Applied Physics

Rewritable High-Density Memories

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Science  07 Jan 2000:
Vol. 287, Issue 5450, pp. 13
DOI: 10.1126/science.287.5450.13a

The storage capacity of magnetic hard disks has increased rapidly from 0.02 gigabits per square centimeter (Gbit/cm2) in 1990 to the current level of around 1 Gbit/cm2. But at this rate of growth (60% a year), the upper limit of a few tens of Gbit/cm2 for this type of magnetic encoding will be reached by 2006, so the search is on for an alternative storage technology. One approach is to modify a surface locally with the tip of a scanning tunneling microscope (STM)or an atomic force microscope (AFM). In an air atmosphere, the tip of the STM or AFM is brought into proximity to a titanium surface, and a voltage pulse is applied at the tip to oxidize the surface. Although a density of 60 Gbit/cm2 has been attained, the resolution of the modified region is limited by the tip geometry.

Cooper et al. have overcome this limitation by attaching a single-walled carbon nanotube, 2 to 5 nanometers (nm) in diameter to the end of the AFM tip. Because the nanotube extends approximately 65 nm beyond the tip, the high electric field necessary for the oxidation process is confined to the region between the end of the nanotube and the surface. They fabricated an array of 8-nm bits spaced 20 nm apart, which corresponds to a storage density of 250 Gbit/cm2.

This write process is, however, a once-only event. Hasegawa et al. used a scanning probe technique, ballistic electron emission microscopy, to reversibly modify the electronic properties of a gold- silicon interface. With a negative voltage applied to the tip for 5 seconds, an area of the interface about 50 nm in diameter was modified and remained so for at least 1 hour; application of a positive voltage then returned the interface back to its original condition. The minimum size of the rewritable unit corresponded to the size of individual grains of gold, about 15 nm.—ISO

Appl. Phys. Lett.75, 3566 (1999); Appl. Phys. Lett.75, 3669 (1999).

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