# Effects of 450 MeV Kr Swift Heavy Ion Irradiation on GaN-Based Terahertz Schottky Barrier Diodes
## 3 Results and Discussion
### The Analysis of Photoluminescence  
It is well known that the \(E_{T}\) of GaN material is verified by the yellow luminescence (YL) in PL spectra [41; 42; 43]. To further investigate the generation of traps in GaN-based THz SBDs induced by Kr SHI irradiation, PL spectra were measured, as shown in Figure 7. The PL spectra of GaN epitaxial wafers were normalized to the peak intensity of the GaN bandgap (3.4 eV), and the wafers exhibited the same PL spectra both before and after Kr SHI irradiation. However, the PL intensity of YL decreases with increasing Kr SHI irradiation fluence, indicating that defects are introduced by the Kr SHI irradiation.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Fluence (Ions/cm\({}^{2}\))** & **Pristine** & \(\mathbf{5\times 10^{6}}\) & \(\mathbf{1\times 10^{7}}\) & \(\mathbf{2.5\times 10^{7}}\) \\ \hline A (\(\times 10^{-13}\)) & 250 & 113 & 42 & 17 \\ \(\tau\) (ms) & 46.7 & 43.6 & 42.9 & 37.4 \\ B (\(\times 10^{-13}\)) & 8.0 & 3.9 & 2.5 & 0.35 \\ \(E_{T}\) (eV) & 0.627 & 0.625 & 0.624 & 0.620 \\ \hline \end{tabular}
\end{table}
Table 2: Extracted parameters for GaN-based THz SBDs with and without Kr SHI irradiation using Equation (4).  
Figure 6: (**a**) The LFN spectra of GaN-based THz SBDs at 0.8 V. (**b**) Current dependences of LFN for GaN-based THz SBDs at 10 Hz.  
The PL spectrum of GaN material includes multiple emission peaks, such as the red luminescence (RL) peak at 1.93 eV and the YL peak at 2.2 eV. To better investigate the effect of Kr SHI irradiation fluence on the PL characteristics of GaN material, two Gaussian functions were employed to fit the PL spectra of the GaN epitaxial layers both before and after Kr SHI irradiation, as presented in Figure 8. The RL and YL emissions involve a possible transition mechanism associated with the recombination of donor-acceptor pairs during trap energy-level transitions [44; 45]. As shown in Figure 8a, unirradiated GaN epitaxial wafer exhibits RL and YL spectra, which are attributable to inherent defects within the GaN material. As illustrated in Figure 8b-d, the intensity of RL and YL increases with the rising Kr SHI irradiation fluence, which is attributed to the defect formation caused by irradiation damage.  
The peak parameters of the RL and YL spectra are summarized in Table 3. The area ratio is defined as the ratio of the integrated intensity of defect band emission to that of bandgap emission. With the increasing Kr SHI irradiation fluence, the area ratio of the RL and PL emission bands increases, correlating with the observed rise in trap states in the LFN spectra. Combined with the studies about the impact of SHI irradiation on GaN under different irradiation fluence [25; 26; 27; 30; 33; 36], these results further demonstrate the significant influence of SHI irradiation on the surface properties of GaN epitaxial layers, which can ultimately impact the performance of a device.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Emission Band** & **Photon Energy (eV)** & **Area Ratio (\%)** & **Intensity (a. u.)** \\ \hline \multirow{2}{*}{Pristine} & RL & 1.90 & 10.8 & 0.024 \\  & YL & 2.20 & 26.6 & 0.059 \\ \(5\times 10^{6}\) ions/cm\({}^{2}\) & RL & 1.93 & 11.9 & 0.028 \\ \(1\times 10^{7}\) ions/cm\({}^{2}\) & PL & 2.21 & 28.8 & 0.065 \\ \(1\times 10^{7}\) ions/cm\({}^{2}\) & RL & 1.92 & 13.1 & 0.030 \\ \(2.5\times 10^{7}\) ions/cm\({}^{2}\) & YL & 2.21 & 30.1 & 0.068 \\ \(2.5\times 10^{7}\) ions/cm\({}^{2}\) & RL & 1.93 & 15.8 & 0.036 \\ \(2.21\times 10^{7}\) ions/cm\({}^{2}\) & YL & 2.21 & 35.3 & 0.080 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The parameters of RL and PL emission bands for the PL spectra in Figure 8.  
Figure 7: PL spectra of GaN-based THz SBDs with and without Kr SHI irradiation.