| 摘要: | 本計畫欲將主持實驗室新開發的高穿透、高導電p型氮化硼(p-type boron nitride, p-BN)，應用在波長為260-nm的UVC LED，取代當前普遍使用的p型AlGaN，希望解決p型AlGaN無法兼顧穿透率與導電度的難題。當UV LED的發光波長短於300 nm時，其外部量子效率大多低於10%。主因是AlGaN的p型摻雜效率很低，讓電洞無法有效注入量子井，難以產生足夠的光子。若要提高p型摻雜效率，必須降低AlGaN的Al含量，以降低能隙(Eg)來提高電洞的活化能。然而，當p型AlGaN的Eg降低時，會吸收量子井發出的紫外光，犧牲了元件的發光效率。因此p型AlGaN很難同時滿足高導電、高穿透的要求。 p-BN是一種二維材料，類似石墨，但它的能隙很大，接近AlN的6.1 eV，而且p-BN的電洞活化能最低僅約30 meV，遠低於AlN的510 meV。換言之，p-BN是極少數具備高穿透、高導電的半導體，有潛力取代p型AlGaN。 本團隊從2017年六月開始以MOCVD成長p-BN磊晶層，對這種獨特的材料已有初步了解。目前，本團隊成長的p-BN在220 nm以上的波長範圍，可維持80 %以上的穿透率。而且，霍爾量測顯示p-BN的電洞濃度超過1E18 /cm3、片電阻低達1200 ohm/sq。這些結果符合文獻紀載，顯示p-BN具備高穿透、高導電的特性。我們計畫將這種新材料，應用在260-nm的UVC LED，並持續優化p-BN在元件上的光電效能，希望能為UVC LED開發出新的p型半導體。 ;This project aims to develop high transparent and high conductive p-type boron nitride (p-BN) epilayer, serving as the p-type contact layer for 260-nm UVC LEDs. p-BN is adopted to replace the mostly used p-type AlGaN, which has been haunted by the trade-off between transparency and conductivity. The low UV transparency or low p-type conductivity of AlGaN is the main culprit for the very low (< 10 %) external quantum efficiencies (EQEs) of UVC LEDs. The development of UVC LED is mainly challenged by the incapability of p-type AlGaN. Depending on the Al composition, the hole activation energy (EA) in AlGaN is usually above 300 meV, leading to low free hole concentrations. Although the concentration can be increased with a reduced bandgap (Eg), the high optical absorption in the p-type layer can severely sacrifice the output optical power in the UV regime, resulting in the low EQEs of UVC LEDs. p-BN is a unique two-dimensional material, like graphite, but with a very high Eg, being close to that (6.1 eV) of AlN. Moreover, the hole activation energy (EA) in p-BN is reported to be merely 30 meV, much less than that (510 meV) of AlN. The high Eg and low EA of p-BN make it the only material that can serve as a high-UV-transparent, yet low-resistive, p-type semiconductor, which is extremely attractive for the applications on UVC LEDs.Since Jun. 2017, we have been growing p-BN with MOCVD, and have learned some characteristics of this unique material. According to the preliminary results, the p-BN produced in our lab exhibits the transmittance above 80 % for the wavelengths above 220 nm. And the Hall measurement showed that the 8-nm-thick p-BN contains the free hole concentration above 1018 cm-3, with the sheet resistance of 1200 ohm/sq. These results indicate the high transparency and high conductivity of p-BN, being consistent with the prediction by other research groups. We plan to integrate the growth of p-BN with that of the 260-nm UVC LED, aiming to develop new p-type semiconductor for UVC LEDs. |
