# Phase-change memory: writing bits as glass or crystal

> An infinity loop for phase-change memory: a heat pulse melts and quenches Ge–Sb–Te into a high-resistance glass (RESET), a gentler one anneals it back into a low-resistance crystal (SET), and both states last at room temperature. Write pulses, record switching speeds and the uses, from rewritable discs to brain-inspired computing, run along the bottom.

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- Updated: 2026-10-14
- Category: Graphical abstracts
- Method: Graphical abstract
- Starting materials: Ge₂Sb₂Te₅ (GST), a chalcogenide phase-change alloy
- Product: A non-volatile memory cell that stores a bit as a glass (high resistance) or a crystal (low resistance)
- Key result: 500 ps crystallization with incubation (Loke et al. 2012); 700 ps writing in ScSbTe vs tens of ns for commercial PCRAM (Rao et al. 2017)
- Figure: 1920 × 1080 px, 86 editable layers (4 generated objects with settings)

## Steps

1. **Two states**: Phase-change alloys switch rapidly and reversibly between amorphous and crystalline phases with very different electrical and optical properties (Lencer et al. 2011); the crystal has an octahedral-like arrangement with many vacancies (Wuttig & Yamada 2007)
2. **RESET: melt and quench**: A high-power heat pulse melts the crystal and quenches it into the high-resistance amorphous state (Ning et al. 2022; Hatayama et al. 2018), over the heater of a mushroom cell (Zheng et al. 2025)
3. **SET: anneal**: A gentler pulse anneals the glass back into the low-resistance crystal; melt, quench and anneal cycles repeat, and both states are long lived at ambient temperature (Mukhopadhyay et al. 2016)
4. **Speed**: Commercial PCRAM writes in tens of nanoseconds, limited by stochastic nucleation (Rao et al. 2017); incubation gave 500 ps crystallization (Loke et al. 2012) and ScSbTe writes in 700 ps (Rao et al. 2017)
5. **Uses**: Rewritable optical discs (Wuttig & Yamada 2007), 3D cross-point memory with threshold-switch selectors (Zhao et al. 2024), synapse-like cells (Kuzum et al. 2012; Ding et al. 2019) and in-memory computing (Syed et al. 2025)

## Uses

- Graphical abstracts on phase-change materials and PCRAM
- Talks on non-volatile and storage-class memory
- Teaching melt-quench amorphization and crystallization

## References

1. Wuttig & Yamada, “Phase-change materials for rewriteable data storage”, Nat. Mater. 2007. https://doi.org/10.1038/nmat2009 (review)
2. Lencer, Salinga & Wuttig, “Design rules for phase-change materials in data storage applications”, Adv. Mater. 2011. https://doi.org/10.1002/adma.201004255 (review)
3. Mukhopadhyay et al., “Competing covalent and ionic bonding in Ge-Sb-Te phase change materials”, Sci. Rep. 2016. https://doi.org/10.1038/srep25981 (primary)
4. Loke et al., “Breaking the speed limits of phase-change memory”, Science 2012. https://doi.org/10.1126/science.1221561 (primary)
5. Rao et al., “Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing”, Science 2017. https://doi.org/10.1126/science.aao3212 (primary)
6. Hatayama et al., “Inverse resistance change Cr2Ge2Te6-based PCRAM enabling ultralow-energy amorphization”, ACS Appl. Mater. Interfaces 2018. https://doi.org/10.1021/acsami.7b16755 (primary)
7. Ning et al., “Low energy switching of phase change materials using a 2D thermal boundary layer”, ACS Appl. Mater. Interfaces 2022. https://doi.org/10.1021/acsami.2c12936 (further reading)
8. Zheng et al., “Extended switching endurance of phase change memory through nano-confined cell structure”, Nat. Commun. 2025. https://doi.org/10.1038/s41467-025-60644-1 (further reading)
9. Zhao et al., “Chalcogenide ovonic threshold switching selector”, Nano-Micro Lett. 2024. https://doi.org/10.1007/s40820-023-01289-x (further reading)
10. Kuzum, Jeyasingh, Lee & Wong, “Nanoelectronic programmable synapses based on phase change materials for brain-inspired computing”, Nano Lett. 2012. https://doi.org/10.1021/nl201040y (primary)
11. Ding et al., “Phase-change heterostructure enables ultralow noise and drift for memory operation”, Science 2019. https://doi.org/10.1126/science.aay0291 (primary)
12. Syed, Le Gallo & Sebastian, “Phase-change memory for in-memory computing”, Chem. Rev. 2025. https://doi.org/10.1021/acs.chemrev.4c00670 (review)

Credit: Drawn by LabCanvas after Wuttig & Yamada, Nat. Mater. 2007 (doi:10.1038/nmat2009); Mukhopadhyay et al., Sci. Rep. 2016 (doi:10.1038/srep25981); Loke et al., Science 2012 (doi:10.1126/science.1221561); Rao et al., Science 2017 (doi:10.1126/science.aao3212); Kuzum et al., Nano Lett. 2012 (doi:10.1021/nl201040y).

