Improving the performance and functionality of future nanoelectronic devices heavily relies on research of innovative materials and material combinations, as well as novel device concepts. One family of emerging materials, the two-dimensional (2D) materials, gathers interest in view of the versatile and exotic properties that arise from its ultra-thin body nature . In particular, the 2D transition metal dichalcogenides such as molybdenum and tungsten disulfide (MoS2, WS2) are three-atom-thin semi-conducting layers and widely applicable, either as complement of Si in ultra-scaled nanoelectronic devices, or in photo- and gas detectors and as catalysts .
To exploit the potential of 2D materials in such a variety of applications, they need to be grown by manufacturable deposition techniques that tailor the structure, crystallinity and properties of the 2D layers towards the desired application, preferably at deposition temperatures compatible with the material environment. To date, the majority of 2D materials are grown by chemical vapor deposition (CVD) at relatively high deposition temperatures (550-1100 °C) for superior crystal grain size and crystallinity . However, these high deposition temperatures are incompatible with back-end-of-line process technology, where 2D transistors hold promise .
Therefore, in view of the envisioned integration approaches for 2D materials, atomic layer deposition (ALD) is benchmarked to other deposition candidates. ALD provides atomistic growth control compatible with temperature sensitive structures, albeit the ALD 2D layers are amorphous or nanocrystalline. However, the crystal grain size, basal plane orientation and number of layers can be tailored towards the desired applications if fundamental insight in the nucleation mechanisms is developed. To date, nucleation behavior of ALD grown 2D layers is hardly studied.
In this presentation, we will provide an overview of existing ALD processes for 2D materials, and demonstrate how the structure, crystallinity and properties can be controlled at comparatively low deposition temperatures (< 450 °C) based on insight in the nucleation mechanisms. Such insight is key to advance the field of ALD of 2D materials. Moreover, we discuss opportunities that ALD offers in view of its strong surface selectivity for self-aligned integration while retaining compatibility with existing Si technology. In that context, we address the persistent challenges that need to be overcome.