A recent study demonstrates the rapid control of
phase-changes in resonantly bonded materials
(July 29, 2015) Rewritable
CDs, DVDs and Blu-Ray discs owe their existence to phase-change materials, those
materials that change their internal order when heated and whose structures can
be switched back and forth between their crystalline and amorphous phases.
Phase-change materials have even more exciting applications on the horizon, but
our limited ability to precisely control their phase changes is a hurdle to the
development of new technology.
One of the most popular and useful phase-change materials is
GST, which consists of germanium, antimony, and tellurium. This material is
particularly useful because it alternates between its crystalline and amorphous
phases more quickly than any other material yet studied. These phase changes
result from changes in the bonds between atoms, which also modify the
electronic and optical properties of GST as well as its lattice structure.
Specifically, resonant bonds, in which electrons participate in several
neighboring bonds, influence the material’s electro-optical properties, while
covalent bonds, in which electrons are shared between two atoms, influence its
lattice structure. Most techniques that use GST simultaneously change both the
electro-optical and structural properties. This is actually a considerable
drawback since in the process of repeating structural transitions, such as
heating and cooling the material, the lifetime of any device based on this
material is drastically reduced.
In a study recently published in Nature Materials,
researchers from the ICFO groups led by Prof. Simon Wall and ICREA Prof. at
ICFO Valerio Pruneri, in collaboration with the Firtz-Haber-Institut der
Max-Planck-Gesellschaft, have demonstrated how the material and electro-optical
properties of GST change over fractions of a trillionth of a second as the
phase of the material changes. Laser light was successfully used to alter the
bonds controlling the electro-optical properties without meaningfully altering
the bonds controlling the lattice. This new configuration allowed the rapid,
reversible changes in the electro-optical properties that are important in
device applications without reducing the lifetime of the device by changing its
lattice structure. Moreover, the change in the electro-optical properties of
GST measured in this study is more than ten times greater than that previously
achieved by silicon materials used for the same purpose. This finding suggests
that GST may be a good substitute for these commonly used silicon materials.