Area-Selective Atomic Layer Deposition of Two-Dimensional WS2 Nanolayers

With downscaling of device dimensions, two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) such as WS2 are being considered as promising materials for future applications in nano-electronics. However, at these nanoscale regimes, incorporating TMD layers in the device architecture with precise control of critical features is challenging using current top-down processing techniques. In this contribution, we pioneer the combination of two key avenues in atomic-scale processing: area-selective atomic layer deposition (AS-ALD) and growth of 2D materials, and demonstrate bottom-up processing of 2D WS2 nanolayers. Area-selective deposition of WS2 nanolayers is enabled using an ABC-type plasma-enhanced ALD process involving acetylacetone (Hacac) as inhibitor (A), bis(tert-butylimido)-bis-(dimethylamido)-tungsten as precursor (B), and H2S plasma as the co-reactant (C) at a low deposition temperature of 250 degrees C. The developed AS-ALD process results in the immediate growth of WS2 on SiO2 while effectively blocking growth on Al2O3 as confirmed by in situ spectroscopic ellipsometry and ex situ X-ray photoelectron spectroscopy measurements. As a proof of concept, the AS-ALD process is demonstrated on patterned Al2O3/SiO2 surfaces. The AS-ALD WS2 films exhibited sharp Raman (E-2g(1) and A(1g)) peaks on SiO2, a fingerprint of crystalline WS2 layers, upon annealing at temperatures within the thermal budget of semiconductor back-end-of-line processing (<= 450 degrees C). Our AS-ALD process also allows selective growth on various TMDs and transition metal oxides while blocking growth on HfO2 and TiO2. It is expected that this work will lay the foundation for area-selective ALD of other 2D materials.