Preparation And Characterization Of BiFeO₃ Ferroelectric Films By Low Temperature Magnetron Sputtering

As a prototype single-phase multiferroic,BiFeO3(BFO)exhibits excellent electrical,magnetic and magnetoelectric properties,appealing to many modem technological applications.One of its overlooked merits is a high piezoelectric performance originating from its large remnant polarization(Pr)and low dielectric constant(?r).Furthermore,its high Curie temperature and large coercive field ensure good stabilities in device applications.However,to achieve close-to-intrinsic properties,a high processing temperature is usually used for the preparation of highly-crystalline BFO films.Proliferation of defects due to loss of volatile Bi2O3 in the high temperature process and its incompatibility with CMOS-Si technologies,have hindered the development of BFO-film based Piezo-MEMS devices.To solve the above problems,the high-quality BFO films with reduced defects were deposited at a low temperature(350?)in the radio-frequency(RF)magnetron sputtering system.By carring out a series of testing methods,the structures and performances of the BFO films were characterized and analysed.The results show that we achieved lowering down the processing temperature and ensuring excellent performances at the same time.Specific research contents and results can be summarized as follows:(1)The BFO film with Pt/Ti bottom electrode and LNO buffer layer was successfully prepared on Si/SiO2 substrate at 350? by RF magnetron sputtering.The serious aging phenomenon i.e.,properties degradation over time,was obversed in the low-temperature prepared BFO films,due to the oxygen vacancies or defective compounds in the bulk and interface regions.The non-intrinsic polarization caused by defects made the initially measured polarization intensity(Ps?100 ?C/cm2)appear arguablely inflated,while the stable polarization intensity(Ps?55 ?C/cm2)after fully aging decreased by as much as 45%.(2)On the basis of(1)above,the thermal insulation process was created and its parameters e.g.,thermal insulation time,atmosphere,and oxygen pressure were optimized.The effects of different paramaters on performance improvement were also explored.It was found that thermal insulation in pure oxygen atmosphere,increasing time and oxygen pressure in the thermal insulation process,can suppresse the formation of bulk and interfacial defects and promote the BFO film's crystalline quality,thus overcome the aging problem mentioned in(1)above.Finally,we obtained the high quality BFO films with a stable polarization intensity(Ps?80-90 ?C/cm2)lifting by?60%.(3)Detailed characterizations to structures and properties of the optimized BFO films were carried out,especially the anti-aging and fatigue resistance of piezoelectric properties for practical applications.A bulk-like rhombohedral phase with a(100)-preferred orientation is identified.The ferroelectric and piezoelectric properties of BFO films stayed stable over at least 6 months.The piezoelectric coefficient e31,f in this work reaches 80%of PZT film prepared at 500? and BFO film prepared at 650?.Furthermore,the cycling performance shows only a small variation(<2.5%)in e31,f,during actuation cycles up to 1.2×106 times.Moreover,in comparison with other lead-free materials(such as BTO,KNN,etc.),this work has the maximum energy efficiency and signal-to-noise ratio.All of these results mean that the optimized BFO film in this thesis has a good application prospect.(4)To systematically study the relationship among the preparation temperature,structure morphology,chemical valence state and electrical properties,and to explore the significance of low temperature preparation,the same processing parameters with(2)were used to prepare BFO films at different deposition temperatures(including 300?,350?,500? and 650?).It was found that the preparation temperature could affect the crystallization degree,crystal orientation,grain size,surface and interface morphology of the BFO films.When the temperature is higher than 350?,with the increase of the preparation temperature,the grain size becomes larger,the surface roughness and defects increases,and the interface diffusion turns to be serious,causing worse rectangle of hysteresis loop,larger dielectric loss and leakage current,reducing the piezoelectric coefficient(e31,f)and the figure of merit(FOM).As a result,the processing parameters in this thesis is suitable for low temperature preparation of BFO films.