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Carbon nanotubes find their way into storage devices

Posted: 20 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Carbon nanotubes? CNTs? CMOS transistor? nonvolatile memory? storage device?

Carbon nanotubes (CNTs) have been of interest as memory and storage device materials since their discovery in 1990, and the potential applications of these materials to a wide spectrum of uses remain to this day. CNTs are members of the fullerene family of materials and are based on the ability of carbon atoms to assemble into long tubes bonded in a graphite-type configuration. Their future, though promising, is relatively open ended at this time.

Two forms of CNTs exist; one exhibits metal-like properties, while the second has semiconductor properties. In addition, there are single-walled and multi-walled CNTs, each with specific properties. Processes that synthesise CNTs produce both forms. For applications where the mechanical strength attributes of CNTs are required, a mixture of CNT forms is useful. For instance, adding CNTs to rubber tire materials has been reported to enhance mechanical strength. Synthesis processes such as chemical vapor deposition (CVD), arc discharge, laser ablation, and plasma torch have been reported to produce CNTs in industrial quantities to make this material economically attractive.

However, applications that require the semiconductor form require a specific separation process. Additionally, some syntheses of CNTs often require a metallic catalyst, usually Co or Ni, and these must be similarly separated. Single-walled CNTs (SWCNTs) are candidates for electronic applications, but synthesis processes that produce high proportions of the single-walled form are in an early stage of development.

There are many types of CNT-based semiconductor applications, but a potentially useful concept employs the property of CNTs to exhibit a mechanical displacement upon application of an electrical current (or voltage). This method employs CNTs as an on-off switch in a mode similar to what other technologies use (such as STT RAM, ReRAM, FeRAM, and Phase Change). These technologies all require an integrated CMOS transistor as the current driver and to prevent a current sneak path in an array configuration. Therefore, maximum cell density depends on this transistor's area. If the CNT device add-on is large, cell density could depend on both the CNT and the CMOS transistor. Operation of these cells relies on a resistance change that correspondingly switches bits between conductive and nonconductive states.

A promising commercial application of CNTs is a type of cell referred to as nanotube RAM (NRAM). Various configurations can produce this switching behaviour, including a simple proposed "fabric of nanotubes" approach. This fabric consists of multiple interlinked nanotubes in a network resistor type configuration positioned between electrodes and directly coupled to the transistor. Electrically biasing this fabric causes the CNTs to contact, thereby lowering the resistance of the cell structure and writing a one. It is reported that the CNTs remain in contact based on Van der Waals attractions when the current is switched off, making the cell a potential example of a nonvolatile memory (NVM). In a RESET operation, a voltage is applied to cause separation of the CNTs, returning the cell to a zero. The exact mechanism for this separation is thought to be phonon (or lattice) excitation. Both switching and RESET voltages are reported to be greater than the read voltage.

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