  A seemingly simple oxide with a rutile structure, RuO2 has been shown to
possess several intriguing properties ranging from strain-stabilized
superconductivity to a strong catalytic activity. Much interest has arisen
surrounding the controlled synthesis of RuO2 films but, unfortunately,
utilizing atomically-controlled deposition techniques like molecular beam
epitaxy (MBE) has been difficult due to the ultra-low vapor pressure and low
oxidation potential of Ru. Here, we demonstrate the growth of epitaxial,
single-crystalline RuO2 films on different substrate orientations using the
novel solid-source metal-organic (MO) MBE. This approach circumvents these
issues by supplying Ru using a pre-oxidized solid metal-organic precursor
containing Ru. High-quality epitaxial RuO2 films with bulk-like
room-temperature resistivity of 55 micro-ohm-cm were obtained at a substrate
temperature as low as 300 C. By combining X-ray diffraction, transmission
electron microscopy, and electrical measurements, we discuss the effect of
substrate temperature, orientation, film thickness, and strain on the structure
and electrical properties of these films. Our results illustrating the use of
novel solid-source MOMBE approach paves the way to the atomic-layer controlled
synthesis of complex oxides of stubborn metals, which are not only difficult to
evaporate but also hard to oxidize.
