He Buffer Gas for Moderating the Kinetic Energy of Pulsed Laser Deposition Plumes
Ryota Takahashi, Takahisa Yamamoto, Mikk Lippmaa
Abstract
We have investigated the effect of helium buffer gas on the moderation of the kinetic energy of pulsed laser deposition (PLD) plumes during the growth of SrTiO3 thin films. Atomically smooth SrTiO3 thin films were grown by PLD using a neodymium yttrium aluminum garnet (Nd:YAG) laser for target ablation. The lattice parameters of the SrTiO3 thin films were found to be different from the cubic bulk value due to the presence of crystal defects, leading to the formation of a tetragonally distorted structure. One of the reasons for the presence of defects was found to be the high kinetic energy of the ablation plumes generated by the Nd:YAG laser operating at high energy densities. The kinetic energy of ionized species in the plume exceeded 100 eV, which means that adatoms arriving on the film surface had sufficient energy to resputter atoms from the surface of the deposited SrTiO3 film, creating point defects. A mixed atmosphere of He and O2 was used to moderate the plume kinetic energy without reducing the deposition rate. A nearly 10-fold reduction of the kinetic energy was obtained at a total pressure of 0.6 Torr, while the O2 pressure was kept at 10–5 Torr. Minimizing the plume kinetic energy and the associated film surface resputtering improved the crystallinity of SrTiO3 thin films. The helium gas pressure was found to be an important process parameter for tuning the functional properties of oxide films. We demonstrate the He moderator effect on two characteristics of SrTiO3 films, the appearance of a polar state and water permeability, both of which are sensitive to the presence of structural defects.