Photonic Crystal Could Better Solar Cells And Lasers!
In a boost that could open new line of approach for solar cells, lasers and meta-materials, researchers at the University of Illinois have demonstrated the first optoelectronically active 3D photonic crystal. Paul Braun, a professor of materials science, engineering and chemistry, who led the research effort said, "We’ve discovered a way to change the three-dimensional structure of a well-established semiconductor material to enable new optical properties while maintaining its very attractive electrical properties."
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Photonic crystal!
Photonic crystals are materials that can govern or manipulate light thanks to their unique physical structures; they can stimulate strange phenomena and affect photon behavior in ways that traditional optical materials and devices can’t. Earlier attempts at making 3D photonic crystals are said to have resulted in devices that are only optically active; they can guide light but are not electronically active, so they can’t convert electricity to light or vice-versa.
The Illinois team’s photonic crystal is said to have both properties. Erik Nelson, a former graduate student in Braun’s lab who now is a postdoctoral researcher at Harvard University said, "With our approach to fabricating photonic crystals, there’s a lot of potential to optimize electronic and optical properties simultaneously. It gives you the opportunity to control light in ways that are very unique to control the way it’s emitted and absorbed or how it propagates."
To produce a 3D photonic crystal that is both electronically and optically alive, the researchers commenced with a template of tiny spheres packed together. Then, they deposited gallium arsenide (GaAs), filling in the gaps between the spheres. The GaAs develops as a single crystal from the bottom up, a process called epitaxy. Epitaxy is common in industry to create flat, two-dimensional films of single-crystal semiconductors, but Braun’s group developed a way to apply it to an intricate three-dimensional structure. "The key discovery here was that we grew single-crystal semiconductor through this complex template," said Braun. "Gallium arsenide wants to grow as a film on the substrate from the bottom up, but it runs into the template and goes around it. It’s almost as though the template is filling up with water. As long as you keep growing GaAs, it keeps filling the template from the bottom up until you reach the top surface."
The epitaxial approach reportedly removes many of the defects brought out by top-down fabrication methods. Another plus point is the ease of creating layered hetero-structures. For example, a quantum well layer could be introduced into the photonic crystal by partially filling the template with GaAs and then shortly switching the vapour stream to another material. When the template is full, the researchers remove the spheres, leaving a complex, porous 3D structure of single-crystal semiconductor. They then coat the entire structure with a very thin layer of a semiconductor with a wider bandgap to improve performance and prevent surface recombination. To test their technique, the group constructed a 3D photonic crystal LED, which is stated to be the first such working device.
Now, Braun’s group is working to make best use of the structure for specific applications. The team issued its advance in the journal Nature Materials.
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