Abstract
This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.
Highlights
Two-dimensional (2D) materials, and the heterostructures that can be created from them, have been widely studied due to their atomic-scale thickness, flexibility and unique electrical/optical properties[1,2,3,4,5]
Atomic layer deposition (ALD) is known to be self-limiting, as the growth rate is dependent on the adsorption of precursor molecules rather than growth conditions such as exposure time[19,20], but as growth occurs through the formation of multi-layer islands it is difficult to achieve the layer controllability needed when compared to other techniques such as CVD21,22
As 2D Transition metal dichalcogenides (TMDCs) have a chemically inactive surface, it is reasonable to expect they will exhibit a unique growth behavior during ALD when compared to conventional materials that are rich in dangling bonds[28]
Summary
Two-dimensional (2D) materials, and the heterostructures that can be created from them, have been widely studied due to their atomic-scale thickness, flexibility and unique electrical/optical properties[1,2,3,4,5]. The self-limiting layer synthesis (SLS) of a 2D TMDC (MoS2) is achieved through ALD by combining precursor exposure, purging, reactant exposure and a final purging into a single cycle. In this way, a point is reached at which the number of layers produced is determined purely by the growth temperature; a unique behavior that is directly attributable to the chemical inactivity of the 2D MoS2 surface. This is concordant with previous results regarding the dependence of the PL signal on the number of layers[14,34,35] and further confirms the growth temperature dependent nature of the SLS of MoS2
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