Thermal oxidation of tungsten disulfide nanocrystals grown by metal organic chemical vapor deposition

Thermal oxidation of two-dimensional (2D) transition-metal dichalcogenide materials is attracting interest for gate-stack formation in advanced electronic devices. However, the oxidation kinetics and mechanism of thermal oxidation of 2D WS2 crystals grown by the industrially relevant metal-organic chemical vapor deposition technique are not yet understood. In this paper, we investigate the thermal oxidation of monolayer-thin triangular WS2 isolated crystals on a SiO2 substrate by O2. In a temperature window between 300 and 500 °C, we observe that the oxidation propagates laterally from the edges to the center of the WS2 crystals. The low oxidation rate enables control of the lateral oxidation length at the nanometer scale. As such, we create WS2/WO3 lateral heterostructures with a WO3 lateral oxidation length that linearly depends on the oxidation time, consistent with a well-controlled reaction-limited process with an oxidation rate of 0.03 nm/s at 375 °C. The Arrhenius energy barrier for oxidation may suggest that S-vacancies near the WS2/WO3 line boundary initiate the oxidation. Finally, we investigate the atomic layer deposition of a high-k dielectric on the WS2/WO3 lateral heterostructures and observe selective deposition of Al2O3 on the WO3 area, while no Al2O3 deposition is detected on the WS2 area of the heterostructure. The obtained insights into the thermal oxidation kinetics and selectivity are relevant to creating various types of heterostructures with 2D materials and gate dielectrics for highly scaled nano-electronic devices.