MoS2 Functionalization with a Sub-nm Thin SiO2 Layer for Atomic Layer Deposition of High-κ Dielectrics

Several applications of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) in nanoelectronic devices require the deposition of ultrathin pinhole free high-kappa dielectric films on 2D TMDs. However, deposition of nm-thin high-kappa dielectric films on 2D TMDs remains challenging due to the inert TMD surface. Here, we demonstrate that the surface of a synthetic polycrystalline 2D MoS2 film is functionalized with SiO2 to enable the atomic layer deposition (ALD) of thin and continuous Al2O3 and HfO2 layers. The origins of nucleation, the growth mode, and layer coalescence process have been investigated by complementary physical characterization techniques, which can determine the chemical bonds, absolute amount, and surface coverage of the deposited material. SiO2 is prepared by oxidizing physical vapor deposited Si in air. The surface hydrophilicity of MoS2 significantly increases after SiO2 functionalization owing to the presence of surface hydroxyl groups. SiO2 layers with a Si content of only 1.5 X 10(15) atoms/cm(2) enable the deposition of continuous 2 nm thin Al2O3 and HfO2 layers on MoS2 at 300 degrees C. This fast layer closure can be achieved despite the sub-nm thickness and discontinuity of the SiO2 nucleation layer. On the basis of the experimental results, we propose a nucleation mechanism that explains this fast layer closure. Nucleation of Al2O3 and HfO2 occurs on the SiO2 islands, and fast layer closure is achieved by the lateral growth starting from the many nm-spaced SiO2 islands. Finally, the dielectric properties of Al2O3 on the functionalized MoS2 are confirmed in a top -gated capacitor that shows a leakage current of 3.8 X 10(-6) A/cm(2) at a 3.4 nm equivalent oxide thickness. To conclude, fast nucleation and layer closure in ALD can be achieved even for a sub-nm thin, discontinuous nucleation layer. We propose that this insight can also be applied to other ALD processes, materials, or applications where thin and fully continuous layers are required.