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.

[1] Amano, H., Sawaki, N., Akaskai, I. & Toyoda, Y. Metal-organic vapor phase epitaxial growth of a high quality GaN film using an Al buffer layer. Appl. Phys. Lett. 48, 353–355 (1986).
[2] Amano, H., Sawaki, N., Akasaki, I. & Toyoda, Y. P-type conduction in Mg-doped GaN treated with low-energy electron beam irradiation (LEEBI). Jpn. J. Appl. Phys. 28, L2112–L2114 (1989).
[3] Nakamura, S., Mukai, T., Senoh, M. & Iwasa, N. Thermal annealing effects on p-type Mg-doped GaN films. Jpn. J. Appl. Phys. 31, L139–L142 (1992).
[4] Narita, T., Yoshida, H., Tomita, K., Kataoka, K., Sakurai, H., Horita, M., Bockowski, M., Ikarashi, N., Suda, J., Kachi, T. & Tokuda, Y. Progress on and challenges of p-type formation for GaN power devices. J. Appl. Phys. 128, 090901 (2020).
[5] Liu, H., Fu, H., Fu, K., Alugubelli, S. R., Su, P.-Y., Zhao, Y. & Fernando, A. Non-uniform Mg distribution in GaN epilayers grown on mesa structures for applications in GaN power electronics, Appl. Phys. Lett. 114, 082102 (2019).
[6] Brochen, S., Brault, J., Chenot, S., Dussaigne, A., Leroux, M. & Damilano, B. Dependence of the Mg-related acceptor ionization energy with the acceptor concentration in p-type GaN layers grown by molecular beam epitaxy. Appl. Phys. Lett. 103, 032102 (2013).
[7] Mayes, K., Yasan, A., McClintock, R., Shell, D., Darvish, S. R., Kung, P. & Razeghi, M. High-power 280 nm AlGaN light-emitting diodes based on an asymmetric single-quantum well. Appl. Phys. Lett. 84, 1046~1048 (2004).
[8] Wang, C.-H., Lai, K.-Y., Li, Y.-C., Chen, Y.-C. & Liu, C.-P. Ultrasensitive Thin-Film-Based AlxGa1−xN Piezotronic Strain Sensors via Alloying-Enhanced Piezoelectric Potential. Advanced Materials, 27, 6289–6295 (2015).
[9] Zheng, T. C., Lin, W., Liu, R., Cai, D. J., Li, J. C., Li, S. P. & Kang, Y. K. Improved p-type conductivity in Alrich AlGaN using multidimensional Mg-doped superlattices. Sci. Rep. 6, 21897 (2016).
[10] Zheng, T., Lin, W., Cai, D., Yang, W., Jiang, W., Chen, H., Li, J., Li, S. & Kang, J. High Mg effective incorporation in Al-rich AlxGa1 − xN by periodic repetition of ultimate V/III ratio conditions. Nanoscale Res. Lett. 9, 40 (2014).
[11] Nakarmi, M. L., Nepal, N., Lin, J. Y. & Jiang, H. X. Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys. Appl. Phys. Lett. 94, 091903 (2009).
[12] Chakraborty, A., Moe, C. G., Wu, Y., Mates, T., Keller, S., Speck, J. S., DenBaars, S. P. & Mishra, U. K. Electrical and structural characterization of Mg-doped p-type Al0.69Ga0.31N films on SiC substrate. J. Appl. Phys. 101, 053717 (2007).
[13] Nam, K. B., Nakamari, M. L., Li, J., Lin, J. Y. & Jiang, H. X. Mg acceptor level in AlN probed by deep ultraviolet photoluminescence. Appl. Phys. Lett. 83, 878~880 (2003).
[14] Jiang, H. X. & Lin, J. Y. Hexagonal boron nitride for deep ultraviolet photonic devices. Semicond. Sci. Technol. 29, 084003 (2014).
[15] Li, X., Jordan, M. B., Ayari, T., Sundaram, S., Gmili, Y. E., Alam, S., Alam, M., Patriarche, G., Voss, P. L., Salvestrini, J. P. & Ougazzaden, A. Flexible metal-semiconductormetal device prototype on waferscale thick boron nitride layers grown by MOVPE. Sci. Rep.7, 786 (2017).
[16] Grenadier, S., Maity, A., Li, J., Lin, J. Y. & Jiang, J. X. Lateral charge carrier transport properties of B-10 enriched hexagonal BN thick epilayers. Appl. Phys. Lett. 115, 072108 (2019).
