Improvement in tunability and dielectric loss of (Ba 0.5 Sr 0.5 )TiO 3 capacitors using seed layers on Pt/Ti/SiO 2 /Si s
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The absence of a low dielectric constant layer at the barium strontium titanate (BST)/Pt interface and a decreased roughness are critical issues in the production of (Ba0.5Sr0.5)TiO3 thin films with high tunabilities and low losses. An improvement in dielectric properties was achieved by the insertion of seed layers at the BST/Pt interface by pulsed laser deposition. The higher tunability can be attributed to (100) texturing of the BST films, which is independent of grain size and grain morphologies, thus leading to a variation in seed layer thicknesses. The tunability and dielectric constant of 1600-Å-thick BST films showed a maximum of 53% and 720, respectively, at a seed layer thickness of 100 Å. Dielectric loss is dependent on the roughness of BST films and reached a minimum of 0.8% at a root mean square roughness of 28 Å. The maximum figures of merit, defined as the ratio of tunability to dielectric loss, of approximately 58 at 100 kHz and 198 kV/cm were obtained at a seed layer thickness of 70 Å. The optimized seed layer thickness for BST deposition onto Pt/Ti/SiO2/Si substrates plays an important role in maintaining the high tunabilities and low loss, which are suitable for microwave device applications.
I. INTRODUCTION
Barium strontium titanate, Ba1−xSrxTiO3 (BST), is being investigated and is of considerable interest for use as a new dielectric material for the generation dynamic random-access memories (DRAMs)1,2 in the future and more currently in tunable microwave applications.3,4 The large electrical field dependent dielectric constant can be used for tunable microwave devices, such as tunable oscillators, phase shifters, and varactors.5,6 In such devices, it is desirable to have a high dielectric tunability over a given electric field range and a low dielectric loss. BST films have been deposited by sputtering,7 chemical vapor deposition (CVD), 8,9 and pulsed laser deposition (PLD).10,11 PLD provides unique advantages for multicomponent oxide films because it allows easy reproduction of the stoichiometry of the target in the deposited films. Various factors, such as the lattice mismatch and differences in thermal expansion between the film and the substrate, as well as the partial pressure of oxygen during the deposition affect the microwave dielectric properties of BST films.12 Park et al. systematically controlled the stress states of BST films by depositing a very thin BST interlayer between the main layer and a MgO (001) substrate.11 The Ba0.6Sr0.4TiO3 films grown on a Ba0.7Sr0.3TiO3 interlayer exhibited a small tensile stress and the best dielectric values. The (Ba,Sr)RuO3 (BSR) conductive J. Mater. Res., Vol. 17, No. 11, Nov 2002
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interfacial layers deposited onto Pt/Ti/SiO2/Si substrates by metalorganic chemical vapor deposition offered an appropriate match in structure with BST and increased the tenability of (100)-textured BST thin films.13 However, BST films with BSR interfacial layers showed an increase of dielectric loss compared wit
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