Nucleation and growth of molybdenum disulfide grown by thermal atomic layer deposition on metal oxides

Abstract

To enable greater control over thermal atomic layer deposition (ALD) of molybdenum disulfide (MoS2), here we report studies of the reactions of molybdenum hexafluoride (MoF6) and hydrogen sulfide (H2S) with metal oxide substrates from nucleation to few-layer films. In situ quartz crystal microbalance experiments performed at 150, 200, and 250 °C revealed temperature-dependent nucleation behavior of the MoF6 precursor, which is attributed to variations in surface hydroxyl concentration with temperature. In situ Fourier transform infrared spectroscopy coupled with ex situ x-ray photoelectron spectroscopy (XPS) indicated the presence of molybdenum oxide and molybdenum oxyfluoride species during nucleation. Density functional theory calculations additionally support the formation of these species as well as predicted metal oxide to fluoride conversion. Residual gas analysis revealed reaction by-products, and the combined experimental and computational results provided insights into proposed nucleation surface reactions. With additional ALD cycles, Fourier transform infrared spectroscopy indicated steady film growth after ∼13 cycles at 200 °C. XPS revealed that higher deposition temperatures resulted in a higher fraction of MoS2 within the films. Deposition temperature was found to play an important role in film morphology with amorphous films obtained at 200 °C and below, while layered films with vertical platelets were observed at 250 °C. These results provide an improved understanding of MoS2 nucleation, which can guide surface preparation for the deposition of few-layer films and advance MoS2 toward integration into device manufacturing.

Thermal Atomic Layer Etching of MoS2 Using MoF6 and H2O

“…This reaction consumes Mo–O bonds and forms Mo–F and MoF x O in agreement with the in situ FTIR measurements and generates no gaseous products in agreement with the in situ QCM measurements. This reaction is consistent with our calculations that MoF 6 can disassociate on surfaces to form fluorides and oxyfluorides . We hypothesize that Mo is in the +4 oxidation state in S 2 Mo–O* and that both species on the right side of eq have Mo in the +5 oxidation state.…”

“…When the mass decrease due to the etching approaches the mass of the initial MoS 2 film, the etch rate decreases toward zero, indicating an “etch stop” (see Figure S1 in the Supporting Information), similar to the etch stop behavior reported previously for Al 2 O 3 ALE . The etch stop behavior observed here is attributed to an aluminum fluoride layer that forms during the initial MoS 2 ALD cycles on Al 2 O 3 but is not etched by the MoF 6 /H 2 O chemistry …”

“…This reaction is consistent with our calculations that MoF 6 can disassociate on surfaces to form fluorides and oxyfluorides. 35 We hypothesize that Mo is in the +4 oxidation state in S 2 Mo− O* and that both species on the right side of eq 5 have Mo in the +5 oxidation state. In eq 6, the H 2 O reaction liberates Mo as MoF 2 O 2 (g) with Mo in the +6 oxidation state, releases S as H 2 S(g), and forms HF(g) and H 2 (g) in agreement with the in situ QMS measurements.…”

“…Here, we report a thermal ALE process for a sulfide, MoS 2 , using MoF 6 as the fluorination source and H 2 O that allows the precise, isotropic removal of both amorphous and crystalline MoS 2 films. In addition to the MoS 2 study reported here, we have previously reported this etching chemistry for TiO 2 and Ta 2 O 5 thin films. − We first characterize the etching process using in situ quartz crystal microbalance (QCM) experiments as a function of temperature on ALD MoS 2 films prepared using alternating exposures of MoF 6 and H 2 S. , Next, we use Gibbs free energy calculations, Fourier transform infrared (FTIR) spectroscopy, and residual gas analysis (RGA) to identify potential mechanisms for the etching reactions. Finally, we measure the etch per cycle using ex situ spectroscopic ellipsometry (SE) and X-ray reflectivity (XRR) measurements and characterize the properties of etched MoS 2 prepared by ALD and mechanical exfoliation using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and atomic force microscopy (AFM).…”

“…40 The etch stop behavior observed here is attributed to an aluminum fluoride layer that forms during the initial MoS 2 ALD cycles on Al 2 O 3 but is not etched by the MoF 6 /H 2 O chemistry. 35 Figure 1b plots the mass changes recorded by in situ QCM during two MoS 2 ALE cycles. The average mass gain during the MoF 6 dose was 42 ng/cm 2 , and the average mass loss during the H 2 O dose is −61 ng/cm 2 .…”