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Posted: December 27, 2007

The perfect computer memory: quantum dots?

(Nanowerk News) Flash storage is one of the main components that makes low power electronics so flexible. Unlike common DRAM, which needs constant refreshing in order to retain its contents, flash memory will stay written for about 10 years without power. However, flash pays for that longevity in access times, which are much slower than those for DRAM. The perfect memory would be nonvolatile like flash yet provide access faster than the current generation of DRAM. Quantum dots, with their nicely tunable electronic properties, look like they may fit the bill.
Researchers in Germany have been exploring the suitability of self-assembled arrays of quantum dots for nonvolatile storage. A quantum dot is a small clump of atoms that is confined in a way that restricts the motion of the electrons, making the whole thing act like a single atom ("106 years extrapolated hole storage time in GaSb/AlAs quantum dots"). The properties of the dot can be modified by changing the size of the clump or the constituent atoms.
In the quantum dot-based storage array, the researchers have been looking at the constituent atoms, trying out silicon and germanium, and more complicated mixtures of gallium, indium, arsenide, aluminum, and antimony (for those of you keeping count, these are III-V materials). Experimentally, they have found that quantum dots can have access times of around 10ns, faster than the current generation of RAM, and they require a refresh rate as low as 0.7Hz. Further calculations show that more suitable combinations would result in a storage time of one million years while maintaining the same access time.
Quantum dots can do this because there is more design freedom in setting them up. Normal flash memory relies on the huge potential barrier created by a silicon oxide layer. The probability of an electron tunneling across the barrier is so low that the data will stay there for 10 years. However, to get electrons across that barrier when writing data to a flash cell requires a lot of energy, energy that destroys the silicon oxide layer. This is why flash memory has a limited number of write cycles in it.
Quantum dots, in contrast, have tunable properties, so the barrier can be kept low. In the current work, the barrier was four times lower than that of silicon dioxide. Additionally, the data can be stored as an absence of electrons, called holes. These holes behave exactly like positively charged electrons, except that they are heavier. The confinement of the quantum dot makes them even heavier than normal, which reduces the chances of them tunneling out of the quantum dot. The result is a very low refresh rate.
Based on the known properties of the materials used and the behavior of quantum dots, the researchers predict that they will be able to make quantum dots that can store data for one million years with an access time of 10ns. If they can make these in volume and at the same data density as standard flash modules, you can say goodbye to hard drives, flash, and RAM. Personally, I won't miss any of them.
Source: ars technica (Chris Lee)
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