Charge ordering and dion-jacobson semiconducting ferroelectric materials towards artificial synapses

Abstract

Ferroelectric materials have demonstrated their great potential in non-volatile memoriesand high-performance artificial synapses. However, in the development of ultrafast and highdensity synaptic devices using novel semiconducting charge ordering and Dion-Jacobson ferroelectrics, basic knowledge of their crystal and domain structures are limited . In this dissertation, I would like to present two of our experimental efforts in pursing the relevant fundamental understanding: First, I present our experimental discovery of the first room temperature ferroelectric semiconductor Ag2BiO3. Its electronic ferroelectricity is induced by coexistence of bond length disproportionation and charge disproportionation hosted by a couple of degenerate low-symmetry phases (Pnn2 and Pc). I demonstrate the existence of a photo-ferroelectric synaptic behavior in Ag2BiO3-based two-terminal device. I further propose a circuit-implementation of such optical synapse in conducting neuromorphic-like computing allowing non-destructive and energy efficient analog read-out. The simulated performance suggests such photo-ferroelectric synapse is promising for high-performance neuromorphic computing. Second, I employ a Dion-Jacobson layered oxide CsBiNb2O7, which was predicted to be a ferroelectric material, as a model system to study the effect of quasi-2D interlayer interaction on ferroelectric domain structure with atomic scale analysis. I reveal the existence of unit-cell thick ferroelectric domain size as well as both 180° and 90° domain walls in free-standing CsBiNb2O7. Our understanding of the quasi-2D ferroelectric material’s domain structure may suggest new material categories to achieve unit-cell thick domain structures and shed light on promising materials solutions for next-generation microelectronic and synaptic devices.