technology  SiGe

coincidence lattice distance. Direct comparison of the two lattice constants, one from the cubic crystal and the other from the hexagonal crystal is not very meaningful because it is an apples-to-oranges comparison although they are often plotted in one diagram. To the contrary, in rhombohedral super-hetero-epitaxy, three <2-20> vectors of cubic crystal that are perpendicular to [111] vector is making coincidence lattice matching with the combination of basal basis vectors of the trigonal crystal. Also, by adding another fact that hexagonal crystals can be epitaxially grown on trigonal crystals such as GaN on Sapphire with in-plane rotation, a combined ab-initio hybrid bandgap engineering diagram was developed and under test now.

Hundreds of new alloys and thousands of new device structures can be fabricated with rhombohedral-trigonal model because this epitaxial scheme is not only limited to SiGe on sapphire but can be extended to other cubic semiconductors on thousands of trigonal crystals in nature. It is expected that not all trigonal crystals can accommodate rhombohedrally aligned cubic crystals, but selected trigonal crystals that have enough difference of formation energies between two cubic crystals rotated by 60° can yield a single crystal epitaxial layer.

To identify other new materials within the rhombohedral super-hetero-epitaxy category, NASA scientists have selected a few candidate materials and they are developing the growth methods. Another benefit of rhombohedral super-hetero-epitaxy in addition to new hybrid crystal structure is that it has unprecedented lattice matching conditions that are different from cubic lattice matching. These new opportunities to create lattice matched and strained semiconductors are under study now. It is also interesting that a cubic semiconductor on trigonal substrate is strained from <1-10> directions and elongated or compressed along [111] direction so that it deforms into a rhombohedron shape while conventional cubic epitaxy creates tetragonal deformation by strains in <100> direction.

Many trigonal crystals are insulators like sapphire. Therefore, it is also possible to create SiGe on Insulator (SGOI) with a possible lattice matching condition. Tables 1 and 2 show how the key features of currently developing lattice matched SiGe on Insulator (LM-SGOI) under our research compare to existing products or technologies: The far right column of table 1 shows the NASA Langley developed SiGe material that is compatible with the conventional insulator silicon oxide. The compatibility of SiGe with the silicon oxide is a very important factor for wafer-based mass production. Table 2 shows the attainable speed of SiGe chipsets based on the gate length and the charge mobility. Lattice-matched SiGe widely opens a possibility of chipset speed improvement, while the single crystal silicon itself has its own intrinsic limit on speed even by miniaturized feature size. From this table, one can easily imagine the great impact of NASA Langley’s rhombohedral lattice-matched silicon-germanium material on the new generation ultrafast chipset development.

20 www.compoundsemiconductor.net April/May 2010

Figure 3. Applications of rhombohedral semiconductors on trigonal crystals

Figure 4. Inter-crystal-structure epitaxy possibility relations with applicability of twin detection XRD methods

NASA’s rhombohedrally single crystal SiGe is the first of its kind ever achieved in the world. Therefore, there is no competition. The highly anticipated increase in charge mobility of the proposed materials technology is unique for the development of ultra fast chipsets. The lattice- matched SiGe is also complemented by silicon oxide as an insulator, unlike the arsenide, antimonide, or other compound semiconductors.

A proper insulation material like SiO2 for lattice-matched SiGe allows fabrication of several hundreds of chips on a

wafer basis. Compound semiconductors, such as zinc- blendes and Wurtzites, do not have proper insulators to allow mass production, instead of a single chip.

Another challenge is to incorporate higher germanium content into SiGe layer to raise carrier mobility. For example, the electron mobility of germanium is 4,000 cm2/V·s while that of silicon is only 1,400 cm2/V·s. By providing a suitable substrate for SiGe layer, transistors of 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