U.S. Department of Energy - Energy Efficiency and Renewable Energy
Building Technologies Office – R&D Projects
Soraa Is Optimizing the Use of Non-Polar and Semi-Polar Substrates to Improve Emitter Efficiency Under High-Current Operation
One of the biggest remaining technological hurdles for LED lighting, efficiency droop is the term given to the rapid drop-off in efficiency as the driving current is increased. Since higher currents produce more light from the LED chip, the ability to drive LEDs efficiently at high current density will have significant effects on cost as well as performance.
Non-polar and semi-polar gallium nitride (GaN), combined with low defect density freestanding bulk-GaN technology, offers significant improvement in optical efficiency and temperature stability, as well as considerably more device design flexibility compared to conventional polar (c-plane oriented) GaN devices. Most notably, semipolar GaN/indium-GaN quantum wells exhibit significantly reduced polarization-related internal electric fields which commonly limit the performance of current c-plane devices.
Soraa is working to find the optimal semipolar orientation for LED epitaxial growth. The project builds on Soraa's GaN-on-GaN technology, which grows the GaN epitaxial layer on low defect density native GaN substrate instead of the conventional approach of growing heteroepitaxially (i.e., on substrates such as sapphire, silicon carbide, or silicon). Soraa has already demonstrated 450 nm emitting nonpolar LEDs with internal quantum efficiencies of almost 90 percent and very low droop characteristics (less than 10 percent at very high current densities).
Soraa is working to find the optimal semipolar orientation (right) for LED epitaxial growth, in contrast to a polar orientation (left).