Improving light extraction in light-emitting diodes using zinc-tin-oxide layers

In this study, we report on the synthesis and applications of zinc-tin-oxide (ZTO) as a light extraction layer for GaN-based light-emitting diodes (LED). The ZTO layers formed on top of an LED epi-structure with a variable Sn-ratio, which was deposited by the spin coating method. The transmission spectra of the ZTO layers with Zn to Sn ratios of 1:1 (ZTO-I) and 1:5 (ZTO-II) exhibited optical transmittances of 98% and 88% in the visible region, respectively. The electroluminescence (EL) and light power-current-voltage (L-I-V) measurements show that double ZTO layers consisting of various Zn to Sn ratios led to enhanced light extraction from blue LEDs. The improvement of light extraction in the LEDs can be attributed to the gradually reduced refractive index of the ZTO layers, which enlarges the photon escape cone and minimizes the Fresnel refraction loss at the interface between air and GaN.