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에피성장한 강유전 나노구조체의 분역구조 및 크기효과에 관한 연구

에피성장한 강유전 나노구조체의 분역구조 및 크기효과에 관한 연구
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The demands on the increase in the density and the decrease in electric power, product cost of ferroelectric electronic devices such as data storage media are driving the miniaturization of ferroelectric thin films. In order to utilize the nanostructured ferroelectrics for the data storage media with ultra-high density, it is highly desired to study on ferroelectric size effects, i.e. how small features can preserve the ferroelectricity and how the domain structure can change with down-scaling because the ferroelectricity is cooperative phenomenon arising from the alignment of local dipole moments through both chemical and physical interactions and the ferroelectric properties are originated from the domain structure. In this context, the size effects and the domain structures in epitaxial Pb(Zr,Ti)O3 (PZT) thin films were systematically investigated in this work. The evolution of epitaxial ferroelectric domain structures was characterized as a function of the material size. The epitaxial PbTiO3 (PTO) nanoislands with size ranging from several hundreds to tens of nanometer were synthesized by chemical solution deposition (CSD), which is one of typical bottom-up approaches. The lateral size and thickness of nanoislands scale down, simultaneously with scaling ratio (f = d/h, d: lateral dimension, h: height) of 3 to 6. PTO nanoislands with average size of 490 nm had a similar domain structures with thin films, i.e., large fraction of c-domain and four-fold symmetric a-domains. As the size decreased, the abundance of a-domains decreased and finally single c-domain structures were observed in 50 nm-sized PTO nanoislands. It was found that the tetragonality of nanoislands was lower than that of thin films, which indicates that the nanoislands are subject to lower misfit strain than thin films, in which strong constraints arising from the rigid substrate exist. It is well known that the piezoelectric property systematically decreases with thickness reduction due to the depolarization field although the tetragonality could be preserved. However, according to electrical measurement (e.g., piezoelectric force microscope: PFM), it was feasible to maintain or even enhance the piezoresponse of nanostructured ferroelectrics through the reduction of a-domains and misfit strains in nanoislands. The piezoresponse was converged to zero dramatically at the nanoislands with thickness of below 8 nm due to the increase of sensitivity to misfit dislocations. The CSD method can provide the opportunity for systematic study on the intrinsic size effects in the extremely small size range. Nevertheless, it still needs to fabricate even smaller sized ferroelectrics with narrow size distributions for more practical applications and precise investigations. In order to fabricate ferroelectric nanocapacitors with size of 65 nm, hard mask approach for physical vapor deposition (PVD), i.e., mask-assisted pulsed laser deposition (PLD) was employed. Ultra-thin anodic alumina (AAO) mask was fabricated as a stencil mask. By taking advantage of excellent thermal stability of alumina, the Pb(Zr,Ti)O3 (PZT) deposition could be performed at crystallization temperature (e.g., 650 °C), resulting in in-situ growth of ferroelectric PZT nanoislands. High resolution synchrotron x-ray diffraction (XRD) with reciprocal space mapping (RSM) confirmed that the 65 nm-sized ferroelectric PZT nanoislands were grown epitaxially on Pt-bottom electrode and contained mostly c-domains, very small fraction of four-fold symmetric a-domains and non-tilted a-domains, which are not presented in the PTO nanoislands fabricated by chemical route. The electrical properties were investigated by PFM. For the accurate electrical measurement, arrays of nanocapacitors with Pt/PZT/Pt stacking configuration (Metal/Ferroelectric/Metal: MFM capacitor type) were prepared by depositing Pt on PZT nanoislands through AAO. It was revealed by local hysteresis measurement that the some of a-domains are switched during the DC bias sweeping and the piezoresponse of nanocapacitors is higher than that of continuous films due to the reduced substrate clamping in nanostructures. Moreover, the nanocapacitors were individually switchable by AFM tip without any interference of electric field among the capacitors while the thin films with same sized-Pt nanoelectrodes showed the significant propagations of electric field to neighboring capacitors. The retention was also tested for 72 hours and showed that the nanocapacitors were stable within the time scale investigated. For the fabrication of perfectly ordered low-dimensional epitaxial ferroelectric nanostructures cover a large area, photolithography process was used. Wafer scale arrays of well-ordered Pb(Zr,Ti)O3 nanodisks and nanorings were fabricated on the entire substrate area (10 mmⅹ10 mm) of SrRuO3 bottom electrode on SrTiO3 single crystal substrate using laser interference lithography (LIL) process combined with pulsed laser deposition. The shape and size of the ferroelectric nanostructures were controlled by the amount of PZT deposited through the patterned holes and the temperature of post crystallization steps. X-ray diffraction and transmission electron microscopy confirmed that (001)-oriented PZT nanostructures were grown epitaxially on SrRuO3 (001) bottom electrode layer covering the (001)-oriented single crystal substrate. The domain structures of PZT nano-islands were characterized by reciprocal space mapping using synchrotron X-ray. Ferroelectric properties of each PZT nanostructure were characterized by scanning force microscopy (SPM) in the piezoresponse mode. If the laser with shorter wavelength becomes available, for example ultra violet (UV) source, then one can realize even smaller sized ferroelectrics for achieving ferroelectric devices with higher density.
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