Growth Of Amorphous Thin Films Research Articles

A hybrid off-lattice kinetic Monte Carlo/molecular dynamics method for amorphous thin film growth

The ability to understand and model the growth of amorphous thin films on solid surfaces is essential to a wide range of industrial applications, from the deposition of wear-resistant coatings to the production of solar cells. Here, a three-dimensional (3D) hybrid off-lattice kinetic Monte Carlo/molecular dynamics (kMC/MD) algorithm is developed to study the growth of thin amorphous films on solid substrates with atomistic resolution over timescales of tens of seconds. We use this method to study the growth of polyphosphate films from tricresyl phosphate (TCP) molecules on an iron substrate. Molecular adsorption/desorption, bond breaking/formation processes, and diffusion of iron ions through the film are simulated in the kMC stage and the film is relaxed during the MD stage. The kMC/MD method is approximately eleven orders of magnitude faster than equivalent reactive force field (ReaxFF) MD simulations. The simulated film growth rate and topology agree well with experimental results and the chemical structure of the film is consistent with previous molecular dynamics simulations of iron polyphosphates. The newly-developed hybrid kMC/MD methodology can be adapted to yield important insights into thin film growth for several other potential applications.

Dynamical properties of a nonlinear Kuramoto–Sivashinsky growth equation

The conserved Kuramoto–Sivashinsky equation can be considered as the one- and two-dimensional evolution equation for amorphous thin film growth. The role of the nonlinear term Δ(∇u2) and the properties of the solutions are investigated analytically and numerically. Analytical results of wavelength and amplitude are provided. Numerical simulations of this equation are presented, showing the roughening and coarsening of the surface pattern and the evolution of the surface morphology over time for different parameter values in one- and two-dimensions.

Atomistic model of ultra-smooth amorphous thin film growth by low-energy ion-assisted physical vapour deposition

The growth of ultra-smooth amorphous thin films induced by low-energy (below 1 keV) ion-assistance processes is studied. The relative contribution of ion-induced smoothening effects is analysed by means of a Monte Carlo model and experimental data. In general, highly rough granular or ultra-smooth (with roughness below one monolayer) films are produced depending on the competition between surface shadowing and ion-induced adatom mobility and sputtering. The ultra-smooth growth regime is experimentally and theoretically consistent with the Edwards–Wilkinson growth mode, which is related to the ion-induced enhancement of surface mobility. Overall, the framework and the fundamentals to analyse this type of growth are developed and discussed.

Onset of shadowing-dominated growth in glancing angle deposition

We demonstrate that shadowing instabilities can dramatically alter the very early stages of growth of amorphous thin films on nominally smooth surfaces. These observations are made by comparing the porosity and morphological evolutions of thin films grown under conditions of normal and glancing incidences of the vapor flux. At conditions of normal incidence, we see evidence of nucleation, followed by coalescence and growth of a continuous film; at glancing incidence, we observe the development of a mounded surface morphology before deposition of the first nanometer, followed by growth of isolated nanopillars.

Roughness evolution in thin-film growth of SiO2 and Nb2O5

We investigate the amorphous thin-film growth of SiO2 and Nb2O5 on glass substrates and quantify the roughness of the film surface by means of a statistical data analysis of atomic force microscopy images. We determine the absolute root-mean-square roughness, the effective growth exponent β, the local roughness exponent α, and the evolution of the lateral correlation length ξa for dual magnetron sputter-deposited thin films.

Control of surface roughness in amorphous thin-film growth

The growth of optical layers of SiO2 and Nb2O5 on amorphous substrates is investigated. We develop a kinetic Monte Carlo model which mimics the amorphous structure by randomly fluctuating binding energies. The resulting surface profiles are characterized by their root-mean-square roughness, height–height correlation functions, and growth exponents. For strong random fluctuations, the growth exponents exceed the value of 0.5, in good agreement with experiment.

Stationary Solutions for a Model of Amorphous Thin-Film Growth

We consider a class of stochastic partial differential equations arising as a model for amorphous thin film growth. Using a spectral Galerkin method, we verify the existence of stationary mild solutions, although the specific nature of the nonlinearity prevents us from showing the uniqueness of the solutions as well as their boundedness (in time).

Growth of vapor-deposited amorphousZr65Al7.5Cu27.5films under oblique particle incidence investigated by experiment and simulation

Amorphous thin film growth is of high scientific interest due to the possibility of identifying growth processes without the superposition of anisotropies. Oblique particle incidence offers the exciting possibility of breaking this isotropy in the plane of the substrate and of creating nanostructure films dependent on the tilt angle, which enters as an additional parameter. We investigate this structure formation and the nature of the underlying atomic-scale processes with experiments and computer simulations. This is achieved by a detailed film characterization using scanning tunneling microscopy and transmission electron microscopy and a numerical analysis of the data as well as with Monte Carlo and continuum growth models. Measures such as rms roughness or spectral power density are utilized for a quantitative characterization and comparison of experiment and theory. This allows us to identify surface diffusion, shadowing, and steering as the main ingredients for oblique film growth.

