George Theodore Dalakos

Role of solid surface structure on evaporative phase change from a completely wetting corner meniscus

The transport processes that occur at small length scales are greatly influenced by interfacial and intermolecular forces. Surface roughness at the nanoscale generates additional intermolecular interactions that arise due to the increased surface area. In this work, we have experimentally studied how the magnitude as well as the shape of surface roughness influences the microscale transport processes that occur in the contact line region of a liquid corner meniscus. The surface roughness contribution to the interaction potential was calculated and a direct relationship between the wetting properties of the liquid and the underlying surface properties was obtained. Since the underlying roughness alters the surface potential, the shape of the meniscus and in turn, the resulting capillary and disjoining pressure forces also changed. Atomic force microscopy was utilized to obtain a detailed characterization of the shape of the prepared surfaces. Surface morphology features were obtained from a height-height c...

Suppressed surface morphology instabilities in amorphous hydrogenated silicon deposition

The surface chemistry associated with a‐Si:H growth by plasma-enhanced chemical vapor deposition is unique in that the hydrogen-passivated surface results in a low threshold energy for surface diffusion relative to the cohesive energies of the material. We show that helium ion bombardment enhances the hopping rate of loosely bound film precursors without substantially affecting the condensed a‐Si:H material. Our investigative approach consists of examination of the temporal evolution of the surface topography under different substrate bias conditions. Without biasing the substrate, the surface morphology becomes unstable, producing mounded-type structures, consistent with shadowing growth instability. Biasing the substrate suppresses these instabilities and allows an initially rough a‐Si:H surface to be smoothed during subsequent deposition.

Surface Roughness Evolution of PECVD Cathodic and Anodic a -Si:H.

Surface or interface roughness can impact optical, electronic, and MEMS applications of thin a-Si:H films. Deposition at lower temperatures can be advantageous for some applications of a-Si:H, but lower temperature deposition generally leads to rougher films. We have found that the evolution of surface roughness growth can be modified substantially by ion bombardment due to the self-bias of the plasma during Plasma-Enhanced Chemical Vapor Deposition (PECVD). Notable differences in the surface roughness evolution and deposition rate are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Suppression of surface roughness growth of a-Si:H can be achieved under conditions of relatively high ion bombardment even at deposition temperatures as low as 75 C. Atomic force microscopy (AFM) was used to measure the relative surface roughness profile as a function of deposition time. Analysis of the power spectral density of the roughness yielded important statistical surface parameter information. Based on these observations, insight is given into growth mechanisms under the two deposition conditions.

Surface Roughness Study of Low-temperature PECVD a -Si:H.

Surface topography of a -Si:H thin films, deposited at 75°C by Plasma-enhanced Chemical Vapor Deposition (PECVD) has been examined using helium/silane feedstock mixtures under different substrate bias conditions. Notable differences in the surface roughness evolution are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Smooth and apparently featureless surfaces result from deposition on RF powered surfaces, upon which a self-bias induces high-energy ion bombardment. Rougher surfaces result from films deposited on electrically grounded surfaces. These anodic films show that after a transition period, surface roughness grows linearly with processing time, exhibiting mounded type growth as evidenced by 2-D power spectral density functions of surface height measurements. Linear growth in roughness has been predicted for shadow growth models assuming film precursor sticking coefficients of one and random angle approach of film precursor species. Growth of this nature has not been reported before in a -Si:H studies, which usually assume directional deposition conditions and sticking coefficients less than unity – occurring even at low processing temperatures.

On the development of CdS properties upon processing CdTe devices

CdS plays an important role in CdTe devices through both the thickness impact on Jsc and the less well understood effect on FF and Voc by impacting the minority carrier lifetime in the CdTe near the interface. In this study, two techniques are explored, chemical bath deposition and radio frequency sputtering, effectively exploring the role of film density. A multitude of characterization techniques have been exploited on CdS films after various stages in the process of device fabrication as the CdS properties change significantly upon processing of the films. For this purpose we compared the properties for as-deposited films, films that are exposed to a CdTe deposition heating profile without the actual CdTe deposition, as well as CdS films as they appear within completed devices. Especially the CdCl2 process turned out to result in huge changes within the CdS film. From this study it is concluded that significant changes occur to the CdS film depending on its starting density. Change in electrical properties and intermixing occur during the CdTe deposition and are thus temperature related, while a change in optical properties and carrier lifetime near the interface is established after CdCl2.