Selective Oxidation Of Two-dimensional Hafnium Selenide In Van Der Waals Heterojunctions

Two-dimensional transition metal dichalcogenides（TMDCs）have wide applications in nanoelectronic devices and novel optoelectronic devices.Most TMDCs materials are easily oxidized by the oxygen in the environment.A class of TMDCs materials represented by HfSe₂,its oxidation product Hf O_xcan be used as a high-κgate dielectric material.Exploring their oxidation process and mechanism and obtaining ultrathin high-κoxides,especially their controllable preparation in van der Waals heterojunctions,are the prerequisites for their application in 2D high-performance devices.In this paper,the oxidation process of HfSe₂under different conditions and in van der Waals heterojunctions is explored in detail.The specific results are as follows:（1）The oxidation process of few-layer HfSe₂under different conditions is revealed.The precipitation and aggregation of Se are observed for both room temperature oxidation and thermal oxidation at150°C.While Se will volatilize at 250°C oxidation as it exceeds the melting point of Se,pure and transparent Hf O_xwithκof 16～is formed.（2）The oxidation process of HfSe₂in the HfSe₂/MoS₂van der Waals heterojunction is revealed.Thermal oxidation at 250°C can completely oxidize HfSe₂to Hf O_xwithout Se residue,but the oxidation of MoS₂at this high temperature remains to be further characterized.Thermal oxidation at 150°C does not destroy the lattice structure of MoS₂,the Se residue only accumulates on Hf O_xplaced on top of Si O₂,while the van der Waals heterojunction part is clean.HfSe₂in the Gr/HfSe₂/MoS₂three-layer van der Waals heterojunction can also be selectively oxidized at 150°C without Se residue in the heterojunction.The controllable synthesis of ultrathin high-κoxides in van der Waals heterojunctions lay the foundation for their future application in complex electronic devices.