Geometrical and energy scaling in the pulsed laser deposition plasma during epitaxial growth of FeSe thin films

Pulsed laser deposition (PLD) is a versatile technique for growing epitaxial heterostructures of a wide variety of novel materials combinations. Achieving low-defect layers with atomically sharp interfaces by PLD requires careful management of crystal growth conditions. Control over the flux of depositing particles, their kinetic energy, and the substrate temperature, is generally sufficient to obtain high-quality single crystal epitaxy in vacuum. In this article, we show that measurements of plasma parameters such as the electron temperature, the electron density, and the Mach number of the plasma fluid expansion may provide additional insights into the tuning of the crystal growth and processing environment of PLD. We report Langmuir probe measurements during growth of FeSe thin films on (100)-oriented SrTiO$_3$. We discuss two distinct plasma regimes that are accessible by KrF laser ablation of FeSe when pulse energies in the 20-630 mJ range are used. The two regimes can be characterized by the relative number of photons to absorbing centers in the laser-plasma interaction volume. Thin films grown under the conditions created by these two distinct plasma regimes are analyzed by x-ray diffraction and x-ray reflectivity and their epitaxial configurations correlated to the PLD conditions associated with these plasmas.