Atomic Layer Deposition of Molybdenum Oxides Atomic Layer

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Atomic Layer Deposition of Molybdenum Oxides with Tunable Stoichiometry Enables Controllable Doping of MoS

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Michael J. Moody, Alex Henning, Titel Jurca, Ju Ying Shang, Hadallia Bergeron, Itamar Balla, Jack N. Olding, Emily A. Weiss, Mark C Hersam, Tracy L. Lohr, Tobin J. Marks, and Lincoln J. Lauhon Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01171 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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Chemistry of Materials

Atomic Layer Deposition of Molybdenum Oxides with Tunable StoichiStoichiometry Enables Controllable Doping of MoS2 Michael J. Moody,†⁺ Alex Henning,†⁺ Titel Jurca,‡# Ju Ying Shang,† Hadallia Bergeron,† Itamar Balla,† Jack N. Olding,§ Emily A. Weiss,‡† Mark C Hersam,†‡∥⊥ Tracy L. Lohr,‡ Tobin J. Marks, ‡† Lincoln J. Lauhon†* † Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, United States ‡ Department of Chemistry, Northwestern University, Evanston, IL 60208, United States § Graduate Program in Applied Physics, Northwestern University, Evanston, IL 60208, United States ∥ Department of Medicine, Northwestern University, Evanston, IL 60208, United States ⊥ Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208, United States ABSTRACT: Metal oxides are a ubiquitous class of materials that have recently attracted interest as dopants for two dimensional (2D) materials. While distinct metal oxides have been used to achieve discrete doping changes, a metal oxide with a tunable oxidation state would allow engineering of carrier concentrations and band alignments in 2D devices. Herein we demonstrate new low-temperature atomic layer deposition (ALD) processes for molybdenum oxides and show that electronic properties can be tuned continuously with composition. We exploit this control to tune the carrier concentration in MoS2 field-effect transistors over a range spanning from depletion to accumulation (i.e., relatively p-type to n-type). Such doping should be broadly useful for 2D semiconductors due to uniform, nondestructive van der Waals nucleation and tunability of carrier concentration. More generally, scalable ALD of molybdenum oxide thin films with controlled oxidation states may have many other applications in catalysis and electronic materials.

Two dimensional (2D) materials are extraordinarily sensitive to their surroundings, and several schemes have been demonstrated to dope 2D semiconductors using nearby ions,1 molecules,2 or compounds3-5 without introducing chemical or structural defects that degrade charge carrier mobility. Of these dopants, oxides are attractive due to their stability and ability to function as dielectrics or passivation layers.6-8 Molybdenum trioxide in particular has been demonstrated as a dopant for multiple 2D semiconductors.4, 9 A practical oxide doping scheme requires not only deposition without damage to the 2D substrate, but also quantitative control over the doping density (e.g., via the oxide Fermi level), which is currently lacking. Oxide work functions are sensitive to composition in general, and this is particularly so for molybdenum oxides.10-11 Thus, control of oxide stoichiometry could enable precise control of the doping level in 2D semiconductors, an approach which we demonstrate here. By using a low-oxidationstate molybdenum precursor (Mo(NMe2)4) for atomic layer deposition,12 we report here a series of molybdenum oxide compositions that continuously tune the carrier density in the 2D semiconductor MoS2 across a range from relative ptype to n-type doping. Atomic layer deposition (ALD) excels at conformal growth of thin films with uniform composition because it

