本計畫延續過去中央光電與泰谷公司的合作關係，希望將 283-nm UVC LED 的輸出功率由 2.12 mW 提升至5 mW 以上。 UV LED有兩個重要的技術瓶頸：一為高品質的AlN磊晶層；一為高穿透、高導電的p型AlGaN。本團隊在2016～2017的產學計畫中，已利用新式的磊晶技術突破第一項瓶頸，此磊晶技術可得到低成本、高品質的AlN磊晶層，並已導入泰谷公司的磊晶系統。相對於第一項技術瓶頸，第二項技術瓶頸的困難度更高，這是因為高能隙半導體的p型摻雜效率非常低，在電洞無法有效注入量子井的情況下，UVC LED很難產生足夠的光子。若要提高p型摻雜效率，必須降低AlGaN的鋁含量，以降低能隙來提高電洞的活化能，然而，當p型AlGaN的能隙降低時，會吸收量子井發出的UV光，犧牲了元件的發光效率。換言之，p型AlGaN很難同時達到高導電、高穿透的要求，這也是目前UVC LED的外部量子效率很難超過10%的主因之一。 為了突破第二項瓶頸，我們將以新開發的氣流中斷技術，成長AlxGa1-xN/AlyGa1-yN超晶格結構，並搭配Ni/Al為基礎的金屬電極，產生高導電、高穿透的p型Al0.5Ga0.5N磊晶層。以氣流中斷法成長的超晶格結構，可有效提升鎂離子(p型摻雜)與五族(氮)離子的結合效率，讓電洞更容易產生。此外，許多研究也顯示：以Ni/Al為底的p型金屬電極，可有效提升覆晶型UVC LED的光翠取效率。然而，超晶格結構與Ni/Al金屬的結合，還有許多需要克服的挑戰，如：金屬與磊晶層之間的高接觸電阻。我們希望以一年的時間，解決這些難題，將p型Al0.5Ga0.5N磊晶層整合至UVC LED的元件結構，提升元件的輸出功率。 ;This project is to continue the collaboration between NCU and Tekcore Inc., and boost the output power of 283-nm UVC LED from 2.12 mW to 5 mW. The development of UVC LED is challenged by two technique obstacles: i) High-quality AlN epitaxial layer. ii) High-transmissive, high-conductive p-type AlGaN. Our previous team work with Tekcore had overcome the 1st obstacle with a unique epitaxial growth method, which can produce high-quality AlN in a cost-effective way and has been implemented in Tekcore’s epitaxy facilities. Compared to the 1st one, the 2nd technique obstacle is often regarded as the more difficult one. This is due to the trade-off between high-transmission and high-conductivity in p-type AlGaN. The hole activation energy in wide-bandgap material is inherently high, leading to the low free hole concentration. Although the concentration can be increased with a reduced bandgap, the high optical absorption in the p-type layer can severely sacrifice the output power. This is the main reason leading to the very low (< 10%) external quantum efficiencies (EQE) of UVC LEDs to date. To address the issue, we plan to adopt the AlxGa1-xN/AlyGa1-yN superlattice, obtained with a pulsed gas injection technique, to increase the incorporation efficiency of Mg (the p-type dopant) in AlGaN. Interrupting the supply of carrier gas is expected to promote the bonding between Mg and nitrogen precursors, and thus results in increased hole concentration. The superlattice is to be integrated with Ni/Al-based ohmic contact to achieve the high-transmissive, high-conductive p-type Al0.5Ga0.5N epitaxial layer, with which we aim to enhance the efficiencies of carrier injection and light extraction of UVC LEDs.
