Alain Estève

Nanopatterning Si(111) surfaces as a selective surface-chemistry route

Using wet-chemical self-assembly, we demonstrate that standard surface reactions can be markedly altered. Although HF etching of Si surfaces is known to produce H-terminated surfaces, we show that up to approximately 30% of a monolayer of stable Si-F bonds can be formed on atomically smooth Si(111) surfaces on HF reaction, when chemically isolated Si atoms are the target of the reaction. Similarly, approximately 30% Si-OH termination can be achieved by immersion of the partially covered F-Si(111) surface in water without oxidation of the underlying Si substrate. Such reactions are possible when H-terminated (111)-oriented Si surfaces are initially uniformly patterned with methoxy groups. These findings are contrary to the knowledge built over the past twenty years and highlight the importance of steric interactions at surfaces and the possibility to stabilize products at surfaces that cannot be obtained on chemically homogeneous surfaces.

Multilayered Al/CuO thermite formation by reactive magnetron sputtering: Nano versus micro

Multilayered Al/CuO thermite was deposited by a dc reactive magnetron sputtering method. Pure Al and Cu targets were used in argon–oxygen gas mixture plasma and with an oxygen partial pressure of 0.13 Pa. The process was designed to produce low stress (<50 MPa) multilayered nanoenergetic material, each layer being in the range of tens nanometer to one micron. The reaction temperature and heat of reaction were measured using differential scanning calorimetry and thermal analysis to compare nanostructured layered materials to microstructured materials. For the nanostructured multilayers, all the energy is released before the Al melting point. In the case of the microstructured samples at least 2/3 of the energy is released at higher temperatures, between 1036 and 1356 K.

Density functional theory study of HfCl4, ZrCl4, and Al(CH3)3 decomposition on hydroxylated SiO2: Initial stage of high-k atomic layer deposition

The initial stage of atomic layer deposition of HfO2, ZrO2, and Al2O3 high-k films, i.e., the decomposition of HfCl4, ZrCl4, and Al(CH3)3 precursor molecules on an OH-terminated SiO2 surface, is investigated within density functional theory. The energy barriers are determined using artificial activation of vibrational normal modes. For all precursors, reaction proceeds through the formation of intermediate complexes that have equivalent formation energies (∼−0.45 eV), and results in HCl and CH4 formation with activation energies of 0.88, 0.91, and 1.04 eV for Hf, Zr, and Al based precursors, respectively. The reaction product of Al(CH3)3 decomposition is found to be more stable (by −1.45 eV) than the chemisorbed intermediate complex compared to the endothermic decomposition of HfCl4 and ZrCl4 chemisorbed precursors (0.26 and 0.29 eV, respectively).

Interfacial Chemistry in Al/CuO Reactive Nanomaterial and Its Role in Exothermic Reaction

Interface layers between reactive and energetic materials in nanolaminates or nanoenergetic materials are believed to play a crucial role in the properties of nanoenergetic systems. Typically, in the case of Metastable Interstitial Composite nanolaminates, the interface layer between the metal and oxide controls the onset reaction temperature, reaction kinetics, and stability at low temperature. So far, the formation of these interfacial layers is not well understood for lack of in situ characterization, leading to a poor control of important properties. We have combined in situ infrared spectroscopy and ex situ X-ray photoelectron spectroscopy, differential scanning calorimetry, and high resolution transmission electron microscopy, in conjunction with first-principles calculations to identify the stable configurations that can occur at the interface and determine the kinetic barriers for their formation. We find that (i) an interface layer formed during physical deposition of aluminum is composed of a mixture of Cu, O, and Al through Al penetration into CuO and constitutes a poor diffusion barrier (i.e., with spurious exothermic reactions at lower temperature), and in contrast, (ii) atomic layer deposition (ALD) of alumina layers using trimethylaluminum (TMA) produces a conformal coating that effectively prevents Al diffusion even for ultrathin layer thicknesses (∼0.5 nm), resulting in better stability at low temperature and reduced reactivity. Importantly, the initial reaction of TMA with CuO leads to the extraction of oxygen from CuO to form an amorphous interfacial layer that is an important component for superior protection properties of the interface and is responsible for the high system stability. Thus, while Al e-beam evaporation and ALD growth of an alumina layer on CuO both lead to CuO reduction, the mechanism for oxygen removal is different, directly affecting the resistance to Al diffusion. This work reveals that it is the nature of the monolayer interface between CuO and alumina/Al rather than the thickness of the alumina layer that controls the kinetics of Al diffusion, underscoring the importance of the chemical bonding at the interface in these energetic materials.

