세미나

Development of High-performance Light-emitting Diodes for General Illumination

*Speaker : Jong Lam Lee (Department of Materials Science and Engineering, Division of  Advanced Materials
                Science Pohang University of Science and Technology)
*Time : 4 : 00  on October 13 (Wednesday )
*Location : E104
*Contact : Prof.  Jeong  Min  Baik (052-217-2324) , jbaik@unist.ac.kr
* Abstract
GaN-based light-emitting diodes (LEDs) are attracting great interest as candidates for next-generation solid-state lighting. However, the external quantum efficiency of LEDs, determined by the internal quantum efficiency (IQE) and the light extraction efficiency, must be further increased. Vertical-structure designs fabricated by laser lift-off technique have been reported to be very effective for increasing the light extraction efficiency of GaN-based LEDs. Here, we present a method of increasing light output power and suppressing efficiency droop in V-LEDs without modifying their epitaxial layers. These improvements are achieved by reducing the quantum-confined Stark effect (QCSE) by reducing piezoelectric polarization that results from compressive stress in the GaN epilayer. This compressive stress is relaxed due to the external stress induced by an electro-plated Ni metal substrate. In simulations, the severe band bending in the InGaN quantum well is reduced and subsequently IQE increases as the piezoelectric polarization is reduced.
We also demonstrate that the combined use of highly reflecive p-contact and nanotexturing of n-face n-GaN with photonic crystal and photochemical etching significantly improved the light extraction efficiency of V-LEDs. Our experimental results that the enhancement of light output power by the nanotexturing is remarkably influenced by the reflectance of the p-contact suggests that not only enhancing light extraction at the air/n-GaN interface but also improving light reflection at the metal/p-GaN one is of vital importance for improving the light extraction efficiency.
Finally, we propose the Ni/Ag multi-layer structure ohmic contact to p-type GaN. The Ni/Ag multi-layer contact exhibits high residual tensile stress in Ag layer, resulting in the reduction of thermal compressive stress during annealing. This stress relaxation reduces the driving force for Ag agglomeration, leading to enhanced thermal stability of the contact.