Paper 2D-TuA10
Nucleation of 2D WS₂ by Plasma Enhanced Atomic Layer Deposition from WF₆, H₂ Plasma and H₂S – Impact on Grain Size and Charge Transport

Tuesday, October 31, 2017, 5:20 pm, Room 15

To exploit the semiconductor properties of two-dimensional (2D) transition metal dichalcogenides in ultra-scaled nano-electronic devices across large area substrates, these materials need to be deposited with a highly crystalline structure and a controlled number of monolayers by manufacturable deposition techniques. Where the majority of the 2D materials in literature are grown by Chemical Vapor deposition (CVD), Atomic Layer Deposition (ALD) is investigated here as an alternative deposition technique. In ALD, (sub-)monolayer growth control is possible as the deposition is based on self-limiting surface reactions. In addition, due to the relatively low deposition temperature, the 2D materials can be grown directly on temperature sensistive structures at Back-End-Of-Line (BEOL) compatible deposition temperatures. As such, a material transfer from the growth to the target substrate can be avoided.

In Atomic Layer Deposition (ALD), the structure of 2D materials is determined by the nucleation mechanisms. However, the nucleation mechanisms of 2D materials by ALD have so far not yet been investigated. In this work, we investigate the nucleation behavior of WS₂ from a recently reported Plasma-Enhanced (PE)ALD process from WF₆, H₂S and H₂ plasma [1]. We show how the crystallinity and domain size of these layers depends on the starting substrate and the deposition temperature, and how they influence the semiconductor properties of WS₂. WS₂ is grown on 300 mm Si substrates covered with either 30 nm amorphous Al₂O₃ or 90 nm thermally grown SiO₂.

At 300 °C, the growth of WS₂ is strongly enhanced on the Al₂O₃ surface. The high nucleation density of (2.2±0.1) ·10¹⁴ /cm² promotes fast closure of the first WS₂ layer. On the other hand, the combination of the high nucleation density with lateral and vertical growth contributions limits the crystal domain size to 5-30 nm. By choosing a substrate that has a lower reactivity towards the PEALD precursors, e.g. SiO₂, the nucleation density decreases to (2.0±0.1)·10¹¹ /cm². An even lower nucleation density of (6±1) ·10¹⁰ /cm² is obtained on SiO₂ by increasing the deposition temperature to 450 °C due to the increasing mobility of the ad-atoms on the surface. By lowering the reactivity of the H₂ plasma to further delay nucleation, the WS₂ crystals grow primarily in a lateral direction, which further increases the crystal grain size to 250 nm. Despite the low deposition temperatures, the WS₂ behaves as a semiconductor in back-gated transistors, that show an I_max/I_min ratio of at least 10⁵ [2].

[1] B. Groven et al., Chem. Mater., 2017, 29 (7), pp 2927–2938

[2] T. Schram, Q. Smets et al. VLSI satellite workshop 2017 (accepted)