Nanostructured BaTiO3 Thin Films for Enhanced Photoresponse

Abstract

Light-driven devices are of great importance among emerging technologies such as artificial intelligence and quantum computing, owing to their remote-control ability, ultrafast response and low energy consumption. Ferroelectric materials have attracted considerable interest in this field. On the one hand, the bulk photovoltaic effect (BPV) allows spontaneous separation of photoexcited electron-hole pairs, simplifying device architectures. On the other hand, switchable polarization endows ferroelectric devices with memory and data processing functions. Despite these advantages, several challenges hinder practical implementation of traditional perovskite ferroelectrics, including the low photovoltage in thin films, slow response, limited absorption in the visible spectrum and substrate-dependent film growth. This thesis addresses these issues by exploring nanostructure engineering strategies using BaTiO3 (BTO) as a representative perovskite ferroelectric. The main achievements and outcomes are highlighted below.

A record high photovoltage was achieved in a thin BTO film. Highly preferred-oriented BTO films were successfully grown on silicon substrates. The film exhibited a two-layer structure with different crystallographic orientations along the growth direction. This unique geometry enabled a high-power conversion efficiency (PCE) of 0.062% under ultraviolet light and a record open-circuit voltage of 1.07 V among pure ferroelectric films below 300 nm thickness. Systematic investigations attributed this enhancement to the BPV. This crystalline rearrangement strategy offers new insights into ferroelectric self-powered electronics demanding high driving voltage.

A high-efficient, broadband, ultra-fast photodetector was demonstrated based on a three-dimensional BTO/SrRuO3 (SRO) heterostructure. The large mismatch strain at interfaces induced the formation of island-like SRO nanostructures. The crystalline and amorphous phases in the BTO layer were responsible for charge separation and transportation, respectively. In addition, the SRO with periodic nanostructured morphology effectively absorbed visible light. Consequently, the Au/BTO/SRO/Ag device had a responsivity of up to 0.166 A/W and fast rise/fall times of 3.7/4.7 microseconds. While SRO is widely used as a conductive electrode, this work demonstrated, for the first time, a synergetic photovoltaic process between BTO and SRO. Remarkably, the device performance surpasses that of reported self-powered photodetectors and is comparable to commercial photodetectors, highlighting the broad application potential of this stable and scalable metal oxide platform.

Ferroelectric devices responsive to visible light were achieved by incorporating narrow-bandgap semiconductors into the ferroelectric matrix. BTO/CoFe2O4 (CFO) composite films with controllable composition were successfully synthesized. The transmission electron microscopy revealed a homogeneous structure, which has not been reported in perovskite/spinel material systems. Ferroelectricity was retained in a low-CFO-ratio composite, enabling polarization-dependent photocurrent under visible light. Utilizing visible-light-induced photocurrent as a readout signal offering a promising route toward energy-efficient ferroelectric memory and computing devices.

Finally, the thickness-dependent charge separation mechanism in free-standing ferroelectric films was elucidated. Polycrystalline free-standing BTO films with lateral dimensions of hundreds of micrometers were fabricated. Kelvin probe force microscopy characterizations revealed a thickness-dependent surface charge behavior: in thinner films, near-surface band bending dominated charge separation, whereas in thicker films, ferroelectric polarization led to opposite carrier accumulation on the two surfaces. This newly identified mechanism provides guidance for exploring free-standing ferroelectrics in photocatalysis and advanced optoelectronic applications. Show more