technology LEDs
Figure 2. (a) (Top) PL spectra from InGaN on planar (blue) and nano- pyramids (green,yellow and red).Eye sensitivity function is overlapped for reference.
(b) (Below) Temperature dependence of the integrated PL intensity for green,yellow, and red InGaN on nano-
pyramids.The intensities are normalized to their values at 10 K
centres, such as point and line defects. One extensively studied, experimentally proven technique for side- stepping the piezoelectric fields is to turn to GaN grown on semi-polar or non-polar substrates. These can be made from either sapphire or GaN. However, stacking faults plague epitaxial non-polar and semi-polar films grown on sapphire, and free-standing GaN substrates of any orientation are too expensive to be used for making LEDs. One promising alternative that is under investigation by many groups, including National Taiwan University, is to reduce the electric field through strain control, such as pre-straining of the multi quantum wells (MQWs).
A more radical idea that has great potential is to build LEDs from InGaN nanostructures. Emission from the blue right through to the red has already been demonstrated with such structures, which have received much attention thanks to their promise to close the green gap and realise polychromatic white LEDs. Strengths of the nanostructures include facets for semi- polar and non-polar GaN growth, enhanced light extraction, and the promise of increased crystal quality, thanks to reduced strain that stems from their small features.
We have used this class of structure – specifically nanoscale pyramids with InGaN layers – to produce epilayers delivering very efficient green, yellow and red emission. In addition, we have fabricated a monolithic LED that produces white light through colour mixing from different quantum wells.
Barriers to monolithic white LEDs The green gap in nitride LEDs stems from a combination of poor crystal quality of indium-rich InGaN and the polar characteristics of III-Nitrides on the c- plane. Macroscopic polarization occurs in this material, giving rise to a piezoelectric field perpendicular to the plane of the quantum well. This field pulls apart electrons and holes in the well, leading to a decline in the radiative recombination rate (called quantum- confined Stark effect).
As the wavelength increases, this efficiency reduction becomes more severe. A hike in indium content is needed to reach these longer wavelengths, and this also increases strain in the quantum well, leading to higher piezoelectric fields that hamper radiative recombination. On top of this, the larger strain and lower growth temperature required to incorporate more indium deteriorate emission efficiency, due to the generation of many non-radiative recombination
34
www.compoundsemiconductor.net November/December 2011
Closing the green gap To produce structures with efficient green to red emission we have used MOCVD to grow InGaN/GaN MQWs or a double heterostructure (DH) on nano-size GaN hexagonal pyramids. These are formed via selective growth on patterned c-plane GaN templates featuring circular openings in a 100 nm-thick SiN film.
After patterning the wafer, we form un-intentionally doped GaN hexagonal pyramids with a proprietary growth process. This growth step concludes with the addition of three InGaN QWs with GaN barriers or an InGaN/GaN DH structure, with the growth condition for the ternary layer carefully selected to control emission wavelength and efficiency. The SiN films are not removed after growth.
The six facets of these arrayed pyramid structures are clearly visible in scanning electron microscopy and transmission electron microscopy (TEM) images (see Figure 1). According to high-resolution X-ray diffraction, all of these facets are semipolar {1122} planes. One of the benefits of this approach is that the threading dislocations are terminated before propagating into the InGaN layer – see the cross-sectional TEM image in
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