Surface and interfacial study of atomic layer deposited Al₂O₃ on MoTe₂ and WTe₂

The atomic layer deposition (ALD) of high-k dielectrics could build an efficient barrier against moisture and O₂ adsorption. Such a barrier is highly needed for MoTe₂ and WTe₂ transition metal dichalcogenides because of the poor structural stability and the fast oxidization in ambient air. In situ x-ray photoelectron spectroscopy and ex situ atomic force microscopy and scanning transmission electron microscopy were employed to report a comparative study between the growth of Al₂O₃ on MoTe₂ and WTe₂ by means of traditional thermal ALD and plasma-enhanced ALD (PEALD). Similar to what has been observed on other 2D materials such as MoS₂ and Graphene, the thermal ALD results in an islanding growth of Al₂O₃ on MoTe₂ due to the dearth of dangling bonds, whereas, a uniform coverage of Al₂O₃ on WTe₂ is observed and likely contributed to the high concentration of intrinsic structural defects. The PEALD behavior is consistent between MoTe₂ and WTe₂ providing a conformal and linear growth rate (∼0.08 nm/cycle), which correlates with the creation of Te-O and metal-O nucleation sites. However, a thin layer of interfacial Mo or W oxides gradually forms, resulting from the plasma-induced damage in the topmost (1-2) layers. Attempts to enhance the Al₂O₃/MoTe₂ interfacial quality by physically evaporating an Al₂O₃ seed layer are investigated as well. However, the evaporated Al₂O₃ process causes thermal damage on MoTe₂, necessitating a more 'gentle' ALD technique for the surface passivation.