Low-resistance Ni/Au ohmic contact to Mg-doped of Al_0.15Ga_0.85N/GaN superlattices

Electrical properties of Ni (6 nm)/Au (14 nm) contacts to Mg-doped strained layer Al_0.15Ga_0.85N/GaN superlattices have been analyzed. The hole concentration and mobility are around 1 × 10¹⁸ cm^-3 and 10 cm²/Vs, respectively. Hall effect measurements for this structure show a high conductivity that the high activation efficiency of Mg is due to the strain-induced piezoelectric field. The temperature-dependence resistivity result suggested that high-hole concentration can enhance tunneling transport for field emission and thereby reducing the contact resistance of metal-semiconductor interface. Before thermal alloying, the current-voltage (I-V) characteristic of Ni/Au contact on p-type Al_0.15Ga_0.85N/GaN SLs shows non-ohmic behavior. As the alloying temperature increases to 600°C, the I-V curve shows a characteristic of ohmic contact. A specific contact resistance as low as 4.0 × 10^-6 Ω cm² was obtained at alloying temperature of 650°C for 5 min in N₂ ambient.