relies on self-limiting half-reactions.13 Alternating doses of precursors saturate the surface so that growth is fixed by precursor selection and temperature, and not by mass transport kinetics. Many ALD oxide processes also operate at low temperatures compatible with polymeric materials such as photoresists. Several molybdenum oxide ALD processes have been reported,14-17 but only those making use of oxygen plasma18 have been demonstrated at temperatures below 100°C. This limits process applicability to the extent that deposition of metallic molybdenum films followed by ex situ oxidation may be necessary.19 Despite the range of possible MoOx stoichiometries, MoOx grown with such strong oxidants is usually constrained to x≈3. Here, we demonstrate two new low-temperature ALD processes and use them to deposit molybdenum oxides with controlled molybdenum oxidation states. The optical bandgaps and resistivities increase with increasing oxygen content, following the same trends as crystalline molybdenum oxides. We then dope MoS2 by depositing a MoOx overlayer and tune the threshold voltage shift in thin film transistors (TFTs) from positive (p-type) to negative (ntype) by tuning the oxide composition. Low hysteresis in sweeps of the transistor gate voltage, an absence of strain, and reversibility upon oxide etching together suggest that this process maintains the van der Waals interface without

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significant chemical changes, and should thus be generally applicable as a doping strategy for 2D systems. ALD of amorphous molybdenum oxides was achieved using Mo(NMe2)4 in a two-step cycle with either water or ozone. Mo(NMe2)4 was prepared by a previously-described method12, 20 and full experimental details are provided in Supporting Information Section S1. Figure 1a shows stepwise mass gain by in situ quartz crystal microbalance (QCM) analysis, and Figure 1b quantifies these growth rates as a function of temperature. Due to the high reactivity of Mo(NMe2)4, growth is possible at low temperatures, here tested down to 60 °C. While growth is observed at higher temperatures, precursor decomposition (red crosses) sets the top of the ALD window at ca. 120 °C. In this temperature range, the deposited molybdenum oxide is amorphous (Supporting Information Section S2). Figures 1c and 1d confirm that the growth in this range is ALD by showing the self-limiting growth rate with increasing precursor dose. The mass growth rate using ozone is almost twice that of growth using water, indicating a higher areal reaction density. Thus, these cases represent two new low-temperature ALD processes for molybdenum oxides. Growth also proceeds with hydrogen peroxide and may constitute a third new ALD process, but determination of the self-limiting nature is complicated by H2O2 physisorption at low temperatures as discussed in Supporting Information Section S3. Indeed, due to the reactivity of this precursor, ALD or chemical vapor deposition (CVD) may even be possible with other oxygen atom transfer agents such as O2. Materials grown by the aforementioned water process and ozone process differ in several ways. As discussed below, ozone leads to higher oxygen content, higher resistivity, and optical absorption consistent with a larger bandgap. Using an ALD tin sulfide21 encapsulation scheme and variable-angle X-ray photoelectron spectroscopy (XPS) (see Supporting Information Section S4), the composition of the

Figure 1. ALD process features showing (a) linear growth with alternating pulses of Mo(NMe2)4 and oxidants, with negligible nucleation delay on amorphous aluminum oxide; (b) growth rate as a function of temperature; (c) saturation of oxidant reaction; and (d) saturation of Mo(NMe2)4 reaction at

a reactor temperature of 60 °C and a supply pressure of ~25 mtorr.

films below the surface can be fit to reveal differences in the Mo:O ratio and Mo binding energy (Figure 2b) that suggest that the oxide grown with water is closer to the MoO2 stoichiometry and the oxide grown with ozone is closer to MoO3. Hence, by using a highly reactive Mo(IV) precursor, we can access molybdenum oxides not previously produced by ALD. The water and ozone processes and supercycles thereof can be used to deposit amorphous oxides with a continuous range of compositions, and thus can be used to produce films with intermediate properties. While ALD is often described as the deposition of a continuous layer of material in each cycle, typically only a fraction of surface sites are covered; the growth per cycle is a fraction of a lattice parameter (here ca. 10%). Consequently, alternating processes with each cycle can in principle produce a uniform solid solution. In particular, the use of supercycles permits access to many small integer-ratio compositions between the end points of the water and ozone processes (Figure 2a). In Figure 2b, the Mo oxidation state is shown to take on intermediate values for supercycles with 4:1 and 1:1 ratios of water to ozone cycles. The optical absorption spectra (Figure 2c) also show a continuous increase in the energy of the absorption edge with increasing oxidation. Using the Tauc plot construction,22-23 this change of slope is consistent with increasing band gap going from the water film (0.8 eV) to the ozone film (2 eV) (Supporting Information Section S5). Resistivity of 10 nm films also varies monotonically, increasing from 6.2×104 to 5.4×107 to 1.6×108 Ω*cm as the water:ozone cycle ratio is varied from 1:0 to 4:1 to 1:1. The trends in electrical and optical properties with stoichiometry for these amorphous films are also observed in crystalline MoOx from MoO2 (metallic, 10-5 Ω*cm resistivity24) to crystalline MoO3 (3.2 eV bandgap, >104 Ω*cm resistivity25). Even in crystalline material reported as MoO3, significant vacancy concentrations can modify the stoichiometry and properties to intermediate values that are consistent with our results.10, 25-26 Here, though, unlike in vacuum evaporation of metal oxide powder, the composition can be controlled to tune electronic structure and transport in the deposited films.

