Preparation And Characterizations Of Several Heterostructures For Ferroelectric Memory Applications

In the past two decades, ferroelectric random access memories (FeRAMs) have attracted much attention. The most intensively studied material is perovskite Pb(Zr,Ti)O3(PZT) thin films. However, PZT thin films, sandwiched in metal electrodes, may suffer from severe polarization fatigue, reduction of remanent polarization due to repetitive switching. Moreover, lead in PZT may revoke health and environment issues. Bismuth-based layered perovskite ferroelectric thin films have been found to show a fatigue-free characterisitic with simple metal electrodes. However, compared with PZT, the bismuth-based layered perovskite show the smaller ferroelectric polarizations, which limits their applications on the high-density FeRAMs.Most recently, multiferroic BiFeO3(BFO) has been investigated intensively due to its high ordering temperatures. BFO, having a ferroelectric Curie temperature of1100K, exhibits a large room temperature remanent polarization (Pr), about60-100μC/cm2for single crystals and thin films. It has been considered as a promising candidate for high-density ferroelectric random access memories. However, due to the existence of Fe2+and oxygen vacancies, BFO suffers from large leakage currents, which results in the difficulty in achieving saturated P-E hysteresis loops.In addition, great technological advances have been achieved in growth of perovskite oxide thin films. And ferroelecticity is found to exist even in thin films of a few nanometers in thickness. This makes it possible to realize a new type of non-destructive readout nonvolatile memories based on ferroelectric tunnel junctions (FTJs). In a FTJ, an ultrathin ferroelectric layer serves as a potential barrier and allows direct quantum mechanical tunneling. The potential height seen by transport electrons can be electrically tuned by polarization reversal. This gives rise to the electrical switching of the tunneling resistance, i.e., the so-called tunneling electroresistance (TER), which manifests itself as a ratio of ON/OFF, where ON is the current density of the low resistance state and OFF that of the high resistance state.This work focus on the Pr and Mn substituted BiFeO3(BPFMO) thin films and the ferroelectric tunneling heterostructures. A series of important results have been achieved, as summarized below,(1) The effects of Pr substitution on the structures, magnetic and electrical properties of BPFMO films are investigated systematically. A phase transition from rhombohedral to tetragonal structure occurs at about15%Pr substitution. Enhanced ferromagnetism at room temperature is observed near the phase boundary composition. The increased remanent magnetization in the15%Pr-substituted film can be understood by taking into account both the modification of the short-range canted G-type antiferromagnetic order at phase boundaries and the suppression of long-range incommensurate spin cycloid of BiFeO3due to Pr substitution. Meanwhile, large ferroelectric remanent polarization in15%Pr substitutied films originates from the suppressed leakage currents and the increased amount of180°domains.(2) Leakage current characteristics of BPFMOx=0.14thin films are studied. Space charge limited current and Poole-Frenkel (PF) emission are found to be the dominant mechanism in the low and the high electric fields, respectively. A negative differential resistivity behavior is observed before the onset of PF emission at93K, which is discussed in terms of the competition between electron trapping and field-assisted thermal emission.(3) Polarization fatigue of BPFMOx=0.15thin films is studied as functions of switching fields and frequencies. Poor fatigue resistance is observed at low switching frequencies and small cycling fields. An increase of dielectric constant is observed in accompany with the suppression of switchable polarization. Both the switchable polarization and the dielectric constant can be restored to their original values simultaneously by high electric field cycling. These characteristics are discussed in terms of migration and aggregation of oxygen vacancies on domain walls to block domain switching. The results support the domain wall pinning scenario for polarization degradation in BPFMO films.(4) Two distinctive resistance states dependent on polarization directions are observed in Pt/BaTiO3/SrRuO3FTJs. Repetitive bipolar switching of the tunneling resistance is achieved at room temperature. Although the largest TER ratio of junctions observed at room temperature is only about130%, a ON/OFF ratio of420%can be obtained at10K. The enhanced TER at low temperature is ascribed to the reduced defect currents in BaTiO3barriers.(5) Giant electroresistance is observed in Pt/BaTiO3/Nb:SrTiO3ferroelectric tunnel diodes we proposed. The ON/OFF ratio of the diodes can reach13000at room temperature. Moreover, the Pt/BaTiO3/Nb:SrTiO3diodes exhibit stable resistance states in retention test and a typical bipolar resistance switching with good reproducibility. An ON/OFF ratio of100can be maintained in the ferroelectric diodes up to3000switching cycles. Current-voltage characteristics of the diodes are investigated as a function of temperature, which indicates that the giant electroresistance in the Pt/BaTiO3/Nb:SrTiO3result from the modification of Schottky barrier height in the Nb:SrTiO3semiconductor by polarization reversal.