Plasma-Assisted Atomic Layer Deposition of SrTiO3: Stoichiometry and Crystallinity Study by Spectroscopic Ellipsometry

Strontium titanate (SrTiO3, STO) films were deposited by plasma-assisted ALD using cyclopentadienyl-based Sr and Ti precursors with O-2 plasma as the oxidizing agent. As determined from Rutherford backscattering spectroscopy (RBS), [Sr]/[Ti] ratios ranging from 0.73 to 2.13 were achieved for 30-40 nm thick films by tuning the [SrO]/[TiO2] ALD cycle ratio. Films deposited at 250 degrees C were amorphous and required post-deposition annealing to crystallize into the ultrahigh-k perovskite structure. The crystallization temperature strongly depended on the film composition as observed by X-ray diffraction (XRD) measurements after rapid thermal annealing (RTA). Using RBS and XRD data as a combined-cross reference, it was shown that the film stoichiometry and the crystallinity can be probed directly by spectroscopic ellipsometry (SE).