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Bendable Memory Made from Nanowire Transistors

A new type of device could ultimately hold more data than flash memory.

Researchers in the U.K. have made a new kind of nanoscale memory component that could someday be used to pack more data into gadgets. The device stores bits of information using the conductance of nanoscale transistors made from zinc oxide.

Memory flexing: Nanowire transistors that switch between four different conductance states can be made on plastic substrates.

The researchers published a paper about a prototype memory device fabricated on a rigid silicon substrate last week in the online version of the journal Nano Letters. They are now testing flexible memory devices in the laboratory, says Junginn Sohn, a researcher at the University of Cambridge Nanoscience Center and lead author of the Nano Letters paper.

The nanowire device stores data electrically and is nonvolatile, meaning it retains data when the power is turned off, like the silicon-based flash memory found in smart phones and memory cards. The new memory cannot hold data for as long as flash, and it is slower and has fewer rewrite cycles, but it could potentially be made smaller and packed together more densely. And its main advantage, says Sohn, is that it is made using simple processes at room temperature, which means it can be deposited on top of flexible plastic materials. Nanowire memory could, for instance, be built into a flexible display and could be packed into smaller spaces inside cell phones, MP3 players, plastic RFID tags, and credit cards.

Flash memory elements contain transistors that store bits of data (a 1 or 0) using the presence or absence of charge on a gate electrode. However, like other silicon-based electronic devices, flash faces physical limits in terms of how much it can be miniaturized. Memory elements are already at 25 nanometers, translating to data densities of one terabit per square inch, and are projected to reach their minimum size limit of about 20 nanometers by late 2011. Companies are increasing flash memory densities by packing twice the amount of data by storing two bits, or four values, in each cell: 00, 01, 10, and 11.

The nanowire device can also store four values, as different levels of conductance. It is based on a zinc-oxide nanowire transistor, which the researchers make by placing a nanowire on a silicon substrate and depositing source and drain electrodes at either end of the wire. They coat the wire with barium-titanate nanoparticles and deposit an aluminum gate electrode layer on top.

A positive gate voltage builds up positive charges on each nanowire and puts the device in a high conductance state. A negative voltage switches it to a low conductance state. The researchers used four different voltages to create four conductance states.

Others are exploring various approaches to make flexible nanoscale memory that could be scaled down beyond silicon. Memory elements based on transistors made from ferroelectric-coated graphene, carbon nanotube, and other nanowires could all eventually be made flexible. Some groups have made flexible memory using organic materials, as well as two-electrode devices based on thin films of titanium dioxide and graphene.

Compared to the zinc oxide transistors, which have three electrodes, two-terminal devices can be packed more densely and potentially even in 3-D, says Curt Richter a researcher at the National Institute of Standards and Technology. Richter has made flexible titanium dioxide memory. But since the electronic read-and-write circuits in today’s flash memory are designed for silicon transistors, “the advantage of nanowire transistors is you wouldn’t have to change the control logic and architecture so much. You could tweak it a little bit and plug the device in.”

A lot of work remains to be done to make the nanowire memory devices practical. Right now, they only retain their conductance state for over 11 hours about 70 times–flash memory can withstand about 100,000 writing cycles. And the nanowires are currently 100 nanometers wide and two micrometers long, although Sohn says they can potentially be scaled down to smaller than flash memory.

The researchers will also have to prove that the memory devices are fast, says James Tour, a chemistry professor at Rice University who is working on ultradense memory using graphite and silicon oxide. In the paper, the researchers rewrite the device every second. Flash memory, by comparison, has a writing speed of microseconds. “At one second, it is not even on the table for being interesting to device folks,” Tour says. “They must increase the speed one million-fold to begin to catch their attention.” Because it is hard to make large batches of nanowires that work uniformly and to align them on surfaces, Tour says, “doing anything on nanowire electronics would not be feasible in mass-production.”

But Georgia Tech materials scientist Zhong Lin Wang, who has made nanogenerators and sensors from zinc oxide nanowires, says that the new memory could be integrated with those devices, paving the way for a completely new kind of electronics technology based on zinc oxide. “This paper demonstrates an exciting application of zinc oxide nanowires as nonvolatile memory, which is a key component for future flexible electronics,” he says.

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