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SiC pyramids yield AlN substrate solution

Hexagonal peaks of SiC can provide 100 mm freestanding AlN wafers with low defect densities.

Researchers in Sweden have revolutionized the growth of AlN substrates on SiC wafers, slashing defect levels by two orders of magnitude.

The Linköping Univerity group produced AlN substrates with less than 1x106 dislocations per cm, starting from pyramids of a rare crystal form of SiC.

The AlN layer only touches the tops of the pyramids, which reduces strain and eases separation from SiC.

The group of Rositza Yakimova has made 100 mm diameter substrates, up to 120 µm thick, at rates as high as 250 µm/h. The sublimation method used stems largely from the PhD of Reza Yazdi, who successfully defended his thesis on August 20.

“The technology is very simple and clean,” emphasized Mikael Syväjärvi, who supervised Yazdi together with Yakimova.

In comparison to Linköping's results, substrate vendor Technologies and Devices International currently offers AlN-on-SiC wafers with dislocation levels above 1x108 cm-2, up to 4-inches in diameter. The company has said its AlN layers are laid down by HVPE at up to 1 µm/min.

Crystal IS produces substrates with defect levels below 1x104 cm-2 up to 2-inches in diameter, cut from AlN boules that it claims to grow at close to 1 mm/h.

Secret of the pyramids
Linköping starts its pyramid process from a commercial 4H-SiC substrate, with a 50µm thick SiC epilayer grown on it. The team mounted a ceramic AlN plate 1 mm away, and ramped the setup's temperature from 1700°C to 1850°C under nitrogen pressures ranging from 300 mbar to 850 mbar.

“In the first stage, some SiC is sublimated from the surface,” Syväjärvi told compoundsemiconductor.net. “This deposits as pyramids, centered on defects in the substrate.”

The pyramids form as the normally-unstable 2H-SiC crystal type, and appear to be stabilized by AlN deposits at the peaks once SiC is depleted from the atmosphere. The AlN deposits begin as discs, growing to become microrods, before finally coalescing to become a continuous layer. 2H is the natural crystal orientation of AlN, and so the transition from 4H- to 2H-SiC also aids final substrate crystal quality.

AlN substrates are crucial for UV LEDs, while their high thermal conductivity, electrical insulation and close lattice match to AlGaN makes them useful in RF devices. The Linköping team now hopes its method could make an impact in these areas.

“We will check out what options we have for bringing this to industry because the process is so promising and the quality is so high,” Syväjärvi said.

This work is patent-pending, and the researchers have submitted a paper based on it for publication in Advanced Materials.

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