We review the recent developments to enhance the external quantum efficiency (EQE) in GaN based vertical light-emitting diodes (V-LEDs). The controlling of side-wall angle by SiO2 nanosphere lithography significantly improved the light extraction efficiency (LEE) of V-LEDs; this result is 6% higher than the photo chemical etching (PCE) method, which is known to have the highest light extraction, and 300% higher than flat surface V-LEDs. Nanostructured V-LEDs with a very dense forest of vertically aligned ZnO nanowires on the surface of N-face n-GaN induce the dramtic improvement in LEE. The structural transformation at the nanolevel by the UV radiation and Ozone (UV-O) treatment contributes the high density of Zn seed on GaN, and then this approach shows an extreme enhancement in LEE (>2.8x) compared to flat V-LEDs. The enhanced LEE was also demonstrated by depositing a spontaneously formed MgO nano-pyramids and ZnO refractive-index modulation layer on the surface of V-LEDs, resulting in the increase of output power by 49 %, comparing with the V-LEDs with a flat n-GaN surface. The enhancement of light output power by the nanotexturing of n-face n-GaN was remarkably influenced by 3-dimentional nanostructures.
We present a review of the recent developments to enhance the external quantum efficiency (EQE) in GaN based vertical
light-emitting diodes (V-LEDs). The combined use of quasi-photonic crystal and photochemical etching significantly
improved the light extraction efficiency (LEE) of V-LEDs by a factor of 5. The enhancement of light output power by
the nanotexturing of n-face n-GaN was remarkably influenced by reflectance of the p-contact. The enhanced LEE was
also demonstrated by depositing a spontaneously formed MgO nano-pyramids and ZnO refractive-index modulation
layer on the surface of V-LEDs, resulting in the increase of output power by 49 %, comparing with the V-LEDs with a
flat n-GaN surface. The thermal stability of Ag-based p-type ohmic contact was siginficantly enhanced by addition of Cu,
In, and Mg atoms to Ag layer, leading to high light reflectance and low contact resisitivity. Finally, we present a method
of increasing light output power and suppressing efficiency droop in V-LEDs without modifying the epitaxial layers.
These improvements are achieved by reducing the quantum-confined Stark effect by reducing piezoelectric polarization
that results from compressive stress in GaN epilayer. This compressive stress is relaxed due to the external stress induced
by an electro-plated Ni metal substrate.
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