Effect of the GaN:Mg Contact Layer on the Light-Output and Current-Voltage Characteristic of UVB LEDs

Abstract

In order to realize UVB LEDs with high wall-plug efficiencies, the light extraction efficiency from the LED heterostructure must be maximized and operating voltages reduced. In this study, we investigate the effect of the GaN:Mg contact layer thickness on the light-output and current-voltage characteristics of UVB LEDs. AlGaN-based LED heterostructures, that are fully transparent for UVB emission except for the GaN:Mg contact layer are grown by metal organic vapor phase epitaxy on c-plane sapphire substrates. From transfer line measurements, it is found that the p-contact resistivity increases rapidly with decreasing GaN:Mg thickness and exhibits a pronounced Schottky behavior for layer thicknesses below 40 nm. At the same time, the emission power increases from 0.1 to 1.5 mW at 20 mA with decreasing GaN:Mg thickness. Ray tracing simulations of the light extraction efficiency of the UVB LEDs show that absorption in the GaN:Mg layer leads to lower emission powers for thicker GaN:Mg layers. Furthermore, with increasing GaN:Mg thickness additional losses occur due to a decrease of the internal quantum efficiency. The electro-optical and the structural properties of the devices show that a 40 nm thick GaN:Mg contact layer is the best compromise due to the low p-contact resistivity (0.01 Ω cm2) and at the same time still sufficient UVB-transmission resulting in UVB LEDs with external quantum efficiencies of more than 1% and a wall plug efficiency of 0.4% (at 20 mA), measured on-wafer.