Strain-free epitaxy of germanium film on mica

November 17, 2017

WASHINGTON, D.C., November 17, 2017 -- Germanium, an elemental semiconductor, was the material of choice in the early history of electronic devices, before it was largely replaced by silicon. But due to its high charge carrier mobility -- higher than silicon by threefold -- the semiconductor is making a comeback.

Germanium (Ge) is generally grown on expensive single-crystal substrates, adding another challenge to making it sustainably viable for most applications. To address this aspect, researchers at Rensselaer Polytechnic Institute in the United States demonstrate an epitaxy method that incorporates van der Waals' forces to grow Ge on mica. Applications could include advanced integrated circuits and high-efficiency solar cells.

"This is the first time strain-free van der Waals epitaxy of an elemental semiconductor has been demonstrated on mica," said Aaron Littlejohn, RPI researcher and co-author of the paper demonstrating the work, published recently in the Journal of Applied Physics, from AIP Publishing.

Growing crystalline film layers on crystalline substrates (called epitaxy) is ubiquitous in semiconductor fabrication. If the film and substrate materials are the same, then the perfectly matched layers form strong chemical bonds for optimal charge carrier mobility.

Layering different materials effectively, however, is a challenge because the crystal lattices typically don't align. To get around this, researchers employed vdW forces, phenomena that are based on the probabilistic nature of electrons, which are not in a fixed position around a nucleus. Rather, they can be anywhere, and the probability that they will be unevenly distributed exists almost all the time. When this happens, there is an induced dipole: a slight positive charge on one side and a slight negative charge opposite. This produces weakly attractive interactions between neutral atoms.

The researchers chose mica as the substrate on which to grow the Ge film because of its atomically smooth surface, which is free of dangling bonds (unpaired valence electrons). This ensured that no chemical bonding would take place during the vdW epitaxy process.

Instead, the materials' interface is held together via weak vdW forces. This allows for the growth of a relaxed film despite the dramatically different crystal structures of the two materials which have a 23 percent difference in atomic spacings. In addition to alleviating the constraints of lattice matching, vdW epitaxy allows the Ge film to be mechanically exfoliated from the mica surface and stand alone as a substrateless film.

"Our Ge film could be used as a thin-film nanomembrane, which could be integrated into electronic devices more easily than nanocrystals or nanowires," Littlejohn said. "It could also serve as the substrate for the subsequent deposition of additional materials for flexible transistors and solar cells, or even wearable optoelectronics."

Geranium films about 80 nanometers thick were grown on millimeter-scale muscovite mica substrates .26 mm thick. By varying the substrate temperature during deposition and annealing in the range of 300-500 degrees Celsius, the researchers found that the crystal lattice stabilizes at about 425 degrees Celsius.

"Previous research implies that elemental semiconductors cannot be epitaxially grown on mica using vdW forces at any elevated temperature, but we have now shown otherwise," Littlejohn said. "With the success of our Ge film grown on mica at a practical temperature, we anticipate that other nonlayered elemental or alloyed materials can be grown on mica via vdW epitaxy."
The article, "van der Waals epitaxy of Ge films on mica," is authored by Aaron J. Littlejohn, Yu Xiang, Elma Rauch, Toh-Ming Lu and Gwo-Ching Wang. The article appeared in the Journal of Applied Physics Nov. 7, 2017 [DOI: 10.1063/1.5000502] and can be accessed at


Journal of Applied Physics is an influential international journal publishing significant new experimental and theoretical results of applied physics research. See

American Institute of Physics

Related Solar Cells Articles from Brightsurf:

Solar cells of the future
Organic solar cells are cheaper to produce and more flexible than their counterparts made of crystalline silicon, but do not offer the same level of efficiency or stability.

A blast of gas for better solar cells
Treating silicon with carbon dioxide gas in plasma processing brings simplicity and control to a key step for making solar cells.

Record efficiency for printed solar cells
A new study reports the highest efficiency ever recorded for full roll-to-roll printed perovskite solar cells.

Next gen solar cells perform better when there's a camera around
A literal ''trick of the light'' can detect imperfections in next-gen solar cells, boosting their efficiency to match that of existing silicon-based versions, researchers have found.

On the trail of organic solar cells' efficiency
Scientists at TU Dresden and Hasselt University in Belgium investigated the physical causes that limit the efficiency of novel solar cells based on organic molecular materials.

Exciting tweaks for organic solar cells
A molecular tweak has improved organic solar cell performance, bringing us closer to cheaper, efficient, and more easily manufactured photovoltaics.

For cheaper solar cells, thinner really is better
Researchers at MIT and at the National Renewable Energy Laboratory (NREL) have outlined a pathway to slashing costs further, this time by slimming down the silicon cells themselves.

Flexible thinking on silicon solar cells
Combining silicon with a highly elastic polymer backing produces solar cells that have record-breaking stretchability and high efficiency.

Perovskite solar cells get an upgrade
Rice University materials scientists find inorganic compounds quench defects in perovskite-based solar cells and expand their tolerance of light, humidity and heat.

Can solar technology kill cancer cells?
Michigan State University scientists have revealed a new way to detect and attack cancer cells using technology traditionally reserved for solar power.

Read More: Solar Cells News and Solar Cells Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to