Resonant Tunneling Of Ferroelectric Tunnel Junctions

A typical ferroelectric tunneling junction（FTJ）is composed of two conducting electrodes separated by a nanometer-thick ferroelectric layer,which serves as a tunnel barrier.The resistance change resulting from polarization reversal of the ferroelectric barrier layer called tunneling electroresistance（TER）effect.Such polarization switching allows the control of two nonvolatile resistance states of an FTJ（low and high）which can be employed in random access memories and other electronic devices.Enhancing the magnitude of TER is beneficial for applications of FTJs in novel electronic devices.In the last 20 years,researchs are focused on the symmetry induced TER of the FTJs.Particularly,the effective barrier height and width of electron tunneling can be adjusted by using different electrodes,composite barrier layers and interface engineering to affect the direct tunneling of two polarization states.This is mainly due to the increase of the effective barrier height or width of high resistance states,because the ferroelectric properties of ferroelectric films will be suppressed with decreasing of the film thickness.In order to combine FTJs with complementary metal oxide semiconductor（CMOS）electronic devices,while maintaining the polarization stability of ferroelectric thin film,it is necessary to reduce the resistance of low resistance state while maintaining the resistance of high resistance state of FTJs.In this thesis,a mechanism of enhancing the TER effect of FTJs by resonant band engineering is proposed.By inserting an ultrathin dielectric layer with a small band gap into the ferroelectric barrier layer,a ferroelectric polarization dependent potential well could be formed.The reversal of polarization changes the position of the potential well relative to the Fermi level,which results in switching of transport between direct tunneling and resonance tunneling and a giant TER effect could be realized.Our theoretical prediction is based on the recent development of thin film growth techniques in which the growth of thin film heterostructures can be controlled with precision at the atomic scale.Using first-principles calculations and quantum mechanical tunneling models,we investigate the embedding of BaSnO₃ monolayer with relatively smaller band gap in the BaTiO₃ barrier of SrRuO₃/BaTiO₃/SrRuO₃ FTJ.In such a composite barrier FTJ,ferroelectric polarization of BaTiO₃ shifts the conduction band minimum（CBM）of the BaSnO₃ monolayer above or below the Fermi energy.For one polarization direction,when the CBM of BaSnO₃ monolayer is under the Fermi level,the transport in FTJ is dominated by the resonant tunneling effect.For opposite polarization directions,the bottom of the conduction band of BaSnO₃ monolayer is higher than the Fermi level,and the transport of FTJ is dominated by direct tunneling.We found that through such band engineering,the ON state resistance is greatly reduced and the large TER effect can be achieved under the premise of maintaining the stability of low scale ferroelectric polarization.Additionaly we propose that the integration of resonant band engineering and FTJs asymmetry can further enhance the TER effect.We studied an asymmetric FTJ SrRuO₃/BaTiO₃/SrTiO₃/SrRuO₃ with composite barriers.The resonant band is integrated into this FTJ by two layers of BaSnO₃ and an elaborated FTJ is formed SrRuO₃/BaTiO₃/BaSnO₃/SrTiO₃/SrRuO₃.Our research shows that in such FTJ,the TER effect induced by asymmetry and the TER effect induced by resonant tunneling can be effectively superimposed together to further enhance the TER.For one polarization direction,the BaSnO₃ and SrTiO₃ dielectric layers work together as barrier to provide large efficient barrier height for direct tunneling and lead to large tunneling resistance in OFF state.For the opposite polarization,the BaSnO₃ layer serves as quantum well to induce resonant tunneling and considerably reduces the tunneling resistance of the ON state.We further consider the ferromagnetism of metal electrodes and investigate the effect of the resonant tunneling effect on the performance of SrRuO₃/BaTi（Sn）O₃/SrRuO₃ multiferroic tunnel junction.We studied four effective resistance states in this resonant multiferroic tunnel junction,two resistance states controlled by polarization inversion and two magnetoresistance states controlled by parallel and antiparallel of electrode magnetic states.The thesis is organized as follows.The first chapter is the introduction of ferroelectric properties and ferroelectric materials;the second chapter introduces the FTJ and its transport properties;in chapter 3,the resonant band engineering controlled by ferroelectric polarization in FTJs is discussed;in chapter 4,the effective integration of resonant band engineering and asymmetry of FTJs is discussed;in chapter 5,multiferroic tunnel junction with resonant tunneling effect is talked about;chapter 6 is summary.