Multidisciplinary Investigations of Chalcogen-based Semiconductor Materials: Structural Heterogeneity, Strain Analysis, and In-Situ Mechanical Testing

Thin films with semiconducting properties can be successfully engineered from chalcogen-basedmaterials, specifically those containing selenium (Se), or/and tellurium (Te). They are vital subjects of study in both their elemental states and various compounds due to their distinct anisotropic properties. These elements play essential roles in compounds like tellurides and selenides, which are crucial in semiconductors, photovoltaic devices, and electronics. However, the quick crystallization tendencies observed in amorphous TexSe1 −x thin films restrict their applicability. It is essential to gain insight into the short- and medium-range structural organization of the amorphous state and comprehend the underlying physics driving film crystallization. Therefore, in the second chapter of the thesis, we provide comprehensive research on the short- to medium-range ordering in amorphous TexSe1 −x thin films through a combination of experimental studies and atomic simulations. This study marked the first instance in which we employed fluctuation electron microscopy (FEM) and Density Functional Theory (DFT) calculations to gain insights into these structural variations across a wide range of compositions, including pure Te. Within the chain network structure, we have identified at least two distinct populations that closely resemble the intrachain distances of Se-Se and Te-Te. In the case of the binary alloy with x greater than 0.61 in TexSe1 −x, we observe an increase in Te-Te-like populations, implying the potential formation of Te fragments. To leverage the anisotropic properties of tellurium, in recent study, the oriented growth of ultra-thin Te layers on a WSe2 substrate is achieved by using a technique called physical vapor deposition (PVD). In this configuration, the atomic chains of Te align with the armchair directions of the substrate, leading to the formation of a moire superlattice. This superlattice consists of micrometer- ´ scale Te flakes positioned on the continuous WSe2 film. Here, we explore the exact orientation relationships and moire lattices by combining electron microscopy with image simulations. We ´ also assess the strain evolution and defects in Te-WSe2 heterostructures with the help of scanning nanodiffraction analysis, shedding light on the complexities of strain transfer and its impact on material properties. In the final chapter of the thesis, we introduce a hybrid approach aimed at addressing challenges in sample preparation for in-situ mechanical testing. It involves depositing samples on flexible, electron-transparent substrates, attaching them to macroscopic copper sheets, and mirroring techniques used in the nanomaterials and thin-film community. With the help of this technique, we were able to study the mechanical responses of sputtered gold (Au) and transition metal dichalcogenides (TMDC), specifically WS2. This method not only streamlines sample preparation but also expands our ability to investigate the mechanical properties of materials at the nanoscale

Advisor: Scott, Mary