P型氮化物半導體大多以鎳金(Ni/Au)作為歐姆接觸電極。對覆晶型(Flip-Chip)深紫外(deep utralviolet, DUV, 波長 ≤ 290 nm) LED來說，金在DUV波段的反射率不到30%，嚴重犧牲元件的光萃取效率。為了解決此問題，我們在DUV LED的P型電極上需克服兩大難題：(1) 提升金屬電極在DUV波段的反射率；(2)降低P型氮化鋁鎵（Al0.5Ga0.5N）表面的接觸電阻。 在本研究中，我們以Ni/Al/Ti/Au取代Ni/Au。我們利用高真空電子束暨熱阻式蒸鍍系統 (E-gun/Thermal)，在P型Al0.5Ga0.5N磊晶層上蒸鍍Ni/Al/Ti/Au，再分析退火溫度對Ni/Al/Ti/Au接觸電阻的影響。我們發現，550℃的退火溫度可將接觸電阻及片電阻分別從2.81x101 ohm-cm2及9.41x108 ohm/sq，降低至3.05x10-2 ohm-cm2及8.79x105 ohm/sq。在反射率方面，Ni/Al/Ti/Au在280 nm的反射率可達57%，遠高於Ni/Au的24%。雖然Ni/Al/Ti/Au的導電度有待改進，但由於Al的高UV反射率，可以補償其在電性上的損失。未來，我們將持續優化Ni/Al/Ti/Au的製程條件，希望能有效提升DUV LED的發光效率。 ;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.