## Questions

**What does the “Phase-change memory: writing bits as glass or crystal” template show?** An infinity loop for phase-change memory: a heat pulse melts and quenches Ge–Sb–Te into a high-resistance glass (RESET), a gentler one anneals it back into a low-resistance crystal (SET), and both states last at room temperature. Write pulses, record switching speeds and the uses, from rewritable discs to brain-inspired computing, run along the bottom.

**What are the steps shown in “Phase-change memory: writing bits as glass or crystal”?** 1. Two states: Phase-change alloys switch rapidly and reversibly between amorphous and crystalline phases with very different electrical and optical properties (Lencer et al. 2011); the crystal has an octahedral-like arrangement with many vacancies (Wuttig & Yamada 2007). 2. RESET: melt and quench: A high-power heat pulse melts the crystal and quenches it into the high-resistance amorphous state (Ning et al. 2022; Hatayama et al. 2018), over the heater of a mushroom cell (Zheng et al. 2025). 3. SET: anneal: A gentler pulse anneals the glass back into the low-resistance crystal; melt, quench and anneal cycles repeat, and both states are long lived at ambient temperature (Mukhopadhyay et al. 2016). 4. Speed: Commercial PCRAM writes in tens of nanoseconds, limited by stochastic nucleation (Rao et al. 2017); incubation gave 500 ps crystallization (Loke et al. 2012) and ScSbTe writes in 700 ps (Rao et al. 2017). 5. Uses: Rewritable optical discs (Wuttig & Yamada 2007), 3D cross-point memory with threshold-switch selectors (Zhao et al. 2024), synapse-like cells (Kuzum et al. 2012; Ding et al. 2019) and in-memory computing (Syed et al. 2025).

**What are the starting materials and the product?** Starting materials: Ge₂Sb₂Te₅ (GST), a chalcogenide phase-change alloy. Product: A non-volatile memory cell that stores a bit as a glass (high resistance) or a crystal (low resistance).

**Which papers is “Phase-change memory: writing bits as glass or crystal” based on?** Wuttig & Yamada, “Phase-change materials for rewriteable data storage”, Nat. Mater. 2007 (doi:10.1038/nmat2009); Lencer, Salinga & Wuttig, “Design rules for phase-change materials in data storage applications”, Adv. Mater. 2011 (doi:10.1002/adma.201004255); Mukhopadhyay et al., “Competing covalent and ionic bonding in Ge-Sb-Te phase change materials”, Sci. Rep. 2016 (doi:10.1038/srep25981); Loke et al., “Breaking the speed limits of phase-change memory”, Science 2012 (doi:10.1126/science.1221561); Rao et al., “Reducing the stochasticity of crystal nucleation to enable subnanosecond memory writing”, Science 2017 (doi:10.1126/science.aao3212); Hatayama et al., “Inverse resistance change Cr2Ge2Te6-based PCRAM enabling ultralow-energy amorphization”, ACS Appl. Mater. Interfaces 2018 (doi:10.1021/acsami.7b16755); Ning et al., “Low energy switching of phase change materials using a 2D thermal boundary layer”, ACS Appl. Mater. Interfaces 2022 (doi:10.1021/acsami.2c12936); Zheng et al., “Extended switching endurance of phase change memory through nano-confined cell structure”, Nat. Commun. 2025 (doi:10.1038/s41467-025-60644-1); Zhao et al., “Chalcogenide ovonic threshold switching selector”, Nano-Micro Lett. 2024 (doi:10.1007/s40820-023-01289-x); Kuzum, Jeyasingh, Lee & Wong, “Nanoelectronic programmable synapses based on phase change materials for brain-inspired computing”, Nano Lett. 2012 (doi:10.1021/nl201040y); Ding et al., “Phase-change heterostructure enables ultralow noise and drift for memory operation”, Science 2019 (doi:10.1126/science.aay0291); Syed, Le Gallo & Sebastian, “Phase-change memory for in-memory computing”, Chem. Rev. 2025 (doi:10.1021/acs.chemrev.4c00670).

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