Atmospheric and Long-term Aging Effects on the Electrical Properties

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Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe2 Transistors Anna Hoffman, Michael G. Stanford, Cheng Zhang, Ilia N. Ivanov, Akinola D. Oyedele, Maria Sales, Stephen McDonnell, Michael Koehler, David Mandrus, Liangbo Liang, Bobby G. Sumpter, Kai Xiao, and Philip D. Rack ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12545 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on October 2, 2018

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ACS Applied Materials & Interfaces

Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe2 Transistors Anna N. Hoffman1, Michael G. Stanford1, Cheng Zhang1, Ilia N. Ivanov2, Akinola D. Oyedele2,3, Maria Gabriela Sales5, Stephen J. McDonnell5, Michael R. Koehler1, David G. Mandrus1,2, Liangbo Liang2, Bobby G. Sumpter2,4, Kai Xiao2,3, Philip D. Rack1,2 * 1

Department of Materials Science & Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA

2

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,USA

3

Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee 37996, USA

4

Computational Sciences & Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831,USA 5

Department of Materials Science & Engineering, University of Virginia, Charlottesville, Virginia 22904, USA.

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KEYWORDS transition metal dichalcogenide, TMD, WSe2, atmosphere, ambient, environment, field-effect transistor

ABSTRACT Atmospheric and long-term aging effects on electrical properties of WSe2 transistors with various thicknesses are examined. While countless published studies report electrical properties of transition metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe2 pristine devices are predominantly n-type which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short term reversible atmospheric effect can be attributed primarily to physisorbed H2O on pristine WSe2, which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 hours demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Additionally, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 hours) is attributed to O2 and H2O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H2O-pristine WSe2. The residual/permanent aging component is attributed to electron trapping molecular O2 and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects demonstrated have the expected thickness


2

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dependence; namely thinner devices are more sensitive to atmospheric and long-term aging effects.

INTRODUCTION Since the demonstration of graphene’s remarkable properties in 20041, layered 2D materials have been the focus of intense research. For many electronic applications graphene is not ideal due to the absence of a bandgap, however this challenge has inspired the investigation of other layered materials. Transition metal dichalcogenides (TMDs) such as MoS2, WS2, MoSe2, and WSe2 have gained interest due to their unique properties applicable in 2D electronics.2–4 Like graphene, these layered materials stack in clean, dangling bond-free surfaces, which allows for easy thin film device fabrication5. Unlike graphene, many TMDs have thickness-dependent bandgaps which make them attractive for electrical and optoelectronic applications. For instance, several TMDs possess a direct bandgap at the monolayer while at bilayer and thicker there is a transition to a smaller, indirect bandgap.6 Layer dependence also impacts carrier concentration and mobility.7,8 These layer-dependent properties can be leveraged to tune the behavior of TMDs in optoelectronic devices. Previous work on the layer dependent electrical characteristics of the Cr/Au contact scheme used in this study has shown a systematic shift in the carrier type with increasing layer thickness3; namely thinner flakes are p-type and as layer thickness increases, n-type conduction increases, which was rationalized based on the shifts in the conduction and valence band positions and subsequently the barrier height for each carrier. Interestingly, scanning tunneling microscopy (STM) of various metal contacts to MoS2 have revealed that intrinsic defects can


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dominate the contact resistance and provide low n- and p-type Schottky barriers for carrier injection.9 Regarding the Cr-WSe2 contact region, the energy band diagram has recently been studied via XPS for bulk crystals.10 The band gap (1.3 eV) and Fermi level position (0.34 eV from conduction band edge) of the pristine WSe2 was initially determined and found to shift ~ 0.34 eV towards the mid-gap after chromium was deposited in ultra-high vacuum, suggesting a larger than expected effective chromium work function (4.92 eV versus 4.5-4.6 eV). The larger effective work function could be due to CrSex formation, which is thermodynamically favorable. The picture is more complicated for high-vacuum contact deposition in which XPS reveals CrOx and WSexOy was detected. Challenges to the realization of widespread TMD applications include controlled large area materials growth with consistent material properties.11–17 Furthermore, the effects of routine semiconductor processing chemistries and environments need to be understood. While many TMDs are touted as “air-stable,” elucidating the effect of atmosphere is necessary as adsorption and oxidation can deleteriously affect the device properties.18–20 Surface layers without dangling bonds are, in general, chemically inert but surface defects and flake edges are still vulnerable to physisorption of polar molecules.21,22 Studies have shown various encapsulation methods can mitigate the atmospheric impact on thin films. However, the various encapsulation methods themselves can alter the properties of the functional thin film, which in what is studied here is the semiconducting channel region. Polymer spin-coated encapsulation has shown reduced structural degradation in TMD FETs23, 4


