Nanocrystalline sputter-deposited ZnMgO:Al transparent p-type electrode in GaN-based 385 nm UV LED for significant emission enhancement

•Polycrystalline ZnMgO:Al films grown by room temperature cosputtering.•Increase of Mg content leads to grain disorientation and Mg precipitation.•The unexpected behavior suggested to be related to Al presence.•ZnMgO:Al transparent electrode to p-GaN: 250% increase in 385 nm LED efficiency.

We demonstrate nanocrystalline ZnMgO:Al transparent conducting films grown by room temperature magnetron sputtering. Unlike in the usual ZnMgO films grown by sputtering, the crystal orientation perpendicular to the substrate surface is strongly disrupted with the addition of Mg, even well below the phase separation threshold of Mg/(Mg + Zn) = 0.43. We argue that the presence of Al in the films promotes disoriented growth. Using transmission, Rutherford Backscattering Spectroscopy and electron microscopy measurements, we prove that not all Mg in the films substitutes Zn in the ZnO lattice, but also forms of precipitates. We apply the highest transmission films as contact electrodes to the p-GaN layer in a GaN-based 385 nm UV LED structure. By replacing the common opaque Ni/Au ohmic contact to p-GaN in ring geometry by a circular ZnMgO:Al electrode we obtain a 250% increase in irradiated power, which is also 148% higher than when Ni/Al is replaced by ZnO:Al.

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