Preparation of freestanding AlN substrates by hydride vapor phase epitaxy using hybrid seed substrates

A novel hybrid seed substrate for preparing a freestanding (0001) AlN substrate was proposed. The seed substrate consists of thin single-crystalline AlN layer and thick poly-crystalline AlN base, which is expected to remove the differences of thermal expansion coefficient and lattice constant between a subsequently grown thick AlN epitaxial layer and the seed substrate. A freestanding AlN substrate (diameter: 20 mm; thickness: 180 μm) was successfully prepared, although the freestanding AlN substrate included a number of inner cracks. The freestanding substrate had no strong specific absorption in the wavelength range 210-1000 nm and showed an absorption number of 52 cm−1 at 265 nm.