AlGaN and GaN sidewalls were turned into Al x Ga 2– x O 3 and Ga 2 O 3 , respectively, by thermal oxidation to improve the optoelectrical characteristics of deep ultraviolet (DUV) light-emitting diodes (LEDs). The thermally oxidized Ga 2 O 3 is a single crystal with nanosized voids homogenously distributed inside the layer. Two oxidized Al x Ga 2– x O 3 layers were observed on the sidewall of the AlGaN layer in transmission electron microscopy images. The first oxidized Al x Ga 2– x O 3 layer is a single crystal, while the second oxidized Al x Ga 2–x O 3 layer is a single crystal with numerous nanosized voids inside. The composition of Al in the first oxidized Al x Ga 2– x O 3 layer is higher than that in the second one. The thermal oxidation at high temperature degrades the quality of the p-GaN layer and increases the forward voltage from 8.18 to 11.36 V. The thermally oxidized Al x Ga 2– x O 3 sidewall greatly enhances the light extraction efficiency of the lateral light of the DUV LEDs by combined mechanisms of holey structure, graded refractive index, high transparency, and tensile stress. Consequently, the light output power of the DUV LEDs increases from 0.69 to 0.88 mW by introducing a 420 nm thick Al x Ga 2– x O 3 sidewall oxidized at 900 °C, in which the enhancement of light output power can reach 27.5%.

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