Study On The Preparation And Properties Of Epitaxial Pb(Zr_0.52Ti_0.48)O₃ Single Crystal Thin Film

Ferroelectric thin films have attracted great attention for their physics and potential applications in various microelectronic devices such as nonvolatile ferroelectric random access memories and ferroelectric field-effect transistors.In order to reduce the dislocations and keep a high performance,conductive oxides with a lattice perfectly matched to the ferroelectric films are highly desirable in ferroelectric devices fabrication;on the other hand,for optical applications in invisible electronic circuits high quality epitaxial ferroelectric heterostructures are also demanded.Among the applications for ferroelectric thin films nonvolatile memories are attractive due to the ability to hold information without requiring an external field,one of the critical features of nonvolatile memory devices is typically destructive reading,requiring the ferroelectric to endure large number of electrical cycles during operation.However decrease of switching charge due to repeated polarization reversal,know as fatigue,is a common problem in bulk and thin film ferroelectrics.Fundamental studies of ferroelectric materials are essential in understanding the origin of the fatigue mechanisms.There have been a number of advances to overcome fatigue in ferroelectric films,such as the utilization of oxide electrode,or using layered type ferroelectrics that exhibit fatigue-free behavior.There still exist certain issues which need to be solved when employing these strategies. Like fatigue,imprint is another failure effect which demonstrates itself by shifting the hysteresis loop along the electric field axis,with a higher coercive electric field and undistinguishable polarization states imprint can cause a failure of the ferroelectric devices,various mechanisms especially electrode-ferroelectric interface effects have been attributed to the appearance of the internal electric field,thus in all oxide epitaxial heterostructures both top and bottom interface states should be considered.In this thesis pulsed laser ablation method was used to deposit thin film. Epitaxial and transparent ferroelectric capacitor was successfully fabricated by employing a new transparent conductive oxide as electrode.Through changing the p/n type electrode ferroelectric capacitors with different electrode configurations were deposited,their fatigue behaviors were carefully investigated,it is shown that not all oxide electrodes can be used to improve fatigue resistance in PZT capacitors,the n type electrode induced fatigue may related to charge injection and the interface capacitor or built-in potential between PZT and n type electrode.The effect of the film quality and the effects of interface contact on permittivity characterization were also discussed.At last,conductive oxide electrodes with different lattice constant were used to fabricate all-oxide ferroelectric capacitors,their process-induce imprint failures were studied,the results indicts that both top and bottom interface strain states are closely correlated to the process-induce imprint,the internal electric field may induced by the oxygen-loss-related lattice strain.The whole thesis consists of five chapters.Chapter 1:The general review of the history and present research situation of the ferroelectric thin film is given.Some related properties,such as ferroelectric thin film devices,crystal structure of perovskite and layered ferroelectric material,doping effect,domain structure,domain growth and switching,reversible and irreversible polarization,are introduced.In the end,we sum up the effects oxygen vacancy and charge injection on the polarization degradation and different models that have been suggested to explain the fatigue behavior,various mechanisms(such as interface screening,built-in electric field and mechanical stress et al.)that given for interpretation of the imprint failure were also summarized.Chapter 2:New conductive and transparent La_0.07Sr_0.93SnO₃(LSSO)film was employed as electrodes for fabrication of epitaxial Pb(Zr_0.52Ti_0.48)O₃(PZT) ferroelectric capacitors.Due to negligible lattice mismatch between LSSO and PZT, the LSSO/PZT/LSSO heterostructures grown on SrTiO₃(001)substrates show excellent single-crystalline quality as revealed by high-resolution x-ray diffraction, and are transparent with transmittance comparable to that of pure SrTiO₃(001) substrates at wavelength of 400-2500 nm.These capacitors show square polarization-electric field hysteresis loops,but smaller polarization,larger coercive field,and especially poor fatigue resistance,in sharp contrast with those observed for the conventional epitaxial all-oxide PZT capacitors.Chpter 3:By using highly conductive p-type La_0.7Sr_0.3MnO₃(LSMO)and n-type La_0.07Sr_0.93SnO₃(LSSO)as electrodes,all-oxide Pb(Zr_0.52Ti_0.48)O₃(PZT)capacitors, LSMO/PZT/LSMO,LSSO/PZT/LSSO,LSSO/PZT/LSMO,and LSMO/PZT/LSSO, have been grown epitaxially on(001)SrTiO₃ substrates,and their structure,switching, fatigue and optical properties were investigated.Strikingly,contrary to the LSMO/PZT/LSMO capacitors,those having the n-type electrodes show poor fatigue resistance especially at lower driving frequencies,which was further confirmed by using another n-type oxide electrode,SrTi_0.9Sb_0.1O₃.The results suggest that with a depletion layer at the PZT/LSSO interface,charge accumulation and injection during switching may be responsible for the fatigue.Chapter 4:Using La_0.7Sr_0.3MnO₃(LSMO)and Pt as bottom and top electrode respectively,high quality Pb(Zr_0.52Ti_0.48)O₃(PZT)ferroelectric thin films with different thickness were epitaxially grown on(001)SrTiO₃ substrates,analyzing the PZT epitaxial thin films as p-type,wide-gap semiconductors and standard Schotty contact with metal electrode through C-V measurement,the intrinsic dielectric constant of this PZT composition seems to be thickness independent and of low value of about 307.The dielectric performances at different fatigue stages of the PZT capacitors with p/n type electrode were also characterized and the primitive results indicate that fatigue behaviors are closely related to the interface capacitance and built-in electric field.Chapter 5:By using La_0.7Sr_0.3MnO₃(L)and SrRuO₃(S)as electrodes, Pb(Zr_0.52Ti_0.48)O₃(PZT)capacitors with different electrode-configurations,i.e., L/PZT/L(from top to L/PZT/L(from top to bottom)(a),S/PZT/S(b),S/PZT/L(c), and L/PZT/S(d)have been grown epitaxially on(LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7(001) substrates,and their process-induced imprint behaviors were investigated.After being annealed at reduced oxygen pressures,the capacitors a(b)have a negative(positive) imprinted polarization(P)state at zero electric field(E),the capacitors c show antiferroelectric-like P-E hysteresis loops,while for d the loops are very stable.We show evidences that the different internal field could be caused by various oxygen-loss-induced lattice strains at the PZT top and bottom interfaces.