Electric field modulation on transport and gas sensing properties in heterostructures composed of PMN-PT and transition metal oxides

Abstract

Owing to the increasing demand in RAM, ferroelectric-based heterostructures, where stable states of polarization can be modulated by electrical fields, have attracted more attention. Among them, thin films of quasi-2-2 structure layered magnetoelectric composite have become the mainstream, owing to its complete polarization ability, epitaxial structure at interface, and strong interfacial stress coupling. It can be constructed by a thin film deposition on a single crystal substrate (like PMN-PT) with high piezoelectric performance. In this thesis, the electric field modulation on transport properties and gas sensing properties were investigated in heterostructures combining PMN-PT and transition metal oxides. The integration of hexagonal ZnO and perovskite PMN-PT was studied. Possible lattice arrangements are proposed on (111)-, (100)- and (110)- oriented PMN-PT single crystal substrates. Successful integrations have been achieved on (111)- and (001)- oriented PMN-PT. XRD -2 scan show the growth of ZnO on both (111)- and (100)- oriented PMN-PT substrates are along [0001] direction, while no signals of ZnO are shown in the (110) PMN-PT based samples. Φ scan was also performed to establish the in-plane epitaxial relationship. TEM images of the microstructures of the hetero-structures formed on the (001) PMN-PT and (111) PMN-PT single crystal substrates were also studied. The growth of ZnO on (111) PMN-PT proved to be epitaxial, while on (001) PMN-PT two domains at the angle of 30 existed in the ZnO films. In the FeFET structure of ZnO/(111)PMN-PT, transport properties were investigated with electric fields applied across the substrates. The applied electric field modulate the resistance of ZnO film in multiple states consistently. The ROFF/RON ratio is superior to other heterostructures of oxide/ PMN-PT. The mechanism can be attributed to the cooperation of charge manipulation induced by the change of ferroelectric polarization and the migration of oxygen vacancies. Aside from ferroelectric field effect, strain effect is regarded as the major mechanism working in the modulation of multiferroics. Here, epitaxial SRO films on (111)-oriented PMN-PT single-crystal substrates were fabricated. Driven by the reversible ferrroelastic strain effects, multiple resistance states can be achieved. The polarization vectors were tuned between in-plane and out-of-plane directions by the electric field applied across PMN-PT, which generate different lattice strain states. This structure shows good potential in nonvolatile information storage. The tunability of gas sensing properties by ferroelectric field has been investigated on ZnO thin films grown on (001)-oriented 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3. With UV LED applied, ZnO films exhibited good sensitivities to oxygen. By applying electric field across PMN-PT substrate, the sensitivity of ZnO was significantly modulated. The behind mechanism could be attributed to the cooperation of charge manipulation and the migration of oxygen vacancy. In addition, the gas sensing properties to reducing gases were also studied in a peculiar structure of V2O5 films. V2O5 strip-and-channel nanostructure formed grid-like texture on SrTiO3 (001) substrate. Prompted by the enlarged surface-to-volume ratio, gas sensors have been developed based on the grid-like films. Reliable sensing properties were exhibited to gaseous ammonia, ethanol and acetone at room temperature.