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ReRAM materials

Materials used to manufacture ReRAMs


In Resistive RAMs (ReRAMs or RRAMs), a metal-insulator-metal structure has high resistance in the OFF state, but low resistance in the ON state. There are a number of proposed material systems for these devices; one of the most studied places a layer of HfO2 between two TiN electrodes. In a crosspoint architecture, the top and bottom wire layers are at right angles, creating a grid. Memory elements lie at the corners of the grid.

As researchers at SEMATECH reported in 2011, oxygen vacancies in the HfO2 layer segregate to the grain boundaries. When a sufficiently high current is applied, the grain boundaries form a percolation path, allowing electrons to propagate through the material by trap-induced tunneling. Hf-O bonds along this path break and Hf-Hf bonds form, ultimately creating a metallic conducting filament between the top and bottom electrodes. This is the ON state. Because it does not depend on stored charge, it is extremely stable.

To reset the memory to the OFF state, the system applies a current, increasing the temperature of the hafnium filament and causing it to re-oxidize. Ultimately, the complete conductive path is lost and the dielectric again behaves as an insulator.

RRAM implementations have struggled with high variability and inconsistent performance. The distribution of grain boundaries in the material is difficult to control at the nanometer scale. Moreover, the dielectric breakdown mechanism is inherently variable, depending on stochastic movements of electrons between defects.

Materials and Physics for Nonvolatile Memories II: Volume 1250 (MRS Proceedings)