Resistive switching behaviors and mechanisms of HfS2 film memory devices studied by experiments and density functional theory calculations

Abstract

Energy band diagrams are widely utilized to explain the switching mechanism of resistance random access memory (RRAM). However, a precise and quantitative band theory is still lacking in this field. Although HfS2 has good applications in many fields because of its good electrical and optical properties, its applications in RRAM have seldom been reported. In this work, the exfoliated nanosheets of HfS2 were utilized to fabricate memory devices with a structure of Pt/Al/HfS2/p+-Si, which show typical bipolar resistive switching behavior with high switching voltage and a small ratio of high and low resistive states (R-ratio). According to the density functional theory (DFT) calculation results of energy band diagrams, instead of conductive filament formation in other resistive switching materials, the doping of sulfur vacancy (VS) of 3.8% is already enough to change the whole HfS2 layer from the semiconductor to the metal. The transition is caused by the change in the VS doping concentration from low to high, which is the result of the generation and movement of VS under an electric field. The DFT also calculated that HfS2 devices utilizing Indium Tin Oxide as the bottom electrode can show bipolar resistive switching behavior with lower switching voltage and a higher R-ratio than those utilizing p+-Si, which is confirmed by the experimental results. The DFT calculation can be utilized for both explaining the switching mechanism and designing the device structure to optimize the switching characteristics.