New insight into the efficiency droop of GaN-based LEDs

 by Simon Li and Joachim Piprek The efficiency of most GaN-based LEDs decreases with higher injection current. Such a reduction in LED efficiency is commonly referred to as the LED efficiency droop. The droop phenomenon is universally observed across a broad wavelength spectrum of InGaN/GaN LEDs and also with ultraviolet AlGaN/AlN LEDs. This efficiency droop is currently subject of intense research worldwide, as it delays general lighting applications of nitride LEDs. Many proposals have been forwarded to explain the efficiency droop (see review in [1]).  One popular explanation is that the Auger recombination dominates the non-radiative loss at high injection current.  Auger recombination is easy to understand and model since its rate increases with the third power of the carrier density, while the light-emission rate scales with the square of the density.  As the carrier density increases with higher current, the Auger process quickly overpowers the light emitting process, resulting in the efficiency droop.    Comparison of Auger recombination and electron leakage within an LED energy band diagram Despite its simplicity and convenience, the Auger droop model is not generally accepted. GaN-based materials have wide bandgaps while the common Auger process is known to be strong only in semiconductors with a much narrower bandgap.  The Auger droop model assumes an Auger coefficient four orders of magnitude larger than predicted by text-book theories of the GaN Auger process. Researchers from the NUSOD Institute LLC (Newark, DE, USA) and Crosslight Software Inc. (Burnaby, BC, Canada) recently undertook investigations that shed new light on the origin of the LED efficiency droop phenomenon [2].  Using the advanced simulation software APSYS, they studied another possible cause of the droop phenomenon: electron overflow into the p-doped layers of the LED. Based on an earlier APSYS simulation study, electron leakage was first suggested in 2007 as possible origin of the efficiency droop [3].  The new study reveals how sensitive the electron overflow is to the properties of the AlGaN electron blocker layer (EBL) typically employed in nitride LEDs.  Not only is the EBL band gap of major importance but also the EBL band offset.  Assuming a reasonably small Auger coefficient, the new APSYS study shows that an EBL band offset ratio of about 50:50 would make the electron leakage current large enough to fully account for the observed efficiency droop. The often assumed AlGaN band offset ratio of 70:30 does not allow for significant overflow. As the exact offset is hard to measure or calculate, a ratio of about 50:50 seems quite reasonable. The electron overflow is more difficult to calculate than the Auger recombination as the former is influenced by the interplay between the built-in polarization charges and the p-type doping in the neighborhood of the EBL. The polarization interface charge tends to reduce the electron barrier of the EBL while the ionized magnesium acceptors screen the interface charge, resulting in a strong influence of the EBL acceptor profile on the leakage current.  However, the EBL acceptor profile is often not well controlled during the epitaxial LED growth process, which may be the reason for some confusing droop observations reported in the literature [1]. The new APSYS simulation study also reveals a quite abnormal thermal behavior: the electron overflow decreases with higher temperature. This is a surprising prediction since most semiconductor devices ranging from transistors to laser diodes perform better at lower temperatures while higher temperature usually causes undesirable power losses. The reason behind this droop abnormality is that the hole transport improves at higher temperatures, leading to reduced electron overflow. So, can this surprising temperature abnormality be observed experimentally? Yes indeed!  In an independent experimental study, a team from Nagoya Institute of Technology (Japan) recently published electroluminescence measurements on 264 nm AlGaN LEDs [4].  The Nagoya team not only found direct evidence for the electron overflow, they also observed that the amount of leakage decreases with increasing temperature.  These measurements serve as an impressive confirmation of the overflow model for the efficiency droop; however, other mechanisms may still be involved [1]. With sophisticated simulation software and increasing computational power, researchers are nowadays able to look beyond the symptoms of the droop and into the interplay of the different internal physical mechanisms, thereby moving towards the ultimate cure for the efficiency droop.   References: [1] J. Piprek, Physica Status Solidi A 270, 2217 (Oct. 2010) [2] J. Piprek and S. Li, Optical and Quantum Electronics, Online First: www.springer.com (Feb. 2011) [3] M. H. Kim, M. F. Schubert, Q. Dai, J. K. Kim, E. F.Schubert, J. Piprek, Y. Park,  Applied Physics Letters  91, 183507 (Oct. 2007) [4] J. Zhang, Y. Sakai, and T. Egawa, IEEE Journal of Quantum Electronics 46, 1854 (Dec. 2010)