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Atomistic growth processes of thin films with and without relaxation
Ulbrandt, Jeffrey G.
Ulbrandt, Jeffrey G.
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Abstract
This dissertation investigates atomistic mechanisms governing thin-film growth under conditions that either permit or inhibit structural relaxation. Real-time in situ x-ray scattering is used to probe how relaxation dynamics shape surface morphology, layer evolution, and internal stress during film deposition.
In Pulsed Laser Deposition (PLD) of crystalline strontium titanate, energetic and thermalized growth modes are compared to reveal the role of relaxation in epitaxial film formation. In the energetic PLD (e-PLD) regime, incident species with kinetic energies near 100 eV disrupt nascent two-dimensional islands, suppressing lateral growth and limiting structural relaxation. In contrast, thermalized PLD (th-PLD), performed in a helium background, enables sub-microsecond interlayer transport and enhanced relaxation of transient surface features. Time-resolved x-ray scattering measurements show that islands nucleate immediately after each laser pulse and subsequently ripen through atom detachment and rearrangement. A kinetic Monte Carlo model reproduces these behaviors and identifies the detachment energy barrier as the dominant factor controlling the recovery time between pulses.
In contrast, nanoparticle film growth by gas aggregation represents deposition without relaxation. Here, nanoparticles are generated in the gas phase using a sputter deposition source operated above a critical pressure, leading to the formation of size-selected clusters 2-7 nm in diameter. Upon deposition, these pre-formed nanoparticles retain their individual identities and assemble into highly porous tungsten silicide films. In situ x-ray scattering confirms that this growth follows the ballistic deposition model, with a packing fraction near 15\% and a tree-like microstructure dominated by a disordered initial monolayer at the substrate interface.
Together, these studies define how relaxation---or its absence---governs atomistic pathways of thin-film formation, linking kinetic energy, adatom mobility, and aggregation dynamics to the resulting crystalline and nanoporous structures.
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2026