With the help of funding from the U.S. Department of Energy (DOE), a team of researchers at Sandia National Laboratories and The Ohio State University have demonstrated the feasibility of using multi-junction cascaded LEDs to increase external quantum efficiency (EQE, a measure of how efficient the LED is) at fixed current density. This approach avoids the common phenomenon known as current droop, in which the efficiency of the LED decreases as the electrical current density increases. Current droop remains a significant barrier to LED efficiency.

The researchers demonstrated low-differential on-resistance gallium nitride (GaN) tunnel junctions grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) and demonstrated linear scaling of EQE for a 3x multi-junction cascaded LED. In addition, they demonstrated complete activation of magnesium (Mg) p-dopants in a buried p-GaN (p-type gallium nitride) layer in a 2x multi-junction cascaded LED.

Hybrid tunnel junctions grown by a combination of MBE and MOCVD achieved 2 mohm.cm² specific on-resistance, and all-MOCVD tunnel junctions achieved 6 mohm.cm² on-resistance at 50 A/cm², which is a relevant current density for LEDs. These results compare well to the best results in the scientific literature. A 3x increase in EQE using a 3x multi-junction cascaded LED with all-MOCVD GaN tunnel junctions was achieved relative to a standard LED. Complete activation of Mg dopants in buried p-GaN:Mg layers using sidewall activation was also observed by uniform LED emission at low current density (2.5 A/cm²) in a 2x multi-junction cascaded LED. This finding shows the feasibility in using sidewall activation for complete Mg activation in multi-junction cascaded LEDs with an arbitrary number of junctions. 

The results of this project pave the way to circumvent droop, which could increase LED power conversion efficiency by as much as 15%. (September 2019)