Visible Light Induced Negative Photo Conduction in Wide Bandgap Oxides Based Memristor for Nonvolatile Opto-Electronic Memory and Photo Sensing

The photo sensor based on wide bandgap oxides relies on the phenomenon of defect generated negative photoconductivity in memristors. Wide bandgap oxides naturally are unresponsive to low energy light, particularly in the visible regime and higher wavelengths owing to their large bandgap energies. Due to the bandgap energy being larger than the energy of the photons in the visible range, they are transparent to visible light and the visible photons are incapable of causing photocarrier generation within the oxide and are therefore naturally photo unresponsive. This has restricted the use of oxide based photosensors to rely on other lower bandgap materials for a photo response or to be restricted strictly to the higher energy ultraviolet band.<br/>However, in the phenomenon studied in this work, through electrically generated defect states within the oxide, it is possible to sensitize wide bandgap oxides to respond to visible light regimes. This is accomplished due to the defect generation creating sub-bandgap states in the forbidden gap which can respond to visible energy photons. The mechanism has been theorized to work as follows; when the oxide system is electrically stressed beyond the breakdown field, the lattice oxygen bonds are severed. If this is done in a constrained manner by means of a current compliance to prevent catastrophic breakdown, the effect is reversible and results in a change in the resistive state of the device and is termed soft breakdown. Such a phenomenon is widely used in memristive devices. During this breakdown, the lattice bonds are broken, and the oxygen is displaced as ions into the interstitial spaces as a metastable vacancy-ion equilibrium. The ions do not naturally recombine with the vacancy and restore the lattice of the oxide as there is an energy barrier that prevents this. However, such an energy may be supplied by means of optical illumination which can cause the vacancy-ion pairs to recombine and return the pristine state of the oxide consequently resulting in a resistance increase in the system. When optical energy greater than the bandgap of the defect state to the conduction band is provided to such a system, a vacancy trap state electron may be excited to the conduction band, resulting in a positive charging of the vacancy state. This creates an electrostatic force between the positively charged vacancy and the negatively charged interstitial ion resulting in a recombination of the two and reformation of the oxide.<br/>In this work, we present a novel bilayer ITO/SiO₂/HfO_x/Pt memristor for optoelectronic nonvolatile memory storage and photo sensing. The device demonstrates good photo sensing characteristics in terms of highly stable endurance at least 200 (electrical SET/optical RESET) cycles for blue light (intensity:10 mW/cm2, wavelength: 405 nm) and excellent retention (10⁵ s) without any disruption. These features of the memristive device would allow us to produce either an optoelectronic nonvolatile memory for data storage or photo sensing in single memristive cell.