Crystals form the basis for the penetrating icy blue glare of car headlights - they could also be fundamental to the future in solar energy technology?
Manufactured alloy crystals, such as indium gallium nitride or InGaN, form the light emitting region of LEDs, for illumination in the visible range, and of laser diodes (LDs) in the blue-UV range.
In an article recently published in the journal Applied Physics Letters, researchers revealed the fundamental aspect of a new approach to growing InGaN crystals for diodes, which promises to move photovoltaic solar cell technology toward record-breaking efficiencies.
The InGaN crystals are grown as layers in a sandwich-like arrangement on sapphire substrates. Typically, researchers have found that the atomic separation of the layers varies; a condition that can lead to high levels of strain, breakdowns in growth, and fluctuations in the alloy's chemical composition.
"Being able to ease the strain and increase the uniformity in the composition of InGaN is very desirable," says Arizona State University Professor Fernando Ponce, "but difficult to achieve. Growth of these layers is similar to trying to smoothly fit together two honeycombs with different cell sizes, where size difference disrupts a periodic arrangement of the cells."
As outlined in their publication, the authors developed an approach where pulses of molecules were introduced to achieve the desired alloy composition. The method, developed by Doolittle, is called metal-modulated epitaxy. "This technique allows an atomic layer-by-layer growth of the material," says Ponce.
Analysis of the atomic arrangement and the luminosity at the nanoscale level was performed by Fischer, the lead author of the study, and Wei. Their results showed that the films grown with the epitaxy technique had almost ideal characteristics and revealed that the unexpected results came from the strain relaxation at the first atomic layer of crystal growth.
"Doolittle's group was able to assemble a final crystal that is more uniform and whose lattice structures match up…resulting in a film that resembles a perfect crystal," says Ponce. "The luminosity was also like that of a perfect crystal. Something that no one in our field thought was possible."
These two seemingly insurmountable defects (non-uniform composition and mismatched lattice alignment) ultimately means that LEDs and solar photovoltaic products can now be developed that have much higher, efficient performance.
"While we are still a ways off from record-setting solar cells, this breakthrough could have immediate and lasting impact on light emitting devices and could potentially make the second most abundant semiconductor family, III-Nitrides, a real player in the solar cell field," says c-author Professor Alan Doolittle of the Georgia Institute of Technology. A patent is pending for the new technology.