review research
LED droop: Lumileds strengthens Auger case
Philips Lumileds has gathered further evidence to support its claim that Auger recombination is the primary cause of LED droop, the decline in a device’s external quantum efficiency at higher drive currents.
Researchers Aurélien David and Michael Grundmann determined that droop is correlated to a shortening of the non- radiative lifetime after studying a 430 nm GaN LED featuring a double heterostructure with a 15 nm thick InGaN layer. “The non- radiative lifetime is quantitatively compatible with Auger scattering, which supports Auger as being this non-radiative mechanism,” explains David.
Although the data presented by Lumileds is in quantitative agreement with Auger scattering, it is still possible that droop is caused by another mechanism. The origin of droop is highly controversial, and many different theories for its cause have been put forward over the last few years.
One alternative explanation for droop is interband absorption, which, like Auger scattering, is a non-radiative process that depends on a cubic power law related to the carrier density. However, David and Grundmann argue that interband absorption is related to optical re-absorption, and is strongly influenced by geometric factors. Droop, however, appears to have a universal, geometry-independent behavior.
Another explanation for droop is based on energetic carriers flying over the active region. However, the Lumileds’ researchers say that this can now be ruled out, because this process would not influence the lifetime of the carriers trapped in the active region. They have shown that this lifetime shortens at higher current densities.
One scenario that they cannot rule out is carrier capture and escape. However, they believe that this is unlikely, because there is no reason why this process should depend on the cube of the carrier density.
Probing the sample
Lumileds’ latest work on droop has focused on determining the evolution of radiative and non-radiative lifetimes by measuring the differential carrier lifetimes in LEDs, and combining this data with an internal quantum efficiency measurement that gave a peak value of 65 percent.
To determine differential carrier lifetimes, the researchers fabricated vertically injected LEDs, and drove them with 3 µs pulses to avoid device heating. A small, AC voltage was superimposed onto this series of pulses, and this provided a probe for the lifetime measurement.
By assuming perfect injection efficiency, the researchers calculated carrier density, and then the radiative and non-radiative lifetimes.
They found that the non-radiative contribution to the lifetime decreases as current increases, indicating the onset of an additional non-radiative process. At high current densities the radiative lifetime
saturates, due to a process known as phase-space filling.
The researchers then compared their data with the standard model for the recombination rate, which involves the sum of: Shockley-Read-Hall recombination; radiative, bimolecular recombination; and an Auger process that depends on the cube of the carrier density. This model provided an excellent fit to the experimental data, and produced a value for the Auger coefficient of 10-29 cm6 s-1.
A. David et al. Appl. Phys. Lett. 96 103504 (2010)
Swiss speed AlInN HEMTs
A Swiss partnership between Colombo Bolognesi’s group at ETH-Zürich and Nicolas Grandjean’s group at EPFL have broken the speed record for AlInN/GaN HEMTs. When deposited on SiC, their devices can deliver a current gain cut-off
frequency (fT) of 144 GHz, an extrinsic transconductance of 480 mS/mm and a maximum current density of 1.84 A/mm. Silicon offers a cheaper, but inferior foundation that leads to a fT of 113 GHz.
AlInN/GaN HEMTs promise faster speeds than their AlGaN/GaN cousins, but until now they have lagged a long way behind. “Our work has closed the gap for 100 nm gate lengths, presumably thanks to progress in the crystal quality of AlInN/GaN materials,” explains Bolognesi.
He claims that the AlInN/GaN HEMT has the potential to outperform its AlGaN/GaN equivalent because its heterostructure can realize a higher channel electron density with a thin top barrier. “This allows one to place the gate electrode closer to the channel and thereby maintain a better electrostatic control of the transistor, so as to minimize the so-called ‘short-channel effects’ that curtail performance in shorter gate devices.” Thanks to the combination of small gate-to-channel distance and a high electron density in the channel, these transistors can realize very high transconductances, and ultimately higher cut-off frequencies.
The AlInN/GaN HEMT fabricated by ETH Zürich and EPFL features Ni/Au T-shaped 100 nm gates. Credit: ETH Zurich
One of the weaknesses of the team’s AlInN/GaN HEMT is its residual gate leakage, which prevents complete channel pinch-off at source-drain voltages above 3V. The source of this leakage might be caused by tunneling through the barrier, or leakage through either dislocation cores or the surface of the structure.
“If the gate leakage problem can be fixed, it will be interesting to explore how these devices behave in power applications,” says Bolognesi. Another target for the partnership is the optimization of the epitaxial and device fabrication processes. “And of course, we are interested in finding out just how fast we can make these devices go.”
H. Sun et al. IEEE Electron Device Lett. 31 292 (2010)
April/May 2010
www.compoundsemiconductor.net 49
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 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255 |
Page 256 |
Page 257 |
Page 258 |
Page 259 |
Page 260 |
Page 261 |
Page 262 |
Page 263 |
Page 264 |
Page 265 |
Page 266 |
Page 267 |
Page 268 |
Page 269 |
Page 270 |
Page 271 |
Page 272