The ever-increasing demand for computing power has driven the aggressive down-scaling of transistor size in microprocessors as well as cell sizes in both volatile and non-volatile memories.
Hence, researchers around the world have put tremendous effort into developing novel storage devices that offer rapid read and write speeds, high storage density and non-volatility.
In recent years, Frost & Sullivan has noted that considerable attention has been given to phase-change materials for such applications. Phase-change materials, as their name suggests, are capable of switching their material phases reversibly from crystalline to amorphous states, under certain conditions such as high temperature. Unlike the floating gate structure in conventional Flash devices, phase-change memory uses the change in resistivity to detect the presence of data.
Conventional memory uses charge or trapped electrons to hold the information. As device dimensions have shrunk, it has become harder to hold the charges and the technology is also more susceptible to noise. Phase-change materials offer a cost-effective and relatively easy way to overcome the scaling issue in conventional Flash memory.
As with other nano-applications, the conventional top-down processing of phase-change materials is highly challenging. Moreover, the intense lithography and etching processing may damage the material and reduce its effectiveness in performance. For phase-change material, it is difficult to scale it down to sub-100 nm, due to the presence of process damages.
Experts in this area are currently developing solutions that could implement phase-change memory at this dimension or below. One promising way is by using a bottom-up approach, which is to employ nanowires. Using the right conditions, different sizes of nanowires can be prepared and they can be defect-free, with a unique geometry in a single-crystalline structure.
Recently, memory effect was reported in germanium tellurium alloy (GeTe) and germanium antimony tellurium alloy (Ge2Sb2Te5) nanowire. However, those reports did not address the critical issues related to memory switching efficiency, cyclability endurance and operation speed that provide the evidence of its feasibility as a true memory device.
With the proof-of-concept from earlier reports, researchers from the University of Pennsylvania developed memory devices based on Ge2Sb2Te5 nanowires and reported the physical and electrical scaling effects in these devices in the September 2007 issue of Nature Nanotechnology. These nanowires are synthesised by using gold catalyst in mediated vapour-liquid-solid process.
Transmission electron micrographs and diffraction patterns showed that these nanowires are single crystalline with trigonal faceted structures. X-ray spectrometry shows the uniform distribution of the three elements and their chemical composition across the dimension of the nanowires. Memory effects were observed when the voltage across the nanowire is swept from 0 V to nearly 5 V. An abrupt increment in the current flow was observed at about 1,8 V, indicating the change from amorphous to crystalline characteristics due to Joule heating.
The conversion to amorphous state and vice versa occurs by treating the nanowire with different stress durations. The conditions were obtained by a thorough investigation using different stress currents and durations. From the endurance cycle test, the device shows little sign of degradation after a hundred thousand cycles. The tests also showed extremely low power consumption for data encoding (0,7 mW per bit), and short data writing, erasing and retrieval time (50 nanoseconds), which is around 1000 times faster than conventional Flash memory.
For more information contact Patrick Cairns, Frost & Sullivan, +27 (0)21 680 3274, [email protected]
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