Deposition (phase transition) | EPFL Graph Search

Deposition is the phase transition in which gas transforms into solid without passing through the liquid phase. Deposition is a thermodynamic process. The reverse of deposition is sublimation and hence sometimes deposition is called desublimation. Applications Examples One example of deposition is the process by which, in sub-freezing air, water vapour changes directly to ice without first becoming a liquid. This is how frost and hoar frost form on the ground or other surfaces. Another example is when frost forms on a leaf. For deposition to occur, thermal energy must be removed from a gas. When the air becomes cold enough, water vapour in the air surrounding the leaf loses enough thermal energy to change into a solid. Even though the air temperature may be below the dew point, the water vapour may not be able to condense spontaneously if there is no way to remove the latent heat. When the leaf is introduced, the supercooled water vapour immediately begins to

Nonequilibrium quasistationary spin disordered state in ?-RuCl3

Zhe Wang

We photoexcite high-energy holon-doublon pairs as a way to alter the magnetic free energy landscape and resulting phase diagram of the frustrated honeycomb magnet ??-RuCl3. The pair recombination through multimagnon emission is tracked through the time evolution of the magnetooptical response originating from ??-RuCl3???s competing zigzag spin-ordered ground state. A small holon-doublon density suffices to reach a spin-disordered state. The phase transition is described within a dynamical Ginzburg-Landau framework, corroborating the quasistationary nature of the transient phase. Our work suggests a new route to reach a nontrivial spin-disordered state in Kitaev-like magnets.

AMER PHYSICAL SOC2022

Etude par STM de la déposition d'agrégats d'or sur TiO2 et mesure de l'émission électronique secondaire induite par l'impact d'agrégats sur une surface

Pierre Convers

This thesis work focuses on impact and diffusion processes as well as equilibrium positions of a metal deposited on a surface. The metal is deposited in the form of clusters containing n atoms in a controlled way. Gold or silver clusters cations (Au+n and Ag+n ) are used. Their size (n ∈ [1, 9]), charge state and deposition energy (E = 10 - 4500 eV) are well defined. The surface being at room temperature during the deposition, can be annealed and transferred in a scanning tunnelling microscope (STM) after deposition. The variable temperature microscope was developed during this thesis. The approach of the tip is carried out by an axial motor, which renders the microscope very stable. The first part of this work treats of the evolution of gold deposited on a TiO2(110) surface. Position and size of gold islands created by cluster or atomic depositions are determined by STM. Compared to atomic deposition at the same energy (10 < E < 100 eV), the cluster deposition produces smaller islands with a large islands density after annealing at 800 K. Regardless the deposition conditions, gold atoms diffuse on the surface already at 300 K. The impact energy is the key parameter to reduce the diffusion by cluster or atomic implantation or defect creation. Upon a high energy deposition, part of the deposited gold is invisible to the STM. The fate of these gold atoms is unclear : they are either buried under the surface or ejected in vacuum at the impact. The control of the islands size should permit the creation of a stable system with an optimum catalytic activity (i. e. CO combustion). Research groups have shown that this activity is strongly influenced by the islands size. It is equal to zero if the diameter is greater than 5 nm. The second part of this work is devoted to the electron emission γ induced by the impact of silver clusters Ag+n on a Pt and HOPG surface. Measurements are made by varying the impact energy (and so the speed v) of size-selected clusters on a defined surface. Impact velocity ranges from 104 to 105 m/s, which is lower than the classical threshold. Nevertheless every cluster size produces an electron emission. A potential emission (γ(v=0) ≠ 0) is observed for the monomer on both surfaces, which is caused by excited ions in a metastable state. The γ(v) curves are increasing, and no mesurable oscillation are observed. This does not confirm earlier work by Meiwes-Broer et al. These authors found oscillations in γ on similar systems relating them to the electronic structure of the clusters and the surface. A model based on heating of the electron gas is developed. This model gives good agreements with the γ(v) curves. The electronic temperature is estimated to 3000-8000 K. Similar behavior on both surfaces (Pt and HOPG) depending of the cluster size is shown. This behavior is probably related to the geometrical structure of the clusters. Finally, a more pronounced molecular effect is observed for the HOPG : the number of electrons emitted by the impact of a Ag+n is up to 7 times higher than the number of electrons emitted by impacts of n independent atoms. This value is smaller than 2 for the Pt surface.

