## 3 Results and Discussion
However, some micro-tears between the gap of source and gate of Device 2 in Figure 5c may be observed, thus resulting in a slightly lower current readout than Device 3. Traces A, B, and C in Figure 5b,d,f correspond to Raman spectra data of graphene collected in the regions on top of the gate, at the edge of the gate electrode, and in the gap between the source and gate electrode, respectively. The graphene supplied by Graphene Inc., Cambridge, MA, USA. was inspected prior to transfer and showed a comparable Raman spectrum with Trace C confirming a monolayer of graphene as shown in Figure S2. The ratio of D (\(\sim\)1350 cm\({}^{-1}\)) and G (\(\sim\)1600 cm\({}^{-1}\)) peaks provides information on the state of disorder in the graphene crystal lattice [63]. A high D peak can be easily observed in Traces A and B in comparison to Trace C, indicating an increase in defect density wherever the Al is present. The increased defect density in these areas correlates to areas with rougher surfaces as observed with SEM in all three devices. In addition, the reduction in the 2D peak (\(\sim\)2650 cm\({}^{-1}\)) with respect to the G peak on the regions over the Al/Al\({}_{2}\)O\({}_{3}\) (Trace A) indicates that the quality of the monolayer nature of the graphene is lost when compared to the regions away from the Al surfaces (Trace A) and even at the step edge (Trace B), as the Raman resonance may be influenced by the rougher surface. Further analysis of the defect density and separation is presented in the Supplementary Materials [64; 65]. Figure 4: (**a**) Top view of a general GFET structure indicating the electrodes, gate length (Lgate) and width (Wchannel), and channel gap (Gaps\({}_{\text{S}}\)/D to Gate). (**b**) Overlay of I\({}_{\text{DS}}\)–V\({}_{\text{GS}}\) of 24 identical devices. V\({}_{\text{DS}}\) = 1 V for all these tests. Devices 1, 2, and 3 were selected for further analysis as representative of low, medium, and high current, respectively. (**c**) Cross-section of GFET channel structure where graphene (black) goes over the gate electrode step. Atomic force microscopy (AFM) results are shown in Figure 6. These measurements were taken on the Al gate electrode (Figure 6a), at the gap between source/drain to gate (Figure 6b), and in an area without graphene (Figure 6c) for a surface roughness analysis comparison. The surface roughness \(\mathrm{R_{a}}\) was 16.19 nm for graphene on the Al/Al\({}_{2}\)O\({}_{3}\) gate electrode and 4.06 nm for the gate-to-source gap on the Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) surface. Additional AFM on the Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) surface without graphene yielded an R\({}_{a}\) value of 1.06 nm. Thus, AFM analysis showed that the Al\({}_{2}\)O\({}_{3}\) layer was not the cause of roughness on the aluminum gate electrode. With the Al gate electrode taking up more than 80% of the channel area, it is evident that the mobility of the devices may be heavily degraded due to the rough Al surface. Such results are in agreement with published work showing a decrease in mobility from 10\({}^{6}\) cm\({}^{2}\)/V\(\cdot\)s to 10\({}^{3}\) cm\({}^{2}\)/V\(\cdot\)s with just a modest increase in substrate surface roughness amplitude from 0.25 nm to 0.3 nm [66]. Figure 5: SEM pictures of fabricated GFETs, and Raman spectroscopic measurements of graphene at each indicated point (A, B, and C) in the fabricated devices for **(a,b)** Device 1 with a low current level of 200 \(\mu\)A (**c,d**) Device 2 with a medium current level of 310 \(\mu\)A, and (**e,f**) Device 3 with a high current level of 379 \(\mu\)A; all devices had V\({}_{\mathrm{GS}}=-10\) V and V\({}_{\mathrm{DS}}=1\) V.  
SEM, AFM, and Raman analyses show that the surface of the aluminum gate electrode is not smooth and presents hillocks and irregularities when compared to the other material surfaces on the devices. In addition, the edge of the aluminum gate electrodes is rough, and the graphene layer does not appear to conform to this edge, resulting in wrinkles and/or a gap in some regions for all three devices.