A high-performance Al0.1Ga0.9N ultraviolet (UV) avalanche photodiode (APD) with a separate absorption and multiplication structure grown on AlN templates is fabricated by employing a triple-mesa structure. The fabricated AlGaN UV-APD exhibits a maximum gain up to 2.3 × 104 at the reverse bias of 67 V and a low avalanche breakdown voltage (<70 V). The triple-mesa structure is confirmed to significantly lower the avalanche breakdown voltage and reduce the sidewall leakage current in comparison with the conventional double-mesa one. These improvements are explained by the simulation of the electric field which shows a significant improvement in the distribution uniformity in the active regions and enhancement in the intensity in the multiplication region. In addition, the scaling effects of various anodes and mesas are investigated, and the dark current is found to decrease with a decrease in the mesa size thanks to the reduced amount in the high-conductivity threading dislocation that crosses the multiplication region.

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A metal-source flow-rate modulation epitaxy method is reported to enhance the hole concentration of Mg-doped AlGaN grown by metal organic chemical vapor deposition. The hole concentration of p-type AlGaN (Al content 0.43) is increased to 2.3 × 1017 cm−3 at room temperature by this method, which is about ten times higher than that of the conventional growth. The resistivity was found to be as low as 12.7 Ω·cm. Furthermore, the effective acceptor activation energy (EA) in the AlGaN films (Al content 0.32–0.43) was determined to be 20–22 meV, several times smaller than EA in p-GaN. Secondary ion mass spectroscopy measurements demonstrated that uniformly Mg-doped AlGaN structures with spatially modulated Al compositions were formed using this technique. It is suggested that the enhancement of hole concentration benefits from the modulation of the valence band edge.

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