Investigation Of Defects In Monolayer WSe₂ And Mechanisms Of ALD Of High-κ Dielectrics For Carbon-based Devices

In the first section,defects in monolayer tungsten disulfide（ML-WSe₂）including both intrinsic and extrinsic were systematically characterized.We selected ML-WSe₂prepared by three so-called main-stream approaches i.e.,mechanical exfoliation（ME）,chemical vapor deposition（CVD）,and molecular beam epitaxy（MBE）as the target samples.Via a conjunction of statistic atomic-resolution annular dark-field scanning transmission electron microscopy（ADF-STEM）imaging,software-based automated defect identification and counting,together with image simulations,defect species and concentrations were quantitatively determined.The experimental results showed that there are up to seven kinds of intrinsic point defects in ML-WSe₂ with V_Se（corresponding to one Se atom missing）as the most dominant defect,irrespective of the synthetic route and growth conditions.Furthermore,exact contents and diversity of point defects depend on the specific preparation method:CVD-WSe₂ is the most defective with the density of V_Se reaching 1.48%in atomic ratio as an example,followed by ME-(～0.85 at%for V_Se)and MBE-WSe₂(～0.49 at%for V_Se).In CVD process,choose of different growth parameters such as precursor content（W-rich vs.Se-rich）and growth substrate（Si O₂/Si vs.sapphire）also affects the distribution of point defects in WSe₂.For example,ML-WSe₂ grown under W-rich condition possess a higher density of V_Se than for Se-rich condition;the same trend applies for sapphire substrate in comparison to that of Si O₂/Si substrate.In addition to point defects,abundant 4|4P grain boundaries（GBs）were also observed in CVD grown ML-WSe₂where the GBs are subject to translation and tilting via the arrangement of non-hexagonal topological defects like pentagon and octagon.Upon in situ excitation like heating and energetic electron beam irradiation,point defects in ML-WSe₂ started to evolve and form MTBs in different configurations such as 4|4P,55|8.These results identify the defect structures in ML-WSe₂ and provide informations to achieving precise control of large-scale sample growth and establishing the structure-property relationship of 2D TMDCs materials.In the second part,preliminary results on our efforts to reveal the mechanism of atomic layer deposition（ALD）of high-κdielectric（HfO₂）on graphene were presented.To favor the ALD and the subsequent ADF-STEM,a"semi-insitu and no-transfer"method was designed where the CVD grown ML-graphene were transferred onto conventional TEM grid and then directly loaded into the ALD system.In this way,such graphene substrate can be placed directly into electron microscope for characterization after HfO₂ deposition,in doing so loss of growth information due to the subsequent transfer process can be minimized.Based on this recipe,as-deposited HfO₂ films with different cycles were characterized by ADF-STEM and the results showed that HfO₂mainly exhibits atomic clusters with an average diameter of 5 nm at the initial stage of nucleation（one cycle only）,and gradually develop into amorphous films as the ALD cycles increases.The carbon adsorption and local folding on graphene surface were found as perferred HfO₂ nucleation sites.Under our typical ALD condition,the development of HfO₂ film follows an island-like growth mode that leads to a nonuniform film thickness.Such an island-like growth may be attribibute to the inhomegeneous distribution of nucleation sites on graphene substrate.This work comprehends the present understanding on ALD mechanisms of high-κHfO₂ for carbon-based electronics.