Benjamin Groven

Nucleation mechanism during WS2 plasma enhanced atomic layer deposition on amorphous Al2O3 and sapphire substrates

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-dimensiona...

WS 2 transistors on 300 mm wafers with BEOL compatibility

For the first time, WS2-based transistors have been successfully integrated in a 300 mm pilot line using production tools. The 2D material was deposited using either area selective chemical vapor deposition (CVD) or atomic layer deposition (ALD). No material transfer was required. The major integration challenges are the limited adhesion and the fragility of the few-monolayer 2D material. These issues are avoided by using a sacrificial Al2O3 capping layer and by encapsulating the edges of the 2D material during wet processing. The WS2 channel is contacted with Ti/TiN side contacts and an industry-standard back end of line (BEOL) flow. This novel low-temperature flow is promising for integration of back-gated 2D transistors in the BEOL.