Fig. 1. (a) Schematic illustration showing the tunnel junction LED structures. (b) Simulated equilibrium bands diagram for an LED representing a tunnel junction using a 5nm GaN layer. Different colors are used to indicate the different layers in the structure. Credit: DOI 10.1364/PRJ.383652
Ayush Pandey, an ECE Ph.D. student, describes the research of Prof. Zetian Mi about high-efficiency ultraviolet light emitting diodes for sterilizing pathogens. The 2020 Editor-in Chief Choice Award for Photonics Research was awarded to this research, "High-efficiency AlGaN/GaN/AlGaN tunnel-junction ultraviolet light-emitting dimers."
Worldwide, millions of dollars and thousands of lives are lost each year due to waterborne and health-care-related illnesses. It is essential to sterilize. There are many methods that can be used, including UV light radiation. Because of the worldwide coronavirus pandemic and the potential spread of infectious diseases, this urgent need has become more pressing.
Mercury lamps, which are currently available, are heavy and contain toxic chemicals. They are also not as flexible in their applications as other sources such as semiconductor light sources. AlGaN, the preferred material for high-efficiency deep UV light sources is the best alternative to mercury lamps. It is also the only technology that can replace mercury lamps in water purification and disinfection. AlGaN-based deep and mid UV LEDs have a very low efficiency. Poor hole injection is a major problem. This is due to inefficient p-type doping by AlGaN alloys using Mg. This is especially true for high Al composition alloys which are critical for the UV-C wavelength ranges (200-280nm).
Ayush Pandey. Credit: University of Michigan
Tunnel junction structures are a promising way to overcome this problem and increase hole injection in the active region of a device. Interband transport of electrons is what drives hole injection in these devices. It takes place between the valence bands of the p and the conduction bands of the the n-type layers.
Professor Zetian Mi's research group at the University of Michigan conducted a thorough investigation of the design and fabrication of tunnel junction UVC LEDs operating at approximately 265 nm. It is difficult to tunnel between the p and n-type layers due to AlGaN's large bandgaps. The team developed unique designs to overcome this problem, including a thin GaN tunnel junction layer that can be used with different thicknesses.
This technique is based on large spontaneous and piezoelectric field polarizations of the III-nitrides. It can be controlled by sandwiching a layer with a different material composition between doped layers. This will dramatically increase the probability of tunneling. A special metal-semiconductor junction-assisted epitaxy method was also developed to improve the Mg doping as well as the hole concentration in Al-rich AlGaN layer.
The current-voltage characteristics of the optimized tunnel junction device were significantly better than those of a conventional LED having a p type AlGaN contact. The tunnel junction device's improved injection resulted in stronger electroluminescence without any defects emission peaks. It was also found that the emission was extremely stable, with very little variation in peak positions across a wide range of injection currents. The maximum external quantum efficiency was 11%, and the wall-plug efficiency was 7.6%. These are the highest ever values for deep UV LEDs operating at 265nm. This provides a viable way to break the efficiency bottleneck in deep UV photonics.
Learn more Understanding the high efficiency of deep-UV LEDs
More information: A. Pandey et al., High-efficiency AlGaN/GaN/AlGaN tunnel-junction ultraviolet light-emitting devices, Photonics Research (2020). A. Pandey et al, High-efficiency AlGaN/GaN/AlGaN tunnel junction ultraviolet light-emitting diodes,(2020). DOI: 10.1364/PRJ.383652
