## 3 Results and Discussions  
Table 1 provides the electrical properties of the ITO and cosputtered ITO-ZnO films as well as the _n_-GaN epilayer examined from the Hall-effect measurements at room temperature. Both the ITO and cosputtered ITO-ZnO films were _n_-type degenerated semiconductors with high electron concentrations of 2.59 \(\times\) 10\({}^{20}\) cm\({}^{-3}\) and 1.54 \(\times\) 10\({}^{21}\) cm\({}^{-3}\), respectively. In company with the carrier mobility of the cosputtered ITO-ZnO film (10.6 cm\({}^{2}\) V\({}^{-1}\)s\({}^{-1}\)), which was about two-fold greater than that of the ITO film (5.4 cm\({}^{2}\) V\({}^{-1}\)s\({}^{-1}\)), the cosputtered ITO-ZnO film (\(\sim\)3.82 \(\times\) 10\({}^{-4}\)\(\Omega\cdot\)cm) possessed a much lower resistivity than the ITO film (\(\sim\)2.46 \(\times\) 10\({}^{-3}\)\(\Omega\cdot\)cm). Figure 1 shows the optical transmittance spectra of the ITO and cosputtered ITO-ZnO films. The incorporation of the Zn atoms into the ITO film led to the apparent red-shift of the absorption edge in the near UV wavelengths, and the optical energy band gap of the cosputtered ITO-ZnO film was decreased to 3.41 eV, as evaluated from the curves of the (\(ahv\))\({}^{2}\)_versus_ the photon energy (eV) shown in the inset figure [13]. Although the absorption onset of the cosputtered ITO-ZnO film occurred at a longer wavelength, it still had good transparency at the visible wavelengths, with an average transmittance at wavelengths of 400-700 nm of about 84.6% (Table 1), even a little better than that of the ITO film (\(\sim\)81.1%). The _I-V_ curves of the as-deposited ITO and cosputtered ITO-ZnO films' contact to the _n_-GaN epilayer, respectively, measured from the contact spacing of 15 \(\upmu\)m are shown in Figure 2. Both of these contact systems exhibited nonlinear characteristics even though the cosputtered ITO-ZnO film's contact to the _n_-GaN epilayer had a lower series resistance at an applied voltage of 2 V (\(\sim\)485 \(\Omega\)) than the ITO/_n_-GaN contact system (\(\sim\)784 \(\Omega\)). The rectifying properties with significant leakage currents measured from these contact systems occurred because of the large discrepancy in work functions between the transparent electrode and the _n_-GaN epilayer [14]. A post-annealing treatment was conducted on these contact systems for the purpose of making these structures become ohmic contacts. Figure 3a,b, respectively, give the _I-V_ curves of ITO/_n_-GaN and ITO-ZnO/_n_-GaN contact \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & \(n\) **(cm\({}^{-3}\))** & \(\upmu_{\textbf{n}}\) **(cm\({}^{2}\)/V\({}_{\textbf{S}}\))** & \(\rho\) **(\(\Omega\cdot\textbf{cm}\))** & _T\({}_{\textbf{ave}}\) **(\%)** \\ \hline ITO & 2.59 \(\times\) 10\({}^{20}\) & 5.4 & 2.46 \(\times\) 10\({}^{-3}\) & 81.1 \\ ITO-ZnO & 1.54 \(\times\) 10\({}^{21}\) & 10.6 & 3.82 \(\times\) 10\({}^{-4}\) & 84.6 \\ _n_-GaN & 4.59 \(\times\) 10\({}^{18}\) & 156.1 & 8.71 \(\times\) 10\({}^{-3}\) & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electrical properties and average optical transmittance of the ITO and cosputtered ITO-ZnO films as well as the electrical properties of the _n_-gallium nitride (GaN) epilayer. Figure 1: Optical transmittance spectra of the ITO and cosputtered ITO-ZnO films (the corresponded optical energy band gaps determined from the _(ahv)\({}^{2}\)versus_ the photon energy (eV) also are provided in the inset figure). systems processed by a rapid thermal annealing (RTA) treatment at various temperatures for 1 min under vacuum ambient, measured from the contact spacing of 15 \(\upmu\)m. The currents of both of these contact systems were apparently increased as the annealing temperature was increased.