LEDs ♦ news digest
photons which are trapped in the device rather than radiating outward as useful light.”
GaN nanowire LED technology offers significant improvements since the wires grow essentially free of strain and defects and should thus enable fundamentally more efficient devices. What’s more, the morphology provided by a “forest” of densely arrayed nanowire LEDs offers improvements in the light-extraction efficiency of these structures compared with their planar counterparts.
says Robert Hickernell, leader of the Optoelectronic Manufacturing Group, which includes the Semiconductor Metrology project. “That’s an advantage for high electrical power applications.” The Group is also studying nanowire field effect transistors (FETs) to accurately measure carrier transport properties. “And we’ve got GaN nanowire FETs that are some of the best research devices in the world.”
In addition, GaN nanowires are mechanically robust. Very robust: Four years ago, a PML- University of Colorado collaboration made headlines by producing nanowires with extraordinarily high quality factors that make them potentially excellent oscillators. “In the distant future,” Hickernell says, “they might be used in cell phone applications as micro-resonators.”
. A «forest» of nanowires
Testing and measuring those and other properties, however, poses significant challenges. “P-type GaN is difficult to grow by any common growth method,” Bertness says. “And what turns out to be very hard is making good electrical contacts to the nanowire, because it is not flat, and its thickness is larger than most of the metal films used to contact planar films.
“This 3D geometry encourages void formation and trapping of chemical impurities near the contacts, both of which degrade the contact, sometimes to the point of being unusable. This is an area we are actively investigating.”
The team is looking at ways to grow nanowires in regular arrays, with careful control of the spacing and dimensions of each individual wire. Recently they found that by creating a grid-like pattern of openings on the order of 200 nanometres wide in a SiN “mask layer” placed over the substrate, they could achieve selective growth of highly regular wires. The ability to produce ordered patterns of uniform GaN devices, Bertness says, “is essential for reliable manufacturing.”
GaN is not only a light source. It also has multiple uses in different fields. “Another nice thing about GaN is that it’s insensitive to high temperatures,”
The combination of high mechanical quality factor and tiny mass also makes them capable of detecting masses in the sub-attogram range. PML collaborators at the University of Colorado are confident that they can extrapolate the present experiments to roughly 0.01 attograms, or 10 zeptograms sensitivity. (For comparison, the mass of a virus is on the order of 1 attogram, or 10
Earlier this year, Bertness, Sanford and CU collaborators used GaN’s native piezoresistance to measure frequency response in nanowires stretched across a 10 micron gap. The results showed that the devices had “immediate utility in high-resolution mass and force sensing applications,” the researchers wrote in their published report.
The team thinks it is possible to make “a new class of electrically-addressable multifunction scanning- probe tools,” Bertness explains. “For example, conventional NSOM relies on a scanning optical tip with an aperture diameter in range of 10 to 100 nanometres which is formed at the tapered end of a passive optical fibre. Those tips are mechanically and chemically fragile and have a very short service life – hours to days. On the other hand, GaN nanowire based NSOM tools can potentially offer electrically-addressable multifunction operation that combines optical emission, optical detection, AFM and RF-AFM functionality.”
Finally, GaN nanowires are also well suited for use in chemical, biological, and gas sensing. Ongoing
November/December 2011
www.compoundsemiconductor.net 63
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