Phase-change materials are the most promising technology for replacing flash memory in laptops, cell phones and many other portable applications. The active material in re-writable DVDs and are usually made of phase-change materials such as chalcogenides like germanium antimony telluride (GST). The materials are switched on and off by the change of state by heating with voltage pulses. The amorphous state is “off" due to a very high resistance and crystalline state is "on" because of a very low resistance, where as these states endure once the power is turned off. But relatively high power levels are usually required to switch between the amorphous and crystalline states in GST memory bits.

Insertion of nanotubes

Researchers of University of Illinois Urbana-Champaign have used carbon nanotubes to construct nanometre-scale Phase-change materials (GST) memory bits. They made tiny gaps varying in size from 20 to 300 nm within the nanotubes usually in the middle of a nanotube using a method called electrical breakdown. The researchers then filled the nanogap with a small amount of GST.

Working

The as-deposited GST bits are amorphous, with a high resistance of around 50 MΩ to get an off state. When a voltage is applied across the nanotube (which effectively acts as a contact or interconnect), an electric field is created across the nanogap and switches the GST bit to the crystalline phase with a low resistance of around0.5 MΩ, which is around 100 times lower. Since the switching only occurs in the small amount of material contained within the nanogap it has extremely low power dissipation compared to state-of-the-art devices that use much larger metallic wires to contact the phase-change material.

Such a power reduction could go very far in extending battery life and portability and could also ultimately lead to many novel applications. Researchers are further working to improve the long-term reliability of the memory bits.