Epitaxy of van der Waals Materials: a Fundamental and Exploratory Study Focused on Molecular Beam Epitaxy of WSe2

The application of new materials in nanotechnology opens new perspectives and enables ground-breaking innovation. Two-dimensional van der Waals materials are a promising class of new materials awaiting their integration into the semiconductor industry. The transition metal dichalcogenides and in more specific the WSe2 compound, are predicted as one of the most interesting van der Waals materials to complement the silicon-based semiconductor technology. However, the integration of van der Waals materials relying on industry-compatible manufacturing processes is still a major challenge. This is currently restricting the application of these materials to the lab environment only. The large-area and single-crystalline growth of van der Waals materials is one of the most appropriate approaches to meet the demanding requirements implied by the semiconductor industry. This PhD thesis contributes to the fundamental understanding on the integration of van der Waals materials through the growth process of epitaxy.Using both molecular beam and metalorganic vapor phase epitaxy methods, the growth of various transition metal dichalcogenides, including WSe2, is studied and found prone to defect formation. This is resulting from the severe and uncontrollable formation of stacking faults such as 60o twins, as observed in systematic and algorithmic analyses of the experimental data. The occurrence of stacking faults is validated by theoretical ab initio calculations and a model is developed to quantify the corresponding defect density. The stacking faults are observed in both quasi van der Waals heteroepitaxy, and surprisingly also in van der Waals homoepitaxy. This highlights the ultimate challenge of defect-free epitaxy of transition metal dichalcogenides and underlines a fundamental limitation of these specific van der Waals compounds.The interfaces of the molecular beam epitaxially grown (quasi) van der Waals homo-/heterostructures are studied and demonstrated to capture important insights into the epitaxy processes. Supported by the application of a simplified diffusion model, various nucleation studies are performed on multiple transition metal dichalcogenide homo-/heterostructures. They reveal a clear correlation between adatom diffusion and surface energy, quantified from ab initio calculations. Furthermore, in-depth reflection high-energy and plan-view transmission electron diffraction analyses demonstrated in specific cases fully relaxed growth or strain formation through lattice matching at the (quasi) van der Waals heterointerfaces.The fundamental understandings attained in this PhD are hence a first step towards a more identified and better controlled growth process of van der Waals materials. These will pursue further the aspiration of large-area, single-crystalline and defect-free epitaxial integration of (quasi) van der Waals homo-/heterostructures, into the great world of the semiconductor industry.

Publication year:2020