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Frist-Principle Calculation And Ferroelectric Properties Of PZT

Posted on:2007-08-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:G PengFull Text:PDF
GTID:1118360242961768Subject:Microelectronics and Solid State Electronics
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Compared with the traditional EEPROM and FLASH memory, the new non-volatile ferroelectric memory (FeRAM) have many advantages, such as low operating voltage, low power consumption, long-time retention, quick writing-operation and outstanding anti-radiation characteristics etc, which are very suitable for the requirements of embedded applications. Therefore, this paper mainly focused on the fundamentally theoretical and experimental research of the ferroelectric film materials which are the key components of the embedded FeRAM.By using the calculation methods of the First Principle, we got the electronic structure of the PTO paraelectric phase and ferroelectric phase, PZT ferroelectric phase with different atomic ratio of Zr/Ti, and the paraelectric phase, ferroelectric phase and anti-ferroelectric phase of PZO respectively. From the calculation results of energy of different phase, at low temperature, PTO, PZT are ferroelectric phase and PZO is anti-ferroelectric phase, which are determined by the principle of minimum binding energy. The electronic structure calculations of PTO, PZT and PZO indicate that the B-sited Ti3d and Zr4d electrons have intensive hybridization with O2p electronics, and the hybridization degree of Ti and O atoms is stronger than that of Zr and O atoms, the hybridization degree in ferroelectric is stronger than that of the paraelectric phase. Similarly, strong intensity of hybridization degree also exists between A-sited Pb6s and O2p atoms, which is strongest in the ferroelectric phase. From the composition of PZT energy band, it is mainly contributed by the electronic energy band of Pb atoms below the valence band, and at the bottom of the valence band it is mainly contributed by hybridized energy bands between the Ti, Zr atoms and O atoms, while at the top of the valence band by the energy band of O atoms, and the bottom of the conduction band by the hybridized energy band of Ti and O atoms. As the ratio of Ti atoms increasing, the hybridization intensity of PZT between B-sited and O atoms enhances, which shows that the ferroelectric characteristics increases and the leakage current increases as well when the energy gap narrows.The PZT37 compound with 1%Ta-doped was chosen as the ideal material for the embedded ferroelectric memory from the material characteristics, technical preparation and the theoretical results, and the LSMO Target, PbO Target and PTZT Target with enriched 20% Pb were prepared by solid state reaction method. The PTZT ferroelectric thin film with LSMO buffer layer deposited at high temperature can meet the needs of FeRAM from the analysis and comparision of the crystal structures, ferroeclectric characteristics and the fatigue characteristics of PTZT ferroelectric film with TiO2, PbO and LSMO buffer layers. Pt/LSMO/PTZT/ LSMO/Pt/TiO2/SiO2/Si is an optimized layer structure and it is suitable for ferroelectric thin film ferroelectric memory. It is indicated that the ferroelectric properties and fatigue characteristics are at the most outstanding when the thickness of the LSMO buffer layer is about 20nm from the analysis of the PTZT ferroelectric thin film with different thickness of LSMO buffer layers.From the analysis of the influence on the ferroelectric properties of the LSMO/PTZT/LSMO sandwich thin film structure, the optimal deposition conditions for PTZT film capacitor are that sputtering work pressure is 1.5Pa, substrate temperature is 200°C, sputtering atmosphere is pure argon gas, annealing temperature is 650°C and annealing time is 20 minutes.Furthermore, LSMO/PTZT/LSMO ferroelectric thin film with a 75mm diameter was successfully prepared on Pt substrate。The surface and section morphology of the film, the dielectric properties and the ferroelectric characteristic were measured. The results show that the films are compact and smooth with uniform thickness (deviation of less than 3.5%); The averaged remanent polarization is about 52.6μC/cm2 at the maximum voltage of 12V, and the average coercive field is about 2V (deviation of less than 3.5%); after 109 switches polarization, the normalized remanent polarization turns to be about 90% of the initial polarization (deviation of less than 2.8%); the film dielectric constant decreases with the increase of the frequency, but this trend turns slower after 1MHz is loaded.Based on the polarization reversal physical behavior, a phenomenological physical model is derived for description of the P-E hysteresis behavior and simulated ferroelectric capacitor. In this model, a differential equation is deduced by making some reasonable assumptions, and the equation can describe not only the saturated and unsaturated hysteresis loops and the loops among any giving arbitrary conditions. To examine the validity of the model our experimental data and the experimental data reported in literatures were used to simulate by our model, and the results shows good consistency, which indicates that the model is valid to model the ferroelectric capacitor for both saturated and unsaturated loops and any arbitrary conditions.
Keywords/Search Tags:Ferroelectric random access memories, First Principle calculation, PTZT ferroelectric thin films, LSMO buffer layer, Ferroelectric capacitor model
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