# High-Temperature Annealing Effects on Atomically Thin Tungsten Diselenide Field-Effect Transistor
## 3 Results and Discussion  
Figure 1b illustrates the Raman spectrum of the device taken by a 532 nm laser. The signature WSe\({}_{2}\) Raman peaks, E\({}^{1}_{2}\)g and A\({}_{1g}\), can be seen at 252 cm\({}^{-1}\) and 261 cm\({}^{-1}\), respectively [37, 38]. The presence of these two peaks not only confirms the material to be WSe\({}_{2}\) but also indicates that the flake thickness is more than four layers [22, 23]. The Rama n spectroscopy was performed after annealing at 673 K as well. The comparison of the Raman spectrum before and after annealing is discussed in the later part. In order to accurately determine the WSe\({}_{2}\) thickness, atomic force microscopy (AFM) was carried out. Figure 1d presents the AFM image of the WSe\({}_{2}\) flake as well as the line profile between the two points shown in Figure 1d. It can be seen that the flake thickness is 5 nm, which corresponds to eight layers of WSe\({}_{2}\). The transfer characteristics curves of the device at a constant source-drain voltage of 1 V measured at 300 K after annealing at different temperatures is shown in Figure 2. The influence of annealing on the device properties was systematically investigated by annealing the device at a target temperature for 30 min in a Nitrogen environment and then measuring its transfer characteristics curve at 300 K. The process was repeated for temperatures ranging from 310 K to 670 K with a step size of 10 K. In Figure 2, however, for better visibility, transfer curves are shown for selected annealing temperatures only. It can be seen that the device exhibits ambipolar behavior with hole-dominated transport for negative gate voltages and electron-dominated transport for positive gate voltages. After annealing at 310 K, the electron current is almost an order of magnitude higher than the hole current. In most studies, WSe\({}_{2}\) exhibit p-dominant transport; however, in the present study, n-dominant transport can be attributed to the Ti contacts. As shown in the schematic band diagram in Figure 2d, Ti has a work function of 4.3 eV, which is closer to the conduction band of WSe\({}_{2}\)[39, 40]. Therefore, Ti can facilitate the injection of electrons in the conduction band of WSe\({}_{2}\) by forming good Ohmic-like contacts. At the same time, Ti forms a large Schottky barrier with the valence band of WSe\({}_{2}\). This Schottky barrier results in electron transport being more prominent than the hole current. The inset of Figure 2a represents the output curve (\(I_{d}\)-\(V_{d}\)) of the WSe\({}_{2}\) device, and Ohmic-like linear characteristics can be seen there. For the positive gate region (electron transport), as the annealing temperature is increased up to 350 K, an increase in the drain current as well as on/off ratio (ratio of on current to the off current of the device) is observed. As annealing temperature increases from 300 K to 350 K, the on/off ratio increases from 8000 to 22,000, whereas maximum on current increases from 14 \(\mu\)A to 37 \(\mu\)A. This observation of enhanced device performance after annealing is consistent with the previous studies in the literature [41]. The annealing improves the device's performance by removing the contaminants, adsorbed moisture and chemical residues from the surface of WSe\({}_{2}\)[41]. Two-dimensional materials possess a high surface-to-volume ratio and are extremely sensitive to the changes at the interface; therefore, annealing can drastically improve the device's performance. However, as the annealing temperature is increased beyond 350 K, the device performance gradually deteriorates. Overall, a decrease in device current as well as on/off ratio can be seen when the annealing temperature exceeds 350 K. The maximum current reduces from 37 \(\mu\)A to 1.5 \(\mu\)A, and the on/off ratio reduces from 22,000 to 1000 as the annealing temperature is gradually increased from 350 K to 590 K. Previous studies reported that WSe\({}_{2}\) oxidizes to WO\({}_{\text{x}}\) at high temperatures [42, 43].