Journal of Alloys and Compounds 516 (2012) 38–40 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds j ourna l ho me pag e: www.elsevier.com/locate/jallcom Non-alloyed Cr/Au Ohmic contacts to N-face and Ga-face n-GaN Liang-Jyi Yan a , Cheng Huang Kuo b , Jinn-Kong Sheu a,c,∗ , Ming-Lun Lee c,d , Wei-Chun Tseng a a Department of Photonics and Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan b Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan County 71150, Taiwan c Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan d Department of Electro-Optical Engineering, Southern Taiwan University Tainan County 71001, Taiwan a r t i c l e i n f o Article history: Received 16 August 2011 Received in revised form 21 November 2011 Accepted 24 November 2011 Available online 2 December 2011 Keywords: GaN Cr–Au Nonalloyed Ohmic a b s t r a c t Non-alloyed Cr/Au Ohmic contacts on N-face and Ga-face n-GaN were studied. The specific contact resis- tances ( c ) of Cr/Au contacts onto the N-face and Ga-face n-GaN were as low as 2.4 × 10 -4  cm 2 and 2.4 × 10 -5  cm 2 , respectively. Native oxide formed on the n-GaN surface was believed to be the key factor for higher  c . The results of X-ray photoelectron spectroscopy confirmed that n-GaN samples with different surface polarities or treated by different chemical solutions exhibited significant differences in gallium oxide content on the surface, which led to a marked difference in the  c of non-alloyed Cr/Au Ohmic contacts to GaN films. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Recently, InGaN/GaN light-emitting diodes (LEDs) have been widely applied to traffic signs, backlighting of liquid crystal dis- plays, and general lighting. White LEDs used as general lighting sources are operated at relatively high power compared with LEDs used in backlighting or traffic signs. At present, the efficiency of GaN LEDs is still not perfect. High-power operation means mass heat generation in LEDs. Therefore, thermal management becomes a critical point in high-power white LEDs. However, conventional GaN/sapphire-based LEDs may be not suitable for next-generation general lighting due to the poor thermal conductivity of the sap- phire substrate. Vertical GaN-based LEDs, whose sapphire substrate was removed by laser lift-off (LLO) method and whose epitax- ial layer was bonded to a good thermal and electrical conductive substrate such as Si and Cu, have demonstrated high-power oper- ation characteristics [1–3]. Generally, III-nitride semiconductors grown by metal organic vapor-phase epitaxy on sapphire substrate with low-temperature-grown GaN or AlN buffer layers exhibit Ga- face surface polarity [4]. Therefore, for vertical LEDs, GaN surface with N-face polarity is created after the removal of the sapphire ∗ Corresponding author at: Department of Photonics and Advanced Optoelec- tronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan. Tel.: +886 6 2758021; fax: +886 6 2758021. E-mail addresses: jksheu@mail.ncku.edu.tw (J.-K. Sheu), minglun@mail.stut.edu.tw (M.-L. Lee). substrate using the LLO process. This means the n-type electrode contacts in the vertical GaN LEDs are formed on the N-face GaN surface. Metal contacts on Ga-face n-GaN surface have been widely studied in the past decade. Different metals, such as Ti/Al- and Cr/Au-based metals, have been proven to exhibit good Ohmic con- tact property and stability when deposited on the Ga-face n-GaN surface [5]. The Ti/Al-based contacts are the most popular metal schemes for n-GaN due to their low work function. However, they do not alleviate the need for high annealing temperatures to form intermetallic alloys further with low work function at the metal/semiconductor interface [5]. The effect of thermal annealing on the electrical property of metal contacts was different between the Ti/Al- and Cr/Au-based contacts on the Ga-face n-GaN sur- face [5]. Several reports have recently revealed that Al-based metal contacts could form low-resistivity Ohmic contacts on the N-face n-GaN surface [6,7]. However, there is still a dearth of studies on Cr/Au-based metals on the N-face n-GaN surface. As the chemical activity of N-face GaN surface was considered to be relatively higher than that of Ga-face GaN surface, the characteristics of metal con- tacts deposited on the Ga-face GaN surface would be substantially different from the contacts on the N-face n-GaN surface. In this study, we performed the non-alloyed Cr/Au-based Ohmic contacts on Ga-face and N-face GaN surfaces to elucidate the properties of both contact schemes. 2. Experiment In this study, Si-doped n-type GaN(n-GaN) epitaxial films with carrier concen- tration of 1 × 10 19 /cm 3 were grown by MOVPE on sapphire substrates. The surface 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.11.106