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Japanese lab creates new GaN substrate

SCAM substrate better than sapphire for crystal defects      

Fukuda Crystal Laboratory, based in Sendai, Japan, has created 2inch-diameter samples of ScAlMgO4 (scandium aluminum magnesium oxide) crystal, also known as SCAM. The idea is to replace sapphire in GaN-based LEDs and laser diodes by reducing crystal defects for GaN-based semiconductors grown on it. As a result, it is expected to boost brightness of GaN-based LEDs.

A research group led by Takashi Matsuoka, professor at the Institute for Materials Research at Tohoku University (also in Sendai), formed an LED structure by using the prototype crystal as a base for stacking GaN-based semiconductor layers, the crystal did indeed improve the crystal structure of the GaN.

1.8 percent lattice mismatch

The lattice mismatch between SCAM and GaN is as small as 1.8 percent, and SCAM reduces the appearance of crystal defects called dislocations. Though SCAM crystals are difficult to make, Fukuda Crystal Laboratory created 2inch high-quality SCAM crystals using the Czochralski (CZ) method. It improved crystal quality by modifying the CZ furnace structure and the conditions for crystal growth.

When the prototype SCAM crystal was cleaved and its C surface examined by X-ray diffraction, the full width at half maximum (FWHM) scatter was 12.9 seconds - equivalent to that produced by a perfect crystal of Si.

Fukuda Crystal Laboratory formed the wafers simply by cleaving an ingot of SCAM crystal without resorting to cutting or polishing, reducing the potential cost of wafer fabrication. When a GaN thin film was grown on the cleaved surface of the SCAM crystal using metal-organic chemical vapor deposition at a temperature of 1040degC, a low-dislocation crystal having a mirror surface was formed. The company considers this as a major achievement.

Fukuda Crystal Laboratory plans to increase the diameter of the SCAM crystals as well as commercialise the process. Specifically, it intends to release a 2 inch -diameter SCAM substrate by the spring of 2015.

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