Laser-ablation deposition and characterization of polycrystalline Nd-modified Pb(Zr,Ti)O3 thin films
1University of Oulu, Faculty of Technology, Department of Electrical Engineering
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|Academic Dissertation to be presented with the assent of the Faculty of Technology, University of Oulu, for public discussion in Kajaaninsali (Auditorium L 6), Linnanmaa, on November 26th, 1999, at 12 noon.
Professor Pekka Hautojärvi
Doctor Timo Kajava
Nd-modified lead-zirconate-titanate (PNZT) thin films were deposited on MgO(100), Si(100) and Al2O3(1102) single-crystal substrates using the pulsed-laser-ablation technique with a XeCl excimer laser. The post-annealing heat-treatment technique was used for the crystallisation of the films. The structural characterization, microstructure and the chemical composition of the thin films and of the ceramic Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets after ablation were studied using x-ray diffraction, scanning electron microscopy and energy-dispersive x-ray spectroscopy, respectively. The formation of the particulates on the target surface during the ablation process and the effect of the particulates on the quality of the thin films were studied. Typically, the ferroelectric PNZT thin films for the capacitor structures were deposited at the laser-beam fluence of around 1.0 J/cm2 and annealed at the temperatures from 600 to 700 °C. The dielectric and, especially, the polarization properties and the residual macroscopic stress state of the PNZT thin films were studied. The relationship between the electrical properties of the films and the nature of the stress state was also investigated.
The average growth rate of the PNZT films increased linearly with increasing laser-beam fluence above the threshold value of around 0.4 J/cm2. The composition of the PNZT films varied strongly with the deposition laser-beam fluence and annealing temperature. The phase structure of PNZT films ablated from Pb0.97Nd0.02(Zr0.55Ti0.45)O3 targets could be adjusted between tetragonal and rhombohedral structures by changing the incident laser-beam fluence on the target surface. The surface of the target after ablation was covered by the laser-cone structure and the topmost layer of the target was amorphous having TiO2 and ZrO2 structures with separate segregated lead droplets.
On MgO substrates, values of the relative dielectric constant er from 430 to 560 and of the remanent polarization εr of the order of 18 μC/cm2 were achieved in PNZT films which were under a compressive stress of the order of 300 MPa. On silicon substrates, εr was around 100 and the polarization properties of the films were modest due to a strong tensile stress of the order of 400 MPa. The Poole-Frenkel conduction mechanism with the activation energy of around 0.2 eV was found responsible for the leakage conductivity in the capacitor structures with PNZT films.
Acta Universitatis Ouluensis. C, Technica
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