Band engineering in rare earth nickelates : physics and applications
Date of Issue2019-04-10
School of Materials Science and Engineering
Transition metal oxides have been the focus of solid state physics research for decades. The Coulomb repulsion energy, orbital bandwidth and Hund’s exchange energy often become comparable in these materials, leading to strong interplay between lattice, electron orbitals and spin, which in turn produces unusual properties such as metal insulator transition, giant magneto resistance, and superconductivity etc. Rare earth nickelates with a chemical formula of RNiO3, where R represents a rare earth element, is a family of transition metal oxides. Much attention has been paid to the metal-insulator transition in this system and the nature of the insulating phase. Oxygen vacancies are intrinsic to RNiO3. Acting as electron donors, they are expected to affect the valance states of Ni ions, thus changing the band structures and transport behaviors of RNiO3, which forms the basis of this study. Both post-deposition vacuum annealing and adjusting oxygen pressure during deposition are used to introduce oxygen vacancies into RNiO3 thin films. This is followed by detailed structural and electrical characterizations. Resistance change of as large as ~10E6 has been observed after introducing oxygen vacancies, and insulating phases with tunable bandgaps are obtained. It is proposed that narrowing of the 3d bands opens a gap between Ni 3d and O 2p bands. The process is reversible and continuously tunable. With their tunable bandgaps, it is expected that the oxygen vacancy engineered RNiO3 thin films may find applications in various optoelectronic devices. As a demonstration, RNiO3/Nb-SrTiO3 heterojunctions are thus produced and their performances as photovoltaic cells are investigated. By varying the oxygen vacancy content, rare earth element and device structure, a power conversion efficiency of 1.1% has been achieved. The same heterostructures can also be used as self-powered ultra-sensitive photodetectors. Such photodetectors show high sensitivity toward a broad spectrum of light with very good long-term stability. Last but not least, the fact that oxygen vacancies can be introduced into RNiO3 thin films via post-deposition vacuum annealing implies relatively weak Ni-O bonds. It is thus possible to dynamically control the amount and distribution of oxygen vacancies in RNiO3 thin films using an electric field. This is particularly interesting because oxygen vacancies affect the electronic phase of RNiO3, so the transport property can be dynamically controlled. Indeed, large resistive switching is observed in various RNiO3/Nb-SrTiO3 heterojunctions. Detailed transmission electron microscopy analysis reveals that oxygen vacancy migration indeed occurs and modulates the interface energy barrier, leading to the observed phenomena. Further studies show excellent retention and fatigue performance of these devices and demonstrate their potential as non-volatile memories. This work improves our understanding on RNiO3 and opens new possibilities for their applications in various optoelectronic devices.