# Improvement of Porous GaN-Based UV Photodetector with Graphene Cladding
## 3 Results  
Figure 2 shows the morphologies of as-received GaN, Figure 2a, and porous GaN, Figure 2b. Prior to PEC etching, the as-received GaN exhibits a smooth morphology. A porous morphology was obtained upon PEC etching, which is exhibited as dark spots that can be seen in Figure 2b,c. In the latter figure, the magnified view enabled some quantitative analysis, i.e., determining pore diameter to be conducted. Through image processing employing Image J, the statistical data about the pore diameter were obtained and are shown in Figure 2d. In that figure, the average pore diameter was revealed to be 35 nm. Figure 3 shows the various types of graphene claddings on porous GaN, and the combination of both also served as the UV sensing layer. In Figure 3a, the area with and without graphene cladding can be seen, with the latter exhibiting a darker contrast. Between CVD-grown and EC-exfoliated graphene, the former shows a continuous film with minor wrinkles (in Figure 3b) that were ascribed to folding when transferring the film onto porous GaN. The latter, as shown in Figure 3c, was exhibited as micron-sized flakes, which showed discontinuity at various areas, and otherwise overlapped onto one another, which can be seen in the yellow circle in Figure 3d. Figure 4 shows the compilation of Raman spectra for various samples. In Figure 4a, Raman spectra between the as-received and porous GaN are displayed. From there, both spectra exhibit identical peak positions at 569 and 737 cm\({}^{-1}\), which attribute to the E\({}_{2}\)(HI) and A\({}_{1}\)(LO) mode of GaN [43]. It should be noted that the GaN is \(c\)-oriented and the Raman configuration adopted was \(z(x,\text{ unpolarized})\overline{z}\); hence, such modes would be allowed. Upon PEC etching, additional modes were revealed. These modes were deemed forbidden under this Raman configuration; however, their existence also infers the successful formation of a porous network, where Raman scattering would take place along the pore walls, which consisted of non-\(c\)-oriented facets. The additional modes detected  
Figure 2: Electron micrographs of (**a**) as-received GaN, (**b**) porous GaN, (**c**) magnified view of porous GaN, and (**d**) pore diameter distribution of porous GaN. were centred around 533 and 561 cm\({}^{-1}\), which were ascribed to the A\({}_{1}\)(TO) and E\({}_{1}\)(TO) modes of GaN, respectively [43]. The Raman spectra of CVD-grown and EC--exfoliated graphene are presented in Figure 4b. Common peaks attributed to graphene can be observed around 1530 and 1589 cm\({}^{-1}\), which are assigned to the D and G bands, respectively [44]. On the other hand, Figure 4c shows the Raman spectra of graphene cladding on the porous GaN surface. Compare to Figure 4b, an additional peak at \(\sim\)1460 cm\({}^{-1}\) attributed to the second-order  
Figure 4: Raman spectra of (**a**) GaN, (**b**) graphene, and (**c**) graphene cladded on porous GaN. Figure 3: Electron micrographs of various graphene claddings (**a**) CVD-grown, showing contrast difference between area with and without graphene cladding; (**b**) CVD-grown, exhibiting continuous film; (**c**) micron-sized EC-exfoliated graphene flakes; (**d**) magnified view of EC-exfoliated graphene flakes. A\({}_{1}\)(LO) mode of GaN was spotted [45]. While no significant peak shift occurred in the D and G peaks relative to that in Figure 4b, the intensity ratio, I\({}_{\text{D}}\)/I\({}_{\text{G}}\), changed. Meanwhile, the 2D bands remained lower than the G bands for both samples. This feature is ascribed to the presence of multilayer graphene. Figure 5 shows the current density-voltage (J-V) curves of all four UV photodetectors taken under dark (Figure 5a) and UV (Figure 5b) conditions. Rectifying behavior was mostly observed in mostly all photodetectors, with the as-received GaN being less significant. From there, photodetectors consisting of porous morphology clearly exhibited lower current compared to that of as-received GaN.