Solving the 'Green Gap' in LED Technology

Photo of a microscope image of a mini deep green LED. Image shows part of a circle with intersecting green bands, green and black shapes, and a smaller glowing circle at the center.

Mini LED 100 µm in diameter as seen through microscope (regular fabricated bare die, no light extraction enhancement)

A line chart with Wavelength represented on the horizontal axis, and EL Intensity on the vertical axis.  A green line representing "on r-sapphire" is in the form of a steep mountain peak.  A red line representing "on a-GaN bulk" starts out more level, then follows the same steep mountain peak form of the green line, only wider and higher up on the scale.

A non-polar homoepitaxial multi-quantum well LED on bulk GaN structure shows tenfold stronger green peak emission than the same growth on sapphire. The research team at RPI will continue this work to further improve green lighting efficiency.

One long-standing high-priority research area for DOE is to increase the efficiency of deep green LEDs. Although most products today use phosphor conversion (PC) to produce white light from a blue LED, having a good green source could lead to color-mixed white sources that would avoid the losses associated with the PC approach.

DOE-funded research at Rensselaer Polytechnic Institute (RPI) has shown the importance of both the epitaxial structure and the substrate material in improving green light efficiency. The RPI team is working on non-polar epitaxial multi-quantum well LEDs on Gallium Nitride substrates that produce deep green light directly. So far they have been able to produce excellent low-dislocation material and to replicate substrate quality throughout the active region, leading to a doubling of green light output for a given input power.

By growing on non-polar Gallium Nitride substrates, the RPI team is developing processes to double or triple the light output power from green and deep green AlGaInN LED dies in reference to the Lumileds Luxeon II. Lumileds Luxeon II dies and lamps have been identified by DOE as the uniform reference of current performance levels and therefore are being used by the research team.

Since LEDs in this spectral region show the highest potential for significant performance boosts, the project is paying particular attention to all aspects of the internal generation efficiency of light. Anticipated results are high output green and deep green (525 - 555 nm) LED chips as part of high efficacy red-green-blue LED modules. Such modules would perform at and beyond the efficacy target projections for white-light LED systems in DOE's accelerated roadmap.

Some key results to date include:

  • Growing good quality green emitting epitaxial material on c-plane, a-plane and m-plane Gallium Nitride
  • No wavelength shift in non-polar green emitting material with increased excitation density
  • Replicating substrate quality throughout the active region
  • Low-dislocation density bulk Gallium Nitride substrate