E. Vasco

Growth evolution of ZnO films deposited by pulsed laser ablation

We study the surface morphology evolution of ZnO films grown by pulsed laser deposition. Atomic force microscopy measurements show the existence of two growth regimes. Initially, the growth morphology is determined by shadowing effects due to the angular spreading of the plume in the 0.1 mbar oxygen working pressure. For longer deposition times a stepped pyramid-like structure is developed, whose roughening and coarsening behaviours are in agreement with those expected for growth systems with step-edge barriers.

Ferroelectric Domain Structure and Local Piezoelectric Properties of La-Modified PbTiO 3 Thin Films Prepared by Pulsed Laser Deposition

La-modified PbTiO 3 (PLT) thin films of different composition (Pb 1 m 3x/2 La x TiO 3 , x=0.15 and 0.20) were characterized by Piezoresponse Force Microscopy (PFM). 001/100-textured films were deposited on (111)Pt/TiO 2 /SiO 2 /(100)Si by Pulsed Laser Deposition (PLD). The spontaneous polarization domain structure was correlated with the grain microstructure and orientation texture. The area ratio between the 001 and m 001 antiparallel polarization domains was estimated. Additionally, the local piezoelectric behavior in PLT (x=0.15) films was characterized.

Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries

The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.

Pulsed laser deposition-type growth kinetics: control by moderate-sized mobile clusters

The nonequilibrium growth kinetics of pulsed laser deposition (PLD) is investigated using a realistic set of rate equations. Our results indicate that growth from a supersaturated pulsed flux of kinetically hyperthermal species is ruled by the kinetics of moderate-sized mobile clusters. On vicinal surfaces (e.g., oxide surfaces), whose high corrugations hinder large-scale diffusions at the moderate temperatures commonly used in PLD, direct impingement and transient mobility play a key role in the nucleation of such mobile clusters. Based on these insights, we propose a new description of the growth kinetics of PLD that should prove valuable in the development of oxide-based nanoscale devices.

Bulk characterization in a Monte Carlo particle-deposition model with a novel adherence-potential barrier

The aim of this work is to analyze in more depth a model of particle deposition by characterizing different parameters such as profile density, bonds and perimeter, and substrate coverage, all being involved in the description of deposits as bulk. Thus, this study is an extension of a previous work on non-equilibrium interface-growth systems where two different interface-growth models, called Standard Adherence Rule Model and Potential Adherence Rule Model, were characterized. In this work, bulk characterization is implemented for the complete range of Peclet numbers. The zones of density profile (Near-Wall, Plateau, and Active-Growth) are studied by proposing an adjustment for each of them and determining the full-setting density profile depending on the Peclet number. The density profiles are compared with other one- and two-stage models. Furthermore, an algorithm is proposed to calculate the number of bonds of the particles and the perimeter that a substrate forms over time. Finally, to analyze the coa...