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GaN Substrates Offer High Performance At A Price


GaN substrates are manufactured by only a handful of companies at prices prohibitive to volume production, but offer great potential for high-performance devices. Richard Stevenson reports.The GaN component market was worth $1.35 billion in 2003 according to market research firm Strategies Unlimited. By 2007 this is predicted to soar to $4.5 billion, with GaN-based LEDs, lasers and electronic devices contributing $4 billion, $402 million and $129 million, respectively.
However, unlike well established GaAs and InP-based components, GaN-based devices are almost exclusively grown on foreign substrates such as sapphire (Al2O3) and SiC. Growth on foreign substrates is not ideal, as differences in lattice constant and coefficients of thermal expansion result in high defect densities and compromised device performance. However, traditional methods of manufacture such as the Czochralski and Bridgeman techniques are not available to GaN substrate manufacturers, due to the extremely high temperatures and pressures required to melt the materials.
Today, GaN substrates are produced by vapor-phase transport or epitaxial-type growth by a small number of companies, including Cree, Kyma, TopGaN and Sumitomo Electric. Wafer sizes are small and prices are high, and it is likely that volume products such as LEDs will continue to be fabricated on sapphire for the foreseeable future. Alternative hybrid options with higher dislocation densities are also available. For example, the US substrate supplier TDI produces GaN-on-sapphire templates, which reduces production times and equipment downtime.
Cree focuses on 2 inch substratesCree, based in Durham, NC, develops and manufactures semiconductor materials and devices using SiC, GaN and Si substrates, and is one of the largest companies with GaN capability. Its technology was acquired by purchasing ATMI's fledgling GaN technology business unit, a deal completed in April 2004 for $10.25 million. Today, it offers 330 and 400 µm thick GaN wafers in three sizes (10 mm, 18 mm and 2 inch) and at various grades, with dislocation densities as low as 3 x 106 cm-2 and usable surface areas of between 80 and 95%.
George Brandes, head of Cree's GaN division, said: "Our aim is to produce GaN substrates for the optoelectronic and electronic device community. Right now we are shipping conductive wafers to a number of optoelectronics manufacturers. We also sell and ship semi-insulating GaN substrates, targeting electronic devices such as high-frequency, high-power amplifiers." Brandes was not willing to disclose Cree's production capacity, the size of the GaN division, or wafer prices. However, Compound Semiconductor has learned that some researchers have paid in the region of 2000 ($2400) for a 0.5 inch GaN substrate.
Cree uses a hydride vapor-phase epitaxy (HVPE) approach to produce GaN material. It has demonstrated production of a 3 inch substrate, although Cree is focused on improving 2 inch material, as it believes most firms want to manufacture at this size. The company has two different processes: a single-wafer approach and a cost-effective boule-based approach, which it claims produces many substrates of higher-quality material. Commercial GaN substrate reactors are not available, and Cree, like other GaN substrate manufacturers, uses a custom-designed reactor. Lack of commercially available reactors arguably stifles development of GaN substrates. Brandes commented: "Clearly, if you could go out and buy a machine today that would yield wonderful results with a click of the button then the field would accelerate. But a lot of know-how goes into these machines. Right now I see substrate manufacturers choosing to keep their technology in-house."
Brandes is optimistic about the technology: "GaN is a viable product right now. You can go out and buy 2 inch material, it is decent quality, and as we go forward prices will be more competitive, and device applications will broaden. Nothing intrinsic in how we make our substrates leads me to believe that the cost will not be competitive. Boules aren't expensive, the materials aren't expensive, and the manufacturing processes are known."
Kyma to produce 4 inch prototypesKyma Technologies, formed in 1998 and based in Raleigh, NC, employs about 20 people. It aims to be a world-class GaN substrate supplier, and has no plans to make epiwafers. Over the last year it has expanded production capacity, shipping more substrates to optoelectronic customers. Kyma has already demonstrated prototype 3 inch substrates, and by the end of the year aims to ship 3 inch substrates to customers, and also to produce prototype 4 inch substrates for microelectronic applications.