[17] Dahal, R., Li, J., Majety, S., Pantha, B. N., Cao, X. K., Lin, J. Y. & Jiang, H. X. Epitaxially grown semiconducting hexagonal boron nitride as a deep ultraviolet photonic material. Appl. Phys. Lett. 98, 211110 (2011).
[18] Narukawa, Y., Ichikawa, M., Sanga, D., Sano, M. & Mukai, T. White light emitting diodes with super-high luminous efficacy. J. Phys. D: Appl. Phys. 43, 354002 (2010).
[19] Kneissl, M., Seong, T.-Y., Han, J. & Amano, H. The emergence and prospects of deep-ultraviolet light-emitting diode technologies. Nature Photon. 13, 233~244 (2019).
[20] Khan, A., Balakrishnan, K. & Rahman, A. III-Nitride-Based Short-Wavelength Ultraviolet Light Sources. Semicond Sci Technol. 6, 1-27 (2011).
[21] Kuo, S.-Y., Chang, C.-J., Huang, Z.-T. & Lu, T.-C. Improvement of Light Extraction in Deep Ultraviolet GaN Light Emitting Diodes with Mesh P-Contacts. Appl. Sci. 10, 5783 (2020).
[22] Wu, D. S., Wang, W. K., Shih, W. C., Hrong, R. H., Lee, C. E., Lin, W. Y. & Fang, J. S. Enhanced Output Power of Near-Ultraviolet InGaN–GaN LEDs Grown on Patterned Sapphire Substrates. IEEE Photon. Technol. Lett. 17, 288~290 (2005).
[23] Wuu, D.-S., Hsu, S.-C., Huang, S.-H., Wu, C.-C., Lee, C.-E. & Horng, R.-H. GaN/Mirror/Si Light-Emitting Diodes for Vertical Current Injection by Laser Lift-Off and Wafer Bonding Techniques. Jpn. J. Appl. Phys. 43, 5239~5242 (2004).
[24] Kim, J. K., Xi, J. Q., Luo, H. & Schubert, E. F. Enhanced light-extraction in GaInN near-ultraviolet light-emitting diode with Al-based omnidirectional reflector having Ni Zn/Ag microcontacts. Appl. Phys. Lett. 89, 141123 (2006).
[25] Cheng, B.-S., Chiu, C.-H., Lo, M.-H., Wu, Y.-L., Kuo, H.-C., Lu, T.-C., Cheng, Y.-J., Wang, S.-C. & Huang, K.-J. Light Output Enhancement of UV Light-Emitting Diodes With Embedded Distributed Bragg Reflector. IEEE Photonics Technol. Lett. 23, 642~644 (2011).
[26] Morita, D., Yamamoto, M., Akaishi, K., Matoba, K., Yasutomo, K., Kasai, Y., Sano, M., Nagahama, S. & Mukai, T. Watt-Class High-Output-Power 365 nm Ultraviolet Light-Emitting Diodes. Jpn. J. Appl. Phys. 43, 5945~5950 (2004).
[27] 1D DDCC, Available at:
http://yrwu-wk.ee.ntu.edu.tw/index.php/ddcc-1d/
[28]1D DDCC, Available at:
http://yrwu-wk.ee.ntu.edu.tw/mediawiki/index.php/%E9%A6%96%E9%A0%81
[29] STREETMAN, B. G., & BANERJEE, S. K. Solid State Electronic Devices. PEARSON (2016).
[30] Hall measurement, Available at:
http://ezphysics.nchu.edu.tw/prophys/basicexp/expnote/hall/hall_97Feb.pdf
[31] Van der Pauw method, Available at: https://en.wikipedia.org/wiki/Van_der_Pauw_method
[32] Rietveld, G., Koijmans, C. V., Henderson, L. C. A., Hall, M. J., Harmon, S., Warnecke, P. & Schumacher, B. DC conductivity measurements in the Van Der Pauw geometry. IEEE Trans Instrum Meas. 52, 447~452 (2003).
[33] Kang, M.-G. & Guo, L. J. Nanoimprinted Semitransparent Metal Electrodes and Their Application in Organic Light-Emitting Diodes. Advanced Materials. 19, 1391~1396 (2007).
[34] Jeon, J.-W., Seong, T.-Y., Kim, H. & Kim, K.-K. TiN/Al Ohmic contacts to N-face n-type GaN for high-performance vertical light-emitting diodes. Appl. Phys. Lett. 94, 042102 (2009).