Conduction mechanisms of ferroelectric La:HfO2 ultrathin films

Abstract

Recently, ion-doped HfO2 thin films are highly desirable for the next-generation nonvolatile memories due to excellent compatibility with current complementary metal-oxide-semiconductor processes and robust ferroelectricity persisted down to the nanoscale. In this work, we study conduction mechanisms of 4 and 8 nm-thick La:HfO2 ultrathin films sandwiched between Pt and (La0.67,Sr0.33)MnO3 (LSMO) electrodes based on band alignments of the Pt/La:HfO2/LSMO, measured by x-ray photoelectron spectroscopy, and temperature-dependent current-voltage curves from 50 to 300 K. In a 4 nm-thick La:HfO2 thin-film capacitor, the conduction mechanism is found to be governed by direct tunneling at 50–100 K and phonon-assisted indirect tunneling when the temperature is further increased to 300 K in which the (LaHf4+3+)′ acceptors are served as localized states, facilitating hole hopping through the La:HfO2 barrier. When the thickness is increased to 8 nm, the tunneling through a La:HfO2 layer is suppressed, and the current-voltage character becomes rectifying, which is regulated by the dominated La:HfO2/LSMO interfacial barrier. The transport for a forward bias of the La:HfO2/LSMO barrier is found to be governed by thermionic-field emission, exhibiting a temperature-independent build-in potential of ∼2.77 V. For the reverse bias, the Fowler–Nordheim tunneling is observed. The revealing of conduction mechanisms in terms of band alignments sheds light on leakage problems and facilitates the design of HfO2-based ferroelectric devices with excellent insulating character for high-performance memory applications.