Low resistance and highly reflective ohmic contacts to p-type GaN using transparent interlayers for flip-chip light emitting diodes

Zn (3 nm)/Rh (100 nm) and oxidized Ag-ITO (100 nm)/Al (200 nm) schemes for the formation of low resistance and highly reflective ohmic contacts for flip-chip light emitting diodes were investigated. It is shown that annealing the samples at 430 and 530°C for 1 min in air results in good ohmic behaviors with specific contact resistances of ∼10^-5 Ωcm² LEDs fabricated using the Zn/Rh contact layers give forward-bias voltages of 3.09-3.12 V at the injection current of 20 mA. Furthermore, the oxidized Ag-ITO film/Al contacts also show good ohmic characteristics with contact resistivity of 10^-5 Ωcm ² when annealed at 330°C for 5 min in vacuum. LEDs fabricated using the oxidized Ag-ITO film/Al contacts layers give forward-bias voltages of ∼3.40 V at 20 mA. It is clearly shown that a transparent contact interlayer is necessary to make high-quality ohmic contacts to p-type GaN for thermally stable flip-chip LEDs using Rh- and Al-reflectors.