Non-thermal fluence threshold for femtosecond pulsed x-ray radiation damage in perovskite complex oxide epitaxial heterostructures

TL;DR

This study identifies a non-thermal damage threshold in perovskite oxide heterostructures caused by femtosecond x-ray pulses, showing damage occurs via ultrafast ionization rather than heating or melting.

Contribution

It provides the first measurement of a non-thermal damage threshold in perovskite heterostructures under femtosecond x-ray irradiation, highlighting ultrafast ionization as the damage mechanism.

Findings

Damage threshold at 2.7 x 10^6 photons/μm^2 per pulse

Damage results in deep cavities and material removal

Damage occurs via non-thermal ultrafast ionization

Abstract

Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO$_{3}$ layer on a SrTiO$_{3}$ substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 x 10$^{6}$ photons ${\mu}$m$^{-2}$ at 9.7 keV photon energy, but remains constant for 1.3 x 10$^{6}$ photons ${\mu}$m$^{-2}$ or less. The damage resulted in the destruction of the BiFeO$_{3}$ thin film within the focal spot area and the formation of a deep cavity penetrating into the STO substrate via the removal of tens of nanometers of material per pulse. The damage threshold occurs at a fluence that is insufficient to heat the absorption volume to the melting point. The morphology of the ablated sample is consistent with fracture rather than melting. Together these results indicate that the damage occurs via a non-thermal process consistent with ultrafast ionization of the absorption volume.