Influence of High-κ Dielectrics Integration on ALD-Based MoS2 Field-Effect Transistor Performance

The integration of high-κ dielectrics on MoS2 field-effect transistors (FETs) is essential for the realization of MoS2 in ultrascaled nanoelectronic devices and circuits. Most studies covering this topic are based on exfoliated MoS2 flakes or chemical vapor deposition (CVD) grown MoS2 films, whereas other techniques, such as atomic layer deposition (ALD), are also gaining attention for the growth of MoS2 in recent years. In this work, we grow large-area MoS2 by means of plasma-enhanced (PE-)ALD and evaluate the influence of high-κ dielectrics on the properties of ALD-based MoS2 FETs through electrical characterization combined with surface-chemical and high-resolution scanning transmission electron microscopy (HR-STEM) analyses. We grow HfOx, AlOx, or both by means of PE-ALD or thermal ALD on our fabricated devices and show that, in addition to the dielectric constant, three other major parameters related to the processing of the dielectrics can simultaneously affect the MoS2 FET electrical characteristics and govern its doping. These parameters are the stoichiometry of the dielectric, its carbon impurity content, and the degree to which the MoS2 surface oxidizes upon the dielectric growth. When grown at 100 °C, our HfOx films are oxygen-vacant whereas our AlOx films are oxygen-rich. In addition, carbon impurities are incorporated into the dielectrics at low deposition temperatures, being one of the likely causes of the MoS2 FET overall n-type performance in all of the studied cases. Our investigations also reveal that PE-ALD of HfOx or AlOx oxidizes the MoS2 surface, whereas thermal ALD AlOx leaves MoS2 almost intact. In this respect, if thermal ALD AlOx of proper thickness is grown between MoS2 and HfOx, it can reduce the degree to which the MoS2 surface oxidizes by HfOx and meanwhile improve the total dielectric constant, altogether leading to the most optimal electrical performance in ALD-based MoS2 FETs.


S.2.1 PE-ALD of HfO x
HfO x films were grown on MoS 2 by using PE-ALD.HyALD and O 2 plasma were employed as the precursor and the oxygen agent, respectively.During the first half cycle, HyALD precursor was delivered into the reaction chamber with a 100 sccm of Ar bubbling flow.The dosing time was 3 s at a pressure of 200 mTorr, followed by a precursor purge step of 2 s with an Ar flow of 100 sccm.During the next half cycle, O 2 plasma was introduced for 8 s, with a flow rate of 100 sccm, a forward power of 400 W and at a pressure of 15 mTorr.The cycle was completed by a plasma purge step using 50 sccm of Ar for 3 s.
During both purge steps, the pressure was adjusted with an automatic pressure-controlled (APC) valve, to ensure that the lowest possible pressure in the chamber was achieved.Further details of the HfO x synthesis process and film specifications were published previously by Sharma et al. 1

S.2.2 ALD of AlO x
During the ALD of AlO x , trimethylaluminum (TMA, Al(CH 3 ) 3 ) vapor and H 2 O were used as the reactants.Throughout the whole process, a constant flow of Ar was introduced with a rate of 100 sccm, and the chamber pressure was adjusted with an APC valve.When the APC valve was fully open (closed), the chamber pressure was the lowest (highest).The dosing time for the TMA precursor was 30 ms, and the subsequent Ar purging was 2 s long.In both steps, the APC was fully open.Next, H 2 O was dosed into the chamber for 100 ms, followed by a reaction step of 1 s.The APC valve was fully closed for these two steps.Finally, the chamber and the manifolds were purged with an Ar flow for 3 s and 1 s respectively, meanwhile the APC valve was fully open.

S.2.3 PE-ALD of AlO x
For the PE-ALD of AlO x , TMA and O 2 plasma were used, and the pressure was adjusted with the APC unit.Throughout the growth process, a 100 sccm of Ar and 50 sccm of O 2 were continuously used, as O 2 does not react with TMA under the employed conditions.The TMA dosing time was 20 ms, and its purging duration was 1.5 s (both with a fully open APC).During the reaction step, an O 2 plasma was initiated into the chamber for 2 s, with a forward power of 200 W and at the highest achieved pressure (a fully closed APC).The subsequent plasma purge step was 0.5 s with a fully open APC.

S.3.1 C 1s Core Level Spectra
Figure S2, provides the C 1s spectrum before and after the growth of ~2.5 nm HfO x on MoS 2 at different deposition temperatures.As can be seen from the plot, C(=O)-OH (carboxyl) species are present after the growth of HfO x at 100 °C.This indicates that carbon impurities are incorporating into the HfO x films at low deposition temperatures, and they can contribute to doping the MoS 2 devices to n-type 2 parallel to the HfO x oxygen vacancies.The latter is already discussed in the main text.However, by elevating the deposition temperature to 200 °C and 300 °C, the FWHM of carboxyl species reduces, suggesting that C-content has less influential role in n-type doping of MoS 2 .

Figure S2
The C 1s core level spectra before and after the growth of HfO x on MoS 2 at various deposition temperatures.

S.3.2 XPS Quantitative Details
To have an estimation on the degree of MoS 2 oxidation upon the growth of HfO x at various temperatures, Mo 6+ /Mo 4+ ratio was determined from the integrated area under the Mo 4+ and Mo 6+ doublets.Table S1 shows the obtained values for the Mo 4+ and Mo 6+ doublet peaks (counts per seconds (CPS)).In addition to the Mo 4+ and Mo 6+ values, the Mo 3d 5/2 and S 2p 3/2 peak positions as well as their relative binding energy (BE) shifts, with respect to bare the MoS 2 case, are provided in this table.
Table S1.The integrated area under the Mo 4+ and Mo 6+ peaks and their ratio, as well as the Mo 3d 5/2 and S 2p 3/2 peak positions and their BE shifts (with respect to the bare MoS 2 case) after the growth of 2.5 nm HfO x at various deposition temperatures.

