Defects boost efficiency of blue LEDs

The performance of blue light-emitting diodes (LEDs) based on InGaN/GaN quantum well (QWs) can be greatly improved by inserting an underlayer made of InGaN. This type of LED is widely used in commercially available solid-state lighting. Adding the underlayer boosts the internal quantum efficiency of the LED to more than 80 percent, although despite being well-documented, it is not precisely known why this improvement occurs.

Several mechanisms have been proposed, but work reported in Applied Physics Letters has now unambiguously determined that this efficiency improvement is due to the way the underlayer reduces point defect density. The reduction occurs independent of dislocation density, the GaN surface morphology and even the strain state of the QWs.

The authors considered several mechanisms that had been proposed in the literature and produced a number of differently structured samples, both with and without an underlayer, to test them. Numeric fits to electroluminescence and photoluminescence measurements, as well as atomic force microscopy scans that directly revealed surface morphology, let them rule out all but one explanation.

The most likely role of the InGaN underlayer is to reduce defects, or impurities, on the surface. The authors further hypothesize that some still-unknown detrimental species is present at the GaN surface and reacts with indium atoms to form nonradiative complexes in the InGaN alloy. The role of the underlayer is to bury this detrimental species before the active QWs are grown, thus avoiding the formation of nonradiative complexes in the QW itself.

The authors plan to continue their investigations of the InGaN underlayer, hoping to gain a better understanding of the point defects and how they improve LED efficiency.

Source: “Burying non-radiative defects in InGaN underlayer to increase InGaN/GaN quantum well efficiency,” by C. Haller, J.-F. Carlin, G. Jacopin, D. Martin, R. Butté, and N. Grandjean, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.5007616 .