Scientists from the University of Rochester have made significant progress in the development of membrane devices that can “remember” their state of resistance even after disconnecting. These devices not only serve as digital memory but also act as building blocks for future neuromorphic computers. The main obstacles in their creation were difficulties associated with mass production and economic efficiency.
The unique feature of these memristors is the mechanism of phase changes that controls the resistance. Traditional phase-change memory utilizes a glass-like material, usually called a halide, positioned between two contacts. The electric current passing through the glass can change its phase between the crystalline state (with low resistance) and the amorphous state (with high resistance). This enables significant differences in the resistance between the two conditions, making them attractive for memory devices. However, the use of this mechanism in thin two-dimensional films presented problems – defects in the glass could prevent consistent phase changes and increase the required operating voltage.
In 2019, researchers from the University of Peredyu and the National Institute of Standards and Technologies proposed a new phase switching mechanism that is more suitable for thin films. They utilized molybdenum disulfide, a material from the class of transition metal dichalcogenides, which can transition between several different phases. For their memristor, the team switched the two-dimensional film of molybdenum ditelluride between the semiconductor phase with high resistance and the half-metal phase with low resistance. This proved to be a more reliable phase change for the compact device, although the required switch voltage was relatively high – over one volt for each phase change, and the switching energy was three femtojoules, moderately high for devices of this type.
The Rochester team addressed these challenges by adding a controlled amount of deformation to the two-dimensional film, causing it to rest on the edge between the two phases. This significantly reduced both the switching speed and switching energy, requiring only 0.1 volts and 120 attojoules for each switch. Furthermore, the ratio of resistance between the two states reached 108, which is the highest value reported for any two-dimensional membrane.