## 3 Results and Discussions
The largest current at a bias of 2 V was obtainable from the 200 \({}^{\circ}\)C-annealed ITO/_n_-GaN contact system, while that of the ITO-ZnO/_n_-GaN contact system occurred at an annealing temperature of 300 \({}^{\circ}\)C. However, although the series resistance of the 300 \({}^{\circ}\)C-annealed ITO-ZnO/_n_-GaN contact system was decreased significantly to about 200 \(\Omega\) at a bias of 2V, its _I-V_ curve still was somewhat insufficient for achieving ohmic contact. In contrast to the ITO-ZnO/_n_-GaN contact system, the as-deposited metallic Ti/Al contact to the _n_-GaN epilayer exhibited a good, linear _I-V_ characteristic without the requirement of a post-annealing process, as shown in Figure 4a. The specific contact resistance derived from the resistance of the Al/Ti/_n_-GaN contact system as a function of the contact spacing in the set figure was approximately \(5.1\times 10^{-5}\)\(\Omega\cdot\)cm\({}^{2}\). The mechanism responsible for the achievement of the nonalloyed ohmic contact behavior using the metallic Ti/Al structure mainly was due to the elemental Ti, which possessed a much lower work function (\(\phi\) = 4.3 eV). This value was only a little higher than the work function of _n_-GaN (\(\phi\) = 4.1 eV) as compared to that of the transparent ITO (\(\phi\) = 5.1 eV) [15; 16]. In addition, since the electron carriers correlated with the nitrogen vacancies (V\({}_{\rm N}\)) at the _n_-GaN surface would be increased as a consequence of the Ti atom, Ti was beneficial for inducing the outdiffusion of the nitrogen atoms at the _n_-GaN surface. Figure 4b shows the elemental distributions over the Ti/_n_-GaN interface conducted from the AES depth profile. The outdiffusion of the N atoms was observed clearly at the Ti/_n_-GaN interface, resulting in the increase in the electron concentration at the _n_-GaN surface associated with large amounts of the V\({}_{\rm N}\) donors [17; 18]. Figure 3: _I-V_ curves of the **(a)** ITO/_n_-GaN and **(b)** ITO-ZnO/_n_-GaN contact systems processed by various annealing temperatures for 1 min under vacuum ambient, measured from the contact spacing of 15 \(\upmu\)m.  
Figure 2: _I-V_ curves of the as-deposited ITO and cosputtered ITO-ZnO films’ contact to _n_-GaN epilayer, respectively, measured from the contact spacing of 15 \(\upmu\)m.  
The above-mentioned structures' contact to the _n_-GaN epilayer demonstrated that the transparent electrodes, such as ITO and cosputtered ITO-ZnO films, have a nonlinear _I-V_ curve even when annealed at an elevated temperature, whereas the opaque Ti/Al system had ohmic contact behavior when contacting the _n_-GaN epilayer without a post-annealing treatment. Therefore, we employed a thin Ti interlayer which was deposited between the interface of ITO-ZnO and _n_-GaN to achieve a true ohmic contact system with optical transparency at the visible wavelengths. Figure 5a shows the _I-V_ characteristics of the ITO-ZnO/Ti/_n_-GaN contact systems as a function of the thickness of the Ti interlayer, measured from the contact spacing of 15 \(\upmu\)m (the _I-V_ curve of the Al/Ti/_n_-GaN contact system also is shown for comparison). The cosputtered ITO-ZnO film directly contacted the _n_-GaN epilayer, which resulted in a nonlinear _I-V_ property that became an ohmic contact behavior when a thin Ti interlayer was deposited prior to the deposition of the cosputtered ITO-ZnO film. The current flow of the nonalloyed ITO-ZnO/Ti/_n_-GaN contact system was increased initially as the thickness of the Ti interlayer increased, but it decreased as the thickness of the Ti interlayer reached 10 nm. It was worth noting that the current of the ITO-ZnO/_n_-GaN contact structure inset by an 8-nm-thick Ti interlayer at a bias of 2 V obviously was higher than that of the metallic Ti/Al structure's contact to the _n_-GaN epilayer. The evolution of the specific contact resistance as a function of the Ti interlayered thickness in the ITO-ZnO/Ti/_n_-GaN contact system is illustrated in Figure 5b.