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Technology  GaN substrates


high-quality GaN seeds that can initiate growth and find a suitable ‘mineralizer’ to aid dissolution of GaN.


The world-leader in this technology is our neighbour, the well-known Polish company Ammono. This firm manufactures 1 cm2


and 1 inch crystals and is on the


Figure 2.a) Schematic presentation of inversion domains in a GaN crystal.They can appear during the rough growth mode (rough growth front) in HVPE technology and hinder the subsequent epitaxy, since a polarity of the (0001) surface is mixed.b) TEM photograph of the inversion domain grown by HVPE (courtesy J.Smalc-Koziorowska)


road to start the production of 2 inch variants. These crystals have many great attributes: They are extremely flat, with bowing radii of the crystallographic planes reaching up to 100 m; defect density is of the order of 104


cm-2 not exceed 2 x 1019


; and free carrier concentration does cm-3


. This material has already


provided a good foundation for making high-power lasers by various groups, including the Polish company TopGaN.


pits, the free-carrier concentration in Sumitomo’s substrates is relatively high – about 5 x 1018


cm-3 . With


HVPE-based approaches, impurities are built into GaN in an anisotropic way (see Figure 3), and the growth of pits occurs in semi-polar directions.


When sapphire is used instead of GaAs as a foundation for making GaN free-standing substrates by HVPE, the best results are obtained with a flat-growth mode and a technique known as Void Assisted Separation (VAS). With this approach that has been pioneered by Hitachi Cable, growth proceeds on a sapphire substrate coated with an ultra-thin layer of MOCVD-deposited GaN and nanometric titanium nitride. 3-inch GaN substrates with a homogeneous dislocation density of about 106


cm-2


can be formed by this technology. Free -carrier concentration is typically 1018


cm-3


bowing radius is below 10 m. Solutions for GaN?


Figure 3.Free carrier concentrations in HVPE material for various directions. Growth on semi-polar planes leads to enhanced incorporation of oxygen and high electron concentration (courtesy I. Grzegory)


It is also possible to form GaN crystals from solution in supercritical ammonia. This approach, which is known as the ammonothermal method, is analogous to hydrothermal crystallization of quartz or oxide crystals such as ZnO. However, ammonia is used in the place of water. One of the biggest drawbacks of the ammonothermal approach is that it is very slow: Growth is at best 0.1 mm per day. It is also necessary to secure


and the lattice


Another way to grow GaN is from solution, using gallium-sodium mixtures held at temperatures from 700- 900 °C and a nitrogen pressure of up to 5 MPa. Osaka University has trail-blazed this approach, and produces bulk single crystals that are a few millimetres thick, have a diameter of up to 3-inch, and exhibit a defect density of the order of 105


cm-2 . Piling up the pressure


But if low defect density is the primary goal, then by far the best approach is the High Nitrogen Pressure Solution (HNPS) method. Dislocations of just 102


cm-2


can be realized via a direct synthesis reaction between a liquid of gallium and nitrogen held at up to 1800 °C, and nitrogen at pressures of up to 2 GPa. A spontaneous reaction yields hexagonal platelets typically 1 cm2 for laser diodes.


in size, which make a good foundation


We are the pioneers of this growth technology, and our experience of making lasers on these hexagonal platelets in conjunction with our spin-off company, TopGaN, has enabled us to determine the most important characteristics for GaN substrates that are used for making laser diodes. Our findings are that GaN crystals must have: High structural quality, including a bowing radius exceeding 20 m and a dislocation density below 106


cm-2 carrier concentration of more than 5 x 1019


; and high electric conductivity with free- cm-3


,


because this enables the preparation of a low- resistance ohmic bottom laser diode contact and also makes the GaN substrate ‘plasmonic’, aiding optical confinement (more about this later).


The HNPS approach yields GaN of great quality, but crystal size is small and material throughput is low. So to address these issues, we have recently developed a method for growing GaN that we call multi-feed-seed (MFS). This involves the conversion of free-standing HVPE-grown GaN crystals to free-standing HNPS GaN, which has a much higher quality than the seed material. The great strength of this approach is that it yields several GaN crystals from one run – what’s more, all of


44 www.compoundsemiconductor.net January/February 2012


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