Light drives the migration of charge carriers (e-, h+) at the juncture between semiconductors with mismatched crystal lattices. Background image shows the bilayer in atomic-scale resolution. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy
It's not easy to make two two-dimensional semiconductors stick together if they have large “lattice misfits”—that is, if the layers of crystalline materials are too differently shaped to form strong bonds. A U.S.-based research team has figured out how to stack such mismatched monolayers to create useful heterostructures (Sci. Advances, doi:10.1126/sciadv.1501882).
Specifically, the researchers stacked two atomically thin layers of gallium selenide (GaSe) and molybdenum diselenide (MoSe2), which together formed a p-n junction that produces a strong, gate-tunable photovoltaic response. This kind of stack could be used to make devices ranging from field-effect transistors to solar cells.
The scientists at Oak Ridge National Laboratory (Tennessee) wanted to build layers of two different kinds of chalcogenides, semiconductors that combine an atom of a metal or transition metal with one or two atoms of group 16 of the periodic table (sulfur, selenium or tellurium). Conventional stacking of chalcogenides with different lattice structures often introduces unwanted contamination at the interfaces. The two substances studied here, the p-type GaSe and the n-type MoSe2, have a lattice mismatch of about 13 percent.
The team used chemical vapor deposition to layer MoSe2 on a quartz substrate and then grew GaSe on top of the other chalcogenide via vapor-phase deposition. Repeating Moiré patterns, seen under the electron microscope, revealed that the two dissimilar materials had settled into a long-range atomic order.
Photovoltaic cells that rely on this semiconductor stack might have rather low efficiency, but the scientists suggest that optimizing the growth conditions under which the GaSe is deposited might produce higher-quality layers of that material, helping to boost efficiency.
Researchers at Vanderbilt University and the University of Utah also contributed to the study.