## 1 Introduction  
Group III-nitride semiconductors are crucial materials for the fabrication of optoelectronic devices [1; 2]. Among them, gallium nitride (GaN) has been the most attractive material during the past decades due to its mature fabrication using epitaxial and doping technologies. One of the important requirements for developing high-quality, GaN-based optoelectronic devices is the formation of ohmic contact with low specific contact resistance and/or Schottky contact with quality rectifying behavior. In regard to the material's contact to GaN, opaque or semitransparent single, bilayer, and multi-metal structural layers have been used extensively as ohmic contact electrodes in contact with GaN. Moreover, since the _n_-GaN epilayer has the advantage of a high carrier concentration, which is beneficial for current flow tunneling the space charge region, and a low work function of about 4.1 eV, many common metals, such as Ti, Al, In, Cr, and their combined structures have been reported to be good contact electrodes [3; 4; 5]. However, although a very low contact resistance was achieved from the above-mentioned electrodes, the resulting optoelectronic devices have very low external quantum efficiency because most of the light that is transmitted is blocked by these electrodes. Accordingly, the development of a transparent conductive oxide (TCO) film for ohmic contact to the _n_-GaN was essential to improve the performance of the device by enhancing the optical transmittance as well as spreading the current transmission. To date, only a few papers have reported the preparation of the TCO electrode contact to _n_-GaN and the structures showed ohmic contact behavior only through an additional process on the contact's structure or using a highly-doped _n_-GaN epilayer [6; 7; 8; 9]. In addition, some researchers have used a thin Ti or In interlayer inset between the TCO/_n_-GaN interface to provide a nonalloyed ohmic contact system [10; 11]. For example, [PERSON] _et al._ reported that ohmic contact behavior with a specific contact resistance of \(5.1\times 10^{-4}\)\(\Omega\cdot\)cm\({}^{2}\) was only obtained when the ITO contacted a highly doped _n_-GaN (\(n=1\times 10^{19}\) cm\({}^{-3}\)) [6]. [PERSON] _et al._ demonstrated an ITO/ZnO/_n_-GaN ohmic contact system with a specific contact resistance of \(3.0\times 10^{-4}\)\(\Omega\cdot\)cm\({}^{2}\) after annealing at 500 \({}^{\circ}\)C in H\({}_{2}\) ambient for 5 min [7]. [PERSON] _et al._ proposed a nonalloyed Ti/ITO system ohmic contact to a plasma treatment _n_-GaN surface with a specific contact resistance of \(3.2\times 10^{-6}\)\(\Omega\cdot\)cm\({}^{2}\)[8]. [PERSON] _et al._ used a thin In interlayer inset between the ITO/N-face _n_-GaN interface to realize a specific contact resistance of \(1.8\times 10^{-5}\)\(\Omega\cdot\)cm\({}^{2}\) after annealing at 300 \({}^{\circ}\)C in N\({}_{2}\) ambient for 60 s [10]. [PERSON] _et al._ used a thin Ti interlayer to optimize the ITO/_n_+GaN ohmic contact behavior with a specific contact resistance of \(4.2\times 10^{-6}\)\(\Omega\cdot\)cm\({}^{2}\) after annealing at 600 \({}^{\circ}\)C in N\({}_{2}\) ambient for 5 min [11].  
In this work, a cosputtered indium tin oxide (ITO)-zinc oxide (ZnO) film, which had a much lower resistivity than the ITO film (as described elsewhere [12]), was deposited onto the _n_-GaN epilayer. In order to produce the nonalloyed ohmic contact system, a thin interlayer of Ti was inset between the cosputtered ITO-ZnO film and the _n_-GaN epilayer. The thickness of the Ti interlayer in the Ti/ITO-ZnO system affected the optical transmittance, and the resulting specific contact resistance was systematically investigated. An optimal ITO-ZnO/Ti/_n_-GaN contact structure was obtained by considering both the behavior of the ohmic contact and the optical transmittance.