Search

Atomically thin, 2D semiconductors, such as transition metal dichalcogenides, complement silicon in ultra‐scaled nano‐electronic devices. However, the semiconductor and its interfaces become increasingly more difficult to characterize chemically and electrically. Conventional methodologies, including scanning probe microscopies, fail to capture insight into the chemical and electronic nature of the semiconductor, albeit vital to understand its impact on the semiconductor performance. Therefore, this work presents a unique and universal in situ approach combining time‐of‐flight secondary ion mass spectrometry and atomic force microscopy to map chemical differences between regions of different electrical conductivity in epitaxially deposited tungsten disulfide (WS2) on sapphire substrates. Surprisingly, WS2 regions of lower electrical conductivity possess a larger amount of sulfur compared to regions with higher conductivity, for which oxygen is also detected. Such difference in chemical composition likely roots from the non‐homogeneously terminated sapphire starting surface, altering the WS2 nucleation behavior and associated defect formation between neighboring sapphire terraces. These resulting sapphire terrace‐dependent doping effects in the WS2 hamper its electrical conductivity. Thus, accurate chemical assignment at a sub‐micrometer lateral resolution of atomically thin 2D semiconductors is vital to achieve a more detailed understanding on how the growth behavior affects the electrical properties. [ABSTRACT FROM AUTHOR]

The structure, crystallinity and properties of as-deposited two-dimensional (2D) transition metal dichalcogenides are determined by nucleation mechanisms in the deposition process. 2D materials grown by atomic layer deposition (ALD) in the absence of a template are polycrystalline or amorphous. Little is known about their nucleation mechanisms. Therefore, the nucleation behavior of WS2 during plasma enhanced ALD from WF6, H2 plasma and H2S at 300 °C is investigated on amorphous ALD Al2O3 starting surface and on monocrystalline, bulk sapphire. Preferential interaction of the precursors with the Al2O3 starting surface promotes fast closure of the WS2 layer. The WS2 layers are fully continuous at WS2 content corresponding to only 1.2 WS2 monolayers. On amorphous Al2O3, (0002) textured and polycrystalline WS2 layers form with grain size of 5 to 20 nm due to high nucleation density (∼1014 nuclei/cm2). The WS2 growth mode changes from 2D (layer-by-layer) growth on the initial Al2O3 surface to three-dimensional (Volmer-Weber) growth after WS2 layer closure. Further growth proceeds from both WS2 basal planes in register with the underlying WS2 grain and from or over grain boundaries of the underlying WS2 layer with different in-plane orientation. In contrast, on monocrystalline sapphire, WS2 crystal grains can locally align along a preferred in-plane orientation. Epitaxial seeding occurs locally albeit a large portion of crystals remain randomly oriented, presumably due to the low deposition temperature. The WS2 sheet resistance is 168 MΩμm, suggesting that charge transport in the WS2 layers is limited by grain boundaries. [ABSTRACT FROM AUTHOR]