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Ultrafast non-volatile flash memory based on van der Waals heterostructures
Lan Liu, Yi Ding, Jiayi Li, Chunsen Liu, Peng Zhou
TL;DR
The paper addresses the need for faster non-volatile memory. It demonstrates a MoS2/h-BN/MLG van der Waals heterostructure flash memory and reports approximately 20 ns writing and erasing, along with retention and cycling performance. The results identify tunneling-layer thickness as a constraint when ultrafast operation and non-volatile retention are both required.
Problem
Millisecond-level non-volatile memory speed is insufficient for next-generation high-speed memory.
Method
The paper demonstrates flash memory based on MoS2/h-BN/MLG van der Waals heterostructures.
Results
~20 ns writing/erasing speed is demonstrated, with 3.15×10^8 s retention measurements and 1185 erase/write cycles reported.
Takeaways & Limitations
The device could support next-generation high-performance memory applications.
Takeaways & Limitations
An appropriate tunneling-layer thickness is required to achieve ultrafast non-volatile memory, while leakage through 2D dielectrics remains not well understood.
Abstract
from arXiv · showhide
Flash memory has become a ubiquitous solid-state memory device, it is widely used in portable digital devices, computers, and enterprise applications. The development of the information age has put forward higher requirements for memory speed and retention performance. Here, we demonstrate an ultrafast non-volatile memory based on MoS2/h-BN/multi-layer graphene (MLG) van der Waals heterostructures, which has an atomic-level flat interface and achieves ultrafast writing/erasing speed (~20 ns), surpassing the reported state-of-the-art flash memory (~100 ns). The ultrafast flash memory could lay the foundation for the next-generation of high-speed non-volatile memory.
Introduction
The paper addresses the need for faster non-volatile memory by demonstrating an ultrafast flash memory based on MoS2/h-BN/MLG van der Waals heterostructures. The device combines an atomic-level flat interface with approximately 20 ns writing and erasing and long retention characteristics.
- 2D-material memories span volatile, quasi-non-volatile, and non-volatile categories with trade-offs involving writing speed and retention.
- Millisecond-level non-volatile memory speed is insufficient for next-generation high-speed memory.
- MoS2 provides a high current ON/OFF ratio and thickness-dependent bandgap suitable for flash-memory channels.
- h-BN serves as an insulating and tunneling layer, while multilayer graphene serves as a floating gate.
- The demonstrated MoS2/h-BN/MLG heterostructure flash memory achieves approximately 20 ns writing and erasing and long retention characteristics.
- The ultrafast flash memory could support next-generation high-performance memory applications.
Results and Discussion
The device uses a MoS2/h-BN/MLG heterostructure in which stored charge in multilayer graphene shifts the MoS2 threshold voltage. Measurements demonstrate large memory windows, nanosecond switching, retention, cycling stability, and thickness-dependent performance.
- Device structure and operation: The MoS2/h-BN/MLG device regulates source-drain current through back-gate and floating-gate control.
- Charge storage: ~4×10^12 cm^-2 stored-electron density is measured in the multilayer-graphene floating gate.
- Ultrafast operation: ~20 ns writing and erasing shifts the threshold voltage through electron tunneling between MoS2 and multilayer graphene.
- Cycling stability: 1185 erase/write cycles retain a state-1/state-0 current ratio of 10^4 under the stated read condition.
- Retention: 3.15×10^8 s retention measurements preserve 50.6% and 50.4% of the initial threshold-voltage difference.
- Tunneling-layer thickness: ~10 nm h-BN provides a better channel-current state 1/0 ratio than 7.5 nm h-BN because the thinner layer has increased leakage current.
- Tunneling-layer thickness: An appropriate tunneling-layer thickness is required for ultrafast non-volatile memory when retention is also needed.
Conclusion
The authors demonstrate ultrafast non-volatile memory using van der Waals heterostructures and a suitably thick h-BN tunneling layer. The device combines ~20 ns writing/erasing, high current ratio, and non-volatile retention, while the measured speed is instrument-limited.
- Conclusion: The demonstrated memory is based on van der Waals heterostructures and is presented for next-generation high-speed non-volatile memory applications.
- Conclusion: ~20 ns writing/erasing time is achieved with a suitable h-BN tunneling-layer thickness.The conclusion identifies tunneling-layer thickness as part of the device design.
- Conclusion: A channel current ratio of state 1/state 0 ~10^6 is reported together with non-volatile retention performance.
- Conclusion: The upper limit of the writing/erasing speed is limited by the instrument response.The authors state that faster memory speed would be further explored.
Experimental Section
The devices were fabricated from mechanically cleaved MoS2, h-BN, and graphene nanosheets, assembled by wet transfer, patterned with electron-beam lithography, and electrically characterized under room- and low-temperature conditions.
- Experimental Section: MoS2, h-BN, and graphene nanosheets were obtained from commercial crystals by Scotch-tape micromechanical cleavage.
- Experimental Section: MoS2 and h-BN were wet-transferred onto graphene using a water-soluble PVA layer.
- Experimental Section: The h-BN tunneling layer was positioned to avoid shorting between the MoS2 channel and multilayer graphene.
- Experimental Section: Cr/Au electrodes were patterned by electron-beam lithography using PMMA polymer.The reported electrode thicknesses are Cr/Au (10 nm/30 nm).
- Experimental Section: Electrical performance was measured with an Agilent B1500 analyzer on a Cascade Summit 11000 platform at room temperature and a Lake Shore probe station at low temperature.
- Experimental Section: Nanosheet thickness was measured using an MFP-3D Origin+ instrument.