technology substrates
Figure 2 Thick, micropipe-free 60 mm
diameter 6H- SiC seed
, yet LED performance is excellent; and commercial manufacturers of optical devices need 2-inch wafers as a minimum size. Taking these factors into account, we have set ourselves the goal of scaling our wafer capability to 2-inch diameter as quickly as possible while maintaining quality that is “good enough.” What is our take on “good enough”? That the density of substrate dislocations is three-to-six orders of magnitude lower than that of epitaxy on sapphire.
– 5 x109 /cm2
our customers use our AlN substrates. Production of our substrates has been governed by two major factors: nitride epitaxy on sapphire has a dislocation density of 5 x 107
We have developed production technology for delivering a range of 15 mm diameter AlN crystals and epi-ready substrates, which can be shipped with the aluminum face epi-ready polished, and the nitrogen-face polished or fine lapped, with US flats. All variants have excellent crystallinity, low dislocation density, high UV transparency and high resistivity. This is borne out by X-ray diffraction maps that show that the single peaks of the best substrates have a full-width half maximum less than 100 arcsec in both asymmetric and symmetric scans. These substrates provide a foundation for growing high-quality AlGaN layers (see Figure 1). We have also recently shown that it is possible to produce larger crystals - we have achieved fully mono-crystalline 2-inch diameter AlN by using low defect SiC seeds that we also grow by PVT.
Figure 3 AlN layer separated from SiC seed and attached to crucible lid
Scaling to 2-inch
Early in our AlN development we realized that AlN can be successfully seeded by PVT on SiC. The growing AlN- and-SiC seed forms a solid solution. Near the interface the concentration of silicon and carbon in AlN can be higher than 5 percent, giving rise to conductive AlN; however, the concentration of these two elements falls rapidly with distance from the interface, where resistivity is far higher. Second-generation (grown on AlN seeds) AlN has resistivity in excess of 5 x 1011
Ohm-cm. We have not
studied the effects of silicon concentrations of more than 5 percent in AlN, but on the basis of the seed growths we speculate that it will be possible to make conductive AlN (or perhaps more correctly AlSiN?).
Figure 4 A crossed
polarizer photo of a typical wafer. The edge striations have not been fully removed by diameter grinding
A micropipe-free, low dislocation seed is required only for the initial growth of a thick AlN layer. Figure 2 shows the initial micropipe-free 6H-SiC seed. Once this thin AlN layer is separated from the SiC, it is then employed for the growth of the AlN bulk crystal. Figure 3 shows the AlN layer separated from the SiC seed and attached to the crucible lid. The pattern of cracks that heal during bulk growth attests to the high quality of the AlN seed layer.
Typically, as shown in Figure 3, the seeds used to produce 2-inch AlN have a diameter that is much larger. This removes edge striations and defects that occur during growth. The high quality of this material is revealed by the lack of features in crossed-polarizer images (see figure 4). Before the AlN crystals are ground to 2-inch diameter, they are used as seeds for fabricating AlN material of this size.
18
www.compoundsemiconductor.net November / December 2010
There are wide variations in the reports of optimum growth conditions for AlN. Hexatech reports that they achieve very high quality crystal growth only on the
Figure 5 Evolution of AlN crystals and substrates from 15 mm to 2-inch diameter
AlN sublimation issues Our outfit, just like the team at CrystAlN, has extensive expertise in the growth of bulk single crystal SiC. However, despite this background, scaling up the growth process to 2-inch has been surprisingly difficult. Unlike SiC, AlN dissociates congruently into aluminum and nitrogen gas; however, aluminum vapor at high temperature is extremely reactive and forms lower- temperature eutectics with many materials that would be otherwise inert. To deal with these issues, we use TaC crucibles, which is a process that Hexatech and CrystAlN have also reportedly adopted. These crucibles are employed for the growth of the initial thick layer of AlN in a graphite system. A combination of TaC and tungsten crucibles, along with tungsten reactors, is then used for growth of bulk crystals. The historical evolution of our AlN crystals from 15 mm diameter to 2-inch diameter is illustrated in Figure 5. In an intermediate phase, the single crystal center was surrounded by a polycrystal ring as we expanded the micropipe-free SiC seeds. Recently, however, we have progressed to the production of fully mono-crystalline material.
Figure 6 VirtualReactor simulation of crystal growth
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