RESEARCH REVIEW Researchers ‘unambiguously assign’
LED droop to Auger processes Electron emission spectroscopy offers new insights into the cause of droop
RESEARCHERS from the University of California, Santa Barbara, and École- Polytechnique, France, say that they have conducted an experiment that enables them to unambiguously assign LED droop to an Auger process.
This is by no means the first time that an Auger process – a non-radiative interaction involving three carriers that leads to the promotion of an electron or hole to a higher energy state – has received the blame for LED droop, the decline in device efficiency as the current through the chip is cranked up. However, up until now, the evidence has been circumstantial, cliams the US-French team.
experimental work carried out by this team, but not all are convinced that the data provides unquestionable proof that Auger recombination causes droop.
A blue LED made by the Taiwanese chipmaker Walsin Lihwa has been used to study the influence of Auger processes on LED droop
“The result is definitely a positive contribution to the droop question,” says theorist Weng Chow from Sandia National Laboratories in Albuquerque, NM. According to him, this work provides a good basis for tying up loose ends: “For example – and speaking as a non-expert – I wonder if the authors can extract from the measurement an Auger electron density relative to the low energy electron density? And from that, perhaps, they can estimate an Auger coefficient?”
According to Claude Weisbush, who is affiliated to both institutions, one of the biggest weaknesses of circumstantial evidence is that it allows droop to be explained by many competing theories. Along with Auger, this efficiency- sapping malady has been attributed to mechanisms such as electron leakage from the quantum well, poor hole injection and localisation of carriers at defects.
Weisbush argues that the experiment that he and his co-workers have performed changes all of this. It provides a direct measurement of hot electrons, which come from an Auger process involving two electrons and a hole.
“Such hot electrons are difficult to produce in semiconductor structures,” says Weisbuch. “Very high electric fields can generate hot electrons, as can energy barriers that launch hot electrons into the semiconductor, but for the LEDs we have, there are no strong electric fields or sufficiently high energy barriers.” So, he argues, the only possible cause of hot electrons is an Auger process taking place in the LED.
To measure the energy of these
electrons, the team performs a very elegant experiment. They place a commercial, conventional LED in a vacuum and bias it at a range of voltages. To measure the hot electrons produced by the device, they add one or two monolayers of caesium to the surface of the p-side of the device, so that electrons passing through the chip can exit it and be detected by a spectrometer. Simultaneous measurements of the power of the light emitted by the LED are also made.
Weishbuch explains that detecting hot electrons is not, in itself, conclusive proof that Auger recombination is the primary cause of droop. What provides this is that the hot electrons start to appear at exactly the same time that droop kicks in.
At injected currents of 4 mA and higher, two high-energy peaks are observed: one at 0.3-0.4 eV and another at about 2 eV. These energies do not correspond exactly with those of Auger electrons at their initial kinetic energies, because these carriers have to first travel through 200 nm of p-GaN, and there is very fast longitudinal-optical phonon emission in this material.
Other research groups admire the
Meanwhile, Fred Schubert from Rensselaer Polytechnic Institute, in Troy, NY, questions whether some of the detected electrons have simply leaked out of the quantum well. Schubert says that Auger recombination is an undisputed effect and it will occurs in LEDs: “But we believe that Auger recombination is not tenable as a major contributor to the efficiency droop.” Four reasons are given to support this claim: the Auger coefficient is too small; the temperature dependence of the Auger process is opposite to the temperature dependence observed for efficiency droop; it’s not possible to fit experimental results to the well-known ABC model, which includes an Auger term; and a more general explanation for droop is needed that accounts for its occurrence in LEDs made from other material systems.
Weisbuch and his colleagues are going to be looking at other material systems, and also other devices. They hope to offer new insights into InP-based telecom lasers, and determine whether the loss mechanism is Auger recombination or intervalence band absorption.
J. Iveland et. al. Phys. Rev. Lett. 110 177406 (2013)
June 2013
www.compoundsemiconductor.net 59
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191