Using an epitaxial approach, the first optoelectronically active 3-D photonic crystal has been demonstrated — an advance that could open new avenues for solar cells, lasers, LEDs, metamaterials and more. "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," said research leader Paul Braun, a professor of materials science and engineering and of chemistry at the University of Illinois. This graphic shows the method for epitaxial growth of 3-D photonic crystals. (Images: Erik Nelson) Photonic crystals can control or manipulate light in unexpected ways with their unique physical structures. However, previous attempts at making 3-D photonic crystals produced devices that could direct light but that could not turn electricity to light or vice versa. The Illinois team's photonic crystal has both properties. To create a 3-D photonic crystal that is both electronically and optically active, the researchers started with a template of tiny spheres packed together, then deposited gallium arsenide through the template, filling in the gaps between the spheres. The GaAs grows as a single crystal from the bottom up, a process called epitaxy. Epitaxy is commonly used 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 3-D structure. "The key discovery here was that we grew single-crystal semiconductor through this complex template," said Braun, who also is affiliated with the Beckman Institute for Advanced Science and Technology and with the Frederick Seitz Materials Research Laboratory at the university. "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." Using an epitaxial approach, University of Illinois researchers developed a 3-D photonic crystal LED, the first such optoelectronic device. The epitaxial approach eliminated many of the defects introduced by top-down fabrication methods, a popular pathway for creating 3-D photonic structures. Another advantage was the ease of creating layered heterostructures. Once the template was full, the researchers removed the spheres, leaving a complex, porous 3-D structure of single-crystal semiconductor. They coated 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, they built a 3-D photonic crystal LED, the first such working device. The group currently is working to optimize the structure for specific applications. The LED demonstrates that the concept produces functional devices, but by tweaking the structure or using other semiconductor materials, researchers can improve solar collection or target specific wavelengths for metamaterials applications or low-threshold lasers. For more information, visit: www.illinois.edu