F. Leroy

Dynamics of solid thin-film dewetting in the silicon-on-insulator system

Using low-energy electron microscopy movies, we have measured the dewetting dynamics of single-crystal Si(001) thin films on SiO2 substrates. During annealing (T>700u2009°C), voids open in the Si, exposing the oxide. The voids grow, evolving Si fingers that subsequently break apart into self-organized three-dimensional (3D) Si nanocrystals. A kinetic Monte Carlo model incorporating surface and interfacial free energies reproduces all the salient features of the morphological evolution. The dewetting dynamics is described using an analytic surface-diffusion-based model. We demonstrate quantitatively that Si dewetting from SiO2 is mediated by surface-diffusion driven by surface free-energy minimization.

Thermal instability of silicon-on-insulator thin films measured by low-energy electron microscopy

Using low-energy electron microscopy (LEEM), we investigate the ultrahigh vacuum annealing of silicon-on-insulator (SOI) samples capped by a chemically-prepared oxide layer. Consistent with previous reports: (1) for T > 750 • C, the capping-oxide decomposes by void nucleation and growth, then (2) for T > 850 • C, the Si thin-film dewets from the SiO2 substrate. Here, we show that the morphological evolution of the surface during the dewetting process is dependent on the preparation of the SOI surface. Two dewetting pathways are evident in recent literature, we find that one evolution is characteristic of clean Si(100)-2 × 1 surfaces, while the other is correlated with surface contamination. Silicon thin-films, capped by ultra-thin oxide layers, are the basic building blocks of microelectronics. Microelectronic device fabrication requires thermal annealing steps, which may induce drastic morphological changes in these building blocks. Here, we explore the annealing behavior of oxide-capped silicon-on-insulator (SOI) films. During annealing at T > 750 • C, the oxide capping layer decomposes (Fig. 1(a)) by void nucleation and growth. Then at T > 850 • C, the Si thin film spontaneously dewets (Fig. 1(b)), forming an assembly of three dimensional Si nanocrystals. Previous works [1, 2, 3, 4, 5] have explored the thermal decomposition of ultrathin Si-oxide, and the subsequent dewetting of the Si (SOI) layer [6, 7]. Here, we report two results which, to our knowledge, have not been reported previously. During thermal decomposition of the capping oxide layer, the radii of the initial isolated voids obeys a r ∝ t 1/3 law. This exponent suggests that void growth is governed by diffusion of the decomposition product (SiO molecules) on the oxide outside the voids. The decomposition of the oxide exposes a 2 × 1 reconstructed surface, characteristic of clean Si(100), ideal for studying the mechanisms of the SOI dewetting process. The dewetting proceeds by the opening of square, crystallographically oriented holes, followed by a finger instability that leads to the formation of self-organized,

Vicinal silicon surfaces: From step density wave to faceting

This paper investigates faceting mechanisms induced by electromigration in the regime where atomic steps are transparent. For this purpose we study several vicinal orientations by means of in-situ (optical diffraction, electronic microscopy) as well as ex-situ (AFM, microprofilometry) visualization techniques. The data show that faceting proceeds in two stages. The first stage is short and leads to the appearance of a step density wave, with a wavelength roughly independent of the surface orientation. The second stage is much slower, and leads to the formation of a hill-and-valley structure, the period of which depends on the initial surface orientation. A simple continuum model enables us to point out why the wavelength of the step density wave does not depend on the microscale details of the surface. The final wavelength is controlled by the competition between elastic step-step interaction and facet edge energy cost. Finally, the surface stress angular dependence is shown to emerge as a coarsed-grained picture from the step model.

Agglomeration dynamics of germanium islands on a silicon oxide substrate: A grazing incidence small-angle x-ray scattering study

Grazing-incidence small-angle X-ray scattering (GISAXS) and grazing-incidence X-ray diffraction techniques are used to characterise the thermally induced solid-state dewetting of Ge(001) thin films leading to the formation of 3D Ge islands. A quantitative analysis based on the Kolmogorov-Johnson-Mehl-Avrami model is derived. The main physical parameters controlling the dewetting (activation energy and kinetic pre-factors) are determined. Assuming that the dewetting is driven by surface/interface minimisation and limited by surface diffusion, the Ge surface self-diffusion reads as Ds,0c0u2009e−Ea/(kBT)u2009∼3×1018u2009e−2.6±0.3u2009eV/(kBT)u2009nm2/s. GISAXS technique enables to reconstruct the mean Ge-island shape, including facets.

