Imaging and controlling epitaxial nucleation and growth using ultra high vacuum transmission electron microscopy

Transmission electron microscopy offers a unique spatially and temporally resolved view of epitaxial nucleation and growth. Continuous imaging of a sample during deposition can provide quantitative measurements of the evolution of strain fields, dislocations, interface structure and surface configurations. Microscope designs that maintain ultra high vacuum at the sample region enable atomically clean surfaces to be prepared on which reactive materials can be deposited. I will describe the application of these techniques to two types of epitaxial process. I will first discuss chemical vapor deposition of Ge or SiGe on Si, where in situ movies show nucleation events, island shape evolution and the formation and motion of dislocations. I will compare the results with observations made during van der Waals epitaxy of metals on 2D materials such as graphene and MoS₂. Evaporation of metals onto UHV-cleaned substrates enables the nucleation sites, equilibrium crystal shapes, interfacial moirés and degree of epitaxy to be quantified. Using patterning and sequential and catalytic deposition, it is possible to form metal and semiconductor nanostructures with complex shapes and compositions and achieve some degree of control over nucleation. Future developments in instrumentation are broadening the scope of in situ electron microscopy to explore the underlying mechanisms of a wider range of epitaxial processes and use the results to create functional epitaxial structures.