In situ transport measurements reveal source of mobility enhancement of MoS2 and MoTe2 during dielectric deposition.

Layered transition metal dichalcogenides (TMDs) and other two-dimensional (2D) materials are promising candidates for enhancing the capabilities of complementary metal-oxide-semiconductor (CMOS) technology. Field-effect transistors (FETs) made with 2D materials often exhibit mobilities below their theoretical limit, and strategies such as encapsulation with dielectrics grown by atomic layer deposition (ALD) have been explored to tune carrier concentration and improve mobility. While molecular adsorbates are known to dope 2D materials and influence charge scattering mechanisms, it is not well understood how ALD reactants affect 2D transistors during growth, motivating in situ or operando studies. Here, we report electrical characterization of MoS2 and MoTe2 FETs during ALD of MoOx. The field effect mobility improves significantly within the first five cycles of ALD growth using Mo(NMe2)4 as the metal-organic precursor and H2O as the oxidant. Analyses of the in situ transconductance at the growth temperature and ex situ variable temperature transconductance measurements indicate that the majority of the mobility enhancement observed at the beginning of dielectric growth is due to screening of charged impurity scattering by the adlayer. Control experiments show that exposure to only H2O or O2 induces more modest and reversible electronic changes in MoTe2 FETs, indicating that negligible oxidation of the TMD takes place during the ALD process. Due to the strong influence of the first <2 nm of deposition, when the dielectric adlayer may be discontinuous and still evolving in stoichiometry, this work highlights the need for further assessment of nucleation layers and initial deposition chemistry, which may be more important than the bulk composition of the oxide itself in optimizing performance and reproducibility.