoluminescence and Afmmentioning

confidence: 99%

“…3͑b͔͒ was used in Ref. 21 to derive quantitative Ge profiles in the WL, which resulted in a maximum Ge content of Ϸ85%. When the domes appear ͑4.25 ML at T g = 700°C and 4.92 ML at T g = 625°C͒ we see an abrupt blueshift of the WL PL signal by 0.06 eV ͑Figs.…”

Section: Experiments: Growth Photoluminescence and Afmmentioning

confidence: 99%

Key role of the wetting layer in revealing the hidden path of Ge/Si(001) Stranski-Krastanow growth onset

et al. 2009

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“…SAM analysis 15 shows that the Ge content of the over-layer is approximately constant within the nucleation region (x < 20 lm) and its average value decreases from $0.8 at 600 C to $0.64 at 700 C. This temperature dependence reflects the thermally activated nature of the Si incorporation. When our results are compared to typical MBE data, where Ge concentration values of the wetting layer greater than 0.86 have been found even at 700 C, 26 it is evident that the thermal diffusion from a local source results in an enhanced intermixing within the OL, whose origin will be discussed in details later.…”

Section: Resultsmentioning

confidence: 72%

“…In MBE, a Ge-rich wetting layer (c WL Ge > 0:86) forms even at high temperatures (T % 700 C). 26 When the WL thickness exceeds its critical value, faceted Ge-rich islands nucleate and grow at the expense of smaller ones via a coarsening process. 38 islands, digging deeply into the substrate.…”

Section: Resultsmentioning

confidence: 99%

Delayed plastic relaxation limit in SiGe islands grown by Ge diffusion from a local source

Vanacore

Nicotra

Zani

et al. 2015

Journal of Applied Physics

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The hetero-epitaxial strain relaxation in nano-scale systems plays a fundamental role in shaping their properties. Here, the elastic and plastic relaxation of self-assembled SiGe islands grown by surface-thermal-diffusion from a local Ge solid source on Si(100) are studied by atomic force and transmission electron microscopies, enabling the simultaneous investigation of the strain relaxation in different dynamical regimes. Islands grown by this technique remain dislocation-free and preserve a structural coherence with the substrate for a base width as large as 350 nm. The results indicate that a delay of the plastic relaxation is promoted by an enhanced Si-Ge intermixing, induced by the surface-thermal-diffusion, which takes place already in the SiGe overlayer before the formation of a critical nucleus. The local entropy of mixing dominates, leading the system toward a thermodynamic equilibrium, where non-dislocated, shallow islands with a low residual stress are energetically stable. These findings elucidate the role of the interface dynamics in modulating the lattice distortion at the nano-scale, and highlight the potential use of our growth strategy to create composition and strain-controlled nano-structures for new-generation devices

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“…5,[10][11][12][13][14][15][16][17][18][19][20][21][22] Atomic-scale modeling of Ge growth on Si(001) suggested that entropy limits the wetting tendency of Ge, thus smearing the interface. 18 High-resolution Rutherford backscattering spectrometry (HR-RBS) studies demonstrated that intermixing extends as deep as four layers upon thermal annealing.…”

Section: Introductionmentioning

confidence: 99%

Dynamic probe of atom exchange during monolayer growth

et al. 2012

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In heteroepitaxy, impinging beam atoms can either wet the surface or swap with substrate atoms. Herein, we present a dynamic study of these phenomena throughout the assembly of the first atomic layer of Ge on Si(111).In situ spectromicroscopic analysis demonstrates that, at a sufficiently high temperature, atom exchange is more significant at the early stages of growth and attenuates as deposition proceeds. Our result highlights the role of propagating monolayer edges in the entropy-driven atom swapping and demonstrates that substitution of Si by Ge is a low-energy pathway to incorporate Ge in the growing monolayer. These observations are confirmed by molecular dynamic simulations.

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Quantitative determination of Ge profiles across SiGe wetting layers on Si (001)

Cited by 29 publications

References 19 publications

Key role of the wetting layer in revealing the hidden path of Ge/Si(001) Stranski-Krastanow growth onset

Key role of the wetting layer in revealing the hidden path of Ge/Si(001) Stranski-Krastanow growth onset

Delayed plastic relaxation limit in SiGe islands grown by Ge diffusion from a local source

Dynamic probe of atom exchange during monolayer growth

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