Effect of graphene/ZnO hybrid transparent electrode on characteristics of GaN light-emitting diodes

In order to reduce the Schottky barrier height and sheet resistance between graphene (Gr) and the p-GaN layers in GaN-based light-emitting diodes (LEDs), conductive transparent thin films with large work function are required to be inserted between Gr and p-GaN layers. In the present work, three kinds of transparent conductive oxide (TCO) zinc oxide (ZnO) films, Al-, Ga-, and In-doped ZnO (AZO, GZO, and IZO), are introduced as a bridge layer between Gr and p-GaN, respectively. The influence of different combinations of Gr/ZnO hybrid transparent conducting layers (TCLs) on the optical and thermal characteristics of the GaN-LED was investigated by the finite element method through COMSOL software. It is found that both the TCL transmittance and the surface temperature of the LED chip reduce with the increase in Gr and ZnO thickness. In order to get the transmittance of the Gr/ZnO hybrid TCL higher than 80%, the appropriate combination of Gr/ZnO compound electrode should be a single layer of Gr with ZnO no thicker than 400 nm (1L Gr/400-nm ZnO), 2L Gr/300-nm ZnO, 3L Gr/200-nm ZnO, or 4L Gr/100-nm ZnO. The LEDs with hybrid TCLs consisting of 1L Gr/300-nm AZO, 2L Gr/300-nm GZO, and 2L Gr/300-nm IZO have good performance, among which the one with 1L Gr/300-nm GZO has the best thermal property. Typically, the temperature of LEDs with 1L Gr/300-nm GZO hybrid TCLs will drop by about 7 K compared with that of the LEDs with a TCL without ZnO film.