On-Demand Visible-Light Sensing with Optical Memory Capabilities Based on an Electrical-Breakdown-Triggered Negative Photoconductivity Effect in the Ubiquitous Transparent Hafnia.

A transparent visible-light sensor may sound like an oxymoron. Indeed, this scenario is often deemed challenging in conventional photosensitive semiconducting materials due to the complementary relationship between absorbance (which determines photosensitivity) and transmittance (which determines transparency). Past studies have relied on photoinduced ionization of vacancy defect states within a wide-band-gap oxide to modulate the flow of current or charge storage in specific device structures such as nanorods, hetero oxide junctions, or thin-film transistors. Here, we demonstrate visible-light-sensing and optical memory functions in a thin, optically transparent wide-band-gap oxide such as the ubiquitous hafnium dioxide, following a soft electrical breakdown. The physical mechanism is distinguished by a persistent current decrease that spans several orders of magnitude, indicating that the breakdown oxide is restored by the visible light. Physical characterization by X-ray photoelectron spectroscopy and the first-principles simulation study based on the density functional theory provide a strong support for the proposed light-assisted recombination of electrically induced Frenkel-pair defects as the underlying mechanism for the observed negative photoconductance response and optical memory effect. By harnessing this alternative mechanism, this work demonstrates a different approach of overcoming the traditional barrier in realizing both transparency and on-demand visible-light sensing with optical memory functions all in a single device.