Electrical and structural properties of low-resistance Pt/Ag/Au ohmic contacts to p-type GaN

A Pt (20 nm)/Ag (50 nm)/Au (30 nm) metallization scheme has been investigated for producing low-resistance ohmic contacts to moderately doped p-type GaN (1.3 × 10¹⁷ cm^-3). It is shown that the as-deposited contacts exhibit a linear I-V characteristic with a specific contact resistance of 4.43 × 10^-3 Ω cm². The Pt/Ag/Au contact produced a specific contact resistance as low as 1.70 × 10^-4 Ω cm² after annealing at 800 °C for 1 min in a N₂ atmosphere. It is further shown that the surface morphology of the contact annealed at 800 °C (RMS roughness of 19.9 nm) became somewhat degraded compared with that of the as-deposited one (RMS roughness of 3.3 nm). Based on the Auger electron microscopy and X-ray diffraction results, possible explanations for the improvement of the ohmic behavior are described.