Preparation of Epitaxial Scandium Trifluoride Thin Films using Pulsed Laser Deposition

Bulk Scandium trifluoride (ScF₃) is known for a pronounced negative thermal expansion (NTE) over a wide range of temperature, from 10~K~to~ 1100~K. The structure of ScF₃ can be described as an ABX₃ perovskite with an empty A-site and a space group of Pm-3m. Growing thin films of ScF₃ allows for tuning the lattice constant, the thermal expansion, and the construction of devices based upon differential thermal expansion. We have investigated the growth of ScF₃ films on oxide and fluoride substrates using pulsed laser deposition (PLD) This letter describes the successful growth recipe for producing high quality epitaxial ScF₃ thin films on positive thermal expansion (PTE) lithium fluoride (LiF) substrates, at substrate temperature, 350∘C with a laser repetition rate of 1~Hz, with an energy per pulse of 600~mJ, under a vacuum of 1.5× 10⁻⁶~ torr, for a growth time of $6$ hours. However, even for films with excellent epitaxy and sharp peaks along the principal axes, diffraction peaks from certain crystallographic directions are extremely broad, with the example of ($104$) reflections, in this work. We attribute this broadening to disorder in the F₆ octahedral rotations that occur as an attempt to accommodate the large temperature-induced lattice mismatch that results in cooling from the growth temperature for this system of a NTE film mated to a PTE substrate.