MYSTERY of NITRIDE LEDs PIONNERING EXPERIMENT might leak through the active region into the p-doped layer where they recombine non-radiatively, failing to produce photons. This electron leakage from the active quantum wells( shown as 3 in fig. 1) translates directly into a loss of luminous efficiency. The solution came in the form of a quantum barrier, an electron blocking layer( EBL), made of the large bandgap AlGaN alloy, inserted between the quantum wells and the p-region to block electron overflow. Thanks to this quantum engineering, nitride LEDs can now reach internal quantum efficiencies of up to 90 %, meaning that 90 % of the injected electrons are converted into photons. These devices are at the heart of the lighting systems found today in our homes, streets, TVs and cars. The LED emission, that use a transition between two energy levels( conduction band and valence band), is by nature monochromatic, at a wavelength that can be adjusted by the Indium content in the InGaN quantum well. With about 20 % In, the LEDs are emitting in the blue. To get white light, one covers the chip with phosphors which absorb part of the blue emission and reemit yellow light, both blue and yellow resulting in white light. The story could have ended there, with a global commercial success and lighting products boasting luminous efficiencies around 150 lm / W, far surpassing competing technologies to the point where competition virtually disappeared. But there was still a small flaw in this triumph: when driven at high current, these LEDs lose efficiency. At high power, their performance can drop by a factor of two or three, a phenomenon known as efficiency droop. In the 2010s, this issue began to stir intense debate within the academic community. Guided by the known challenges of p-type doping and electron leakage, many researchers believed the problem once again lay in these areas: under high injection, the proportion of electrons crossing the active region and even the electron blocking layer( EBL) was thought to increase( 3 in Fig. 1). To counter this detrimental effect, various structural modifications were attempted, such as adjusting the height of the EBL, changing the number of quantum wells, or altering doping profiles. These studies, often carried out in low
Figure 1. LED band profile and schematic illustrating the Auger effect, which can populate the higher valleys, and electron emission into the vacuum through a cesiated surface. Various mechanisms at work in an LED are illustrated, respectively electron 1 and hole 7 injection, electron capture in the light-emitting quantum well 2, electron overflow from the active quantum well 3, Auger non-radiative recombination generating hot electrons 4, hot electron energy relaxation into Γ and L bands 5, surface energy relaxation generating the electron energy distributions observed outside the semiconductor 6.
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