Figure 2. Effect of oxidant on film showing (a) schematic of


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Chemistry of Materials deposition process (precursor pressure vs. time, resulting composition); (b) film composition information derived from XPS; and (c) optical absorption, both for 10 nm thick films.

MoOx with controllable oxidation state could be useful in many applications such as catalysis27-29 and semiconductor contacts,26, 30 and here we demonstrate its use as a charge transfer dopant for the transition metal dichalcogenide MoS2. Oxide charge transfer dopants are conventionally assumed to transfer a fixed amount of charge, but here, both accumulation and depletion can be produced by exploiting the MoOx composition range. Figure 3a schematically illustrates monolayer CVD31 FETs with an ALD MoOx doping layer, and Figure 3b shows an optical micrograph of such a device. AFM images in Figures 3c and 3d indicate that MoOx deposits uniformly on a monolayer MoS2 channel. Figure 3e shows transfer characteristics of back-gated monolayer MoS2 FETs before and after deposition of 10 nm MoOx with a 10 nm Al2O3 capping layer. Pre-deposition threshold voltages are similar in all samples (standard deviation 5 V, Supporting Information S6). Mobilities and on/off ratios are provided in Supporting Information Table S4. By depositing MoOx of different stoichiometries, threshold voltages are readily shifted tens of volts towards accumulation or depletion. Beyond tunability, a major advantage of this process is nucleation without covalent bonding to the semiconductor, which could degrade charge carrier mobility.6 The low temperature of this ALD process likely enables physisorptive nucleation, which is known to promote conformal growth and preserve van der Waals structures, and thus electronic properties.32-33 Films nucleate reversibly on inert CVD graphene, providing evidence that covalent bonding is not necessary (Supporting Information Section S7). Furthermore, the changes in MoS2 properties are reversible with a mild oxide etch (10 mM NaOH) as shown in Figure 3f. Raman spectroscopy suggests that strain due to encapsulation is no more than ~10-4 (Supporting Information Section S6), which is consistent with predominantly van der Waals bonding of the oxide. Further support is provided by AFM in that growth by reaction at defects typically results in island growth,34-35 but here the MoS2 rms roughness actually slightly decreases from before (0.39 nm, Figure 3c) to after (0.3 nm, Figure 3d) oxide deposition. This uniform growth is a further advantage of physisorptive nucleation when growing ultrathin films on 2D materials. To analyze the doping effects of the MoOx overlayer on the MoS2 carrier concentration, the change in carrier density was estimated using both the threshold voltage shift (via Q = V*Cox/A) and also changes in the Raman spectra (Figure 4a and Supporting Information Section S8).36-38 The electron densities calculated by the two methods are well correlated and decrease with higher MoOx oxidation state as expected for surface charge-transfer doping. In this way, controlling MoOx stoichiometry allows us to modify the carrier density in MoS2 over intermediate values in a technologically useful range. Here, relative p-type doping is observed as depletion of electrons. In fact, the ozone oxide produces a complete disappearance of the electron branch (Supporting Information Section S9), which may be due to p-type doping into inversion. Further modulation of

p-type transport should be possible with suitable contacts for hole injection.

