Non-volatile memory and negative photoconductivity in a metal-insulator-semiconductor diode with embedded Co nanoparticles

We describe a new metal-insulator-semiconductor (MIS) device in which cobalt based nano particles (NPs) in a core-shell structure (Co-core and Co₃O₄-shell) are embedded between a thermally grown SiO₂ layer and a HfO₂ film deposited by atomic layer deposition. Two additional structures were prepared for comparison. One had no NPs and the other included the Fe NPs, prepared using the same procedure as used for the Co film. All devices exhibited the classic behavior of a voltage variable MIS capacitor with or without a large hysteresis as in non-volatile memory (NVM) systems. However, only the device with the Co core-shell structure exhibits a negative photoconductivity (NPC) effect as well as NVM capabilities in both the capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The dependence of C-V and current voltage I-V characteristics on illumination intensity and wavelength (from ultraviolet to near infrared) as well as on temperature was characterized. Illumination enhances the NPC effect as well as the flat-band voltage shift determined from C-V characteristics and hence the memory width. Illumination in the wavelength range of 735-780 nm caused a current decrease, at a given voltage, by up to a factor of two. The NPC effect stimulates an annihilation of the stored charges and therefore erases the system instantly at a small applied bias. The main cause of the NPC effect under illumination is the photo excitation of supplementary trap channels in the Co₃O₄ shell, which lowers the free carrier density and hence the conductivity of the MIS structure.