# AlGaN Ultraviolet Metal-Semiconductor-Metal Photodetectors with Reduced Graphene Oxide Contacts
## 3 Results and Discussion  
Figure 2 shows the high-resolution XRD measurements of the three AlGaN/GaN samples that were used to obtain the accurate Al mole fraction. Peaks corresponding to the lattice parameters of GaN and AlGaN are clearly shown in the figure. An enlarged view of the AlGaN peaks is shown in the inset of Figure 2. The lattice parameters (\(c\)) of the samples were calculated by the Bragg relation, \(c=\frac{\lambda I}{2\sin\theta_{\mathrm{B}}}\), where \(\lambda\) is the wavelength of the X-ray radiation used for the measurement, \(\theta_{\mathrm{B}}\) is the Bragg angle, and \(l\) is the Miller index. The Al mole fractions (\(x\)) of the samples were then calculated by [PERSON]'s rule: \(C_{AlGaN}(x)=xC_{AlN}+(1-x)C_{GaN}-\delta_{c}x(1-x)\), where \(C_{AlN}\) (4.9792 A) and \(C_{GaN}\) (5.1855 A) are the bulk lattice constants of AlN and GaN, respectively, and \(\delta_{\mathrm{c}}\) is the bowing parameter available from previous studies [20; 21; 22]. The Al mole fractions of the three samples were thus confirmed as 9.5%, 13.4%, and 17.9% (henceforth given in decimal form as 0.095, 0.134, and 0.179, respectively). The GO film shows a very high transmittance over a broad wavelength range, as shown in Figure 3. A slight decrease in transmittance occurs after thermal reduction at 800 \({}^{\circ}\)C, and a gradual decrease occurs as the wavelength decreases from 350 nm, which is well matched with the previous report in the literature [23]. Regardless, the transmittances of GO and rGO are >90% even for wavelengths <300 nm. The Raman spectral investigation of the GO film on the AlGaN/GaN substrate showed clearly the primary in-plane vibrational mode of \(sp^{2}\)-bonded carbon atoms (G peak) and the defect-induced breathing mode (D peak) [23]. The samples showed \(I_{\mathrm{D}}/I_{\mathrm{G}}\) peak intensity ratios of <0.8, indicating that the GO layers were successfully formed on the substrates and that defect generation was well suppressed, even during the reduction process. In addition, the sheet resistance of rGO in our study was in the range of 6 \(\times\) 10\({}^{3}\)  
Figure 2: High-resolution XRD measurements of the (0002) planes of AlGaN samples with various Al mole fractions (inset: enlarged view of the AlGaN peaks). Figure 4 shows the current-voltage (_I-V_) characteristics of the rGO/AlGaN MSM photodiodes under dark and under illumination at 330 nm. The measurements are done on MSM photodiodes (the total area of 350 \(\times\) 450 \(\upmu\)m\({}^{2}\)) with the finger length of 200 \(\upmu\)m, width of 10 \(\upmu\)m, and spacing of 10 \(\upmu\)m. Noise-like dark currents of less than 10 pA are measured, indicating that the dark current is below the measurement limit of our measurement system. Under the 330-nm illumination and the bias voltage of \(-\)5.5 V, the photocurrent increases to 24 nA, 58 nA, and 70 nA for the samples with the Al mole fractions of 0.179, 0.134, and 0.095, respectively. The increase of more than three orders of magnitude between the dark current and photocurrent implies a high photo/dark current contrast ratio of the devices. Slight increases in photocurrent with the increase of bias are observed in Figure 4. When defects in the active layer are involved, the trapping of photoexcited carriers might be influenced by the induced bias. The photocurrent under illumination is lower for samples with higher Al mole fractions, namely, in samples with higher energy bandgaps, which is caused by the lower excited carrier concentrations in the samples with higher Al mole fractions. Figure 4: Current–voltage (_I–V_) characteristics of the rGO/AlGaN MSM photodiodes under dark and under 330 nm of illumination. Figure 3: Transmittance spectra of the as-sprayed GO and rGO reduced at 800 \({}^{\circ}\)C.  
Figure 5 shows the spectral photoresponses of the samples with three different Al mole fractions from 300 nm to 410 nm.