| Lead-zirconate-titanate(Pb(ZrxTi1-x) O3, 0<x<1) is one of the traditional ferroelectric materials. Devices based on PZT ferroelectric thin films have attracted our attention because of their unique ferroelectric, pyroelectric and piezoelectric properties. The crystal structure and electrical properties can be changed greatly by changing the ratio of doped zirconium and titanium or doping some other elements. The application of Pb(ZrxTi1-x) O3 thin films in MEMS sensor, small electronic devices(MEMS) and nonvolatile memory(NVM) are also in great demand. That’s why most people are still studying Pb(ZrxTi1-x) O3 thin films. PZT is perovskite structure, which can be written as ABO3. Materials have strong ferroelectricity because the electric binding forces are different for different ions and the electrons bounded by weaker electric forces can be polarized by external electric field. The PZT thin films are fabricated on non-conducting substrate with SRO on it and conductive substrate respectively to explore the properties of PZT thin films.Main steps are as follows:1. The PZT target is made by solid state reaction method and Zr/Ti ratio is 52/48. The morphotropic phase boundary(MPB) is formed between trigonal and teragonal phase near x=0.52 at room temperature. The above two phases co-exsist and the crystal structure of the ferroelectric material will be mutated in the morphotropic phase boundary. A high pure PZT ceramic target has been prepared by powder embedded sintering method and excessive proportion method to avoid the losses of lead element during the preparation of PZT target.2. PZT thin films are prepared by pulsed laser deposition(PLD) method on strontium titanate(STO) and Nb-doped strontium titanate(NSTO) substrates respectively. The strontium ruthenate(SrRuO3) layer is prepared on STO substrate as bottom electrode before depositing PZT thin films, which is useful for the preparation of PZT thin films. In addition, SRO can improve the electrode fatigue properties. Platinum(Pt) is prepared as top electrode by small ion sputtering instrument on PZT thin films and weld indium(In) is prepared under the NSTO substrate as bottom electrode.3. The experiment condition is difficult to control when SRO thin films are prepared by pulsed laser deposition technique directly because SRO target is hygroscopic. The water vapor of SRO target and some other gases are removed by high temperature and the SRO target is putted in a tube furnace for 12 hours. Then SRO thin films are prepared successfully and the resistivity is measured by four-probe method. The results showed that SRO thin films with good conductivity can be obtained through the pre-treatment of SRO target.4. The structure of PZT/SRO/STO is called MFM. The resistive switching mechanism of PZT thin films is understood by analyzing its XRD pictures, SEM figures, P-V, C-V and I-V curves.5. PZT thin films deposited on NSTO substrate constitutes a typical MFS(metal-ferroelectricsemiconductor) structure which is the basic cell of FFET(ferroelectric field effect transistor). Its structural characterization of X-ray diffraction and phi-scan curves are presented in our paper respectively. It is found that the PZT thin film prepared on NSTO substrate is four fold symmetry epitaxial structure, showing tetragonal phase because of lattice stress. According to the I-V tests, a typical rectification effect below 3V, an abnormal bipolar resistance switching(BRS) between 3V and 8V, and normal bipolar resistance switching above 8V. Based on piezoelectric force microscopy(PFM) tests, the rectifying property originates from p-n junction but not polarization and the bipolar resistive switching is related to the polarization of PZT thin films. The DC I-V tests showed that 8V is the critical point of transformation between abnormal and normal bipolar switching modes and the electric field is 1.66×104 kv/m. According to the PFM-phase and PFM-amplitude hysteresis loops, the coercive field is 1.51×104 kv/m. It is the critical point of polarization of PZT thin films which is similar to the electric field at the change point between abnormal and normal bipolar switching modes. It implied that the ferroelectric polarization induced the charge compensation at the semiconductor surface, leading to polarization resistive switching storage.Through the analysis, the resistance mechanisms of PZT thin films grown on different substrates are different. Large leakage currents still exist even though a conductive SRO buffer layer is deposited on STO substrate when the preparation condition of PZT thin films is the same. So we need to explore the optimized preparation and testing scheme. PZT thin films grown on NSTO substrate have a better crystalline and the deposition structure and fabrication procedure are simple. The system exhibited good bipolar resistive switching characteristics which had different modulation effects under different scanning voltages. |
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