This page contains a Flash digital edition of a book.
Technology  GaN substrates


pressures of 6 GPa and temperatures of almost 2500 K – a combination of conditions that prevents the growth of GaN from its stochiometric melt, which is the method used to make silicon and GaAs boules.


A handful of alternative methods have been developed for making GaN crystals, including HVPE deposition on foreign substrates and the growth of GaN in solution. All these approaches have their weaknesses – either in material deposition rate or crystalline quality. Minimizing these drawbacks holds the key to making affordable, high-quality GaN, and at the Institute of High Pressure Physics we believe that we have developed a novel approach that can do just that: It involves using a high nitrogen pressure solution to convert multiple GaN seed crystals grown by HVPE to free-standing GaN. Reliable blue and violet lasers have been formed on these high- quality crystals.


HVPE: Pros and cons Today HVPE is the most common approach for manufacturing GaN substrates. This involves crystallization from the vapour phase at ambient pressure, with GaN deposited on a foreign substrate through the reaction of ammonia with gallium chloride at temperatures of about 1300 K. Etching, laser lift-off and self-lift-off techniques can all be used to remove the nitride film from the foreign substrate – typically sapphire or GaAs – and yield a large-diameter, free- standing GaN substrate.


This technique has a relatively fast growth rate of up to 500 µm per hour, but suffers from a phenomenon known as parasitic nucleation. Superfluous GaN nucleation takes place within the reactor, often leading to uncontrolled changes in crystal growth conditions during the crystallization run, such as variations in the flow rate of reactants. Reducing growth time prevents degradation to the crystal, but this limits the thickness of HVPE-grown GaN to typically below 1 mm.


Another drawback is that the material quality degrades, due to either introduction of donors, such as silicon or germanium, or the addition of an acceptor, iron. This makes it difficult to produce highly n-type or semi- insulating substrates, thereby limiting the typical free- carrier concentration for free-standing GaN to 1018


cm-3 .


Deposition on a foreign substrate enables the growth of large-diameter GaN crystals, but these suffer from lattice bowing. This stems from significant differences between the lattice constant and thermal expansion coefficient of the foreign substrate and the nitride film (see Figure 1). When GaN is grown by HVPE on sapphire, the bowing radii of crystallographic planes is below 10 m. This relatively low number means that there is little benefit in using HVPE-grown GaN as a seed for subsequent crystallization runs. It is possible to overcome these issues and grow crystallographically flat, free-standing


Figure 1.(a) Simplified scheme of GaN wurtzite structure with nitrogen and gallium atoms’positions; one can distinguish two polar directions +c and –c and therefore two polar GaN surfaces: gallium (0001) and nitrogen (0001),respectively


.(b) schematic view of typical orientation


of a single crystalline GaN substrate; crystallographic planes are flat. c) schematic view of the crystallographic planes in GaN lifted-off from sapphire giving a curvature.d) schematic view of the crystallographic planes in GaN substrate.Although the substrate can be geometrically flat it is not flat from crystallographic point of view


HVPE-GaN by switching the growth mode during the HVPE process from a flat one to a rough one. But there is a penalty to pay: inversion domains that hamper subsequent epitaxy (see Figure 2 for details).


The best HVPE ‘free-standing’ GaN technology has been developed by Sumitomo Electric Industries. This Japanese firm can produce very good quality, free- standing GaN crystals of up to 6 inches in diameter via deposition of this wide bandgap semiconductor on GaAs wafers. The curving of GaN is not that severe, thanks in part to the similar thermal expansion coefficient of this material and GaAs. In addition, Sumitomo’s growth process aids the fabrication of crystallographically flat substrates – it is based on selective growth of the nitride, followed by re-growth on the surface containing large inverse pyramidal pits (a process described as either Dislocation Elimination by Epitaxial growth with inverse-pyramidal Pits (DEEP), or an Advanced variant known as A-DEEP). With this method the crystal is grown in the controlled rough growth mode, probably with presence of the inversion domains.


Sumitomo’s crystals feature 400 µm wide stripes that have alternating regions with defect densities of typically 104


cm-2 and 5 x 108 cm-2 . High-quality lasers for Blu-Ray


players are manufactured on these crystals by positioning the chips on low defect density stripes that are free from inversion domains. Due to the growth and re-growth of


January/February 2012 www.compoundsemiconductor.net 43


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