X-Ray Diffraction and Raman Spectroscopy Study of Strain in Graphene Films Grown on 6H-SiC(0001) Using Propane-Hydrogen-Argon CVD

We have grown graphene films on 6H-SiC(0001) using propane CVD and evidenced the strong impact of the hydrogen/argon mixture used as the carrier gas on the graphene/SiC interface and on the orientation of graphene layers. By studying a set of samples grown with different hydrogen/argon mixture using Raman spectroscopy and grazing incidence X-ray diffraction, we evidence the links between graphene/SiC interface and strain in graphene.

Simple views on surface stress and surface energy concepts

Some aspects of the thermodynamics and mechanics of solid surfaces, in particular with respect to surface stress and surface energy, are reviewed. The purpose is to enlighten the deep differences between these two physical quantities. We consider successively the case of atomic flat surfaces and the case of vicinal surfaces characterized by surface stress discontinuities. Finally, experimental examples, concerning Si surfaces, are described.

Dewetting of patterned solid films: Towards a predictive modelling approach

Owing to its ability to produce an assembly of nanoislands with controllable size and locations, the solid state dewetting of patterned films has recently received great attention. A simple Kinetic Monte Carlo model based on two reduced energetic parameters allows one to reproduce experimental observations of the dewetting morphological evolution of patterned films of Si(001) on SiO2 (or SOI for Silicon-on-Insulator) with various pattern designs. Thus, it is now possible to use KMC to drive further experiments and to optimize the pattern shapes to reach a desired dewetted structure. Comparisons between KMC simulations and dewetting experiments, at least for wire-shaped patterns, show that the prevailing dewetting mechanism depends on the wire width.

Shape transition in nano-pits after solid-phase etching of SiO2 by Si islands

We study the nano-pits formed during the etching of a SiO2 film by reactive Si islands at T≈1000u2009°C. Combining low energy electron microscopy, atomic force microscopy, kinetic Monte Carlo simulations, and an analytic model based on reaction and diffusion at the solid interface, we show that the shape of the nanopits depend on the ratio R/xs with R the Si island radius and xs the oxygen diffusion-length at the Si/SiO2 interface. For small R/xs, nanopits exhibit a single-well V-shape, while a double-well W-shape is found for larger R/xs. The analysis of the transition reveals that xs∼60 nm at T≈1000u2009°C.

Oxygen-induced inhibition of silicon-on-insulator dewetting

We report that solid state dewetting of Si thin film on SiO2 can be reversibly inhibited by exposing the Si surface to a partial pressure of dioxygen (∼10−7Torr) at high temperature (∼1100K). Coupling in situ Low-Energy Electron Microscopy and ex situ atomic force microscopy we propose that the pinning of the contact line induced by the presence of small amounts of silicon oxide is the main physical process that inhibits the dewetting.

Surface-dependent scenarios for dissolution-driven motion of growing droplets

Nano-droplets on a foreign substrate have received increasing attention because of their technological possible applications, for instance to catalyse the growth of nanowires. In some cases the droplets can move as a result of a reaction with the substrate. In this work we show that the substrate orientation, the surface morphology and the shape of the pits etched in the substrate by the droplets affect the droplet motion, so that a single mechanism (droplet-induced substrate dissolution) may lead to several unexpected droplet dynamics. The experiments are carried out by low energy electron microscopy on Au-Si and Au-Ge, which are model systems for studying liquid droplet alloys. Studying in-situ the behaviour of Au droplets on various Si and Ge surfaces, we describe a subtle interplay between the substrate orientation, the surface defects, and the droplet motion. Our observations allow a deep understanding of the interfacial mechanisms at the origin of the alloy formation and the associated droplet motion. These mechanisms are based on events of substrate dissolution/recrystallization. The outcomes of this work highlight the importance of the etching anisotropy on the droplet-substrate behaviours, and are essential in the perspective of positioning liquid alloy droplets used for instance as nanowire catalysts.