Defect-Mediated Van der Waals Epitaxial Growth of WS2 Nanosheets at Low Temperatures for Back-End-of-Line Integration

Abstract Epitaxial growth plays a crucial role in the current state-of-the-art semiconductor technology, providing a path for processing complex and high-quality thin films, heterostructures, and nanomaterials, which are used in a wide range of applications. Similarly, van der Waals (vdW) epitaxy, the epitaxial growth of layered compounds on top of a different 2D material, is recognized as a promising method to synthesize high-quality atomically thin films and their heterostructures. Indeed, research on vdW epitaxy has been intense, resulting in very promising results for many different material systems. Chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) are the leading synthetic methods used for this purpose, operating at a growth temperature above 500 °C. Reducing the thermal budget and processing temperatures below 450 °C is essential for integrating these materials in many technologies, such as back-end-of-line (BEOL) silicon processing and flexible electronics, to name a few. Here, we demonstrate vdW epitaxial growth of nanodomains of WS2 on previously deposited MoS2 monolayers at a temperature range of 295−420 °C. The resulting heterostructures were characterized by means of high-resolution transmission electron microscopy (HRTEM), selected area diffraction (SAD), and energy dispersive X-ray spectroscopy (EDS), in which the registry between the transition metal dichalcogenides (TMDs) shows a clear epitaxial relation. With the help of Raman spectroscopy and processing variations, defect-induced vdW epitaxial growth was revealed. Finally, a proof-of-concept photodetector was fabricated on the WS2/MoS2 and on pristine WS2 and MoS2 references, clearly showing an improved performance for the heterostructure device. A twelve-fold increase in the photoresponsivity is explained in the context of the WS2/MoS2 heterostructure band offsets and surface passivation. This work may lead to further research on the low-temperature vdW epitaxial growth for integrating a wide range of 2D materials within BEOL Processing, i.e., monolithic 3D integration (M3D) and ultra-thin diffusion barriers.