Thermal Annealing Effects on the Performance of a Ga-Doped ZnO Transparent-Conductor Layer in a Light-Emitting Diode

To identify the individually optimized thermal annealing conditions for reducing the contact resistivity between highly Ga-doped ZnO (GaZnO) and doped-GaN and for improving the electrical and optical properties of GaZnO, the results of the one-step and two-step growth/annealing processes of GaZnO at various growth and thermal annealing temperatures are compared. The one-step (two-step) process corresponds to the condition of thermal annealing for the whole GaZnO layer (only for a 10-nm GaZnO layer of the first-step growth). The two-step process always results in lower contact resistivity on either p-GaN or n-GaN at any annealing temperature. Lateral and vertical light-emitting diodes (LEDs) with GaZnO layers on the top are fabricated to show that the LED samples with the two-step process have the lower device resistance levels, higher emission efficiencies, and weaker efficiency droop effects, when compared with those with the one-step process. In the lateral LED sample with the two-step process, the combination of the effective atomic interdiffusion at the GaZnO/p-GaN junction under the optimized annealing condition and the preservation of the superior electrical property in the major GaZnO layer without annealing leads to its better performance, when compared with that of the lateral LED sample with the one-step process.