Effects of oxygen plasma treatments on the formation of ohmic contacts to GaN J. Yan * , M.J. Kappers, Z.H. Barber, C.J. Humphreys Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK Available online 7 July 2004 Abstract Oxygen plasma treatments have been performed prior to contact deposition on both n- and p-type GaN, and the effects of plasma pressure, rf power and treatment time on the contact characteristics are discussed. By exposing the surface of n-type GaN to an oxygen plasma prior to metal deposition, the as-deposited Ti/Al contacts change from rectifying to ohmic, and further improvements are observed after rapid thermal annealing (RTA). A specific contact resistivity better than 10 7 O cm 2 was obtained using a plasma treatment of 20 s at 30 Wand 0.2 mbar, followed by RTA at 500 8C in argon. The I–V characteristics of the Ti/Al contacts degraded when plasma treatments were performed for a longer time, at increased plasma pressure, or at higher rf power. However, unlike in the case for n-type GaN, oxygen plasma treatment prior to metal deposition deteriorated the electrical properties of the Ni/Au contacts to p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in order to help elucidate the mechanism behind these effects. # 2004 Elsevier B.V. All rights reserved. 1. Introduction High quality ohmic contacts are required for appli- cations in GaN-based optoelectronic and electronic devices. According to the literature, a specific contact resistivity (r c ) as low as 10 6 O cm 2 can be readily achieved by using Ti-based metallisation to n-type GaN [1]. The value of r c is further reduced after a chlorine-based reactive ion etching (RIE) treatment prior to contact deposition. The contact improvement has been explained as the result of preferential sput- tering of nitrogen from the GaN surface, and because nitrogen vacancies (V N ) act as donors in GaN, the sputtering process results in the surface becoming highly n-type [2–5]. The same RIE treatment deterio- rated the contact performance for p-type GaN, which would support the argument of V N formation [3,4,6]. However, Chen et al. performed a similar chlorine- based RIE treatment on n-type GaN and observed a deterioration in the contact characteristics, which was attributed to lattice disordering at the surface [7]. In this work, we demonstrate that excellent ohmic Ti/Al contacts to n-type GaN can be achieved by treating the surface with oxygen plasma prior to contact deposition and a rapid thermal annealing (RTA) treatment post-deposition. We investigated the effects of treatment time, rf power and plasma pressure on the contacts to n- and p-type GaN. X-ray photoelectron spectroscopy (XPS) was used in an attempt to understand the mechanism underlying the change in contact resistance which occurs follow- ing the plasma treatment. Applied Surface Science 234 (2004) 328–332 * Corresponding author. Tel.: þ44 1223 334404. E-mail address: jy223@cam.ac.uk (J. Yan). 0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.066