Explaining LEDs' Diminishing Returns
New concept pulls popular theories together, drives researchers apart
The LED community is ensnared in a long-running and contentious debate over the origin of a phenomenon called droop, the decline in the efficiency of blue and white emitters as their current is cranked up. Solving this mystery will enable the design of droop-busting LED architectures that will make brighter, cheaper solid-state lighting.
The droop debate has recently heated up, with a handful of competing conjectures winning significant backing from groups of optoelectronics experts. Now a theorist at Sandia National Laboratories, in Albuquerque, has found a way to draw the models of light emission together. Weng Chow calculates the behavior of LEDs from their band structure, which specifies the energies that charge carriers can have within devices and the likelihood of finding carriers with those energies.
Every LED, regardless of its color, operates by injecting electrons into the device from one side and driving in their positive counterparts—holes—from the other. Both types of carriers meet in a narrow trench, known as a quantum well. Here they are trapped, bind together through electrostatic attraction, and recombine to emit light.
In red LEDs, these quantum wells are just like those in an elementary quantum-mechanics textbook, and the probability of finding electrons and holes in the trenches is very high. But blue and white LEDs are plagued by strong internal electric fields. Chow explains that when the current is merely trickling through the LED, electrons and holes rarely form the bound states needed for light emission: "The field just rips them off."
In one model, as you crank the current up, electron and hole populations rise, partially offsetting the internal electric field. However, even though most of the carriers are now in the well, droop sets in because the internal field is still strong enough to yank electrons to one side of the trench and holes to another, hampering light emission. Cranking the current even higher in this model negates the effects of the internal field, improving emission efficiency and leading to a recovery from droop, a phenomenon that contradicts what is seen in real LEDs.
Blue and white LEDs are riddled with defects in their crystal lattices, which are more prevalent in their indium gallium nitride quantum wells. When Chow includes this detail in his model, it can quash droop recovery and mimic the real behavior of these devices.
In this form, Chow's model unites two leading conjectures for the cause of LED droop—defects and inefficient carrier recombination in quantum wells. But, crucially, he took it one step further: Chow incorporated the most popular droop theory of all—Auger recombination, a non-light-emitting interaction of three charge carriers that propels either an electron or a hole to a higher energy state. With this addition, Chow's theoretical predictions for LED efficiency mirror the major trends seen in experimental results.
According to Ümit Özgür, from Virginia Commonwealth University, Chow's work warrants attention. He believes that one of its strengths is that it models real physical phenomena known to take place during light emission. A simpler, very widely used model to explain LED behavior and account for droop fails to do this. Özgür now wants Chow to fit experimental data from a range of real devices to his model.
Manos Kioupakis and Chris Van de Walle from the University of California, Santa Barbara, are more critical of Chow's work. They point out that in Chow's model, droop is sensitive to temperature and argue that this finding is inconsistent with experimental results.
The Sandia researcher counters by claiming this inconsistency does not exist, because his model is not as sensitive to temperature as the UCSB team suggests. He also points out that work by Jörg Hader and his colleagues from the University of Arizona can replicate LED behavior at various temperatures with a model, like Chow's, that includes defects.
What is clear is that the droop debate shows no signs of abating. While Chow's work is drawing together several leading theories, it's failing to unite all the theorists responsible for them.