Germanium 'Sponge' Shows Promise in Photonics

Researchers have known for years that porous silicon has different properties than crystalline silicon but both work well in microelectronics. Now two research groups have discovered ways to make another semiconductor used in microelectronics -- germanium -- porous as well, making it potentially adaptable to applications including solar cells, photodetectors, sensors and light-emitting devices.

The two research groups, one at Michigan State University (MSU) and one at the University of California, Los Angeles (UCLA), have developed methods for honeycombing germanium with microscopic channels. These tiny holes alter the electronic properties of the now spongelike material, and suggest new ways to "tune" its behavior for specific applications, the researchers said. Silicon has previously been turned into a light-emitting material by using an acid to etch it into a holey, sponge-like form.

MSU chemistry professor Mercouri Kanatzidis and his MSU colleague Gerasimos Armatas used a mixture of magnesium germanide, containing negatively charged germanium atoms, and germanium tetrachloride, containing positively charged germanium atoms, to make a nanoporous form of germanium templated by a surfactant (wetting agent). The material is threaded by an ordered but intricate network of pores, and absorbs light at shorter wavelengths than ordinary crystalline germanium, the researchers said.

UCLA assistant professor Sarah Tolbert of the department of chemistry and biochemistry and her team used a similar approach, involving a compound of germanium and potassium containing small clusters of nine germanium atoms that become linked into chains. Again, these were combined with a surfactant that created a network of holes in the germanium, giving it a honeycomb-like pattern.

Tolbert's team said they also discovered that the porous material absorbs light at shorter wavelengths than bulk germanium, and they can tune this wavelength by altering the thickness of the walls between pores, converting some of the germanium to its oxide. They said the method also works with a mixture of germanium and silicon: such semiconductor "alloys" are commonly used in microelectronics and optoelectronics, because their electronic and optical behavior can then be tuned by varying its composition.

Both the MSU paper, "Mesostructured Germanium with Cubic Pore Symmetry", and the UCLA paper, "Hexagonal Nanoporous Germanium Through Surfactant-Driven Self-Assembly of Zintl Clusters", were published in the journal Nature this week. For more information, visit: www.nature.com/nature