Key Findings
A team reporting in Applied Microscopy has demonstrated that germanium-telluride (GeTe) can function as both a memory cell and a selector depending on its tellurium content. By stacking a near-stoichiometric GeTe memory layer on a tellurium-rich GeTe6 selector layer, the researchers created a monolithic one-selector one-resistor (1S1R) device. This architecture achieved a stable memory window of about 0.2 volts and survived approximately eight million switching cycles.
Technical Details
The researchers used radio-frequency magnetron co-sputtering to create GexTe1-x thin films, varying germanium power from 20 to 50 watts while keeping tellurium target power at 15 watts. This produced compositions ranging from Ge0.11Te0.89 to Ge0.77Te0.23. The devices featured 20-nanometer-thick functional layers between 100-nanometer-thick tungsten electrodes, with hole sizes between 34 by 34 nanometers and 1921 by 1921 nanometers.
- Memory Cell: Near-stoichiometric GeTe functioned as a phase-change memory cell. RESET operations worked with pulse widths from 70 to 300 nanoseconds, while SET operations stabilized at 150 nanoseconds and above. Standalone cells survived roughly five million program-erase cycles.
- Selector: Tellurium-rich GeTe6 acted as a bidirectional threshold switch. It maintained a high-resistance OFF state above one gigaohm and dropped below one kilohm at one volt when above the threshold. It showed a delay time under 55 nanoseconds and selectivity exceeding ten thousand, surviving one hundred million cycles.
Chemical analysis via X-ray photoelectron spectroscopy and Raman spectroscopy revealed that excess tellurium forms homopolar tellurium-tellurium bonds, increasing localized trap states that enable threshold switching.
Background and Significance
High-density 3D cross-point arrays typically require a nonlinear selector to prevent sneak currents from corrupting readouts. Traditionally, phase-change memory is paired with an Ovonic threshold switch, but these usually require different materials and separate manufacturing optimizations. Using a single binary material allows both layers to be deposited in one cluster system and patterned in a single monolithic lithographic step, addressing the physical scaling limits and process difficulties facing charge-based technologies like DRAM and flash.
Source: https://bioengineer.org/single-germanium-telluride-material-yields-both-memory-cell-and-selector/
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