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Using a hetero-epitaxial lateral overgrowth (ELO) technology with low-temperature deposited AlN interlayer, high-quality crack-free Al 0.22 Ga 0.78 N with low dislocation density is realized. The dislocation density in the Al 0.22 Ga 0.78 N was reduced to as low as the order of 10 7 cm --2 over the whole wafer. Applying the high-quality AlGaN layer to a UV light-emitting diode (LED), high output power of more than 0.1 mW at a forward current of 50 mA has been demonstrated with the emission peak wavelengths from 323 to 352 nm. The highest output power of 0.6 mW is obtained for the 352 nm LED with GaN/AlGaN multiple quantum well active layer. The emission efficiency was dependent on the wavelength, and the 323 nm LED has the lowest output power of 0.18 mW at 50 mA bias. One of the factors determining the external quantum efficiency is thought to be an inferior hole spread in highly resistive p-type layers.

Grown-in stress and subsequent fracture in Al x Ga1-x N/GaN heterostructures with or without impurity doping were studied in situ. It was found that the critical thickness of Al x Ga1-x N depends not only on its composition but also on the concentration of impurities such as Si or Mg. Increase in tensile stress during growth at a constant AlN molar fraction can be explained by the increase in the biaxial modulus of Al x Ga1-x N due to impurity doping.