## 3 Results and Discussion
For quantitative comparison, we define the difference of threshold voltage (\(\Delta V_{\text{th}}\)) by subtraction between the \(V_{\text{th}}\) values in the forward and reverse transfer characteristic curves, which are estimated to be 20.1 V and 0.5 V for the pristine and Al\({}_{2}\)O\({}_{3}\)-passivated devices, respectively. However, \(I_{\text{off}}\) in the Al\({}_{2}\)O\({}_{3}\)-passivated MoS\({}_{2}\) TFT increased by about one order of magnitude, resulting in a decrease in the \(I_{\text{on}}/I_{\text{off}}\) from \(10^{6}\) to \(10^{5}\). The results may be attributed to the fact that excess electrons could be induced in the MoS\({}_{2}\) active channel due to the positive fixed charges in the Al\({}_{2}\)O\({}_{3}\) layer [23; 28]. According to transport physics in TFTs, the relation among \(I_{\text{ds}}\), \(V_{\text{gs}}\) and \(V_{\text{ds}}\) are expressed for a linear regime (\(\,\text{$V_{\text{ds}}$}\,\text{$<$}\,\text{$V_{\text{gs}}$}-\text{$V_{\text{th }}$}\,\text{$\mid$}\)) as in [29]:  
\[I_{\text{ds}}=\frac{\mu_{\text{eff}}W\text{C}_{\text{GI}}}{L}\bigg{[}(V_{\text{ gs}}-V_{\text{th}})\,V_{\text{ds}}-\frac{V_{\text{ds}}^{2}}{2}\bigg{]}\, \tag{1}\]  
where \(C_{\text{GI}}\) is capacitance per unit area of the gate insulator, and \(\mu_{\text{eff}}\) is the field-effect mobility, one of the figure-of-merit for evaluating TFT characteristics. Based on the standard model of  
Figure 2: Atomic force microscopy (AFM) images of Al\({}_{2}\)O\({}_{3}\) surfaces on MoS\({}_{2}\) active channel: (**a**) without and (**b**) with ultraviolet-ozone (UV-ozone) treatment for 5 min before atomic-layer-deposited (ALD) procedure. RMS: root-mean-square. metal-oxide-semiconductor (MOS) FETs and a parallel plate model of gate capacitance [8; 29; 30], \(\mu_{\rm eff}\) of the TFT can be expressed in terms of transconductance (\(g_{\rm m}\equiv\partial I_{\rm ds}/\partial V_{\rm gs}\)) as  
\[\mu_{\rm eff}=\frac{L_{\rm gm}}{W\rm C_{\rm ox}V_{\rm ds}}, \tag{2}\]  
where \(C_{\rm ox}\) is capacitance per unit area of gate insulator (\(1.15\times 10^{-8}\)\({\rm Fm}^{-2}\)). The maximum transconductance of the pristine MoS\({}_{2}\) TFT was \(2.31\times 10^{-7}\) S, which was increased to \(3.25\times 10^{-7}\) S after Al\({}_{2}\)O\({}_{3}\) passivation. As a result, the \(\mu_{\rm eff}\) values for the pristine and Al\({}_{2}\)O\({}_{3}\)-passivated MoS\({}_{2}\) TFTs were calculated to be \(40.9\)\({\rm cm}^{2}\)\({\rm V}^{-1}\)\({\rm s}^{-1}\) and \(57.6\)\({\rm cm}^{2}\)\({\rm V}^{-1}\)\({\rm s}^{-1}\), respectively. Figure 3b compares the output characteristic curves (\(I_{\rm ds}-V_{\rm ds}\)) of the MoS\({}_{2}\) TFTs without and with Al\({}_{2}\)O\({}_{3}\) passivation layers under the application of \(V_{\rm gs}\) ranging from \(-30\) V to \(0\) V with a step of \(5\) V. At a low \(V_{\rm gs}\) range from \(-30\) V to \(-15\) V, the \(I_{\rm ds}\) values of the Al\({}_{2}\)O\({}_{3}\)-passivated MoS\({}_{2}\) TFTs were slightly lower than those of the pristine MoS\({}_{2}\) TFT. However, the clear enhancement of the \(I_{\rm ds}\) of the passivated device has been observed at higher \(V_{\rm gs}\) than \(-10\) V, which agrees well with Figure 3a. It is well known that MoS\({}_{2}\) seems to be very sensitive to oxygen and water when exposed to an ambient environment [31]. Oxygen and water molecules can be adsorbed on the defect sites of MoS\({}_{2}\) to induce charge traps, thus leading to a relatively larger and clockwise hysteresis as well as mobility degradation [13; 32]. High-\(k\) dielectric passivation through ALD can be suggested as an efficient method to isolate MoS\({}_{2}\) from atmospheric environments that include external contaminants [27; 28; 33]. The enhanced \(I_{\rm ds}\) is also attributed to the annealing effect, which decreases the contact resistance between the S/D electrodes and MoS\({}_{2}\) surface.