Power/Performance Bits: Aug. 9

Phase-change memory

Researchers at Stanford are working on phase-change memory technology, which could deliver the best of volatile and non-volatile memory.

Phase-change materials can exist in two different atomic structures, each of which has a different electronic state. A crystalline, or ordered, atomic structure, permits the flow of electrons, while an amorphous, or disordered, structure inhibits electron flows. Researchers have developed ways to flip-flop the structural and electronic states of these materials by applying short bursts of heat, supplied electrically or optically.

Phase-change materials are attractive as a memory technology because they retain whichever electronic state conforms to their structure. Once their atoms flip or flop to form a one or a zero, the material stores that data until another energy jolt causes it to change.

This animation shows how data is stored using phase-change memory technology. Phase-change materials can exist in two atomic structures, disordered or ordered. An electric jolt flip-flops these structures back and forth. (Source: Tricia Seibold)

The new research focused on the brief interval when an amorphous structure began to switch to crystalline. This intermediate phase – where the charge flows through the amorphous structure like in a crystal – is known as “amorphous on.”

In the presence of a sophisticated detection system, the Stanford researchers jolted a small sample of amorphous material with an electrical field comparable in strength to a lightning strike. Their instrumentation detected that the amorphous-on state – initiating the flip from zero to one – occurred less than a picosecond after they applied the jolt.

Showing that phase-change materials can be transformed from zero to one by a picosecond excitation suggests that this emerging technology could store data many times faster than silicon RAM for tasks that require memory and processors to work together to perform computations.

Space is always a consideration in design, and previous experiments have shown that phase-change technology has the potential to pack more data in less space, giving it a favorable storage density.

Taking energy into account, researchers say the electrical field that triggered the phase change was of such a brief duration that it points toward a storage process that could become more efficient than today’s silicon-based technologies.

Finally, although this experiment did not establish precisely how much time would be required to completely flip an atomic arrangement from amorphous to crystalline or back, these results suggest that phase-change materials could perform superfast memory chores and permanent storage – depending on how long the thermal excitation is engineered to stay inside the material.

Switching liquid light

Researchers at University of Cambridge in collaboration with researchers from Mexico and Greece built a miniature electro-optical switch which can change the spin of a liquid form of light by applying electric fields to a semiconductor device a millionth of a meter in size.

Current methods of converting between electrical and optical signals are both inefficient and slow, and researchers have been searching for ways to incorporate the two.

The switch utilizes a new state of matter called a Polariton Bose-Einstein condensate in order to mix electric and optical signals, while using miniscule amounts of energy. Polariton Bose-Einstein condensates are generated by trapping light between mirrors spaced only a few microns apart, and letting it interact with thin slabs of semiconductor material, creating a half-light, half-matter mixture known as a polariton.

Polariton fluid emits clockwise or anticlockwise spin light by applying electric fields to a semiconductor chip. (Source: Alexander Dreismann)

Putting lots of polaritons in the same space can induce condensation – similar to the condensation of water droplets at high humidity – and the formation of a light-matter fluid which spins clockwise (spin-up) or anticlockwise (spin-down). By applying an electric field to this system, the researchers were able to control the spin of the condensate and switch it between up and down states. The polariton fluid emits light with clockwise or anticlockwise spin, which can be sent through optical fibers for communication, converting electrical to optical signals.

“The polariton switch unifies the best properties of electronics and optics into one tiny device that can deliver at very high speeds while using minimal amounts of power,” said Dr Alexander Dreismann from Cambridge’s Cavendish Laboratory.

While the prototype device works at cryogenic temperatures, the researchers are developing other materials that can operate at room temperature, so that the device may be commercialized. The other key factor for the commercialization of the device is mass production and scalability. According to Professor Pavlos Savvidis of the FORTH institute in Crete, Greece, “Since this prototype is based on well-established fabrication technology, it has the potential to be scaled up in the near future.”