# Vertical-Type Ni/GaN UV Photodetectors Fabricated on Free-Standing GaN Substrates
## 3 Results and Discussion
As can be seen, the electrical current drops by 4 orders of magnitude within 0.3 s (the limitation of the measurement system) once the UV light is mechanically turned off, indicating a response time of much lower than 0.3 s. It is noted that a slow component with a much lower amplitude is observed during the decay process. This means a very weak persistent photoconductivity (PPC) still exist after the dark current goes to the relative steady state value. The PPC is probably originated from the defects at the interface of Ni/GaN. The slow components in the time response property can be quantitatively analyzed by the bi-exponential relaxation equation, which is expressed by [21]:  
\[J=J_{s}\left[exp\left(\frac{q(V-IR)}{nkT}\right)-1\right] \tag{1}\]2  
where \(I_{0}\) is the initial dark current in the slow components, \(t\) the time. \(A\) and \(B\) are fitting constants. \(\tau_{1}\) and \(\tau_{2}\) are relaxation time constants, corresponding to the shallow and deep traps, respectively. Figure 3c-f depict the slow components in PPC fitted by the above equation. The dependence of the relaxation time constants on the bias voltage is extracted in Figure 3b. For the slow component (I), the relaxation time (\(\tau_{1}\)) shows a slight reduction with the bias voltage increasing, indicating a faster electron capture speed by the shallow traps at a higher bias voltage. On the other hand, the relaxation time \(\tau_{2}\) exhibits an increasing trend with the applied voltage increasing from 0 to 5 V. The longer relaxation time (\(\tau_{2}\)) is suggested to be derived from the deep traps, which are excited under the high electric field, resulting in a longer relaxation time to fill in the deep traps after the UV light is turned off [4]. The same structured Schottky photodetectors fabricated on the GaN layers with different growth rates from 2.61, 4.72, to 7.78 um/h (named as device-A, C and D) are further investigated and the performances are compared in \[J=J_{s}\left[exp\left(\frac{q(V-IR)}{nkT}\right)-1\right] \tag{1}\]3 The device-A fabricated on the GaN epilayer with the lowest growth rate shows the best characteristics, including the highest specific detectivity, highest signal-to-noise ratio and shortest relaxation time. These results indicate the relatively lower growth rate for the GaN layer is preferred for the Schottky photodetector to achieve a high responsivity and spectral selectivity [11]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Detector.** & **Responsivity** & **Specific Detectivity** & \multirow{2}{*}{\(R_{350}/R_{630}\)} & \multirow{2}{*}{\(I_{350}/I_{dark}\)} & **Relaxation Time** \\ \cline{5-5}  & **(A/W)** & & & & \(\tau_{1}\) **(s)** & \(\tau_{2}\) **(s)** \\ \hline A & 0.10 & \(1.47\times 10^{12}\) & 1492 & 8885 & 1.07 & 11.20 \\ C & 0.10 & \(6.19\times 10^{11}\) & 1659 & 1542 & 1.33 & 13.59 \\ D & 0.20 & \(1.19\times 10^{11}\) & 891 & 30.37 & 1.59 & 20.9 \\ \hline \end{tabular}
\end{table} 
Table 2: The key detector parameters extracted from detector A, C and D. The responsivity, specific detectivity and relaxation time are obtained at \(-\)3 V under the 350 nm illumination. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Detector.** & **\(R\) @\(-\)5 V (A/W)** & **Peak D\({}^{*}\) (Jones)** & \(R_{350}/R_{630}\) @\(-\)5 V** & **Peak QE (\%)** \\ \hline Device-A & 0.16 & \(1.47\times 10^{12}\) & 1530 & 56.4 \\ R1\({}^{[16]}\) & 0.15 & \(\sim\)10\({}^{10}\) & \(\sim\)1000 & 51.8 \\ R2\({}^{[18]}\) & \(\sim\)0.11 & \(2.96\times 10^{9}\) & \(\sim\)100 & 50.0 \\ \hline \end{tabular}
\end{table} 
Table 1: A comparison of GaN UV detectors from present study with reports from the literature.