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.
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.