Review of Transition-Metal Diboride Thin Films

TL;DR

This review discusses the growth, properties, and applications of transition-metal diboride thin films, emphasizing their unique structure, high-performance characteristics, and the importance of contamination control during deposition.

Contribution

It provides a comprehensive overview of TMB2 thin films, highlighting recent advances, challenges in deposition, and potential applications in coatings and electronics.

Findings

TMB2 films exhibit high melting points, hardness, and corrosion resistance.

Contamination control is crucial for optimizing film properties.

Sputter deposition is reaching a pivotal point for diverse applications.

Abstract

We review the thin film growth, chemistry, and physical properties of Group 4-6 transition-metal diboride (TMB2) thin films with AlB2-type crystal structure (Strukturbericht designation C32). Industrial applications are growing rapidly as TMB2 begin competing with conventional refractory ceramics like carbides and nitrides, including pseudo-binaries such as Ti1-xAlxN. The TMB2 crystal structure comprises graphite-like honeycombed atomic sheets of B interleaved by hexagonal close-packed TM layers. From the C32 crystal structure stems unique properties including high melting point, hardness, and corrosion resistance, yet limited oxidation resistance, combined with high electrical conductivity. We correlate the underlying chemical bonding, orbital overlap, and electronic structure to the mechanical properties, resistivity, and high-temperature properties unique to this class of materials. The review highlights the importance of avoiding contamination elements (like oxygen) and boron segregation on both the target and substrate sides during sputter deposition, for better-defined properties, regardless of the boride system investigated. This is a consequence of the strong tendency for B to segregate to TMB2 grain boundaries for boron-rich compositions of the growth flux. It is judged that sputter deposition of TMB2 films is at a tipping point towards a multitude of applications for TMB2 not solely as bulk materials, but also as protective coatings and electrically conducting high-temperature stable thin films.