TECHNOLOGY GaN SUBSTRATES
Hybrid approach yields the best GaN crystals and wafers
Marrying HVPE and ammonothermal growth techniques will accelerate the production of ultra-high quality GaN
BY MICHAL BOCKOWSKI FROM THE INSITUTE OF HIGH PRESSURE PHYSICS, POLISH ACADEMY OF SCIENCES AND ZLATKO SITAR FROM THE WIDE BANDGAP LABORATORY AT NORTH CAROLINA STATE UNIVERSITY
IF A PERFECT PROCESS were to exist for forming boules of GaN, it would create material that is completely free from dislocations, impurities and bow. In addition, this process would quickly yield crystals with very large dimensions, so that substrates sliced from them would provide a cheap, ideal foundation for the manufacture of LEDs, lasers and power electronics.
However, as we all know, such a process does not exist. Instead, makers of GaN devices have to choose between a far-from-perfect native platform that is very pricey – a 2-inch GaN substrate retails for $2000 or more – and several options for larger, foreign substrates that have the drawback of leading to inferior material quality. Makers of laser diodes don’t actually have this second choice, because they must work with
very high quality material, so GaN is used. However, for the manufacturers of LEDs and transistors, GaN is nearly always deemed to be too expensive, so sapphire, silicon and SiC are widely adopted. These foreign substrates all have a lattice mismatch to GaN that leads to the generation of copious defects in the epistructures, resulting in the loss of device performance and lifetime.
It should be possible to address the lack of an affordable, ultra-high-quality GaN substrate with a process that has been developed by partnership between the Institute of High Pressure Physics of the
Polish Academy of Sciences (IHPP PAS), working in close partnership with both Ammono S.A. and scientists at the Wide Bandgap Laboratory at North Carolina State University (NCSU). This team that we are part of employ a technology that involves uniting two existing techniques: HVPE and ammonothermal growth.
Virtues of HVPE Strengths of the former, HVPE, include a relatively high growth rate, which can exceed 100 μm/h, and a possibility to crystallize high-purity material. Thanks to these attributes, HVPE is a well- established method for making GaN substrates – this growth technique is employed by the likes of Sumitomo Electric Industry, Hitachi Metals, Furukawa Co, Mitsubishi Chemical, and Saint Gobain (formerly Lumilog).
These leading manufacturers of GaN begin by placing a foreign substrate, typically sapphire or GaAs, into a HVPE reactor, before heating the chamber to around 1300K. Introducing ammonia and gallium chloride at ambient pressure leads to the crystallization of GaN on foreign substrates, and when this thick film is removed via etching or self- separation techniques, a GaN substrate results.
Fig. 1: a) A polished 2-inch GaN wafer produced by Ammono S.A. (courtesy of Ammono S.A.); b) An atomic force microscope image of the epi-ready (0001) surface of the Ammono-GaN wafer after chemo-mechanical polishing; the image shows the desired bi-layer steps; root-mean-square roughness is below 0.1 nm (courtesy of G. Kamler and G. Nowak, IHPP PAS).
48
www.compoundsemiconductor.net June 2014
Manufacture of these GaN substrates is well established. For example, market leader Sumitomo already demonstrated 6-inch freestanding material back 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