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however, another report suggests that the TMDs transform from semiconducting to metallic and the electrical properties degrade after extended time in ambient24. Molecular doping can passivate surface defects, however this process obviously modifies intrinsic conduction and has been shown to degrade over time in atmosphere25,26. Encapsulation graphene or hexagonal boron nitrate (h-BN) is a common method, which preserves device performance over extended exposure to atmosphere, but the stacking fabrication process is complex with high probability of cracks and is not practical for large scale fabrication27,28. Even as encapsulation methods improve, it is vital to understand of atmospheric and aging effects on these materials as inevitably the materials are exposed to ambient conditions during device processing. The effect of physisorbed species and oxidation on electrical properties of MoS2 has been studied,19,20,29–33 but limited information is available for WSe2. WSe2 can be ambipolar11 which provides the unique opportunity to observe the effects of ambient environment on n-type and ptype charge carriers. Additionally, it was shown that adsorbed atmospheric species and oxidation effects on selenides are more significant than on sulfides.34 Here we demonstrate the reversible effect of physisorbed atmospheric gas on n- and p-type transport as a function of WSe2 layers, by measuring the transfer characteristics of field-effect transistors (FET) in air and vacuum. The reversible characteristics are attributed to H2O/O2 physical adsorption, which preferentially depletes electron carriers.35 Subsequently, we study the effects of long term exposure to atmosphere on the electrical properties of WSe2. With prolonged exposure to atmosphere, n-type on-current decreases and p-type on-current increases. Density functional theory (DFT) 5



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calculations suggest that the observed changes can be attributed to chemisorption of molecular O2 at selenium vacancies and isoelectronic O chemisorption of Se vacancies. DISCUSSION To elucidate the atmospheric and aging effects on WSe2 multilayer devices, we performed a series of electrical tests in air and vacuum as a function of atmosphere exposure time. Five bottom gate devices were fabricated with a Cr/Au contact scheme on 10 layer (L), 9L, 7L, 4L, and 3L flakes. To distinguish the relative activation energy of adsorbed species, both short-term (~20 minute) and long-term (~48 hours) vacuum hold electrical measurements were performed. In what follows, we will first show atmospheric effects on as-fabricated devices. Subsequently, by distinguishing the electrical characteristics of the short-term and long-term vacuum pumping at specific aging times, we assign various physisorbed and chemisorbed species to the observed changes in the electrical characteristics. Figure 1a shows a set of transfer curves for a 7-layer FET measured in air, then held in vacuum for two-days and re-measured in vacuum, as a function of atmospheric exposure time. The device was measured the day it was fabricated (as-fab) then aged for 300 hours, about 13 days, and re-measured in air, vacuum, and additionally an N2 environment. The device was then aged for an additional 600 hours, about 25 days, and re-measured in air and vacuum. The representative curves for other WSe2 thicknesses can be found in supporting information. As will be described throughout this work, Figures 1b and 1c depict the schematic of as-fabricated devices in air and vacuum, respectively; changes in the electrical properties of the as-fabricated sample in atmosphere and after a short vacuum pump-down is attributed to H2O physisorption on 6


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pristine WSe2. Figures 1d and 1e depict schematically the devices in vacuum after 900hr and 3600hr of atmospheric aging, respectively, illustrating how H2O/O2 physisorption on selenium vacancies dominates initially and chemisorbed molecular O2 and atomic O at selenium vacancies dominate at longer aging times. The data shows that for all thicknesses and aging times, there is an increase in n-type conduction in vacuum compared to atmosphere. Furthermore, for the devices tested after 300 hours of aging, there is no significant change in the transfer characteristics when the pumped vacuum chamber is flushed with N2 gas indicating this n-type conduction increase is not pressure dependent and not affected by N2 adsorption. To observe the short-term conduction shift associated with physisorbed atmospheric gases, transfer measurements were taken on a pristine as-fabricated 3-layer device during a 20-minute pump-down cycle (Fig. 2a) and venting cycle (Fig. 2b) after the device was held in vacuum for two-days (see Methods for details and comprehensive electrical data set for additional flake thicknesses can be found in SI). The n-type conduction increases >200x during the 20-minute pump-down and reaches a stable value, which remains unchanged after a two-day exposure to vacuum. The n-type current reversibly decreases toward the original value when subsequently exposed to air for 20-minutes. The largest change occurs in the first 2 minutes of both cycles during which we were unable to collect data due to system vibrations, hence the large gap between the first and second curves in both Fig. 2a and Fig. 2b. The p-type conduction modestly decreases during pump-down and reversibly increases during venting. A plot of the ratio of oncurrent (at +/-70V gate voltage) measured after the two-day vacuum exposure, to the atmosphere 7



