Preparation of Epitaxial Scandium Trifluoride Thin Films using Pulsed Laser Deposition

Abstract: Bulk Scandium trifluoride ($\mathrm{ScF_3}$) is known for a pronounced negative thermal expansion (NTE) over a wide range of temperature, from $10~\mathrm{K}~\text{to}~ 1100~\mathrm{K}$. The structure of $\mathrm{ScF_3}$ can be described as an $\mathrm{ABX_3}$ perovskite with an empty A-site and a space group of Pm-3m. Growing thin films of $\mathrm{ScF_3}$ 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 $\mathrm{ScF_3}$ films on oxide and fluoride substrates using pulsed laser deposition (PLD) This letter describes the successful growth recipe for producing high quality epitaxial $\mathrm{ScF_3}$ thin films on positive thermal expansion (PTE) lithium fluoride ($\mathrm{LiF}$) substrates, at substrate temperature, $350^{\circ}\mathrm{C}$ with a laser repetition rate of $1~\mathrm{Hz}$, with an energy per pulse of $600~\mathrm{mJ}$, under a vacuum of $1.5\times 10^{-6}~ \mathrm{torr}$, 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 $\mathrm{F_6}$ 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.