We discuss the growth mechanism of GaN films and report very high two-dimensional electron gas (2DEG) mobility in AlGaN/AlN/GaN heterostructures fabricated on sapphire using BGaN micro-islands as novel buffers by metalorganic vapor phase epitaxy. The three-dimensional growth of BGaN (formation of BGaN micro-islands) occurs due to the phase separation of BGaN. However, the surface of the overgrown GaN on the BGaN micro-islands becomes smooth and continuous through the epitaxial lateral overgrowth process. The threading dislocations (TDs) in GaN consist mainly of pure edge-type ones and are effectively annihilated using single and double layers of BGaN micro-islands from 2×1010 to 2×109 and 2×108 cm-2, respectively. An n-type GaN film shallowly doped with Si exhibits an electron concentration and high Hall mobility of 3.0×1016 cm-3 and 669 cm2/Vs at room temperature (RT). Very high Hall 2DEG mobility in an Al0.10Ga0.90N/AlN/GaN heterostructure is obtained: 1910 and 20,600 cm2/Vs at RT and 77 K, respectively. The sheet carrier density had almost constant values of 6.9-5.7×1012 cm-2 in the temperature range from 77 to 500 K, indicating that the parallel conduction due to the residual electrons in the GaN underlying layer was negligible.
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