Chemical thermodynamics overtakes epitaxy

Saturday 27th March 2010
Schematic of hybrid core-shell growth process

A historic new semiconductor processing development is emerging from research work at Maryland University. "Nonepitaxial growth of hybrid core-shell nanostructures with large lattice mismatches," from Jiatao Zhang, Yun Tang, Kwan Lee, and Min Ouyang, appearing in 'Science,' holds a revolutionary promise for the industry.

"By controlling soft acid-base coordination reactions between molecular complexes and colloidal nanostructures, we show that chemical thermodynamics can drive nanoscale monocrystalline growth of the semiconductor shell with a lattice structure incommensurate with that of the core."

"Our method doesn't require a clean room facility and the materials don't have to be formed in a vacuum the ways those made by conventional epitaxy do," says (right) Min Ouyang, an assistant professor in the Department of Physics and the Maryland NanoCenter "Thus it also would be much simpler and cheaper for companies to mass produce materials with our process."

The team has created a process that uses chemical thermodynamics to produce, in solution, a broad range of different combination materials, each with a shell of structurally perfect mono-crystal semiconductor around a metal core.

They say their thermodynamic approach offers a host of benefits over epitaxy, currently used to create single crystal semiconductors and related devices. The biggest advantage of their chemical process may be that it avoids two key constraints of epitaxy - a limit on deposition semiconductor layer thickness and a rigid requirement for "lattice matching."

"Our process should allow the creation of materials that yield highly integrated multi-functional microelectronic components; better, more efficient materials for photovoltaic cells; and new biomarkers," said Ouyang, who noted his team is in the process of applying for a patent.

(Left: 3D-TEM images of hybrid Au-CdS core-shell nanostructures possessing monocrystalline CdS shell and various unequal Au core lattice structure).

"We envision for example that we can use this method to create new types of photovoltaic cells that are ten times more efficient in converting sunlight to electricity than current cells."

The new method also can be used to design and fabricate artificial quantum structures that help scientists understand and manipulate the basic physics of quantum information processing at the nanoscale, said Ouyang, noting that he and his team have a separate paper on the quantum science applications of this method that they expect to be published in the near future.

"This is a major, major advance that shows it is possible to do something that was impossible to do before," said Massachusetts Institute of Technology Associate Professor Francesco Stellacci, (right) whose work focuses on discovery of new nanoscale material properties and development of new nanofabrication schemes but was not involved in the study. "This research actually shows that it's possible at the nanoscale for two materials to happily coexist at their interface, two materials that would not coexist otherwise." 

The work was supported by the Office of Naval Research, the National Science Foundation (NSF) and the Beckman Foundation. Facility support was from Maryland Nanocenter and its Nanoscale Imaging, Spectroscopy and Properties Laboratory, supported in part by the NSF as a Materials Research Science and Engineering Centers shared experiment facility.

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