Tunnel-injected UV LEDs
- P-AlGaN has long been critical problem for the application in optoelectric devices, because of the low hole conductivity and high contact resistance, arrised from the high acceptor activation energy.
- Our research focuses on non-equilibrium hole injection into UV LEDs.
- Holes are suplied using interband tunneling injection through a AlGaN/ InGaN tunnel junction interconnected to the p-AlGaN cladding layer.
- This could potentially solve multiple problems in UV LED applications which are caused by poor p-type AlGaN layers.
- Yuewei Zhang, Sriram Krishnamoorthy, Fatih Akyol, Andrew A Allerman, Michael W Moseley, Andrew M Armstrong, Siddharth Rajan, "Design of p-type cladding layers for tunnel-injected UV-A light emitting diodes", Appl. Phys. Lett. 109, 191105 (2016)
- Yuewei Zhang, Sriram Krishnamoorthy, Fatih Akyol, Andrew A Allerman, Michael W Moseley, Andrew M Armstrong, Siddharth Rajan, "Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions", Appl. Phys. Lett. 109, 121102 (2016).
- Y. Zhang, S. Krishnamoorthy, J. M Johnson, F. Akyol, A. Allerman, M. W Moseley, A. Armstrong, J. Hwang, S. Rajan, ''Interband tunneling for hole injection in III-nitride ultraviolet emitters'' Appl. Phys. Lett. 106, 141103 (2015).
- Yuewei Zhang, Sriram Krishnamoorthy, Fatih Akyol, Zane Jamal-Eddine, Andrew Allerman, Michael Moseley, Andrew Armstrong and Siddharth Rajan, "Engineering of Hole Transport in Tunneling Injected UV-A LEDs", International Workshop on Nitride Semiconductors (IWN 2016), October 2-7, 2016, Orlando, Florida.
III-Nitride Tunnel Junctions
Polarization charge at GaN/InGaN heterointerface ultilized to create band bending in GaN p+n+ junction, to design inter-band tunnel junctions
Low resistance tunnel junctions (1x10-4 ohm-cm2) realized using 4 nm In0.25Ga0.75N barrier sandwiched between p+ GaN and n+ GaN.
Low resistance tunnel junctions can enable a large range of devices such as hole injection in visible/UV LEDs, lasers, multiple active region emitters and multi-junction solar cells.
1) S. Krishnamoorthy, D. Nath, F.Akyol, P. S. Park, S. Rajan, " Polarization-engineered GaN/InGaN/GaN tunnel diodes", Applied Physics Letters, 97, 203502 (2010).
2) S. Krishnamoorthy, P.S. Park. and S. Rajan, " Demonstration of forward inter-band tunneling in GaN by polarization engineering", Applied Physics Letters, 99, 233504 (2011).
3)Sriram Krishnamoorthy , Thomas Kent, Jing Yang, Pil Sung Park, Roberto Myers, and Siddharth Rajan, "GdN Nanoisland-Based GaN Tunnel junctions", Nano Letters. 13 (6) ,pp 2570 (2013).
4)Sriram Krishnamoorthy , Fatih Akyol, Pil Sung Park, and Siddharth Rajan, "Low Resistance GaN/InGaN/GaN tunnel junctions ", Applied Physics Letters 102, 113503 (2013).
Cascaded emitters to overcome efficiency droop
Efficiency droop is a major bottleneck to adoption of solid state lighting technology. It refers to the reduction in efficiency of LEDs when operated at high input current that is required for high light output power.
Epitaxial cascading of multiple LEDs using tunnel junctions , operated at low current density can be used to overcome efficiency droop. Electrons and holes are regenerated at the tunnel junction sites and one electron-hole pair can lead to multiple photon emission.
Proof-of-concept epitaxial cascading of GaN p-n junctions demonstrated by MBE.
Calculations indicate huge advantage in using cascaded LED structures.
1) Fatih Akyol, Sriram Krishnamoorthy, and Siddharth Rajan, "Tunneling-based carrier regeneration in cascaded GaN light emitting diodes to overcome efficiency droop", Applied Physics Letters, 103, 081107 (2013).
LEDs along N-polar GaN orientation can supress electron overflow and hence potentially improve the efficiency of green emitters.
1) F. Akyol, D. N. Nath, E. Gur and S. Rajan, "N-Polar III- Nitride green(540 nm) light emitting diode", Japanese Journal of Applied Physics 50 , 052101 (2011).