Lee's demonstration of nanowires enables nanoscale strain engineering
Laura Schmitt, MNTL
3/6/2017 9:41:42 AM
ECE ILLINOIS faculty member Minjoo Lawrence Lee and his students have demonstrated a novel crystal growth method for making semiconductor nanocomposites that could someday lead to better performing devices like CMOS chips and lasers. Nanocomposites are formed when nano-sized particles are embedded in a matrix in order to improve certain material properties.
Lee’s team, working in the Micro and Nanotechnology Lab, used molecular beam epitaxy (MBE) to grow germanium (Ge) nanowires in a matrix made of indium aluminum arsenide (InAlAs), a well-studied semiconductor material used in telecom applications. At a certain point of adding the germanium, it stopped dissolving and coalesced into self-assembled nanostructures.
According to Lee, MBE provides a great deal of control and is easier to monitor than other crystal growth methods. “MBE was ideally suited for this research because it enabled us to monitor the surface in situ—while we’re growing it—and we can control the germanium deposition down to the sub-monolayer level,” said Lee about the growth innovation known as spontaneous phase separation.
The resulting nanocomposites enable strain engineering at the nanoscale, resulting in novel and enhanced optoelectronic properties. Today, strain engineering is widely used to make quantum well lasers for long-haul telecommunications and silicon chips for digital electronics products. In both cases, strain induces changes to the active material’s bandgap and “density of states”, leading to properties and performance that would be otherwise unattainable.
“This growth mechanism allows us to impose new strain states that are difficult to get to in almost any other way,” said Lee, who is an associate professor of the department of Electrical Computer Engineering. “This enables us to expand the tool box of strain engineering.”
Lee’s group, which recently published its findings in Nature Communications, achieved the highest biaxial tensile strain in Ge (5.3%) reported to date.
In the coming months, Lee will try to extend the phase separation technique to other semiconductor materials—both for the matrix and the nanostructure. “We can basically draw from other III-V materials for the surrounding matrix,” he said. “We’re also interested to see if this phenomenon (creating strained states by phase separation) might also apply to other materials systems.”
Beyond that, Lee envisions making new types of optoelectronic devices that take advantage of the new strain states—for example, laser diodes, LEDs, or detectors that can take advantage of how light interacts with nanostructures.