Application of 4D-STEM and Multi-Slice Ptychography for Depth Sectioning of Lateral and Vertical 2D Heterostructures Grown by CVD

Two-dimensional (2D) materials have the potential to transform semiconductor technology. Their rich compositional and stacking diversity, especially when deposited as heterostructures, allows tailoring of material properties to enable a wide range of device applications.⁽¹⁾ A prominent class of these 2D materials are the transition metal dichalcogenides (TMDCs). With improved synthesis and fabrication capabilities, heterostructure concepts have been developed that show promising material properties for ultrathin optoelectronic devices. While these heterostructures can be fabricated by mechanical pattern transfer for small areas with high precision, these approaches are difficult to up-scale. Therefore, alternative approaches such as chemical vapor deposition (CVD) have been developed to fabricate 2D materials and their heterostructures.⁽²⁾ Depending on the process parameters, several growth modes can dominate and thus a variety of structure combinations can be created. Since optical and electronic properties typically depend on the orientation, high-resolution measurements of the number of layers and the atomic structure of each material can help to understand the macroscopic optical measurements and guide the optimization of growth processes towards the desired layer structures.<br/><br/>We have studied CVD grown vertical and lateral heterostructures of WS₂/MoS₂ and WSe₂/MoSe₂. These structures are transferred to electron-transparent grids specialized for scanning transmission electron microscopy (STEM) either by a PMMA⁽³⁾ transfer process or directly by growth of the material on SiN grids. The samples are characterized in an aberration-corrected JEOL 2200 FS STEM operating at 80 kV and 200 kV in conjunction with a fast pixelated pn-CCD detector, allowing the samples to be analyzed by 4D-STEM⁽⁴⁾. In addition to conventional high-resolution STEM imaging, the chemical composition is determined by energy dispersive X-ray spectroscopy (EDX) and the orientation alignment within the heterostructures is resolved by scanning nanobeam diffraction (SNBD).<br/><br/>Although EDX, HR-STEM and SNBD provide a huge insight into the actual structure of the heterostructure, including the formation of defects such as vacancies and anti-sites within the 2D layers, the formation of grain boundaries between domains of different crystal orientations, the epitaxial relationship between the 2D layers in the heterostructure^(5-6) as well as elemental interdiffusion at the lateral heterointerfaces, these techniques only provide a projected view of the total structure. Therefore, to further analyze the vertical heterostructures, we explore the possibilities of multi-slice ptychography using the py4DSTEM⁽⁷⁾ software package to obtain depth sectioning of the atomic structure within the vertical and lateral heterostructures in bi- and tri-layer regions of the samples. The depth sectioning provides useful additional information on the formation of defects such as screw dislocations originating from buried layers and clarifies the location of lateral interfaces within the vertical heterostructures.<br/><br/>Our recent progress in the analysis of vertical and lateral 2D heterostructures and our improved understanding of the growth of the 2D heterostructures by CVD will be discussed in the presentation.<br/><br/>References:<br/><br/>(1) K. S. Novoselov et al., Science <b>353</b>, 6298 (2016). doi: 10.1126/science.aac9439<br/>(2) S. Tang et al., MRS Advances <b>7</b>, 751–756 (2022). doi: 10.1557/s43580-022-00312-4<br/>(3) F. Zhang et al., Nanotechnology <b>29</b> 025602S (2018). doi: 10.1088/1361-6528/aa9c21<br/>(4) C. Ophus, Microscopy and Microanalysis <b>25,</b> 563–582 (2019). doi: 10.1017/S1431927619000497<br/>(5) O. Maßmeyer et al., Small, 2402155 (2024). doi: 10.1002/smll.202402155<br/>(6) O. Maßmeyer et al., Adv. Mater. Interfaces<b>,</b> 2400158 (2024). doi: 10.1002/admi.202400158<br/>(7) B. H. Savitzky et al., Microscopy and Microanalysis <b>27,</b> 712–743 (2021). doi: 10.1017/S1431927621000477