Effect of the Oxygen Partial Pressure on the Microstructure and Properties of Barium Strontium Titanate Thin Films Synth
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ABSTRACT Barium strontium titanate (BSTO) films were synthesized by the pulsed laser deposition technique (PLD) on silicon substrates at room temperature. The BSTO film synthesis took place at constant laser energy, 500 mJ, and partial oxygen pressure of 3, 15, 30, 45, 100 mTorr. All films were post annealed at 750 'C in a tube furnace in an oxygen atmosphere. The microstructure, crystallinity and lattice constant of the BSTO films were studied with the aid of scanning electron microscopy (SEM), photon tunneling microscopy (PTM) and Glancing Angle X-ray Diffraction analysis (GAXRD). The hardness and modulus of elasticity of the films were studied with the aid of a nanohardness indenter. The film stoichiometry was determined with the aid of Rutherford Back Scattering (RBS). The results of this research will be combined with the results of our previous work [1] on the effect of substrate temperature on the microstructure and mechanical properties of the BSTO films in order to construct a structural zone model (SZM) of the BSTO films synthesized by PLD. INTRODUCTION Thin films of novel barium strontium titanate (BSTO), deposited by the pulsed laser deposition (PLD) technique exhibit excellent electronic properties including tunable dielectric constants and low electronic loss. The dielectric constant of the BSTO depends on the applied electric field. This variable dielectric constant results in a change in phase velocity in the device allowing it to be tuned in real time for a particular application. The dielectric requirements for tunable BSTO thin film are (a) loss less than 0.01; (b) high tunability; (c) dielectric constant between 30 and 100; (d) low leakage current; and (e) good frequency and temperature stability of dielectric properties [1]. A tunable BSTO film must be a single phase and crystalline. It must also have a smooth, defect and crack free surface, uniform microstructure and exhibit good thermal stability with the substrate. The microstructure of the film influences the electronic, and mechanical properties (internal stresses and adhesion), important factors affecting the mechanical integrity and reliability of a device made of these thin films, which in turn influences the performance of the film. The concept of electronic thin film deposition at lower temperatures is desirable because it enables the integration of several device fabrication steps. However, the inherent material properties required for any application have to be maintained at the lower deposition temperatures. In the case of electronic thin films used for tunable dielectric applications, these properties include dielectric constant, voltage tunability, and the electronic loss tangent. However, the mechanical integrity of the thin film (i.e., the adhesion and cohesion) is just as important.
157 Mat. Res. Soc. Symp. Proc. Vol. 603 C 2000 Materials Research Society
It is well known that the substrate temperature Ts, the ambient gas pressure, and the energy of any incoming ions influence the growth conditions and therefore, the film s
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