2,025 research outputs found
Thermal instability of silicon-on-insulator thin films measured by low-energy electron microscopy
No full textInternational audienceUsing 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
Spatial inhomogeneity and temporal dynamics of a 2D electron gas in interaction with a 2D adatom gas
No full textInternational audienceFundamental interest for 2D electron gas (2DEG) systems has been recently renewed with the advent of 2D materials and their potential high-impact applications in optoelectronics. Here, we investigate a 2DEG created by the electron transfer from a Ag adatom gas deposited on a Si(111) 3 × 3-Ag surface to an electronic surface state. Using low-energy electron microscopy (LEEM), we measure the Ag adatom gas concentration and the 2DEG-induced charge transfer. We demonstrate a linear dependence of the surface work function change on the Ag adatom gas concentration. A breakdown of the linear relationship is induced by the occurrence of the Ag adatom gas superstructure identified as Si(111) 21 × 21-Ag only observed below room temperature. We evidence below room temperature a confinement of the 2DEG on atomic terraces characterised by spatial inhomogeneities of the 2DEG-induced charge transfer along with temporal fluctuations. These variations mirror the Ag adatom gas concentration changes induced by the growth of 3D Ag islands and the occurrence of an Ehrlich-Schwoebel diffusion barrier of 155 ± 10 meV
Interplay between deoxidation and dewetting for ultrathin SOI films
No full textInternational audienceSince a few years there have been many efforts for studying solid state dewetting that means the spontaneous agglomeration of a metastable film into an assembly of 3D crystallites [1–3]. Since the size of the so-formed 3D crystallites depends on the initial thickness of the film [3–6], solid state dewetting of ultra-thin films is a promising method for producing nanodots [7, 8]. Many studies thus focused on the effect of the film thickness on the dewetting morphologies. Generally thicker films dewet by void nucleation and opening process forming then well-organized 3D islands [1–3]. In contrast to this thinner films tend to form more complex structures [9–14]. For instance Kinetic Monte Carlo simulations have shown that a 3 ML-thick film dewets by void nucleation whereas a 1 ML-thick film dewets by forming a labyrinthine pattern of bilayer islands [10, 11]. In the literature the specific behavior of the thinner films has been attributed to barrierless void nucleation [15], spinodal dewetting (enhanced sensitivity to thickness or thermal fluctuations) [16], short range effect of the wetting potential [17, 18] or local stresses [14, 19]. Recently we investigated dewetting properties of Silicon-on-Insulator (SOI) samples capped by a chemically-prepared oxide layer. We thus reported that films thicker than 8 nm dewet in four steps and break down into self-organized 3D crystals whereas films thinner than 6 nm dewet by labyrinthine formation [9]. In this paper we show that this thin film effect is actually due to a subtle interplay between film deoxidation and film dewetting. Indeed the temperature at which a film dewets (T dew (h)) depends on its thickness h whereas the deoxidation temperature (T deox) is a constant. We thus show that if T dew (h) > T deox SOI dewetting occurs by void opening and leads to 3D crystals whereas if T dew (h) < T deox SOI dewetting leads to labyrinthine morphologies. In other words, the dewetting of the thicker films does not depend on the oxide cap (that decomposes before dewetting and thus only depends on the surface energy gain due to the disappearance of the Si surface with respect to the uncovered bare substrate) while the dewetting of the thinner films depends on the deoxidation (since only the clean deoxidized parts of the silicon film can dewet). We use LEEM to investigate in-situ and in real time the decomposition of the oxide as well as the dewetting of the SOI films. We define the deoxidation temperature T deox as the temperature at which LEEM enables to see void opening in the oxide cap layer with subsequent silicon film appearance. In a similar manner, the dewet-ting temperature T dew is defined as the temperature at which the amorphous SiO 2 substrate starts becoming visible by retraction of the silicon film. Obviously both temperatures T deox and T dew do not correspond to phase transitions and thus depend on the observation means. However since measured in the same experimental conditions they can be compared without any ambiguity. The SOI samples used in the experiments (12 nm Si on top of 25 nm SiO 2) were prepared as described elsewhere [20]. By means of repeated etching and oxidation cycles a stepwise thinning of the samples is possible to produce films of a certain thickness with a very high accuracy ending with a final oxidation step. Since this protective oxide layer is produced with the same wet chemical oxidation for each Si film thickness the thickness of the oxide is the same for all samples. Ellipsometry measurements have proven that the oxide thickness is constant with approximately 1 nm. FIG. 1. Temperature at which the initial deoxidation (black) and dewetting (red) was observed depending on the Si film thickness. Dewetting velocities are in the range of 1 to 8 nm s. The data point at 21.6 nm was obtained using a SOI sample of 22 nm Si on top of 150 nm SiO2. In Figure 1 are thus reported T deox and T dew as a function of the Silicon film thickness. As expected, the decomposition temperature of the capping oxide does not depend on the thickness of the silicon film and is rather constant at T deox =760 • C whereas T dew decreases as h n with n = 0.08 ± 0.01 comparable to [21, 22] where exponents n = 0.15 ± 0.01 have been reported. In the following we will report dewetting morphologie
Self propelled motion of Au-Si droplets on Si(111) mediated by monoatomic step dissolution
No full textInfluence of facets on solid state dewetting mechanisms: Comparison between Ge and Si on SiO2
No full textSurface diffusion of Au on root 3 x root 3 Si(111)-Au studied by nucleation-rate and Ostwald-ripening analysis
No full textInternational audienceThe surface diffusion energy of Au on the root 3 x root 3 Au-Si(111) reconstructed surface is determined from low energy electron microscopy experiments. We have used two methods, one based on the nucleation of Au particles, the other one on the island growth by Ostwald ripening. The two methods give E-d = 1.10 +/- 0.34 eV and E-d = 1.56 +/- 0.31 eV respectively and with a weighted mean we obtain E-d = 1.3 +/- 0.2 eV. We suggest that this high activation energy could be due to a mechanism of diffusion by exchange. (C) 2015 Elsevier B.V. All rights reserved
Reflections on the effect of an external flux in surface physics
No full textInternational audienceSublimating surfaces are out of equilibrium. It has been proposed that sublimation can be compensated by an impinging atomic ux to obtain equilibrium. This work concerns the eect of such an impinging ux on the stability of surfaces in various situations. For this purpose we combine Kinetic Monte Carlo Simulations with analytical developements based on the Burton-Cabrera-Frank (BCF) classical theory. We show that a perfect compensation of the sublimation is possible for vicinal surfaces but not when 2D islands are present on a surface. We thus study the eect of an impinging ux on the dynamic of a 2D island on a surface. We show that the 2D island area generally varies with time t as −t α. In absence of any impinging ux the value of the exponent α enables to identify the main mechanism at work (diusion limited or attachment-detachment limited). On the contrary, in presence of an impinging ux the value of the exponent α is not enough to identify the main mechanism limiting the area change
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