“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 eliminates many of the defects introduced by top-down fabrication methods, a popular pathway for creating 3D photonic structures. Another advantage is the ease of creating layered heterostructures. For example, a quantum well layer could be introduced into the photonic crystal by partially filling the template with GaAs and then briefly switching the vapor stream to another material.
Once the template is full, the researchers remove the spheres - leaving a complex, porous 3D structure of single-crystal semiconductor. Then they 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 built a 3D photonic crystal LED – the first such working device. Braun’s group 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.
“From this point on, it’s a matter of changing the device geometry to achieve whatever properties you want,” Nelson said. “It really opens up a whole new area of research into extremely efficient or novel energy devices.”