  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.
