Genre: Information/Explanation

Direct Auger recombination causes droop , but its impact can be diminished inserting graded layers into quantum wells to smooth the confining potentials . Calculations by a partnership between at Technion-Israel Institute of Technology and the US Naval Research Laboratory show that a ' softer ' potential reduces Auger recombination , leading to an increase in LED efficiency . The debate over the origin of LED droop has taken yet another twist , with a partnership between researchers at Technion-Israel Institute of Technology and the US Naval Research Laboratory claiming that direct Auger recombination is to blame . This view is at odds with that of many other theorists , who account for droop - the reduction in light efficiency as current is cranked up - with either more complex forms of Auger recombination or models involving defects . For example , Chris Van de Walle's computational science group at the University of California , Santa Barbara , claims that the primary causes of droop are Auger-related processes involving phonons and alloy disorder . And a partnership between researchers at Boston University and Politecnico di Torino , Italy , argue that although Auger-related processes contribute to droop , carrier leakage , compositional fluctuations and threading dislocations may also play significant roles . These differences over the cause of droop stem from differences in the structures under study , according to Roman Vaxenburg from Technion-Israel Institute of Technology . He has his co-workers have looked at quantum well structures , while others have focused on calculations for bulk material . " In general , in bulk material , Auger recombination is not efficient due to strict energy and momentum conservation requirements , " explains Vaxenburg . " On the contrary , in quantum-confined systems , such as quantum wells , the momentum conservation requirement if lifted and Auger processes are enhanced . " Values calculated by the US-Israeli team for the Auger coefficient in quantum wells are in the range 10 -31 - 10 -30 cm 6 s -1 . The structures studied by this team feature symmetric quantum wells , which are found in non-polar LEDs . That does not mean , however , that their findings offer no insights into the vast majority of devices made today , which are polar LEDs grown on c-plane sapphire . " We expect that the Auger rate will be further accelerated in quantum wells grown in the polar direction , " says Vaxenburg . For their calculations , he and his co-workers use the well-established Pidgeon and Brown model which was proposed in the 1960s . This is often used for calculating the characteristics of direct bandgap semiconductors with a band-edge at the Gamma point of the Brillouin zone . " It takes into account the eight band-edge sub-bands , " explains Vaxenburg . " Adding more bands will only accelerate the rate of Auger recombination , because it will increase the density of final states . " The code employed for these calculations has been written from scratch . It is a few tens of thousands of lines long , and when it is run on a powerful personal computer , it takes 8-10 hours to calculate the total Auger recombination rate for a given quantum well . Auger rates have been calculated in a wide variety of InGaN / GaN quantum well structures with different confining potentials . One class of structures features a conventional active region , with In 0.25 Ga 0.75 N barriers surrounding a GaN quantum well with a thickness of either 1.5 nm , 2.0 nm , 2.2 nm or 2.5 nm . Other types of structure under study feature ' softer ' potentials , resulting from the insertion of layers with intermediate compositions . The conclusions of the team should not be questioned due to their assumption of zero temperature . " At temperatures higher than zero , the Auger rate is expected to accelerate even further , " says Vaxenburg . Calculations reveal that using 11 layers to form a quantum well , rather than the conventional number , 3 , can lead to a three-fold reduction in the Auger recombination rate . This is claimed to spur a 20 percent increase in LED efficiency , shift droop onset to a higher current density , and reduce the droop development rate at higher current densities . The next goal for the team is to extend their model to include piezolelectric polarization .