Impact of Silver Surface Morphology on the Wall Plug Efficiency of Blue Vertical Light-Emitting Diodes

Abstract

Due to merits of good thermal conductivity and vertical current distribution, vertical light-emitting diodes (VLEDs) have been widely studied for many years. However, the wall plug efficiency (WPE) remains limited by the light trapping effect in GaN cavity and difficulties to achieve low-resistance ohmic contacts to p-GaN. As one of the solutions, Ni-/Ag-based mirror layers as ohmic contacts have attracted much attention due to the high work function of Ni and high reflectivity of Ag for blue light. In this paper, a Ni/Ag/Ni/Ag/Ti metal stack was deposited separately on the surface of p-GaN. A high WPE of 45.7% for GaN-based blue VLEDs on Si substrates has been achieved at the mean diameter (69 nm) of Ag grains and root-mean-square (rms) surface roughness (2.06 nm) of Ag mirror layers. There are three impacts on VF: formation of NiO, growth of Ag grain, and increases of Ag oxide. The impacts of surface topography on optical properties are investigated by the Mie scattering theory for Ag grains and surface scattering theory for roughness surface of Ag mirror layer. The conclusion shows that: 1) with growth of Ag grain, the absorption of Ag grains is heightened obviously and the intensity distribution of scattering light with scattering angle is changed from uniformity to irregularity and 2) since the magnitudes of specular reflectance are dramatically larger than the scattering about 1011 times, the specular reflectance of Ag roughness surface totally covers the attribution of scattering and is only decided by rms values and wavelength of incident light.