Tunable Encapsulation and Doping of Monolayer MoS2 by In Situ Probing of Excitonic Properties During Atomic Layer Deposition

Here, it is shown that in situ spectroscopic ellipsometry (SE) is a powerful method for probing the effects of reactant adsorption and film formation on the excitonic properties of 2D materials during atomic layer deposition (ALD), thus allowing optimization of both film growth and opto(electronic) characteristics in real time. Facilitated by in situ SE during ALD on monolayer MoS2, a low temperature (40 degrees C) process for encapsulation of the 2D material with a nanometer-thin alumina (AlOx) layer is investigated, which results in a 2D/3D interface governed by van der Waals interactions rather than chemical bonding. Charge transfer doping of MoS2 by AlOx is found to be an interfacial phenomenon that initiates from the earliest stages of film formation, but saturates upon deposition of a closed layer. However, the lack of chemical binding interactions at the 2D/3D interface enables physical removal of the AlOx that results in a reversal of the charge transfer doping effect. Overall, it is demonstrated that in situ SE of 2D materials during ALD can precisely probe the impact of film formation on sensitive optoelectronic characteristics of 2D materials, which is of key importance in the development of integrated 2D/3D systems.