Electroresistance Effect Of Ferroelectric Tunnel Junctions Based On The BNT Ultrathin Film

Ferroelectric memories have attracted much attention for application prospects inelectronic information and military industry because of their many advantages such asnon-volatility, low power consumption, high reading and writing speed, high density, andexcellent antiradiation. The conventional ferroelectric memories are mainly based on thecapacitance-type memories. However, the conventional capacitive ferroelectric readout isdestructive. Ferroelectric tunnel junctions (FTJs) provide a new quantum tunneling effect toenable nondestructive resistive readout in non-volatile ferroelectric devices. Recently, FTJshave attracted giant attention as potential systems for next-generation information technologydevices. Currently, the FTJ is based on the electrostatic effect. However, except for usingdifferent top and bottom electrodes, the concerning retention time and polarization switchingare becoming obstacles in the progress of FTJ. In this thesis, the BNT-based FTJs wereprepared by pulsed laser deposition (PLD) methods. A FTJ based on the piezoelectric effecton electroresistance properties was discussed. Moreover, the electron transport properties ofthe BNT-based FTJs intergrated with Si technology were studied. The main research work andresults are summarized as follows:1. Preparation and electroresistance properties of a ferroelectric tunnel junction basedon piezoelectric effectThe Bi3.15Nd0.85Ti3O12(BNT) thin films with different thicknesses (100nm,35nm,18nm,10nm,7nm and4nm) deposited on Pt/Ti/SiO2/Si substrates were prepared by PLD. X-raydiffraction (XRD) and piezoresponse force microscopy (PFM) were used to study themicrostructure, ferroelectric and piezoelectric properties of the prepared films. Furthermore,conductive atomic force microscopy (CAFM) was used to study electroresistance propertiesof the FTJs. The PFM experiment results show that the ferroelectric properties of the BNTfilms reduce with decreasing the fillm thickness. The BNT films deposited on Pt/Ti/SiO2/Sisubstrates are ferroelectric and retain ferroelectric properties down to a thickness of4nm. Inaddition, the4nm BNT ultrathin films also exhibit good piezoelectric properties. The CAFManalysis results show that the TER effect is found in the bilayer perovskite structure BNT thinfilm. In addition, the results demonstrate that the piezoelectric-related strain effect is theorigin of the resistive switching of FTJs and produces the giant TER effect.2. Preparation and electron transport properties of the ultrathin film intergrated with SitechnologyThe BNT films with different thicknesses (25nm,13nm,7nm and2nm) were deposited on heavily doped Si substrates by PLD. XRD, atomic force microscopy (AFM), ferroelectricand semiconductor test system were used to study the microstructure, surface topography,ferroelectric properties, leakage current, fatigue and retention properties of the prepared BNTfilms with different thicknesses. The results show that the saturated Prvalue is6.15μC/cm2when the thickness of the BNT film is25nm. In addition, all films show low leakage current,good fatigue and retention properties. Moreover, the Vcvalue of the BNT films decreases withscaling down the film thickness. By performing direct electrical measurements, the Pr=3.15μC/cm2is observed for the2nm thick BNT film.PFM were used to study the domain structure, the phase and amplitude loops of the BNTfilms. Furthermore, conductive atomic force microscopy (CAFM) was used to study electrontransport properties of the FTJs. By PFM measurements, the25and2nm thick BNT filmsshow the striking domain contrast indicating the excellent ferroelectric switching polarizationcharacteristic. By CAFM measurements, the Fowler-Nordheim (FN) tunneling is occurred inthe25nm thick BNT film, but the2nm ultrathin film transports with direct tunneling. Theresults exhibit the BNT films with different thicknesses integrated with Si technology possessthe different electron transport properties.