Compositionally-Driven Formation Mechanism of Hierarchical Morphologies in Co-Deposited Immiscible Alloy Thin Films.

Max Powers,Rémi Dingreville,Amit Misra,James A Stewart,Benjamin K Derby

doi:10.3390/nano11102635

Abstract

Co-deposited, immiscible alloy systems form hierarchical microstructures under specific deposition conditions that accentuate the difference in constituent element mobility. The mechanism leading to the formation of these unique hierarchical morphologies during the deposition process is difficult to identify, since the characterization of these microstructures is typically carried out post-deposition. We employ phase-field modeling to study the evolution of microstructures during deposition combined with microscopy characterization of experimentally deposited thin films to reveal the origin of the formation mechanism of hierarchical morphologies in co-deposited, immiscible alloy thin films. Our results trace this back to the significant influence of a local compositional driving force that occurs near the surface of the growing thin film. We show that local variations in the concentration of the vapor phase near the surface, resulting in nuclei (i.e., a cluster of atoms) on the film’s surface with an inhomogeneous composition, can trigger the simultaneous evolution of multiple concentration modulations across multiple length scales, leading to hierarchical morphologies. We show that locally, the concentration must be above a certain threshold value in order to generate distinct hierarchical morphologies in a single domain.

Highlights

Plasma-based, physical vapor deposition (PVD), such as sputter deposition, for instance, is a fabrication technique widely used to deposit nanostructured thin films
We simulated the time-dependent deposition and growth of Cu-Mo thin films as a function of deposition conditions using our previously developed PVD phase-field model [13,18,19]. This phase-field model is a mesocale model that captures the dynamics of film growth and the associated evolution of concentration modulations comprising the growing film’s microstructure by describing the microstructure with a system of continuous variables that evolve both in space and time
The model explicitly accounts for major aspects of the PVD process, including: (i) the deposition of the incident vapor phase of a binary alloy onto the substrate, (ii) surface interdiffusion, and (iii) the subsequent phase ordering in the resulting inhomogeneous thin film

Summary

Introduction

Plasma-based, physical vapor deposition (PVD), such as sputter deposition, for instance, is a fabrication technique widely used to deposit nanostructured thin films. Depending on the elemental composition and deposition conditions, PVD is known to produce self-organized, phase-separated, nanoscale regions during the co-deposition of immiscible alloys at an elevated temperature, where the concentration modulation (CM) direction may be oriented laterally (LCM), vertically (VCM), or randomly (RCM) [1,2] with respect to film growth direction. These concentration modulations are classified as monomodal, maintaining a consistent microstructure morphology throughout the film.

Methods

Results

Conclusion