Info
Info
News Article

UCSB Blames Auger For Droop

The debate over the origin of LED droop rages on, with recent simulations contradicting earlier calculations and claiming that direct Auger recombination is the primary cause. Richard Stevenson investigates.

The pertinent question regarding the cause of droop – the decline in GaN LED efficiency at high currents – is this: Auger or not Auger? And if it is Auger, then the obvious follow-up question is what is the exact form of this non-radiative recombination process that limits the output power of high-brightness LEDs.

 




In the fall of 2007, when Philips Lumileds claimed that Auger is the cause of LED droop, this chip maker failed to elaborate on the finer details. But last summer a partnership between theorists at the universities of Arizona and Marburg claimed that droop was not caused by direct Auger recombination, the interaction between an electron, a hole and a third carrier. Instead they postulated that either defect-assisted Auger recombination or phononassisted Auger recombination might be the cause.

Another group has now entered the debate. Computational scientists at the University of California, Santa Barbara (UCSB), believe that droop is caused by a form of direct Auger recombination – interband Auger recombination (figure 1). Its simulations can even explain the origin of the “green gap", the decline in GaInN LED efficiency as the wavelength is stretched from the blue to the green.

The discrepancy between the results of the academic teams stems from differences in the modelling of the nitride band structure – the UCSB researchers identified and included a second conduction band. “When we identified this second conduction band we were thrilled," said team member Patrick Rinke. “Then it was just a matter of working through the theory and putting the calculations of the Auger recombination rate on a rigorous footing."

The UCSB team has not quantified LED loss due to Auger, because this would also require calculations of the radiative recombination rate and possibly other non-radiative recombination rates. “However, we think that it is significant that our Auger coefficient agrees well with that measured by Philips Lumileds," said UCSB’s Chris Van de Walle.

One of the limitations of these simulations is that they are based on bulk structures rather than quantum wells, which are the emitting layers in LEDs. “Full first-principle calculations for realistic device structures are beyond the capabilities of current computers," admitted Van de Walle, “but we are looking into other types of modelling."

Arizona–Marburg team member Joerg Hader is supportive of the UCSB effort, and says that he has no reason to doubt their results. His team did not consider the transitions to higher conduction bands because the experimental results that they are aware of claim that the separation between the lowest and higher conduction bands in GaN is about 3.5 eV. “This means that for our structure, the higher conduction band cannot be reached by Auger transitions."

Hader believes that there is an approach that can determine whether interband Auger transitions are the cause of droop. This involves the growth of an AlGaN/InGaN structure with AlGaN barriers that are tuned to prevent these transitions. “If the droop disappears, it would be a strong indication that their assumption is correct," he said. Getting to the bottom of the cause of droop would be beneficial in several ways – it would allow closure on this debate and it would also enable the design of LED architectures that are efficient at high current densities.



AngelTech Live III: Join us on 12 April 2021!

AngelTech Live III will be broadcast on 12 April 2021, 10am BST, rebroadcast on 14 April (10am CTT) and 16 April (10am PST) and will feature online versions of the market-leading physical events: CS International and PIC International PLUS a brand new Silicon Semiconductor International Track!

Thanks to the great diversity of the semiconductor industry, we are always chasing new markets and developing a range of exciting technologies.

2021 is no different. Over the last few months interest in deep-UV LEDs has rocketed, due to its capability to disinfect and sanitise areas and combat Covid-19. We shall consider a roadmap for this device, along with technologies for boosting its output.

We shall also look at microLEDs, a display with many wonderful attributes, identifying processes for handling the mass transfer of tiny emitters that hold the key to commercialisation of this technology.

We shall also discuss electrification of transportation, underpinned by wide bandgap power electronics and supported by blue lasers that are ideal for processing copper.

Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.

Having attracted 1500 delegates over the last 2 online summits, the 3rd event promises to be even bigger and better – with 3 interactive sessions over 1 day and will once again prove to be a key event across the semiconductor and photonic integrated circuits calendar.

So make sure you sign up today and discover the latest cutting edge developments across the compound semiconductor and integrated photonics value chain.

REGISTER FOR FREE

VIEW SESSIONS

Info
×
Search the news archive

To close this popup you can press escape or click the close icon.
×
Logo
×
Register - Step 1

You may choose to subscribe to the Compound Semiconductor Magazine, the Compound Semiconductor Newsletter, or both. You may also request additional information if required, before submitting your application.


Please subscribe me to:

 

You chose the industry type of "Other"

Please enter the industry that you work in:
Please enter the industry that you work in:
 
X
Info
X
Info
{taasPodcastNotification}
Live Event