ALD Assisted 2D Monolayer Transition Metal Dichalcogenides and Their Applications in Optoelectronics

Recently, 2D Transition Metal Dichalcogenides (TMDCs) have received significant attention in the field of optoelectronics due to their extraordinary optical and electrical properties [1], [2]. Among TMDC family, molybdenum disulfide (MoS
₂
) has intensively been studied. Bulk and monolayer MoS
₂
have bandgap energy of ~ 1.3 e V and ~ 1.8 e V, respectively [1], [3]. Different growth techniques such as Atomic Layer Deposition (ALD) are alternatively used aiming at uniform growth of MoS
₂
. For ALD Mo and S precursors such as Mo(CO)
₆
, MoCl
₅
and H
₂
S, dimethyl disulfide respectively are used on SiO
₂
/Si or sapphire substrate [4]-[7]. Using ALD, the synthesis of wafer scale uniform bilayer MoS
₂
film has been reported. However, in the experiment, H
₂
S precursor is used both in the reaction cycle [4], [5] and high-temperature post-deposition annealing performed as necessary step to reduce impurity contents and increase crystal quality in the films [6], [7]. H
₂
S is highly toxic and has a high vapor pressure at room temperature. As a result, it requires a special container pressure controller and gaskets which increase considerably the cost and risk for use in ALD process. For 2D MoS
₂
growth by ALD, uniformly distributed small-size flakes are reported in [4], [5]. In order to control the growth process of 2D MoS
₂
, the flake quantity and distribution needs to be taken under control. Interestingly, in CVD, it is possible to grow MoS
₂
layer using MoO
₃
thin film instead of powder. The control of MoS
₂
flake size, density, number of layer and cleanness can be achieved by controlling the MoO
₃
thin film. Plasma Enhanced Atomic Layer Deposition technique should be most suitable for such a uniform deposition. Successful growth is reported in [8] but it is not optimized for MoS
₂
growth.