A vertical AlGaN DUV light-emitting diode fabricated by wafer bonding and sapphire thinning technology

Significance 

Deep ultraviolet (DUV) light-emitting diodes (LEDs) made from AlGaN exhibit numerous advantages, including high reliability, low power consumption, small dimensions and wavelength tunability. To this end, they are promising alternative to traditional mercury lamps in various applications like sensing, polymer curing and water purifications. For instance, DUV LEDs have been extensively produced and used to destroy coronavirus disease viruses. Nevertheless, the widespread application of DUV LEDs is restricted by their self-heating effect, current crowding and low light extraction efficiency (LEE).

Enhancing LEE and EQE is critical in improving the performance of DUV LEDs. Compared to lateral structures, DUV LEDs of vertical structures have drawn significant research attention for fabricating high-performance AlGaN DUV LEDs. Vertical structures have a large active region, low turn-on voltage, improved heat dissipation and are easier to fabricate in a thick n-doped AlGaN layer. To date, nearly all research that attempts to fabricate vertical AlGaN DUV on sapphire substrates mainly uses AIN as the template layer due to its higher wavelength transparency and small lattice mismatch. Unfortunately, the resulting Al metal is often difficult to remove and can lead to cracking due to mechanical stress.

Although absorbing the laser power by inserting superlattice or GaN sacrificial layers can alleviate these challenges and mitigate the mechanical stress, the inserted sacrificial layers often decrease the epilayer quality leading to lower LED EQE. Herein, Dr. Jiabin Yan, Dr. Jialei Yuan, Dr. Yan Jiang, Professor Hongbo Zhu, and Professor Yongjin Wang from Nanjing University of Posts and Telecommunications in collaboration with Professor Hoi Wai Choi from the University of Hong Kong fabricated a submicron vertical DUV LED to improve the LEE and EQE by facilitating the removal of more photons from the device. Their work is currently published in the journal, Applied Physics Express.

In their approach, the vertical AlGaN DUV LED was fabricated using sapphire thinning and wafer bonding technology. A combination of adequately-controlled sapphire thinning processes and low-stress eutectic bonding were employed to remove sapphire substrates. A submicron thickness, emission wavelength of 272 mm, roughened n-doped AlGaN surface, and vertical-emitter architecture were adopted to improve the device LEE. The epilayers were grown on 2-inch double-side-polished c-sapphire substrate via either organic or metal-based chemical vapor deposition. Finally, the advantages and practical significance of this scheme in comparison to other schemes were discussed.

The authors reported the fabrication of a vertical AlGaN DUV LED with ultrathin epilayers thickness of about 670 nm – the thinnest ever to be reported to date. This was attributed to the effective thinning and wafer bonding techniques. This technique also allowed the realization of the light-emitting surface with a better root mean square of 74.7 nm without requiring extra surface surface-roughening treatments. The proposed sapphire substrate removal scheme is much easier to realize in mass production and is more suitable for controlling the inherent mechanical stress than that of laser lift-off technology, among other advantages. Overall, the resulting LED devices exhibited improved LEE and EQE desirable for different applications.

In summary, the efficiency and robustness of thinning and wafer bonding technologies in fabricating high-performance vertical thin-film DUV LEDs were demonstrated. An n-contact electrode and bottom metal electrode functioned to expose the emission region and a reflector to reflect the light propagation in an upward direction, respectively. In a statement to Advances in Engineering, Dr. Jiabin Yan first author said that the newly presented design and scheme offer a feasible scheme for enhancing the LEE and extending the application of AlGaN DUV LEDs.

About the author

Jiabin Yan received the B.S. degree in microelectronics from Shanghai University, Shanghai, China and the Ph.D. degree in microelectronics and solid state electronics from Southeast University, Nanjing, China, in 2013 and 2018, respectively. He is currently a lecturer with the Grünberg Research Centre, Nanjing University of Posts and Telecommunications, Nanjing, China. The discipline of his research focuses on the GaN-based optoelectronic devices, optoelectronic integrated circuit (OEIC), thermoelectric/photoelectric energy harvesters, and RF MEMS devices. (https://orcid.org/0000-0003-2460-5832

Reference

Yan, J., Yuan, J., Jiang, Y., Zhu, H., Choi, H., & Wang, Y. (2022). A vertical AlGaN DUV light-emitting diode fabricated by wafer bonding and sapphire thinning technologyApplied Physics Express, 15(3), 032003.

Go To Applied Physics Express

Check Also

Reversible Mn valence state switching in submicron α-Al2O3:Mn by soft X-rays and blue light – a potential pathway towards multilevel optical data storage - Advances in Engineering

Reversible Mn valence state switching in submicron α-Al2O3:Mn by soft X-rays and blue light – a potential pathway towards multilevel optical data storage