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

Abstract

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.

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“…Inverse (or negative) photoconductivity has been observed since the early age of solid state electronics [9]. More recently it has been found in InAs nanowires [10][11][12], ZnO nanowires [13], Si nanowires [14], InAs/AlSb coreshell nanowires [15], Tellurium nanosheets [16], SiO 2 films containing cobalt nanoparticles grown on GaAs [17], core-shell cobalt nanoparticles embedded between SiO 2 and HfO 2 [18], Au nanoparticle arrays in the Coulomb blockade regime [19], Au nanoparticles embedded in silk protein fibroin hydrogels [20], diamond-like carbon films with embedded Ag nanoparticle [21], metal nanoparticle based self-assembled monolayers [22], p-type…”