## 3 Results and Discussion  
Figure 1a shows a three-dimensional (3D) schematic illustration of the back-gated TFT encapsulated with Al\({}_{2}\)O\({}_{3}\) on top of the multilayer MoS\({}_{2}\) active channel, which was also confirmed in a top-view optical microscope image, as shown in Figure 1b. MoS\({}_{2}\) and contact holes in the S/D electrodes are indicated by red dashed and green dotted lines, respectively. It should be noted that we could not find any pinholes or cracks in the Al\({}_{2}\)O\({}_{3}\) layer resulting from UV-ozone pre-treatment, which will be further discussed in relation to Figure 2. Channel length (\(L\)) and width (\(W\)) of the MoS\({}_{2}\) device were 13.06 and 20.32 \(\upmu\)m, respectively, used for calculating the TFT performance in relation to various temperatures. Figure 1c shows a cross-sectional TEM image of the Al\({}_{2}\)O\({}_{3}\)-encapsulated MoS\({}_{2}\) TFT with UV-ozone pretreatment. It can be seen that the Al\({}_{2}\)O\({}_{3}\) with an average thickness of approximately 42.0 nm uniformly covered the entire MoS\({}_{2}\) surface. In addition, the thickness of the MoS\({}_{2}\) multilayer was estimated to be approximately 64.6 nm, indicating its nearly bulk-like energy band characteristics. The fast Fourier transform patterns and high-angle annular dark-field imaging of the MoS\({}_{2}\) are shown in Figure S1 in the Supplementary Materials. Figure 1: (**a**) 3D schematic structure of multilayer molybdenum disulfide (MoS\({}_{2}\)) thin-film transistor (TFT) passivated with aluminum oxide (Al\({}_{2}\)O\({}_{3}\)); (**b**) optical microscope and (**c**) cross-sectional transmission electron microscopy (TEM) images of the MoS\({}_{2}\) TFT with Al\({}_{2}\)O\({}_{3}\) passivation, respectively. Figure 2 compares the surfaces morphologies of the Al\({}_{2}\)O\({}_{3}\) deposited on MoS\({}_{2}\) with and without UV-ozone pretreatment, which were measured by AFM scanning in a 1 \(\upmu\)m \(\times\) 1 \(\upmu\)m region. Many clusters and boundaries can be observed on the surface of the Al\({}_{2}\)O\({}_{3}\) (thickness of approximately 40 nm) deposited on the pristine MoS\({}_{2}\) (Figure 2a). However, in the case of Al\({}_{2}\)O\({}_{3}\) deposited on the MoS\({}_{2}\) with UV-ozone pretreatment, it shows complete coverage with high uniformity (Figure 2b). The root-mean-square surface roughness (\(R_{\text{RMS}}\)) of the directly deposited Al\({}_{2}\)O\({}_{3}\) is 0.672 nm, but it decreases to 0.190 nm with a 5-min UV-ozone treatment on MoS\({}_{2}\). These results indicate that UV-ozone exposure is an efficient way to achieve uniform growth of Al\({}_{2}\)O\({}_{3}\) on a MoS\({}_{2}\) surface. Previous studies have found that the electrical properties and device performance of MoS\({}_{2}\) transistors can be enhanced by high-\(k\) dielectric encapsulation [27; 28]. Therefore, we measured the electrical properties of the MoS\({}_{2}\) TFTs to elucidate the effect of Al\({}_{2}\)O\({}_{3}\) passivation on multilayer MoS\({}_{2}\) with UV-ozone pretreatment. Figure 3a compares the transfer characteristic curves \(I_{\text{ds}}-V_{\text{gs}}\) of the MoS\({}_{2}\) TFTs without and with Al\({}_{2}\)O\({}_{3}\) passivation layers with the application of a source-to-drain voltage (\(V_{\text{ds}}\)) of 1 V. Before Al\({}_{2}\)O\({}_{3}\) passivation, the pristine MoS\({}_{2}\) TFT exhibits a large hysteresis (black-solid lines in Figure 3a). However, after Al\({}_{2}\)O\({}_{3}\) passivation with UV-ozone pre-treatment, represented by the red solid lines in Figure 3a, the transfer curve obtained from a forward \(V_{\text{gs}}\) scan closely approached that obtained from a reverse \(V_{\text{gs}}\) scan and vice versa, which indicates a distinct reduction of hysteresis behavior (see also Figure S3). It should be noted that reduction of hysteresis was not observed in the MoS\({}_{2}\) TFTs with only UV-ozone treatment, i.e., without Al\({}_{2}\)O\({}_{3}\) passivation (Figure S4).