Scanning cathodoluminescence images of two InGaN MQW samples with InGaN ULs grown at 790°C (left) and 880°C (right). The small-scale dark spots evident in the sample with the lower growth temperature UL indicate the formation of v-defects, while the near absence of these dark regions suggest effective suppression of v-defect formation with higher temperature ULs.
The efficiency droop studies involved the exploration of a number of heterostructure designs and it was observed that particular designs reduced the anomalously high ideality factors of InGaN LEDs. In particular, through the study of a series of InGaN LEDs in which differing numbers of the GaN quantum barriers were Si-doped, it was observed that barrier doping led to reduced operating voltages and ideality factors. These experimental insights, along with numerical simulations, led to the conclusion that the anomalously high ideality factors in InGaN LEDs are caused by polarization-induced triangular band profiles of the GaN quantum barriers and that dopant-induced screening of polarization fields aids in reducing those potential barriers.

In addition to the studies on threading dislocations, the research team addressed the existing controversy of the impact of v-defects on InGaN multiple quantum well (MQW) and LED efficiency. The researchers developed a series of InGaN MQW and LED samples that employed InGaN “underlayers” (ULs) beneath the quantum wells (QWs) as a method to controllably nucleate these defects on existing threading dislocations. Through temperature-dependent PL studies of InGaN MQW samples both with and without v-defects, it was determined that v-defects were not contributing to increased IQE in the samples, but growth on underlayers themselves leads to increased IQE. Given the special importance of improving efficiency of LEDs at longer wavelengths, the team further investigated the potential for improving green LED efficiency through the application of underlayers. Although underlayer-induced efficiency enhancements were greater when probed by cathodoluminescence, electroluminescence measurements of green LEDs still revealed a ~ 40% improvement in LED efficiency with the application of underlayers.

While the first project thrust focused on investigating InGaN materials properties to improve internal quantum efficiency, a second thrust involved a variety of nanoscale engineering methods to enhance light extraction from InGaN LEDs. A major emphasis was the development and application of graded refractive index (GRIN) dielectric coating materials to eliminate Fresnel reflection from the GaN-air interface of the LED chip. As an example, the team applied oblique-angle deposition techniques to realize GRIN indium tin oxide (ITO) nanorod coatings which were shown to enhance LED output efficiency by 24%. It was found that additional lateral nano-patterning and micro-patterning of GRIN dielectric layers helps to the extract waveguided modes and leads to even higher efficiencies.

Beyond dielectric materials, the researchers also investigated nanostructured metal coatings and the potential for surface-plasmon-induced improvements to LED efficiency. The studies confirmed strong photoluminescence enhancement from InGaN QWs with various metal coatings and further demonstrated both enhanced LED output and reduced PL lifetimes of LEDs employing metal coatings in close proximity to QW layers. The teams notes that while the main focus of this thrust area was to explore methods to enhance light extraction, the coupling of QW excitation to surface plasmon modes in nearby metal coating layers is likely most advantageous in its potential to increase internal quantum efficiency of InGaN LEDs rather than light extraction. Thus, surface-plasmon-enhanced emission is most promising for LEDs that presently have low internal quantum efficiencies, including green and longer wavelength LEDs.

In summary, the joint project yielded a number of valuable insights and direct demonstrations of approaches to improve internal quantum efficiency and extraction efficiency of InGaN-based LEDs. These results indicate the continued importance of both nanoscale materials studies and nanoscale engineering of materials to maximize LED efficiencies and to realize ultra-efficient solid-state lighting.

This work was done by Mary H. Crawford, Arthur J. Fischer, Daniel D. Koleske, Stephen R. Lee, and Nancy A. Missert of Sandia National Laboratories, and E. Fred Schubert, Christian Wetzel, Shawn-Yu Lin of Rensselaer Polytechnic Institute.

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