Enhancing the Reactivity of Al/CuO Nanolaminates by Cu Incorporation at the Interfaces

In situ deposition of a thin (∼5 nm) layer of copper between Al and CuO layers is shown to increase the overall nanolaminate material reactivity. A combination of transmission electron microscopy imaging, in situ infrared spectroscopy, low energy ion scattering measurements, and first-principles calculations reveals that copper spontaneously diffuses into aluminum layers (substantially less in CuO layers). The formation of an interfacial Al:Cu alloy with melting temperature lower than pure Al metal is responsible for the enhanced reactivity, opening a route to controlling the stochiometry of the aluminum layer and increasing the reactivity of the nanoenergetic multilayer systems in general.

Multiscale Modeling of the Atomic Layer Deposition of HfO2 Thin Film Grown on Silicon: How to Deal with a Kinetic Monte Carlo Procedure.

An original integrated approach developed within a multiscale strategy, which combines first-principles quantum simulations and kinetic Monte Carlo (KMC), is presented to investigate the atomic layer deposition (ALD) of HfO2 on Si(100) surface. Density functional theory within the hybrid functional is used to determine the detailed physicochemical mechanisms and associated energetics of the two half cycles taking place during the initial stage of film growth. A kinetic Monte Carlo model is then proposed that deals with the stochastic nature of the calculated DFT mechanisms and barriers. Beyond the chemical information emanating from DFT calculations, the lattice-based KMC approach requires preliminary physical considerations issued from the crystal structures that the system is intended to adopt. This is especially critical in the case of heterogeneous systems like oxides deposited on silicon. We also describe (i) how atomistic configuration changes are performed as a result of local events consisting in elementary reaction mechanisms occurring on specific lattice sites, (ii) the temporal dynamics, governed by transition probabilities, calculated for every event from DFT activation barriers, and (iii) the relation of KMC with the ALD experimental procedure. Some preliminary validation results of the whole multiscale strategy are given for illustration and pertinence with regard of the technological main issues.

Difficulty for oxygen to incorporate into the silicon network during initial O2 oxidation of Si(100)-(2×1)

First principles calculations and scanning tunneling microscopy studies of the oxidation of Si(100)-(2x1) surfaces by molecular oxygen reveal that the surface silanone (O)(Si=O) species is remarkably stable, constituting the key intermediate for initial oxidation. The propensity for oxygen to remain within the top surface layer as opposed to incorporating within Si-Si backbonds is surprisingly high. This resistance to incorporation into a cubic lattice even at higher coverages could be a factor to facilitate surface amorphization in subsequent steps.

On the early stage of aluminum oxidation: An extraction mechanism via oxygen cooperation

We propose a barrierless mechanism for describing the oxidation of Al(111) in which oxygen atoms located on the outer surface extract aluminum atoms of the surface layers through local cooperation of other pre-adsorbed oxygen atoms. We show the details of this complex chemical process that kinetically competes with the non-destructive formation of an oxygen monolayer onto the Al surface, thus elucidating the initial aluminum oxidation regime. We demonstrate that further stripping of the complete surface Al layer is consistent with both (i) the formation of a defective alumina structure and (ii) an oxide capping layer preventing further oxidation at low temperature.

Nucleation and growth of atomic layer deposition of HfO2 gate dielectric layers on silicon oxide: a multiscale modelling investigation

We apply our recently developed approach, combining advanced ab initio density functional theory (DFT) methods with a probabilistic kinetic Monte Carlo (KMC) scheme, to quantify the properties of mesoscopic size systems operating in real experimental conditions. The application concerns the investigation of the atomic layer deposition (ALD) of HfO2 film growth on Si(100) surface. We show that the proposed models offer guidance in the optimization of the experimental deposition processes, in terms of OH density on the substrate, optimal growth temperature, pulse durations, and finally growth kinetics.

A computational chemist approach to gas sensors: Modeling the response of SnO2 to CO, O2, and H2O Gases

A general bottom‐up modeling strategy for gas sensor response to CO, O2, H2O, and related mixtures exposure is demonstrated. In a first stage, we present first principles calculations that aimed at giving an unprecedented review of basic chemical mechanisms taking place at the sensor surface. Then, simulations of an operating gas sensor are performed via a mesoscopic model derived from calculated density functional theory data into a set of differential equations. Significant presence of catalytic oxidation reaction is highlighted.