Breakdown of the approximations of small perturbations in continuum modeling of amorphous thin film growth

Morphology evolution of thin film growth can be quantitatively modeled by employing rate equations for two-dimensional, single-valued functions within a treatment of small perturbations. This description can be expected to be valid in an intermediate film thickness regime, where substrate influences and three-dimensional effects are minor. The breakdown of the mathematical description in its upper film thickness limit is systematically investigated by a detailed comparison of experiments and simulation. Possible reasons for the failure are discussed and improvements are suggested.

Tailoring the surface morphology of amorphous thin films by appropriately chosen deposition conditions

Amorphous thin film growth on a substrate by cocondensation is dominated by growth instabilities arising from self shadowing and surface diffusion, and these lead to pronounced three dimensional growth. It is possible to influence structure formation profoundly through systematic variation of the deposition characteristics of the particles, i.e., the deposition energy, the deposition angle, and the angle distribution. Experimentally, this can be achieved by varying the deposition technique from vapor deposition to sputtering, and changing the deposition angle with or without simultaneous rotation of the substrate. While roughening can be enhanced by oblique particle incidence, sputtered amorphous films have smooth surfaces. This behavior can be understood in terms of shadowing and energy transfer effects as illustrated by Monte Carlo and continuum growth models.

Amorphous thin film growth: effects of density inhomogeneities.

A nonlinear stochastic growth equation for the spatiotemporal evolution of the surface morphology of amorphous thin films in the presence of potential density variations is derived from the relevant physical symmetries and compared to recent experimental results. Numerical simulations of the growth equation exhibit a saturation of the surface morphology for large film thickness originating from the inclusion of the density inhomogeneities. Furthermore, we argue why moundlike surface structures observed on vapor deposited amorphous films are not the result of the Grinfeld instability.

Amorphous thin film growth: minimal deposition equation

A nonlinear stochastic growth equation is derived from (i) the symmetry principles relevant for the growth of vapor deposited amorphous films, (ii) no excess velocity, and (iii) a low-order expansion in the gradients of the surface profile. A growth instability in the equation is attributed to the deflection of the initially perpendicular incident particles due to attractive forces between the surface atoms and the incident particles. The stationary solutions of the deterministic limit of the equation and their stability are analyzed. The growth of the surface roughness and the correlation length of the moundlike surface structure arising from the stochastic growth equation is investigated.

Structural and mechanical properties of amorphous Zr–based alloy thin films

AbstractThe evolution of surface structures of coevaporated and sputtered amorphous Zr65Al7.5Cu27.5films with varying deposition conditions is investigated primarily with scanning tunneling microscopy (STM). While vapor deposited thin films reveal pronounced structure formation, depending on parameters, such as substrate temperature, film composition (variation of the Al versus the Cu content) and the angle of incidence, comparable sputtered films hardly show any structure formation. With the help of a numerical analysis of the STM data, surface diffusion, self–shadowing and energy transfer in the case of sputtering can be identified as the main structure forming mechanisms. Presuming these atomic processes, it is possible to model the main experimentally observed features of amorphous thin film growth by the use of stochastic continuum growth equations, which are numerically solved. Additionally, the connection to intrinsic stress formation during film growth is discussed.

Fractal models for the autocatalytic growth of amorphous thin films

Fractal models are proposed for the autocatalytic growth of amorphous thin films. The models give good agreement with the growth kinetics measured for electroless deposited films of amorphous nickel-phosphorus. A universal result obtained from the models is discussed, and only self-similarity is found to be important; randomness is effectively eliminated in the model.

Morphology and structure of TiO 2 thin layers vs. thickness and substrate temperature

TiO 2 thin layers are often used in multilayer coatings for optical filters and this makes their study especially interesting. Different TiO 2 thin layers corresponding to different thicknesses and deposited at different temperatures have been prepared by vapour deposition onto amorphous substrates. Most of these layers are amorphous but small crystals of anatase and rutile can also exist. These crystals are more frequent (i) if the substrate temperature is raised (350 °C) and/or (ii) if the thickness of the thin layer is higher. The surface roughness of TiO 2 thin layers has also been investigated by electron microscopy by two different and complementary methods: direct imaging by transmission electron microscopy and carbon-platinum replica observation. The thin layer surfaces are rather irregular, and this practically independently of the film thickness and of the substrate temperature during the deposition. Surface irregularities have been measured to reach about 10 nm. They have been shown to be slightly reduced on increasing the thin layer thicknesses from 35 to 70 nm. Each layer is made of blocks about 40 nm in size and each block is made of smaller blocks (about ten times smaller). The origin of this phenomenon has been sought. The formation of the smaller blocks is in agreement with existing theories of nucleation and growth of amorphous thin films. An attempt to find the origin of the larger blocks has also been carried out. This could be related to the permanent chemical transformation occuring during the thin layer formation. Finally, we relate the optical properties of the TiO 2 thin layers to their morphology and structure.

Local equilibrium model of morphological instabilities in chemical vapor deposition

We construct a continuum model of morphological instabilities encountered in the growth of amorphous thin films by chemical vapor deposition. Both the effects of surface diffusion and the re-evaporation of material into the gas phase are incorporated in our description. These processes are modulated by variations in the chemical potential along the surface. We characterize the stability of a growing film with a planar interface using linear stability analysis. We also develop techniques for calculating the dynamics of surfaces well into the non-linear regime at longer times and examine the growth behavior of several non-planar interfaces for both two-dimensional and three-dimensional systems.