||Deep ultraviolet (DUV) LEDs traditionally employ Ni/Au as the ohmic contact to p-type AlGaN. However, for flip-chip devices, the reflectivity of gold at DUV wavelengths is below 30%, severely sacrificing the light extraction efficiency of DUV LEDs. To address the issues, there are two challenges to overcome with the p-type ohmic contact: (1) Increasing reflectivity in the DUV regime; (2) Decreasing the contact resistance on p-type Al0.5Ga0.5N. |
In this study, Ni/Au is replaced with Ni/Al/Ti/Au as the ohmic contact to p-type Al0.5Ga0.5N. The Al-based alloy was deposited by a high-vacuum electron-beam/thermal evaporation. The ohmic contact structure is then annealed at varied temperatures, with the attempt to reduce contact/sheet resistance. It is found that the annealing at 550 °C effectively reduces the specific contact resistance and the sheet resistance from 2.81x101 ohm-cm2 and 9.418x108 ohm/sq to 3.057x10-2 ohm-cm2 and 8.791x105 ohm/sq, respectively. For optical characterization, the reflectance of Ni/Al/Ti/Au at 280 nm reaches 57 %, significantly higher than that (24%) attained with Ni/Au. Although the conductivity of Ni/Al/Ti/Au on p-type Al0.5Ga0.5N remains to be improved, the high UV reflectivity is expected to compensate the loss in electrical performance. In the future, the fabrication of Ni/Al/Ti/Au will be optimized to enhance the external quantum efficiency of DUV LEDs.
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