(100)- and/or (001)-preferred-oriented and (111)-preferred-oriented polycrystalline Pb(Zr,Ti)O3 (PZT) films with Zr/(Zr+Ti) ratios of 0.35 and 0.62, corresponding to tetragonal and rhombohedral PZT, respectively, were compared with that of epitaxially grown ones with the same composition and similar orientation from the viewpoint of crystal structure and ferroelectricity. (100)- and/or (001)-preferred-oriented and (111)-preferred-oriented polycrystalline PZT films were deposited on (111) Pt/Ti/SiO2/Si substrates at 580 and 415 °C, respectively, by pulsed-metalorganic chemical vapor deposition (pulsed MOCVD). On the other hand, epitaxially grown PZT films with the same composition and similar orientation were deposited on SrRuO3‖SrTiO3 substrates at 580 °C also by pulsed MOCVD. The difference in ferroelectricity between the polycrystalline and epitaxial films was found to be mainly due to the film composition rather than to the film orientation. Polycrystalline tetragonal PZT films deposited by pulsed MOCVD showed almost the same remanent and saturation polarizations (Pr and Ps) as the epitaxially grown ones corrected for film orientation even though the films were deposited at a low temperature of 415 °C. Moreover, the coercive field (Ec) values were almost the same. On the other hand, the polycrystalline rhombohedral films showed almost the same Ps value as the epitaxially grown ones, but their Pr value was smaller than that of the epitaxial ones. Moreover, the Ec value of the polycrystalline films was smaller than that of epitaxially grown ones. The difference in ferroelectricity between tetragonal and rhombohedral PZT is strongly related to the film composition’s dependence on the domain wall stability when the electric field is released.
The crystal structure and the electrical properties were systematically compared for tetragonal and rhombohedral Pb(Zr,Ti)O3 [PZT] films prepared at various deposition temperatures from 580 °C to 395 °C on (111)Pt/Ti/SiO2/Si substrates by pulsed-metalorganic chemical vapor deposition (pulsed MOCVD). Film orientation changed from (111) to (100) and/or (001) with the decrease in the deposition temperature, but the well-crystallized PZT phase was obtained down to 395 °C. The lattice parameter was almost constant with decreasing deposition temperature, suggesting that the Zr and Ti elements in the films were incorporated into the PZT phase. When the deposition temperature decreased, the leakage current density decreased together with a decrease in surface roughness, especially for tetragonal PZT films. Remanent polarization (Pr) continuously decreased with decreasing deposition temperature down to 395 °C, but was above 20 μC/cm2 even at 395 °C. Specifically, the tetragonal film shows good squareness down to 415 °C. These results show that PZT films with low leakage current density and a large Pr were obtained even at 395 °C by pulsed MOCVD irrespective of the film composition.
Fabrication of PbTiO3/Pt/yttria-stabilized ZrO2(YSZ) heteroepitaxial films on Si substrate was investigated with the aim of applying them to electric devices such as integrated infrared sensors and ferroelectric random-access memories (FRAMs). First, a (100)YSZ buffer layer was epitaxially grown on a (100)Si substrate using reactive electron-beam evaporation. In this process, a metallic Zr film was deposited before YSZ deposition in order to prevent a Si substrate surface from oxidizing. Then, Pt film was sputtered on the buffer layer. A rough YSZ surface attained by slightly etching the Si substrate surface is found to be essential to obtain (100)-oriented Pt films, while (111)-oriented Pt films were obtained on a smooth YSZ surface. This indicates that we can control the orientation of Pt film by controlling the morphology of the YSZ surface. Lastly, a (001)-oriented PbTiO3 film was epitaxially grown on (100)Pt/(100)YSZ/(100)Si using metalorganic chemical vapor deposition (MOCVD).
The effect of the atmosphere during the heating of a substrate before starting the deposition on the characteristics of the deposited Pb(Zrx,Ti1−x)O3 (PZT) films was investigated. Rhombohedral PZT films were prepared on Pt/Ti/SiO2/Si substrates by metalorganic chemical vapor deposition (MOCVD) from 415 to 580 °C. PZT films with smooth surfaces and low leakage current densities were obtained when the substrate was heated under an Ar atmosphere at the heating rate of 7 °C/min. Moreover, a crystalline PZT film having good ferroelectricity was obtained at as low as 415 °C when the substrate was heated under the Ar atmosphere, while the film consisted of an amorphous phase when the substrate was heated under an O2 atmosphere. These results clearly show that heating the substrate under an Ar atmosphere before starting the deposition is effective for obtaining a film with a large ferroelectricity at a low deposition temperature for the MOCVD process.
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