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Developers Seek New Nitride Platform

With no immediate bulk GaN on the commercial horizon, engineers have started to look beyond silicon and SiC to composite materials and metals as a platform for nitride growth. Richard Stevenson investigates.
If our community was granted a single wish, it would probably ask for an affordable, large-size bulk GaN substrate. That s because today s GaN substrates are small and pricey, which is limiting the development of every nitride device. The difficulties associated with producing GaN bulk material have driven many firms to pioneer alternative platforms for nitride growth.

One company that has been pursuing this route for the best part of a decade is French epiwafers specialist Picogiga, a division of Soitec. "We are the only company in the world that can provide GaN HEMT epiwafers on three different substrates," explained Picogiga s chief operating officer Jean-Luc Ledys.



The first wide-bandgap product launched by Picogiga was a GaN-on-SiC epiwafer. This offers great performance, but at a high price due to the costs associated with using single-crystal SiC. To offer a cheaper option, the company then added GaN-on-silicon epiwafers, which are suitable for less demanding applications due to their poorer thermal management characteristics. A third solution to nitride growth has also been added more recently, silicon-on-poly-crystalline-SiC (SopSiC), which combines an intermediate price with power-handling characteristics similar to crystalline SiC.

"SopSiC is a thin silicon layer, bonded and transferred onto polycrystalline SiC," explained Ledys. The widely available polycrystalline SiC base was chosen because it s 10–20% of the cost of its crystalline brother, it is compatible with the company s MBE processes, it is widely available and it can be produced in sizes of up to 12 inches.

Ledys believes this platform is ideal for producing high-voltage Schottky diodes for rectification because devices built on silicon are limited to 250 V. "[The alternatives are silicon] bipolar devices, which are not very efficient, or SiC devices, which are very, very expensive."

The Smart Cut technology that holds the key to SopSiC production has also been used to fabricate SiC-on-insulator (SiCOI) material. The company first produced 2 inch SiCOI substrates several years ago, but since then they have put development on the back burner. "SiCOI does not give either us or our customers a good solution in terms of supplier, because we have to rely on SiC vendors. This does not help to expand the source supply chain," explained Ledys.

Picogiga is also developing GaNOI material. This could be used for microelectronic applications, but the company sees it primarily as a potential platform for homo-epitaxial growth of high-brightness white LEDs. "There is a serious bottleneck – similar to SiC – because very few people can grow bulk GaN," admitted Ledys. "However, we are ready to put in more effort and even look at growing GaN ourselves."

Wafer bonding is also at the heart of Aonex Technologies prototype substrates, which feature thin layers of either sapphire or GaN attached to polycrystalline AlN, a relatively low-cost material with comparable power-handling capabilities to GaN.

"For technology and market reasons we re focusing on the GaN-on-AlN right now," revealed the California start-up s co-founder James Zahler. This composite could produce material with an epitaxial quality equivalent to that produced by growth on single-crystal GaN, but at lower cost, making it ideal for lasers and high-performance LEDs.

Zahler regards the sapphire-on-AlN composites as a second, more affordable platform for vertical LEDs. According to him, these wafers enable LED production with a flip-chip technique that is much simpler than laser lift-off, which is normally used to separate sapphire from the nitride epilayers. In addition, the wafer is less prone to bowing and warping during growth, thanks to a closer match between the thermal expansion coefficients of the nitride epilayers and the underlying AlN.

Another candidate for nitride growth is metallic substrates, which are being developed at the Naval Research Laboratory in Washington, DC. "Metallic substrates are attractive to us because they have an ohmic contact and very good thermal conductivity," explained team-member James Freitas.

Nitride films grown on TiC have a similar quality, in terms of morphology and electrical properties, to those obtained on sapphire and SiC. The electrical properties of TiC show great promise for producing LED and laser structures, which would benefit from excellent carrier injection. However, the defect densities in the nitride epilayers are currently too high. Commercialization of this potential substrate would also require an increase in size from the 5 mm × 5 mm pieces used by Freitas to standard manufacturing sizes. "Unfortunately, there is not a lot of interest in single-crystal TiC material," said Freitas, but if this technology was pursued, it might offer yet another solution to the awkward problem of nitride growth.



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