technology LEDs
Figure 2:
(a) Scanning electron microscopy image (SEM) reveals a stack of GRIN layers made up of TiO2
and SiO2
with an indium tin oxide layer on top.
(b) An SEM image of an array of GRIN pillars
In our case, we use sputter deposition to form a GRIN stack containing five layers with different refractive indices, each of which was formed by carefully selecting the powers applied to the SiO2
and TiO2 targets. We add a thin
indium tin oxide (ITO) layer onto this stack that acts as a hard mask for the subsequent dry etch (See Figure 2 (a)).
GRIN patterns are defined in the LED wafers by a combination of contact lithography and an inductively coupled-plasma (ICP) dry etch. (The ITO is dry etched under a CH4
, H2 and Cl2 mask, before the GRIN layers are etched under CHF3
Putting theory into practice To produce pillars that can extract all the optical modes of the LED, it is essential to work with a pair of transparent materials with vastly different refractive indices. To this end, we employ the high-refractive-index material TiO2 conjunction with a low-refractive-index partner, SiO2
. Films
of these oxides can form a composite dielectric layer with any desired refractive index from 1.46 to 2.47 – its value just depends on the ratio of the two materials.
in
environment to pattern the hard ).
The sidewalls of the pillars that are formed can extract all the trapped optical modes because they are smooth, vertical and contain minimal residues such as particles (see Figure 2(b) for an example).
normal angles of incidence. Consequently, all the trapped optical modes can be extracted out of the LED chip through appropriate design of the refractive index and height of the layers forming the GRIN stack.
Eliminating light trapping due to total internal reflection is possible by selecting the refractive indices of the layers so that the critical angle at the boundary of consecutive layers is complimentary to the critical angle of the sidewall-air interface. Note that the bottom layer must be chosen to have a refractive index closest to that of the top of the LED chip to ensure no coupling loss at this interface. Each layer in the GRIN stack extracts light incident on the layer’s surface at a specific range of incident angles.
For example, in our structure the first layer, which has a refractive index of 2.47, extracts light striking the top of the LED chip between 66° and 90°. In comparison, the second layer, which has a refractive index of 2.26, enables light extraction for emission incident between 54° and 66°.
With our approach it is possible to prevent light from bouncing back into the semiconductor without striking the sidewall through careful design of the height-over- width ratio of the individual layers that form the pillar. These constructions are arranged in an array to form a GRIN pattern, and choosing the spacing of these pillars is a delicate balancing act. If they are too far apart, not enough light enters the pillars and gains in LED extraction efficiency are modest; but put them too close together, and a significant proportion of the light exiting one pillar enters its neighbour, rather than leaving the device.
40
www.compoundsemiconductor.net November/December 2011
We have put our LED design to the test by fabricating GRIN patterns on the planar top surface of thin-film GaInN/GaN blue LEDs. This modification increased light output power by 131 percent and boosted the light extraction efficiency to around 70 percent. Performance is influenced by the type of pillar employed, and we have found that LEDs with diamond-shaped GRIN pillars are brighter than those made from cylindrical pillars. The reason: Light entering cylindrical pillars can be trapped inside these structures, and bounce around their edges in whispering gallery modes, a frailty that does not afflict diamond-shaped structures.
Figure 3: LEDs with GRIN structures can deliver superior light output compared to a planar reference LED and a roughened reference LED
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