S.3.3 MoS 2 Raman Analysis
To verify that MoS 2 is structurally intact upon the growth of HfO x on MoS 2 , Raman analysis was employed.The set-up used for this purpose was Renishaw InVia confocal Raman microscopy, which was equipped with a 514 nm laser, an integrated switchable grating with 600 or 1800 lines/mm and a charge-coupled device (CCD) detector.During the Raman scans, 5 accumulations with an acquisition time of 10 s were taken, using a (laser power of <0.2 mW focused on a ∼1 μm region).Figure S3 shows the obtained data before and after the growth of 30 nm HfO x films at various deposition temperatures.As can be seen, the characteristic MoS 2 Raman modes (A 1g and E 1 2g peaks) are yet present after the growth of HfO x , irrespective of the deposition temperature, providing no significant evidence that the growth of HfO x damages the entire underlying MoS 2 .

Figure S3
. Raman analysis before and after the growth of 30 nm HfO x films at various deposition temperatures.Figure S5 shows the Mo3d spectrum after the growth of ~5 nm HfO x at 100 °C, which is considered as our reference case.Here, the plot is provided separately for better visualization.

Figure S5
The Mo 3d spectrum after the growth of ~5 nm HfO x on MoS 2 at 100 °C.

S.4.3 XPS Quantitative Details
Table S2 provides the details of the Mo 6+ and Mo 4+ chemical states, the Mo 3d 5/2 and S 2p 3/2 peak positions as well as their relative BE shifts, after the growth of 5 nm HfO x and AlO x dielectrics on MoS 2 .
Table S2 CPS area under the Mo 4+ and Mo 6+ doublet peaks, their ratio, the Mo 3d 5/2 and S 2p 3/2 peak positions and their BE shifts after the growth of 5 nm of HfO x and AlO x .Figure S7 shows the S 2p, C 1s and Si 2p core level spectra after the growth of 2.5/2.5 nm AlO x /HfO x on MoS 2 .The data are compared with respect to the 5 nm of HfO x grown of MoS 2 .The Si 2p spectrum is provided without any extra fitting because only the peak intensity was of interest.Table S3 provides the acquired data from the XPS analysis (Mo 6+ /Mo 4+ , Mo 3d 5/2 and S 2p 3/2 ), after the growth of 2.5/2.5 nm of AlO x /HfO x on MoS 2 films.

CPS
Table S3 CPS area under the Mo 4+ and Mo 6+ doublet peaks, their ratio as well as the Mo 3d 5/2 and S 2p 3/2 peak position and their binding energy shifts after the growth of 2.5/2.5 nm of AlO x /HfO x bilayers on MoS 2 .

S.5.4 HR-STEM Analysis
Figure S8 displays the HAADF-STEM images of 2.5/2.5 nm PE-ALD HfO x /ALD AlO x on MoS 2 in various magnification modes, to better visualize that the nucleation of the dielectric stack occurs initially at the grain boundaries and defect sites.The images also illustrate that 2.5/2.5 nm dielectric does not lead to a completely closed dielectric layer on the MoS 2 surface.As can be seen from Figure S9, it is only upon the growth of 5/25 nm ALD AlO x /PE-ALD HfO x on MoS 2 FETs that I ON substantially improves and increases close to 1 µA/µm (on average and compared with the reference).

S2S. 1
Figure S1(a) and (b) show the top view microscopic image and the schematic cross-section of the fabricated MoS 2 devices capped with a layer of high-κ on top, respectively.

Figure S1 .S. 2
Figure S1.(a) Microscopic top view image of the fabricated devices, under measurement with probe tips.The visible patterns are the Ti/Au contact pads, (b) cross-sectional schematics of the fabricated MoS 2 devices.
Figure S4(a), (b), (c) show the average statistical data for I ON, maximum µ FE mobility, and I OFF of the MoS 2 FETs, respectively, capped with ALD and PE-ALD of AlO x as well as PE-ALD of HfO x .

Figure S4 . 2
Figure S4.Average data for (a) I ON , (b) the maximum µ FE and (c) I OFF upon ALD and PE-ALD of AlO x as well as PE-ALD of HfO x on the fabricated MoS 2 FETs.

2 FET
Figure S6(a), (b), (c), (d) and (e) illustrate linear transfer curve, average max µ FE , I ON , I OFF and I ON /I OFF data of the MoS 2 FETs, respectively, capped with 2.5/27.5 nm of AlO x /HfO x bilayers.

Figure S6 .
Figure S6.MoS 2 FET (a) linear transfer curve, (b) average maximum µ FE , (c) average I ON , (d) average I OFF and (e) average I ON /I OFF ratio after being capped with 2.5/27.5 nm of AlO x /HfO x .

Figure S7 .
Figure S7.The S 2p, C 1s and Si 2p core level spectra, after the growth of 2.5/2.5 nm of AlO x /HfO x bilayer on MoS 2 , with respect to 5 nm of HfO x on MoS 2 .

Figure S8 .
Figure S8.HAADF-STEM image of 2.5/2.5 nm PE-ALD HfO x /ALD AlO x on MoS 2 in various magnification modes for further visualizing that the dielectric nucleation initially starts at the grain boundaries and/or defects sites.

Figure S9 . 2 Figure
Figure S9.The average I ON of the MoS 2 FETs capped with 5/25 nm AlO x /HfO x

Figure S12 .
Figure S12.The HAADF STEM images of the ALD AlO x /PEALD HfO x on MoS 2 with various AlO x interlayer thicknesses.