Special optoelectronic properties make 3-D photonic crystals shine

Ashley N. Paddock, ashley.paddock@photonics.com

A newly demonstrated 3-D photonic crystal with exceptional optical and electrical properties will open new avenues for solar cells, metamaterials, lasers and more.

“For many years, people have demonstrated 3-D photonic crystals with exceptional optical activity; however, these structures have almost universally been electrically inert,” said Paul Braun, professor and lead scientist for the study at the University of Illinois. “Given that many of the exciting applications for 3-D photonic crystals require a system with both electrical and optical activity, we decided to focus on a path to enable that.”

A depiction of the method for epitaxial growth of 3-D photonic crystals. Images courtesy of Erik Nelson, Harvard University.

To create a 3-D photonic crystal that is both optically and electronically active, the researchers began with a template of tiny spheres packed together. They deposited gallium arsenide (GaAs) through the template, filling in the gaps between the spheres. The GaAs grows as a single crystal from the bottom up in a process called epitaxy that creates flat, 2-D films of single-crystal semiconductors. Braun’s group, however, developed a way to apply it to an intricate 3-D structure by growing single-crystal semiconductors through the complex template. The behavior of GaAs makes it want to grow as a film on the substrate from the bottom up, but as it runs into the template, it goes around it, essentially acting as a filler around the template until it reaches the top surface.

The epitaxial approach enables the researchers to eliminate many defects common to top-down fabrication methods. In addition, it facilitates creation of layered heterostructures. For example, a quantum well layer can be introduced into the photonic crystal by partially filling the template with GaAs and then briefly switching the vapor stream to another material.

When the template has been filled, the scientists remove the spheres, resulting in a complex, porous 3-D structure of a single-crystal semiconductor. They then coat the entire structure with a very thin layer of a semiconductor with a wider bandgap to improve its performance and prevent surface recombination.

The team tested its technique by building a 3-D photonic crystal LED, the first working device of its kind.

Using an epitaxial approach, University of Illinois researchers developed a 3-D photonic crystal LED, the first such optoelectronic device.

“Now that we have demonstrated that indeed optical and electrical activity can be combined in a structure formed from a direct bandgap semiconductor, we expect rapid advancement in using photonic crystal for both light harvesting and light emission applications,” Braun said.

The researchers are at work to improve the structure of the template they created for active 3-D photonic crystals.

“The current colloidal assembly template fabrication route results in structures with an undesirable number of defects,” Braun said. “Other optical patterning methods have the potential to solve this issue. We are looking to expand the space of materials we can work with. Nitrides in particular could be interesting.”

Findings of the study were reported online July 24 in Nature Materials (doi: 10.1038/nmat3071).