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."
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