Photonic crystal could lead to improved solar cells and lasers

‘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 Paul Braun, a professor of materials science, engineering and chemistry, who led the research effort.

Photonic crystals are materials that can control or manipulate light thanks to their unique physical structures; they can induce unusual phenomena and affect photon behaviour in ways that traditional optical materials and devices can’t.

Previous attempts at making 3D photonic crystals are said to have resulted in devices that are only optically active; they can direct light but are not electronically active, so they can’t turn electricity to light or vice versa.

The Illinois team’s photonic crystal is claimed to have both properties.

‘With our approach to fabricating photonic crystals, there’s a lot of potential to optimise electronic and optical properties simultaneously,’ said Erik Nelson, a former graduate student in Braun’s lab who now is a postdoctoral researcher at Harvard University. ‘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 create a 3D photonic crystal that is both electronically and optically active, the researchers started with a template of tiny spheres packed together. Then, they deposit gallium arsenide (GaAs), filling in the gaps between the spheres.

The GaAs grows 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 eliminates many of the defects introduced by top-down fabrication methods. 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 vapour 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, which is claimed to be the first such working device.

Now, Braun’s group is working to optimise the structure for specific applications.

The team published its advance in the journal Nature Materials.