Study On The Fabrication And Electrical Properties Of Epitaxial BiFeO₃ Thin Films

With the rapid development of computer,communication and network,as well as the pursuit of low cost,high speed and large capacity,requirements in the device integration,miniaturization and multi-functional are getting higher and higher,some multi-functional materials with electrical and magnetic properties have attracted researchers interests.Therefore multiferroic materials have become the research hotspot.As the only single-phase multiferroic material at room temperature,much attention has been paid to BiFe O₃?BFO?.In this thesis,epitaxial BFO thin films have been deposited on single crystal SrTiO₃?STO?and Nb-SrTiO₃?Nb-STO?substrate by pulsed laser deposition,the strain effects on crystal structure and piezoelectric properties of thin films have been investigated,then ferroelectricity and leakage current of 200 nm BFO thin films under different temperature were also studied.The main experimental results are as follows:1.The deposition of BFO thin films have been optimized by adjusting the deposition parameters such as laser energy,deposition temperature and oxygen pressure in?001?-oriented STO single crystal substrate by pulsed laser deposition,the optimal deposition parameters are as follows:the deposition temperature and oxygen pressure are 680?,13 Pa respectively,laser energy in the chamber is 60 mJ and the laser frequency is 5 Hz,the distance between target and sample is 6 cm,the as-grown sample was kept at 680?and 1200Pa oxygen pressure for 60 minute,then was cooled down to room temperature by 5?·min^-1.XRD results demostrate that BFO thin films are epitaxially deposited on?001?STO,AFM show that the morphology of BFO thin films are flat and uniform,and there are no particles and holes.The ferroelectric test results show that BFO film has good ferroelectricity with a remonant polarization of 102?C/cm².2.Epitaxial BFO thin films of various strain states were grown on?001?-oriented Nb-STO single crystal substrate by pulsed laser deposition,the strain effects on crystal structure and piezoelectric properties of BFO thin films have been investigated.XRD and TEM results show that BFO thin films were epitaxially grown along the substrate orientation.The out-of-plane strain in BFO thin films decreased from 2.778%to 2.045%with the increase of thickness,BFO thin films also evolved from full strained to a partially relaxed one.Piezoresponse force microscopy measurements revealed that all the as-grown BFO thin films possess high piezoelectric response and the piezoelectric coefficient d₃₃ decreased from 259pm/V to 136 pm/V with the decrease of the film thickness.3.200 nm BFO thin films were epitaxially grown on?001?-oriented STO single crystal substrate with 40 nm La_0.67Sr_0.33MO₃?LSMO?buffer layer as bottom electrode.We investigate its ferroelectricity and leakage current under 300 K?250 K?200 K?150 K respectively,and futher the leakage mechanisms under different temperature were also analyzed.XRD results show that BFO thin films were epitaxially grown along the substrate orientation.Transmission electron microscope?TEM?shows the interface between BFO and LSMO is relatively clear and smooth,and no dislocation was observed.The epitaxial relationship was determined to be?001?BFO//?001?LSMO//?001?STO,[010]BFO//[010]LSMO//[010]STO.Ferroelectric test results showed that the BFO thin films have good ferroelectricity.When the test temperature ranges from 300 K to 150 K,its remanent polarization P_r decreases from 132?C/cm² to 114?C/cm²,while its coercive field increases from 181 kV/cm to 207 kV/cm.Leakage current test results show that the leakage current decreases when reducing the temperature.There is a peak in the leakage current curves at various temperature,which represents the transition between two leakage mechanisms.The leakage mechanism changes from Ohmic conduction to space-charge-limited current when the temperature decreases from 300 K to 200 K under low electric field,while in the high electric field range,the leakage mechanism is dominated by Fowler-Nordheim tunneling behaviror and does not chang with the test temperature.