In situ transport measurements reveal source of mobility enhancement of MoS2 and MoTe2 during dielectric deposition
Ju Ying Shang, Michael J. Moody, Jiazhen Chen, Sergiy Krylyuk, Albert Davydov, Tobin J. Marks, Lincoln J. Lauhon
Layered transition metal dichalcogenides (TMDs) and two-dimensional (2D) materials are widely studied as complements to Si complementary metal-oxide-semiconductor 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 concentrations 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, calling for in situ or operando methodology. We demonstrate that exposures to the oxidants H2O and O2 at temperatures and pressures relevant to ALD induce reversible electronic changes on MoTe2 FETs, indicating negligible oxidation takes place during exposure. Furthermore, we report in situ electronic measurements on MoS2 and MoTe2 FETs during ALD MoOx growth, which reveal that significant field- effect mobility improvement occurs within the first five cycles of growth. Further experiments confirm that this mobility enhancement is effected by modulation of the channel, rather than the contacts, and suggest that it is primarily due to screening of charged impurity scattering by the adlayer, rather than a reduction in scattering centers through reactions or an increase in screening by free carriers. Due to the strong influence of the first <2 nm of deposition, when the 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.