A Schottky-to-Ohmic Transition in Epitaxial Ferroelectric Hafnia Devices
The development of analog resistive non-volatile memory elements is of great interest for future information storage technology. These devices can be used to emulate the electronic behaviour of biological synapses and neurons for neuromorphic computing or sensing applications. Hafnia is a promising material to use in resistive memory systems due to its high industrial compatibility. Both the migration of oxygen ions and ferroelectricity are two mechanisms actively being explored to drive resistance change in hafnia based devices. This thesis explores the interplay between interfacial redox chemistry and ferroelectricity and how these influence the resistance switching behaviour in epitaxial hafnia films. A device stack is engineered to elicit large resistance changes through both ferroelectricity and interfacial redox reactions. Ultra-thin epitaxial Y doped HfO2 (YHO) is grown with a controlled oxygen stoichiometry. To stabilise the polar phase in YHO and provide a resistance-tuneable bottom interface, YHO is grown on La0.66Sr0.33MnO3 (LSMO) buffered Nb doped SrTiO3 substrates. In-depth X-Ray diffraction based structural analysis identified the polar nature of the film and a strain induced rhombohedral distortion. Ferroelectricity in the film was confirmed by piezoresponse force microscopy. In device configuration, using an Au top electrode with an ultra-thin Ti adhesion layer, a resistance switching mode with large on-off ratio was observed. The pristine device was stabilised in an intermediate resistance state and did not need an electro-forming process. The mode reversibly switched between a purely capacitive (Schottky) and resistive (Ohmic) state, a Schottky-to-Ohmic transition (SOT). A series of voltage pulse trains were used to explore the intermediate resistance states of the system. Both analog and integration behaviour was demonstrated, depending on the pulse profile. The SOT may therefore find application as both a synapse and a neuron respectively in neuromorphic applications. Capacitance-voltage measurements were employed to correlate the ferroelectric polarisa- tion reversal with the SET process and not the RESET process. Therefore, ferroelectricity was indicated to only be partially responsible for the observed resistance change. Electrode area dependent current measurements suggested that in the low resistance state, current transport occurred through a localised conductive filament. A detailed analysis of the inter- face chemistry suggested the importance of precisely designed oxygen scavenging at both interfaces to stabilise the SOT. Furthermore, interface stoichiometry changes were observed during resistance switching by a combined hard photo-electron spectroscopy and electron energy loss spectroscopy study. Impedance spectroscopy measurements were used to correlate the stoichiometry changes at the LSMO|YHO interface to resistance changes during the SOT. The conductive pristine resistance state was shown to predominantly be limited by the LSMO|YHO interface and was proposed to occur due to oxygen scavenging at the top electrode, by-passing through-grain conduction. Analysis of the SOT analog switching behaviour showed that both the YHO layer and the LSMO|YHO interface switch separately during the RESET operation, but simultaneously during the SET process. Furthermore, stack modifications demonstrated the importance of using an ultra-thin LSMO to stabilise the SOT. It was hypothesised to occur due to interface strain enhanced defect formation, oxygen transfer or symmetry changes at the LSMO|YHO interface.
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