Ed Pugh, Kyma's CEO, told Compound Semiconductor that demand for substrates is very high, with enquiries from all over the world. "Customers have approached us for applications that they can't achieve on SiC or sapphire. In particular, they seek out GaN for low dislocation densities and subsequent improvements in device performance. They will at least evaluate GaN." Pugh revealed that optoelectronic customers were only interested in 2 inch material, better suited to development than production. He acknowledged that Si and sapphire already have a foothold, and that GaN substrates will find it difficult to compete in terms of price, but believes substrate prices will fall due to process improvements, and greater supply and demand.
Kyma uses a fast-growth vapor-phase process to fabricate GaN substrates. The technology combines a chemical vapor-phase process to grow bulk GaN material and a vapor-phase process, licensed from North Carolina State University, to provide a starting template. According to Kyma, the process has a growth rate up to 10 times faster than any other commercialized deposition process for nitride-based materials.
Extreme growth conditionsTopGaN is a spin-off from Unipress, the High Pressure Research Center operated by the Polish Academy of Sciences in Warsaw. Founded in 2001, it employs about 20 people. It has retained an academic outlook, with universities forming the majority of its customers.
TopGaN has produced GaN substrates using relatively extreme growth conditions, with pressures of 15,000 atm and temperatures of 1600ºC. One growth can produce 20-30 crystals of 10 mm in diameter, exhibiting dislocation densities of only about 100 cm-2. Mike Leszczynski, vice-president and head of epitaxy, explained: "It's not a growth technique for millions of wafers, but just for special devices." Recently, laser diodes with a 15 µm x 500 µm cavity and output powers per laser facet of 1.89 W have been made using these substrates (figure 1). TopGaN claims that this is the highest power reported for nitride-based lasers on any substrate.
However, TopGaN is unable to extend this growth technique to 2 inch material. Instead, it begins with a 5 µm thick MOVPE-grown layer of GaN on a 2 inch sapphire substrate, patterned for epitaxial layer overgrowth (ELOG). The high-pressure growth conditions used result in a liquid-phase epitaxy process, enabling growth of stripes with a dislocation density of 106 cm-2 over more than 20 µm, compared with 2-3 µm for ELOGs grown from the gas phase by MOCVD or HVPE.
Template solution?Today GaN substrates are too expensive for both commercial production and academic research. For example, despite nitride research dominating the recent 12th International Conference on MOVPE, only a handful of groups used GaN substrates. An interim solution for growers is offered by TDI, which sells GaN-on-sapphire templates in 2, 3 and 4 inch sizes. TDI employs about 20 people and occupies a 3200 m2 facility in Silver Spring, MD. It currently ships hundreds of 2 inch wafers per month, and also some 4 inch material.
Vladimir Dmitriev, TDI's president, explained the benefits of using templates over sapphire substrates for growth of LEDs. Growth times and material costs are reduced, since all the steps up until the growth of the LED structure are removed (figure 2). In addition, reactors require less maintenance due to the decreased deposition of GaN, reactor downtimes are reduced, and lifetimes of reactor components are increased. Templates also circumvent problems associated with growth of the foundation layer, which may be subject to intellectual property issues. A typical growth sequence for blue LED production with sapphire substrates involves pre-treating the substrate by heating it to between 1000 and 1100ºC, depositing a nucleation layer at 600ºC, and then growing a 2-4 µm GaN foundation layer at 1000ºC, before finally growing the LED structure, which has a thickness of about 1 µm. For LED growth, TDI's GaN templates can offer several benefits.
TDI uses a HVPE process to deposit 2-5 µm of Si-doped GaN on sapphire substrates with a deposition rate 100 times that of MOCVD growth. It believes that the material quality associated with the HVPE process, which has a yield of over 90%, is actually higher than that of MOCVD. Dislocation densities are 108 cm-2, and are expected to decrease as the growth process is improved.
Laser diode possibilitiesUntil GaN substrate prices fall substantially, sapphire, which costs $40-50 for a 2 inch substrate, will remain the first choice for volume production. In fact, for GaN-based LEDs, which are today successfully produced on foreign substrates, manufacturers may never transfer to GaN. Competition could instead come from TDI's substrates, which cost $120 for a 2 inch wafer. However, for laser diodes, GaN substrates are a much more attractive proposition, and here a market for GaN substrates may develop.


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