High-quality Pt (20 nm)/Ru (50 nm) Ohmic contacts to surface-treated p-GaN with lowest resistance and thermal stability are reported. The electrical properties and interfacial reactions of the contacts to GaN were investigated using current±voltage, Auger electron spectroscopy, and glancing angle X-ray diffraction measurements. It is shown that the as-deposited contact produces a specific contact resistance of 7.8(AE2.2) Â 10 ± ±4 Wcm 2 , while the contact annealed at 600 C for 2 min yields a resistance of 2.2(AE2.0) Â 10 ± ±6 Wcm 2 . The latter is believed to be the lowest value reported so far for Ohmic contacts to p-GaN. It is further shown that the contacts exhibit excellent thermal stability during annealing at 600 C.Introduction Due to the rapid progress in the growth and processing of GaN-based semiconductors, the electronic and optoelectronic devices have been successfully fabricated on these semiconductors [1,2]. In order to achieve low power loss and allow high temperature operation of such devices, control of the contact metallurgy with low resistance and thermal stability is of great technological importance.For the contacts to p-GaN, despite the problems such as the difficulty in achieving a carrier concentration !10 18 cm ±3 and the absence of appropriate metals having work functions larger than that of p-GaN (6.5 eV) [3], reasonable electrical properties have been achieved [4 to 11]. Most of the contacts studied for p-GaN are Au-based schemes such as Ni/Au [4 to 6], Pt/Au [7], Cr/Au [4], Pd/Au [8], Ni/Pt/Au [9, 10], and Pt/Ni/Au [11]. However, the Au-based contacts generally have poor thermal stability. Several groups have employed Au-free contact schemes such as W [12], WSi x [12], and Ti/Ta/p-GaN[13] to obtain thermally stable and low resistance Ohmic contacts on p-GaN. However, the W-based contacts produced a rather high contact resistance of %10 ±2 Wcm 2 , indicating that these schemes are not suitable for applications on blue LDs. Furthermore, the Ti/Ta/p-GaN contacts were found to experience atmospheric degradation although the contacts had a low contact resistance of 3 Â 10 ±5 Wcm 2 when annealed at 800 C for 20 min in vacuum ( 4 Â 10 ±4 Pa).Native oxide on p-GaN is detrimental to Ohmic properties. It is, therefore, important to remove native oxide thoroughly for achieving low resistance and thermally stable Ohmic contacts. In a previous work [14], we showed that two-step surface treatment is effective in removing native oxide and increasing a near-surface carrier concentration. In this paper, we report on a new metallization scheme for the formation of low resistance and thermally stable Ohmic contacts to the surface treated-p-GaN.