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Self-Separation of a Thick AlN Layer from a Sapphire Substrate via Interfacial Voids Formed by The Decomposition of Sapphire

A technique for separating a thick AlN layer grown by hydride vapor phase epitaxy (HVPE) on a sapphire substrate was developed. By heat treatment at 1450 °C in a gas flow containing H2 and NH3, many voids could be formed at the interface between a thin (100 nm) AlN layer grown at 1065 °C and the sapphire substrate due to the preferential decomposition of sapphire. During the cooling process after the subsequent growth of a thick (85 µm) AlN layer, the thick AlN layer separated from the sapphire substrate with the aid of the interfacial voids. The freestanding AlN substrate thus obtained had a smooth surface, a dislocation density of 1.1×109 cm^-2, and an optical transparency for wavelengths above 208.1 nm.

The growth temperature of AlN layers is one of the most important factors in metal-organic vapor phase epitaxy (MOVPE) growth. AlN layers were grown using our customized high-temperature MOVPE system. The crystalline quality was discussed on the basis of X-ray diffraction, atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements. The samples grown at a temperature of 1400 °C had much improved crystalline quality in terms of the X-ray rocking curve full width at half maximum values and AFM root-mean-square roughness. In addition, according to TEM analysis, edge type dislocations caused by small-angle grain boundaries were predominant under a low growth temperature, whereas these dislocations became much fewer with increasing growth temperature.


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