# Improvement of Porous GaN-Based UV Photodetector with Graphene Cladding
## 3 Results
This was ascribed to the mechanism of porous GaN formation, where doped regions were selectively etched. Consequently, this resulted in a higher resistance, which translates to a significant decrease in current upon biasing. Figure 6 shows the sensitivity-transient profile of all four UV photodetectors. The sensitivity, \(S\), can be evaluated using the following formula:  
\[S=\frac{I_{UV}-I_{dark}}{I_{dark}} \tag{1}\]  
where \(S\) is sensitivity, \(I_{UV}\) is the current measured under UV illumination, and \(I_{dark}\) is the current measured under a dark condition. The bias voltage was set to 5 V for all measurements. Among the four photodetectors, CG-GaN gave the highest sensitivity, about 4.6, followed by that of EC-GaN (-4.2), then Pr-GaN (-0.95), and finally as-received GaN (-0.01). Following Equation (1), the responsivity, \(R_{I}\), and external quantum efficiency (EQE) of the sensors can be determined using the following formulae [22]:  
\[R_{I}=\frac{I_{ph}}{P} \tag{2}\]  
\[EQE=\frac{hc}{e\lambda}R_{I} \tag{3}\]  
Figure 5: J–V characteristics of UV photodetectors operating under (**a**) dark and (**b**) UV ambient. where \(I_{ph}\) is the photocurrent, \(I_{ph}=I_{UV}-I_{dark}\); \(P\) is the incident light power impinged on the sensor's surface; \(h\) is the Planck constant; \(c\) is the speed of light; \(e\) is the electronic charge; and \(\lambda\) is the incident wavelength. Since both responsivity and EQE are dependent on the magnitude of photogenerated current, they scaled accordingly to that of sensitivity. The as-received GaN possessed the lowest responsivity of about 0.066 A/W, with 23% EQE. Both of these parameters experienced a significant increase when the morphology of GaN was made porous, with responsivity and EQE of 0.59 and 206% respectively. By cladding the porous GaN surface with graphene, improved responsivity and EQE was noted; however, when comparing among the types of graphene claddings, their differences were lesser. The highest recorded responsivity and EQE in this work were 1.23 A/W and 430%, respectively, when CVD-grown graphene was used as cladding. From Figure 6, besides the responsivity and EQE, the response and recovery time of each photodetector can also be estimated. The response and recovery time can be defined as the time taken for the sensor sensitivity to rise from 0 to 63.2% and from 100 to 32.8%, respectively. To summarize the obtained results, they were tabulated in Table 2.  
The as-received GaN exhibited the greatest response time, about 3.3 s, compared to those with porous morphology. This is ascribed to the separation of photogenerated carriers occurred at the surface, where the higher surface area in Pr-GaN would be advantageous \begin{table}
\begin{tabular}{c c c c c c} \hline
**Type of Sample/** & **Sensitivity** & **Response** & **Recovery** & **Respusivity** & **EQE (\%)** \\
**Nomenclature** & & **Time (s)** & **Time (s)** & **(A/W)** & **EQE (\%)** \\ \hline As-received & 0.01 & 3.3 & 1.8 & 0.066 & 23 \\ Porous GaN/ (Pr-GaN) & 0.95 & 0.9 & 7.8 & 0.59 & 206 \\ EC-graphene-GaN/EC-GaN & 4.2 & 2.6 & 4.8 & 1.11 & 390 \\ CVD-graphene-GaN/CG-GaN & 4.6 & 1.2 & 8.7 & 1.23 & 430 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of photodetector parameters in this work. Figure 6: Sensitivity-transient profile of UV photodetectors. The sensitivity for the as-received UV photodetector was multiplied by 20 times for better clarity. The applied biased voltage used in this measurement was set to 5 V.  
in producing more photocurrent, providing a more responsive response. The addition of graphene cladding raises the response time, with EC- and CG-GaN having response times of 1.2 and 2.6 s, respectively. Meanwhile, the recovery times for Pr-, CG-, and EC-GaN remained higher than their response time, varying from 4.8 to 8.7 s.