Nanometer scale control of solid phase epitaxy within ion beam-induced amorphous LiNbO3 using electron irradiation

Abstract

Thin film LiNbO­3 (TFLN), an important material for nonlinear optics, quantum devices, and microwave photonics, has a conductive layer that can affect low-frequency device performance. Here, we show using transmission electron microscopy (TEM) that unetched commercial x-cut TFLN has a thin (∼5 nm) amorphous LiNbO3 layer at the ion sliced surface and that this amorphous layer crystallizes under electron-beam irradiation. Further investigation on a ∼35 nm thick amorphous LiNbO3 layer reveals that while 300 keV electron-beam irradiation causes no noticeable structural change in the amorphous LiNbO3 layer, both 120 and 60 keV electron-beam irradiations cause amorphous LiNbO3 to crystallize starting from the amorphous/crystal LiNbO3 interface. Atomic resolution scanning TEM images and electron energy loss spectra confirm crystal orientation and stoichiometry in the crystallized LiNbO3. As the possible temperature rise due to beam heating is calculated to be negligible and the solid phase epitaxy rate of amorphous LiNbO3 inversely correlates with the electron-beam energy, an ionization-driven process, i.e., radiolysis, rather than an elastic interaction-driven (knock-on damage) process is hypothesized as a primary cause of solid phase epitaxy occurring in the amorphous LiNbO3. This study demonstrates that crystallinity restoration within amorphous LiNbO3 can be controlled with nanometer scale precision using electron-beam irradiation, potentially offering a route to controllable device performance improvements.