Scaling behavior and mechanism of formation of Si O2 thin films grown by plasma-enhanced chemical vapor deposition

This paper reports a study of the kinetic roughening of Si O2 thin films prepared by plasma-enhanced chemical vapor deposition (PECVD). Tetramethylsilane has been used as a precursor, and the synthesis has been carried out under remote and in-plasma configurations. The analysis of surface topography of the films by atomic force microscopy shows an anomalous scaling behavior that cannot be represented by the Family-Vicsec scaling relation of dynamic scaling theory. The application of different methods for obtaining the roughness exponent α yields different values of this exponent (α=0.7 for the height-height correlation function and α=1.3 for the power spectral density function for long deposition times) in all experimental conditions. Moreover, a strong variation of the α exponent with deposition time has been determined for the samples grown in remote mode. This correlates with the presence of a crossover region of the growth exponent β, which varies from a first value of 1.3 for low deposition times to another of 0.3 for longer deposition times. Such a variation is not found for the samples grown in the plasma, characterized by a β value of 0.28. The results obtained can be explained by the combined effect in the growth process of a low diffusivity of the physisorbed species along with the existence of nonlocal interactions due to shadowing effects. These two assumptions are in agreement with the empirical knowledge existing about the kinetics of the growth of Si O2 thin films by PECVD and establish a link between the scaling properties of the films with the surface chemistry during the film growth.Ministerio de Educación y Ciencia MAT2004-01558 y MAT2007-6576

Relationship between scaling behavior and porosity of plasma-deposited TiO2 thin films

The growth of TiO2 thin films prepared by plasma enhanced chemical vapor deposition has been studied by analyzing their roughness with the concepts of the dynamic scaling theory. Differences in the growth and roughness exponents have been found depending on the composition of the plasma by using either O2 or mixtures Ar+ O2 as plasma gas and titanium isopropoxide as the precursor. The slope of the representations of the film roughness against the deposition time yielded values of the exponent β of 0.45 and 0.32 for, respectively, thin films prepared with plasmas of O2 or mixtures Ar+ O2. Meanwhile, values of the exponent α of 1.15 and 1.89/0.35 were deduced from the power spectral density representations for the films prepared under these two experimental conditions. These values are congruent with growth processes dominated, respectively, by shadowing or diffusion processes. A columnar microstructure was observed by scanning electron microscopy for the thin films prepared with pure oxygen. Meanwhile, homogeneous films were obtained with mixtures of Ar+ O2. The open porosity of the films was determined by measuring water adsorption-desorption isotherms with a quartz crystal monitor. This analysis showed that in the samples prepared with mixtures of Ar+ O2 the porosity consisted exclusively of micropores (d2 nm). It is concluded that the different growth mechanisms found by just changing the chemistry of the plasma are responsible for the quite distinct microstructures, porosities, and optical properties obtained for the films.Ministerio de Educación y Ciencia NAN2004-09317-C04-01 y MAT2007-6576

Ethanol reforming in non-equilibrium plasma of glow discharge

The results of a detailed kinetic study of the main plasma chemical processes in non-equilibrium ethanol/argon plasma are presented. It is shown that at the beginning of the discharge the molecular hydrogen is mainly generated in the reaction of ethanol H-abstraction. Later hydrogen is formed from active H, CH2OH and CH3CHOH and formaldehyde. Comparison with experimental data has shown that the used kinetic mechanism predicts well the concentrations of main species at the reactor outlet.Comment: 16 pages, 8 figure

A Domain-Agnostic Approach for Characterization of Lifelong Learning Systems

Despite the advancement of machine learning techniques in recent years, state-of-the-art systems lack robustness to "real world" events, where the input distributions and tasks encountered by the deployed systems will not be limited to the original training context, and systems will instead need to adapt to novel distributions and tasks while deployed. This critical gap may be addressed through the development of "Lifelong Learning" systems that are capable of 1) Continuous Learning, 2) Transfer and Adaptation, and 3) Scalability. Unfortunately, efforts to improve these capabilities are typically treated as distinct areas of research that are assessed independently, without regard to the impact of each separate capability on other aspects of the system. We instead propose a holistic approach, using a suite of metrics and an evaluation framework to assess Lifelong Learning in a principled way that is agnostic to specific domains or system techniques. Through five case studies, we show that this suite of metrics can inform the development of varied and complex Lifelong Learning systems. We highlight how the proposed suite of metrics quantifies performance trade-offs present during Lifelong Learning system development - both the widely discussed Stability-Plasticity dilemma and the newly proposed relationship between Sample Efficient and Robust Learning. Further, we make recommendations for the formulation and use of metrics to guide the continuing development of Lifelong Learning systems and assess their progress in the future.Comment: To appear in Neural Network