Figure 3. Deposition on monolayer MoS2 as a dopant showing (a) schematic of doped FET structure, (b) optical image of typical device, morphology via AFM images of lithographically-processed CVD MoS2 flakes (c) before (rms roughness 0.39 nm) and (d) after (rms roughness 0.3 nm) water oxide deposition, (e) FET threshold voltage shifts after deposition of 10 nm MoOx / 10 nm Al2O3, and (f) reversibility of doping via wet etching. Z-scale is 2.5 nm and scale bar is 500 nm in (c) and (d).

In terms of doping mechanism, the present observations are consistent with charge-transfer doping,39 as shown schematically in Figure 4b, and with the expected trends in oxide electronic structure. In this model, equilibration of the semiconductor and dopant Fermi levels results in surface accumulation or depletion, with a higher work function dopant causing electron depletion as seen experimentally. Since the process does not rely on reactive functionalization or semiconductor-specific mechanisms, this scheme should in principle be applicable to any 2D material. In conclusion, we present here new ALD processes for molybdenum oxides with a range of stoichiometries, and thus a range of properties. We show that the use of new chemistry for MoOx ALD enables low temperature deposition under mild conditions, allowing uniform growth on 2D materials without damage. Such growth conditions are also enabling for growth on polymer substrates and resists. We also note possibilities for related ALD and CVD processes using Mo(NMe2)4 with other oxidants such as O2. Finally, we use MoOx to controllably dope CVD-grown monolayer MoS2. A range of applications for molybdenum oxide have been reported in areas such as catalysis,28 semiconductor contacts,26, 30 organic and 2D semiconductor doping,4, 9 and memristors,40 and its tunability has been recognized.11 Nonetheless, this work is the first to control the stoichiometry to leverage a range of useful electronic properties. The



scalable deposition methods presented here could there-

fore lead to further advances in each of these applications.

Figure 4. Effects of oxide doping, comparing (a) inferred carrier density changes (colored bars) and Raman shifts (black points) and (b) schematic of charge transfer doping. Although band bending is illustrated for clarity, the MoS2 is likely fully depleted/accumulated because the monolayer thickness is much less than the Debye length.

Supporting Information. Additional experimental details, auxiliary spectroscopic and electrical data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

national Institute for Nanotechnology. A.H. acknowledges the support of a Research Fellowship from the Deutsche Forschungsgemeinschaft (Grant HE 7999/1-1). H.B. acknowledges support from the NSERC Postgraduate ScholarshipDoctoral Program and National Science Foundation Graduate Research Fellowship.

AUTHOR INFORMATION

REFERENCES

Corresponding Author

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ASSOCIATED CONTENT

*E-mail:[email protected]

Present Addresses Addresses # Present address: Department of Chemistry and Cluster for the Rational Design of Catalysts for Energy Applications and Propulsion, University of Central Florida, Orlando, Florida 32816, United States

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ⁺ These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT M.J.M. acknowledges numerous helpful discussions with Dr. Jonathan D. Emery. M.J.M. also acknowledges Dr. Kan-Sheng Chen for supplying CVD graphene and Spencer A. Wells for discussion. This work was supported by the NSF via EFRI1433510 (M.J.M., T.J.) and DMR-1720139 (A.H., J.N.O.) and by U.S. Department of Commerce, National Institute of Standards and Technology under financial assistance award number 70NANB14H012 (J.Y.S, H.B, I.B.). It made use of the J.B.Cohen X-Ray Diffraction Facility and the Keck-II, EPIC, and SPID facilities of the NUANCE Center at Northwestern University, which have received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. M.J.M. gratefully acknowledges support from the Ryan Fellowship and the Northwestern University Inter-


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Atomic Layer Deposition of Molybdenum Oxides Atomic Layer

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