EPFL2005

Design, characterisation and modelling of a high current DC arc plasma source for silicon and silicon carbide processing at low pressure

Lukas Derendinger

In the frame of this thesis, two similar high current DC arc (HCDCA) plasma sources were investigated in a low gas pressure regime (10-3-10-2 mbar). One of them was initially designed for the epitaxial growth of silicon and silicon germanium (LEP), the other for the industrial deposition of diamond (BAI). The LEP source was analysed using pure argon plasmas. Measurements of the ion saturation current were performed with a custom-built multi-Langmuir probe to analyse plasma density homogeneity. Plasma instabilities at 50 Hz were observed and studied by different means. One source of the instability is the AC current used for the filament heating, whereas lower frequency instabilities are due to the use of a ring shaped anode. A sensitive Hall sensor was used to measure the magnetic field induced by the discharge current. It was found that depending on the plasma parameters gas pressure and external magnetic field the current tends to attach at different points on the anode, leading to a complete loss of reactor symmetry. The installation of an additional cusp field around the reactor chamber lead to an increase of the overall homogeneity of the plasma density, but it could not resolve the problems of current attachment. In the following the ring anode was replaced by a point anode and the maximum external magnetic field strength was increased by a factor of ten. With a novel multi-Hall probe the current density of the now columnar shaped plasma was measured, showing a strongly peaked current density profile, when an external magnetic field is applied. Together with ion saturation current measurements made with a Langmuir probe, the electron temperature inside the plasma column was estimated to be about 4 eV. In earlier works made on the BAI reactor the high dissociation efficiency of the HCDCA plasma source has already been shown. Optical emission spectra were compared between RF plasmas, low pressure (1.5mbar) HCDCA plasmas and our very low pressure (10-3-10-2 mbar) HCDCA plasmas. The dominating species found in RF plasma spectra are molecular, the spectra of low pressure HCDCA plasmas are dominated by atomic species and the spectra of very low pressure HCDCA plasmas are dominated by ions, emphasising the high dissociation efficiency of this system. Silane and Methane was used as precursor gases in the BAI reactor for the deposition of silicon carbide films. Promising high deposition rates up to 9 nm/s were found, but FTIR spectroscopy showed high hydrogen and oxygen concentrations in the porous films, making the not optimised deposited material useless for the application of wear-resistant coatings. Microcrystalline hydrogenated silicon (µc-Si:H) is viewed as a cost- and energy-effective alternative to crystalline silicon for the production of solar cells. A detailed analysis of the deposition rate and the Raman crystallinity of µc-Si:H films deposited in the BAI reactor showed deposition rates up to 6.5 nm/s and a wide range of crystallinity from 0-80%. Film thickness inhomogeneity in silicon solar cells has to be less than 5%. To meet this standard a first substantial improvement was achieved with the installation of a linear gas injection along the plasma column. Tests on large surface glasses (47 × 37 cm2) revealed strong diffusive effects, which could be reproduced with a simple gas diffusion model. The model showed the necessity to reduce the dead volume around the plasma and to set the substrates as close as possible to the plasma column in order to minimise film thickness inhomogeneity due to diffusion. Deposition rate measurements made in these conditions assured the results of the model. The development of a method to estimate the dissociation efficiency of the plasma by simple pressure measurements showed also an important increase of the silane dissociation from 75-92% when the plasma is confined in a smaller volume. Therefore, compared to the reactor with a large dead volume, only a third of the initial silane is lost to the pumps.

EPFL2008