深紫外發光二極體(deep ultraviolet light-emitting diodes, DUV LEDs, 波長 ? 290 nm)需要高穿透、高導電的P型寬能隙半導體，才能發出更多的DUV光子。目前，多數團隊使用氮化鎵(GaN)或氮化鋁鎵(AlGaN)作為P型接觸層。然而，GaN的能隙(3.4 eV)太小，會吸收波長短於 360 nm 的深紫外光；AlGaN雖然能隙較大(3.4-6.1 eV)，可減緩吸光，但是卻有低導電的問題。為了解決穿透率與導電度之間的兩難，我們嘗試以高能隙、高導電的N型 AlGaN (n-AlGaN)，利用在 n-AlGaN/Ni 介面形成的二維電洞氣(Two-dimensional hole gas, 2DHG)，來製作高穿透、高導電的P型接觸層。 在本研究中，我們還以Ni/Al取代Ni/Au、Ti/Al取代Ti/Au。我們利用高真空電子束暨熱阻式蒸鍍系統 (E-gun/Thermal)，在n-AlGaN磊晶層上蒸鍍不同的金屬電極，比較其光電特性。根據霍爾量測的結果，鍍上Ni/Al的N型Al0.3Ga0.7N可產生穩定的電洞訊號，且電洞遷移率可達10.4 cm2/V?s。由於Ni/Al在N型半導體表面會形成蕭特基介面，可累積高濃度的電洞，從而形成2DHG。我們也將2DHG的技術應用在n-Al0.7Ga0.3N/MQW的結構上，利用電激發得到 330 nm 的紫外光訊號。未來，我們將持續優化Ni/Al的製程條件，希望能有效提升DUV LED的發光效率。 ;Deep ultraviolet light-emitting diodes (DUV LEDs, λ ? 290 nm) require high-transparent and high-conductive p-type semiconductor to produce adequate photons. P-type gallium nitride (p-GaN) or aluminum gallium nitride (p-AlGaN) are currently the most used contact layer for p-type electrodes. Nevertheless, p-GaN suffers severe UV absorption owing to her small bandgap (3.4 eV). Although the absorption issue can be alleviated by p-AlGaN with larger bandgaps (3.4-6.1eV), increasing the aluminum content in p-AlGaN leads to low conductivity. To circumvent the trade-off between transparency and conductivity faced by p-AlGaN, we propose a two-dimensional hole gas (2DHG) induced at the n-AlGaN/Ni interface.
In this study, the p-type metal electrodes of Ni/Au and Ti/Au were replaced with Ni/Al and Ti/Al. In order to evaluate the electrical properties, the Al-based alloy was deposited by a high-vacuum electron-beam/thermal evaporation. We obtained the hole mobility of 10.4 cm2/V?s and a stable hole concentration, by the Hall measurement performed on the Al/Ni/n-Al0.3Ga0.7N structure. The result was attributed to the Schottky contact formed at the Ni/n-AlGaN interface, whose upward band bending traps high concentration of free holes. We also grew the 2DHG structure on an AlGaN-based quantum wells and obtained a 330 nm emission peak in electroluminescence spectra. The concept presented in this study has the potential to enhance the external quantum efficiency of DUV LEDs.
