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TECHNOLOGY LEDs


WE ARE ON THE CUSP of a lighting revolution. LEDs are no longer limited to just backlighting displays, but are now being deployed in lamps boasting incredibly long lifetimes and unprecedented efficiency. However, these solid-state sources are still to take a significant share of the lighting market from incandescents and compact fluorescents. Why? Because they retail for eye-watering prices.


Given that the battalion of packaged LED chips that are mounted in the bulb account for about half its cost, the obvious route to driving down the price of this form of lighting is to use fewer chips, and crank up the current through them. This plan would work for red-emitting arsenide and phosphide based LEDs, but III-nitride emitters are plagued by a mysterious malady known as droop: As the current rises, the internal quantum efficiency (IQE) and, as a consequence, the light output efficiency, diminish considerably. This is a major setback for general lighting because nitride-based LEDs lie at the heart of these solid-state bulbs. White light stems from mixing blue-emission from the chip with yellow emission from a phosphor, which is pumped by the LED.


Understanding what causes droop holds the key to developing new LED architectures for combatting this efficiency-sapping mechanism. To try and uncover its origin, experimentalists and theoreticians, both in academia and industry, have been racking their brains to develop theories for droop. This has led to many conjectures, and today droop is a highly controversial topic.


Listing the suspects


The first potential candidate for the origin of droop appears in the equations describing the three standard recombination processes in an LED. In this mathematical description of LED behaviour, there is a term for the light-generation process, which is proportional to the square of the carrier density, and two other terms: one for Shockley- Read-Hall recombination, which is unlikely to cause droop because it scales linearly with density; and another for Auger recombination, a process that is proportional to the third power of the carrier density, and involves an interaction of three carriers to promote one of them into a higher energy state.


At first sight, Auger recombination is a very promising contender for explaining droop, because the strength of this processes increases as the current through the device is cranked up. However, it is debatable whether the Auger coefficient – the proportionality factor between the Auger recombination rate and the carrier density – is large enough for this mechanism to be the primary cause of droop. Some theorists have calculated that the Auger coefficient is not large enough to cause droop, but recently a team from the University of California,


Figure 1. Light output power for a red InGaAsP- based LED and for a blue GaN-based LED. For a red LED the power increases almost linearly with the current, whereas the curve for a blue LED droops, lagging behind a linear dependency


Santa Barbara and Ecole Polytechnique, France, have reported spectrally resolved measurements identifying energetically elevated carriers as a relevant current component [1].


Another obvious suspect for causing droop is carrier leakage. Cranking up the current increases the carrier density in single or multiple quantum wells, making them less likely to trap carriers. Instead, these charged particles – particularly electrons – can fly over the active region, rather than contributing to radiative recombination. This makes them a source of droop and a contributor to the leakage current.


In addition to Auger recombination and carrier leakage, other sophisticated explanations have been proposed, such as defect-assisted mechanisms or a saturation of spontaneous emission [2]. The multitude of theories highlights the enduring quest within the scientific community to fathom the origin of droop.


Bringing it together


Our team at the University of Kassel, Germany, has also studied the cause of droop. Our efforts have not focused on developing yet another entirely new theory for droop, but looking again at the two leading traditional approaches, re-thinking how to model


Figure 2. Radiative and Auger recombination coefficient dependency on the current density for a green, single-quantum- well LED. The coefficients are plotted relative to their respective values at 0.1 A cm-2


. At 300 A cm-2


the Auger coefficient is more than two times higher than the reference value. The inset illustrates the reason for this effect: In c-plane quantum wells the polarization effect tilts the conduction and valence band edges (CB, VB) resulting in a reduced overlap integral of electron and hole wave functions (black curves). At high bias, the polarization field is screened. The overlap integral, as well as the radiative and Auger recombination coefficients, increase. In a-plane devices no interface polarization is present, resulting in a high radiative as well as in a high Auger coefficient


July 2013 www.compoundsemiconductor.net 47


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