# Improvement of Porous GaN-Based UV Photodetector with Graphene Cladding
## 4 Discussions
This region is not only better at impeding charges flow in the reverse bias condition but also provides a shorter path length for charge transportation upon UV illumination. This would translate into higher sensitivity for such sensors. Meanwhile, when comparing both CG- and EC-GaN photodetectors, their difference in sensitivity was less notable. However, CG-GaN would retain a higher sensitivity compared to EC-GaN, which can be attributed to the distribution of graphene cladding. CVD-grown graphene is regarded as a continuous film, hence providing better cladding compared to that of electrochemical exfoliation, which requires control over surface tension in order to aggregate graphene flakes that could still provide small voids upon successful SIT deposition. The response time for the photodetectors was reduced when made into porous morphology, additionally cladded with graphene. The rise can be ascribed to the additional time required to overcome the space-charge region between graphene and the porous GaN surface. Meanwhile, the slight difference between EC- and CG-GaN was likely due to the coverage of graphene on porous GaN. Since CVD-grown graphene provides better coverage, this also inferred more space-charge regions between interfaces. Despite the improved response time, the recovery time behaved otherwise. The increase in recovery time was attributed to the resistive nature of the photodetectors. While being responsive towards photogenerated carriers, the photodetectors have become more resistive, while the presence of space-charge regions in graphene-cladded photodetectors has to be taken into account. This inferred more time required to dissipate the remaining photogenerated carriers when sensing the ambient changes from UV light illumination to dark. Based on the obtained experiment results, the sensing mechanism of the UV sensor in this work was proposed and translated figuratively in Figure 7. Since the UV sensors require an external bias to operate, they can be regarded as operating under a photoconductive mode. For as-received GaN, as shown in Figure 7a, two metal contacts consisting of conductive silver paint were applied on both ends of the sample. The exposed surface would serve as an active layer where photogenerated carriers would be induced upon UV light illumination. Since GaN is capable of absorbing UV light owing to its wide bandgap, photogenerated carriers would preferably form near the surface. This also implies the number of photogenerated carriers would be lesser compared to the other sensors in this work; hence, lower sensing would be expected. The J-V characteristics of this sensor were almost linear, with little signs of rectifying behaviour with large leakage current. This was likely due to the symmetrical placement of the silver contacts on the GaN surface; hence, charge flow was possible in either direction. In addition, silver has a lower metal work function; hence, the barrier between silver/GaN was less notable, enabling charge flow with lesser resistance. For Pr-GaN, the surface of GaN has been etched to reveal a porous morphology. This has significantly increased its surface area, which leads to higher absorption of UV light along the pore channels. Consequently, the additional photogenerated carriers would lead  
Figure 7: Suggested sensing mechanism of the UV photodetectors based on (**a**) as-received GaN, (**b**) Pr-GaN, (**c**) both EC- and CG-GaN, and (**d**) band diagram illustrating the conduction of charges at the graphene/porous GaN heterojunction. to higher sensitivity. The J-V characteristics for this sensor exhibit discernible rectifying behaviour. This was likely due to the surface passivation of porous GaN, where a rather thin layer of oxide formed on the surface of GaN. The passivated layer would induce a potential barrier between the porous GaN interface, trapping photogenerated carriers until sufficient energy was supplied to properly conduct them. Upon introducing graphene cladding to porous GaN, an additional mechanism was included.