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CdS homoepitaxy growth was performed by molecular dynamics using different substrate orientations and structures in order to analyze the CdS crystallinity. As anticipated from thermodynamics of homoepitaxy, highly crystalline films with only point defects were obtained on substrates with rectangular surface geometries, including [112¯] zinc blende (ZB), [101¯0] wurtzite (WZ), [112¯0] WZ, [110] ZB, [010] ZB, and [1101110] ZB. In contrast, films grown on substrates with hexagonal surface geometries, corresponding to the [0001] WZ and [111] ZB growth directions, showed structures with a large number of defects including; anti-sites, vacancies, stacking faults, twinning, and polytypism. WZ and ZB transitions and grain boundaries are identified using a lattice identification algorithm and represented graphically in a structural map. A dislocation analysis was performed to detect, identify, and quantify linear defects within the atomistic data. Systematic simulations using different temperatures, deposition rates, and substrate polarities were perform to analyze the trends of dislocation densities on [0001] WZ direction and showed persistent polytypism. The polytypism observed in the films grown on the substrates with hexagonal surface geometry is attributed to the similar formation energies of the WZ and ZB phases.

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TYPE Journal Article YEAR 2016

DOI OSTI Scopus

Tensile cracking of a brittle conformal coating on a rough substrate

International Journal of Fracture

Bays, Nathan R.

This note examines the effect of interfacial roughness on the initiation and growth of channel cracks in a brittle film. A conformal film with cusp-like surface flaws that replicate the substrate roughness is investigated. This type of surface flaw is relatively severe in the sense that stress diverges as the cusp-tip is approached (i.e., there is a power-law stress singularity). For the geometry and range of film properties considered, the analysis suggests that smoothing the substrate could substantially increase the film’s resistance to the formation of the through-the-thickness cracks that precede channel cracking. Furthermore, smoothing the substrate’s surface has a relatively modest effect on the film stress needed to propagate a channel crack.

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TYPE Journal Article YEAR 2016

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Size effects on the thermal conductivity of amorphous silicon thin films

Physical Review B

Bays, Nathan R.; Braun, Jeffrey L.; Baker, Christopher H.; Elahi, Mirza; Artyushkova, Kateryna; Norris, Pamela M.; Leseman, Zayd C.; Gaskins, John T.; Hopkins, Patrick E.

We investigate thickness-limited size effects on the thermal conductivity of amorphous silicon thin films ranging from 3 to 1636 nm grown via sputter deposition. While exhibiting a constant value up to ∼100 nm, the thermal conductivity increases with film thickness thereafter. The thickness dependence we demonstrate is ascribed to boundary scattering of long wavelength vibrations and an interplay between the energy transfer associated with propagating modes (propagons) and nonpropagating modes (diffusons). A crossover from propagon to diffuson modes is deduced to occur at a frequency of ∼1.8 THz via simple analytical arguments. These results provide empirical evidence of size effects on the thermal conductivity of amorphous silicon and systematic experimental insight into the nature of vibrational thermal transport in amorphous solids.

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TYPE Journal Article YEAR 2016

DOI OSTI Scopus