Composition, structure, and dielectric tunability of epitaxial SrTiO3 thin films grown by radio frequency magnetron sputtering

Abstract

Epitaxial (001) oriented SrTiO3 films have been deposited on LaAlO3(001) substrates by off-axis radio frequency magnetron sputtering in Ar:O2 gas mixtures at substrate temperatures ranging from 650 to 850 °C. For the deposition conditions used, stoichiometric targets yielded 20% Sr-deficient films, whereas Sr-enriched targets (Sr1.1Ti0.9O3.0) resulted in stoichiometric films. The Sr-deficient films had a mosaic structure and a larger lattice parameter in comparison to bulk SrTiO3. The stoichiometric films on the other hand had a much higher crystalline quality in the as-deposited condition. The mosaicity of the latter films was primarily limited by the crystalline quality of the LaAlO3 substrates. The lattice parameters of the stoichiometric films were also smaller than the Sr-deficient ones and closer to the bulk value. The dielectric properties of the stoichiometric films were superior to the Sr-deficient films. For films with a thickness of ∼300 nm, the typical dielectric constants as measured at ∼77 K and 1 MHz were determined to be 820 and 500, for the stoichiometric and Sr-deficient films, respectively. Also the capacitance change, as a direct current bias voltage was applied to an interdigital capacitor, was higher for the stoichiometric film, 27.3% as compared to 8.6% when applying a bias of 300 V at 77 K. We also demonstrate the effectiveness of thermal annealing in improving both crystalline quality and dielectric properties, especially for the Sr-deficient films.

Influence of sputter deposition parameters on the properties of tunable barium strontium titanate thin films for microwave applications

“…21,17,35 The decreased relative permittivity and tunability of the BST thin films deposited at the largest distances of 8.8 cm and 10 cm can be attributed to their smaller grain size compared to the sample deposited at d = 7.5 cm as measured by AFM and XRD (see Table 1). Furthermore, thin films prepared with high sputter pressure and distance tend to have lower density and higher porosity, 36 thereby lowering the effective relative permittivity.…”

“…22 The cause for the excess Ti appearing in the BST thin films is believed to be resputtering and/or re-evaporation. 17 Yamamichi et al have found a pronounced dependence of the dielectric properties of BST thin films on stoichiometry. 21 Increasing the Ti excess of a sputtered BST thin film from y ≈ 0 to ≈ 0.33, a decrease of ε r from ≈ 500 to ≈ 250 could be observed.…”

“…Depositing stoichiometric films (y = 0) by sputtering from ternary oxides and their mixtures can be challenging as shown for strontium titanate (STO) 17,20,21 and BST. 18 Typically an excess of Ti is observed in these oxide films which could be counterbalanced by using SrO enriched targets and by increasing the sputter pressure.…”

“…This versatile method allows the control of the thin film properties as composition, morphology and structure. The dielectric properties of the BST thin film varactors 17,18,19 are affected by the deposition parameters as target composition, sputter power, sputter pressure and gas atmosphere as well as target to substrate distance and substrate temperature.…”

Effect of (Ba+Sr/Ti) ratio on the dielectric properties for highly (111) oriented (Ba,Sr)TiO3 thin films

“…Moreover, the sputtering deposition of oxide films has the potential for film degradation by neutral and negative-ion bombardment of the growing film. For multicomponent materials such as BST, this resputtering can lead to off-stoichiometric films and degradation of electrical properties, which has been studied in other examples for YBa 2 Cu 3 Ox and SrTiO 3 [15,16]. Im et al investigated composition-control of magnetron-sputter-deposited polycrystalline BST thin films for voltage tunable devices, and found that (Ba + Sr)/Ti ratio of these BST films could be adjusted from 0.73 to 0.98 by changing the total (Ar + O 2 ) process pressure [17].…”