# High-Temperature Annealing Effects on Atomically Thin Tungsten Diselenide Field-Effect Transistor
## 3 Results and Discussion
The oxidation of 2D materials differs from bulk materials as these materials show higher reactivity at the edges and defect sites only. The effects of oxidation on the electrical characteristics of the WSe\({}_{2}\) device can be seen in this temperature range (350 K to 590 K), where not only the on/off ratio and total current reduced but a decrease in the subthreshold swing can also be observed. It is safe to say that higher thermal and electrical stress has induced structural and chemical changes in the WSe\({}_{2}\), which is further explained in the latter part. From 590 K to 670 K, the device behavior changes from semiconducting to fully conducting with very little gate control. The on/off ratio in the region is less than 10, and the off current has increased several orders of magnitude. Inorder to obtain further insight into the device behavior, we extracted field-effect mobility and hysteresis of the device. Figure 3 illustrates the field effect mobility as well as hysteresis as a function of annealing temperature. The field-effect mobility was extracted for electrons as well as holes using the following equation:  
\[\mu=\frac{L}{W}\frac{g_{m}}{C_{ox}V_{d}}\]  
In this equation, \(L\) and \(W\) are the total channel length and width, respectively, and \(C_{ox}\) is the back gate silicon dioxide capacitance per unit area, calculated by \(C_{ox}=\epsilon_{0}\cdot\epsilon_{r}/d\). Here, \(\epsilon_{0}=8.85\times 10^{-12}\) Fm\({}^{-1}\) is the permittivity of free space, and \(\epsilon_{r}=3.9\) is the relative permittivity of SiO\({}_{2}\) and \(d\) is SiO\({}_{2}\) thickness (300 nm). \(g_{m}\) is the trans-conductance obtained from the slope of each transfer curve, \(V_{d}\) is the applied drain voltage and \(\mu\) is the field effect mobility. A similar trend for electron mobility can also be seen in Figure 3, where initially, electron mobility increases from 4 cm\({}^{2}\)/Vs to 9 cm\({}^{2}\)/V\(\cdot\)s as the annealing temperature is increased from 300 to 350 K. The mobility in 2D materials is limited by the scattering, and an improvement in mobility indicates a reduction in scattering. All the measurements were performed at 300 K, so it was safe to rule out any change in phonon scattering. Therefore, the increase in electron mobility can be attributed to a cleaner WSe\({}_{2}\) surface enabled by thermal annealing. As observed previously, in the temperature range of 360 K to 590 K, a consistent drop in electron mobility can be seen. One can also observe three distinct regions in the mobility curve: the mobility increases from 300 K to 350 K and from 350 K to 590 K, mobility gradually reduces from 9 cm\({}^{2}\)/Vs to 2.5 cm\({}^{2}\)/Vs and it reaches 0.1 cm\({}^{2}\)/V\(\cdot\)s at 670 K. This trend of mobility and device current reflects that 350 K is the optimal annealing temperature for WSe\({}_{2}\). Until this temperature, annealing improves the  
Figure 2: (**a**) Transfer characteristics curve of the WSe\({}_{2}\) device measured at 300 K after annealing at different temperatures on a linear scale (legend shows the annealing temperature in Kelvin, and inset represents the \(l_{d}\)–\(V_{d}\) curve of the device). (**b**) Transfer characteristics curve of the WSe\({}_{2}\) device measured at 300 K after annealing at different temperatures on a semi-logarithmic scale (legend shows the annealing temperature in Kelvin). (**c**) The ratio of on current to the off current of the device as a function of annealing temperatures. (**d**) Schematic band diagram showing Ti and WSe\({}_{2}\) before and after contact. device's performance by removing the contaminants, adsorbed moisture and chemical residues from the surface of WSe\({}_{2}\)[41]. Beyond 350 K, the high temperature starts to oxidize WSe\({}_{2}\) and alter its chemical composition. As WSe\({}_{2}\) becomes oxidized to WO\({}_{\text{x}}\), new traps are created at the WSe\({}_{2}\)-WO\({}_{\text{x}}\) interface, which results in a reduction in electron mobility [42; 43].