# Improvement of Porous GaN-Based UV Photodetector with Graphene Cladding
## 4 Discussions
Compare to Figure 4b, an additional peak \(\sim\)1460 cm\({}^{-1}\) was attributed to the second-order A\({}_{1}\)(LO) mode of GaN [45]. It was noted that the I\({}_{\text{D}}\)/I\({}_{\text{G}}\) ratios for both samples have increased relative to that without GaN. By using the Tuinstra-Koenig relation, the L\({}_{\text{D}}\) for both EC- and CVD-graphene cladding can be determined, i.e., 8.5 and 7.4 nm, respectively. The decrease of L\({}_{\text{D}}\) was expected as it signified the presence of interfacial interaction between graphene and porous GaN. Graphene in general acted as a topology insulator, where charge conduction performed better within the basal plane. This leaves the outer \(\pi\) clouds uninterrupted. As the porous GaN surface was resistive due to losses of carriers through PEC etching, having the \(\pi\) clouds from the graphene layers resulted in interfacial interaction. This effect was exhibited when electrical characterizations were performed. The analysis of Figure 5a leads to some interesting findings. The current magnitude at the 5 V reference point was shown to decrease according to the photodetectors of as-received GaN, Pr-GaN, EC-GaN, and CG-GaN. While the decrease was attributed to the resistive nature of porous GaN, the graphene claddings in general were electrically conductive, which suggests higher current compare to that of Pr-GaN. However, the contrary occurred. This implies that graphene cladding served other roles as well. One plausible role is the formation of a space-charge region at the graphene/GaN interface, which impedes the flow of charge unless sufficient energy was supplied. When comparing EC- to CG-GaN, the latter shows even lower current. This can be correlated to the distribution of graphene on the porous surface of GaN. CVD-grown graphene possessed continuous domains that provide good surface coverage compared to that of EC-GaN, which in turn led to more interfacial interactions. Another interesting feature is the negative bias region, where the current magnitude was significantly lower for photodetectors with porous morphology. This feature can be attributed to leakage current, where lower magnitude is desirable for photodetectors. From there, the as-received GaN would have a higher leakage current compared to others, i.e., \(-13.7\) mA at \(-5\) V. This was attributed to the carriers in as-received GaN being higher compared to the other samples. Upon undergoing PEC, the leakage current significantly decreased, being \(-70.9\)\(\mu\)A or \(0.52\%\) relative to that of as-received GaN. The inclusion of graphene claddings seemed to contribute lesser changes towards leakage current, with EC- and CVD-graphene cladding contributing to \(-83.5\) and \(-63.6\)\(\mu\)A, respectively. In Figure 5, some changes in current magnitude were observed for photodetectors upon UV light illumination. Interestingly, in the negative bias region, the leakage current seemed to increase for photodetectors with porous morphology, with the exception of CG-GaN having the least leakage current among the others. This substantiates the role of graphene cladding with good coverage to impede the flow of charges in the negative bias region. Although the photodetector exhibited responses towards UV light illumination in the form of an increase in current magnitude, current-transient responses would be more suitable for providing a clearer picture. The variations of sensitivity for various photodetectors as shown in Figure 6 remained intriguing. By comparing as-received and Pr-GaN, it was expected the latter would have higher sensitivity compared to the former due to the significant increase in surface area. Interestingly, upon cladding the porous GaN surface with graphene, the sensitivity was further enhanced. Given the morphological aspects of graphene, the enhanced sensitivity was less likely contributed by an increase in surface area. Based on the earlier results from Figures 4 and 5, interfacial interaction between porous GaN and graphene has led to the formation of a space-charge region.