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Investigations of sputtered (Ba,Sr)TiO3 thin films for tunable microwave applications

Abstract : This dissertation (thesis) discussed the deposition and characterization of perovskite (Ba,Sr)TiO3 (BST) thin films using radio-frequency (RF) magnetron sputtering with the goal of studying the feasibility of fabricating high quality BST films in an economical manner for tunable microwave devices. We optimized the sputtering parameters to synthesize perfectly (001)-and (111)-oriented BST thin films directly on Pt(111)/TiOx/SiO2/Si substrates without buffer layers and to achieve best possible dielectric properties for tunable microwave devices. The O2/(Ar+O2) mixed ratios (OMR) during deposition were found to have significant effects on the crystallographic orientation, microstructure and dielectric properties of the prepared BST films. The samples with OMR=0 % showed perfect (001) orientation, while those with OMR ranging from 10 % to 50 % showed preferential (111) orientation. Electrical measurements showed that high tunability of 52 % and 68 % can be achieved for (001)-and (111)-oriented BST films, respectively, measured at 10 kHz and 400 kV/cm applied field. We have also studied the effect of ultrathin TiOx (0~12 nm) seeding layers on the degree of texturing, surface morphology and dielectric behaviors on BST thin films. The ultrathin TiOx layer was found to act as an initial template for the BST films in the first nucleation stage, resulting in a significant influence on crystalline orientation and electrical characteristics of the resultant BST films. With increasing the TiOx layer thickness from 0 to 1.2 nm, the orientation of the BST films switched from (001) to (111) preferred orientation, and highly (111)-oriented BST film [α(111)~97 %] was obtained as TiOx layer thickness was ~ 5 nm. As a result, the BST(111)/TiOx(5 nm) film exhibited a shifted Curie temperature (Tc=275 K) and an enhanced tunability of 61.16 % at 400 kV/cm, when compared with (001)-oriented BST without TiOx layer (Tc=250 K and tunability=50.05 %). It would be desirable for the ferroelectric thin films to have moderate dielectric constant while having low losses in order to obtain high figure of merit, hence composite thin films of high tunability films such as perfectly BST(111) films with low loss material like Bi1.5Zn1.0Nb1.5O7 (BZN) were prepared. We have therefore studied the effects of thickness of BZN layer on dielectric properties of BST (111) films. It was observed that the BZN layer thicknesses play an important role in improving the dielectric properties. A significant improvement in the figure of merit (~46.8) was obtained for BZN (50 nm)/BST(450 nm) bilayered films at frequency of 10 kHz and 400 kV/cm bias field. Our studies showed that perfectly BST (111) films on Pt(111)/TiOx/SiO2/Si substrates exhibited excellent dielectric properties, which have more advantages for microwave tunable devices application than (001)-oriented BST films. Thus, we further investigated the variations in the permittivity with film thickness and measurement temperature of perfectly BST (111) thin films with thicknesses ranging from 45 to 800 nm. Our results revealed that BST (111) films had unusual Curie temperature independent of thickness, about 305±5 K. A model incorporating a bulk film and an interfacial capacitance was employed to explain the thickness-dependent permittivity at the given temperature. Since tunable devices are operated at in the microwave range (300 MHz to 300 GHz). To test BST thin films for practical purpose, the material should be characterized at microwave frequencies. Epitaxial (111)-oriented BST thin films on Al2O3 (0001) substrates have been grown by inserting an ultrathin TiOx (~1.2 nm) seeding layer. Microwave properties of the epitaxial and polycrystalline BST thin films were evaluated up to 40 GHz. The dielectric dispersion demonstrated that the complex permittivity (ε=ε′-jε′′) is well described by a Curie-von Scheidler power law with an exponent of 0.40. Moreover, the high permittivity (~428) combined with a high tunability (~40.88%, 300kV/cm) up to 40 GHz suggest that epitaxial (111)-oriented BST thin film could be well suited for microwave tunable devices. Finally, microwave properties of BZN/BST bilayered thin films directly on high resistivity Si toward 50 GHz were also investigated. As expected, the BZN helped in tailoring the dielectric constant and reducing the loss tangent significantly. The BZN/BST/HR-Si heterostructured films have moderate permittivity (~224), low microwave loss (~0.167) and a high tunability (~7.7 %, 200kV/cm) until 50 GHz, which demonstrated a great promise application for integration into tunable microwave devices.
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  • HAL Id : tel-03004410, version 1

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Lihui Yang. Investigations of sputtered (Ba,Sr)TiO3 thin films for tunable microwave applications. Engineering Sciences [physics]. Université de Valenciennes et du Hainaut-Cambrésis, 2010. English. ⟨NNT : 2010VALE0016⟩. ⟨tel-03004410⟩

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