Crystallization Study by Transmission Electron Microscopy of SrTiO₃Thin Films Prepared by Plasma-Assisted ALD

Abstract

The crystallization behavior of thin strontium titanate (SrTiO3, STO) films with ∼15 nm thickness was studied by Transmission Electron Microscopy (TEM). Amorphous STO films with [Sr]/([Sr]+[Ti]) ratio ranging from 0.50 to 0.63 were deposited at 350°C by plasma-assisted ALD and subsequently treated by rapid thermal annealing in flowing N2 for crystallization. Different temperatures and annealing durations were employed to fully characterize the crystallization process. TEM analysis showed that transrotational crystals were formed and evidenced the influence of the STO composition and of the thermal budget applied on the grain size, crack and void formation. In particular, Sr-rich layers ([Sr]/([Sr]+[Ti] ≥ 0.59) showed a finer crystalline structure which was imputed to a higher nucleation probability at the onset of the crystallization process. Crystallization into the perovskite structure was confirmed for all the film compositions studied. By tuning the STO composition and the thermal budget of the annealing step it was demonstrated that it is possible to control the microstructure of the crystallized film as a further step in optimizing the STO film properties.

Impact of composition and crystallization behavior of atomic layer deposited strontium titanate films on the resistive switching of Pt/STO/TiN devices

“…The cracks seem less pronounced for the Sr-rich films. This is consistent with the previous TEM study 34 which revealed an increased nucleation density leading to a higher density of smaller grains for the Sr-rich films as compared with the stoichiometric STO films. The Ti-rich STO films showed altogether a different surface morphology.…”

“…3(c) and 3(f)). Referring to previous crystallization studies, 34 the morphological differences were explained by a locally incomplete crystallization process. In particular, additional AFM analyses (see Fig.…”

“…While the [Sr]/([Sr] þ [Ti]) ratio in the as-deposited amorphous films could be well controlled by means of the ALD pulsing sequence, the microstructure and morphology of the polycrystalline STO thin films were found to depend in a complex manner on various external parameters, like the substrate material, the cation stoichiometry, the film thickness, and the annealing conditions. 15,34 We will show that, in addition, the RS behavior of the ALD STO based device is affected by the 2 ] ALD cycle ratio, while the film thicknesses were controlled by the numbers of executed ALD cycles. Details on the growth process can be found in a recent publication.…”

“…The thin film crystallization is controlled by the kinetics of nucleation and grain growth. On similar thin films deposited on Si 3 N 4 grids, Longo et al 34 showed by means of transmission electron microscopy (TEM) that the crystallization of the films yielded transrotational perovskite-type crystals. In addition, the important effect of the STO composition and the annealing temperature on the final grain size, the crack, and the void formation has been demonstrated.…”

Growth and characterization of nanocrystalline SrTiOxfilms: room temperature deposition using RF sputtering system in a pure argon environment

“…Furthermore, in the VIS range, the refractive indices of nanocrystalline STO films are found to be 2.36 and 2.31 (@550 nm) for as-deposited and annealed films, respectively. Its value decreased with annealing, however, the observed values is larger than the reported amorphous (1.83-2.06) and polycrystalline (2.05-2.12) films [28,[32][33][34]. It should be noted that the film thickness which is generally lower than 100 nm has affects film properties, however, according to our best knowledge there is no comparative information on the effect of film thickness on thin STO films.…”