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measurement for n-type and p-type conduction, respectively, is plotted as a function of layer thickness in Figure 2c. It is clear that the environmental impact on n-type conduction has a strong thickness dependent while p-type conduction is minimally affected. The reversible, increase of n-type conduction upon exposure to vacuum suggests adsorbed molecules are modulating electron transport. Adsorbed surface molecules intuitively affect thinner materials more strongly due to their higher surface to volume ratio which is consistent with our observed layer dependence. DFT calculations by Wang et al. estimated that water molecules have the highest binding energy of atmospheric gases of about 45 meV on pristine WSe2 and that physisorbed H2O molecules act as an electron trap with a charge transfer of 0.0186 e- per adsorbed H2O molecule.36 Thus, the observed n-type pressure dependent conduction is attributed to electron transfer from WSe2 to physisorbed water molecules. Importantly, the binding energy of H2O-WSe2 is much lower than H2O-stainless steel chamber (~ 1 eV) 37 ; thus the coverage on the WSe2 is dictated by the residual H2O partial pressure, which after the initial volume of gas is pumped is limited by the desorption from the chamber walls. Similarly, Qiu et al. showed that the n-type conduction of bilayer MoS2 increased by a factor of ~ 10 in 10-6 Torr vacuum and increased another factor of 5 upon vacuum annealing at 350K. The vacuum annealing reduced the O1s XPS peak by ~ 43% and the observed increase in n-type conduction were attributed to either or both H2O and O2 adsorption.20 More recently, Shimazu et al. studied the hysteresis behavior of MoS2 devices under different ambients and noted that H2O generated the largest hysteresis; though hole trapping was attributed to the ~ +28 V threshold 8


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voltage shift during the reverse sweep.38 Finally, H2O and O2 adsorption has been shown to reversibly act as a photoluminescence gate modulator in n-type TMD materials; the photoluminescence modulation was attributed to the adsorbed species depleting the n-type carriers, which weakens electrostatic screening that would otherwise destabilize excitons. 39 In addition to the impact of adsorbed atmospheric molecules, various aging phenomena were observed. Figure 3a and 3b show the n-type and p-type on-currents at +/-70V gate bias, respectively, measured in air after over 900hrs of aging time in atmosphere (1200hrs for the 10L device). A similar plot with on-currents in vacuum added can be found in Fig. SI5. The conduction changes for both n-type and p-type over time are consistent with the effects of the adsorbed molecules, namely: with increasing aging time the n-type conduction decreases while p-type conduction increases. While adsorbed H2O additionally affects the transport of aged devices, the long-term aging effects produce a different behavior during the pump-down and venting cycles as discussed ahead. The ratio of aged to as-fab on-currents measured in atmosphere are plotted in Figure 3c, which shows the expected general trend that the long-term aging effects thinner flakes more, note exceptions to the data trends exist for instance the 3 layer n-type ratio and 4 layer p-type ratio. A plot including the 10L device, which was aged for a different amount of time, can be found in SI. Furthermore, hole conduction increases more than electron conduction is suppressed. Figure 3d shows the normalized Raman spectrum of an exfoliated monolayer flake, which was measured immediately after exfoliation and after 700 hours of ambient aging (see SI for other thicknesses). Significant degradation and disorder in 9



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TMDs can cause the Raman peaks to broaden, shift in position, and quench.40,41 Monolayers are particularly susceptible to these effects. In this study, there is no discernable shift in the Raman spectra for the monolayer or multilayer WSe2 after aging, and there is no evidence of oxidationinduced changes. This is due to the thermodynamic mechanisms of aging in ambient, as described below, in which rather than crystalline tungsten oxide formation, H2O/O2 physisorption, and molecular O2 and atomic O chemisorption, occur at chalcogen vacancies and therefore we did not observe WOx in the Raman spectra. To further elucidate the long-term aging effects on electrical properties of WSe2, DFT simulations were performed similar to previous reports.42,43 Figure 4 illustrates the electronic band structure of (a) single layer WSe2 and (b) WSe2 with a Se vacancy. Consistent with previous DFT calculations the Se vacancy introduces two almost degenerate gap states approximately 0.37 eV from the conduction band. Clearly, a high concentration of Se vacancies will shift the Fermi level towards the conduction band rendering it n-type, which is common for many TMDs. Figure 4c shows the band structure of an O2 molecule chemically adsorbed to the Se vacancy, which reflects that the dangling bonds are saturated; 1.37 e- are transferred from the WSe2 to the O2 molecule and the adsorption energy has been calculated to be 2.36 eV.44 The O2 molecule can also be physically adsorbed above the Se vacancy with the adsorption energy of ~ 0.24 eV (where about 0.26 e- transferred from the WSe2 to the O2 molecule), and the barrier energy to be chemically adsorbed at the vacancy site has been calculated to be between 0.45-0.91 eV depending on calculation methods.45 This energy barrier implies that the oxidation of the 10


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vacancy site is possible after long-term aging. In contrast, for the H2O molecule, DFT simulations suggested that it can only be physically adsorbed above the Se vacancy site: only 0.05 e- are transferred from the WSe2 to the H2O and the adsorption energy is about 0.3 eV.46 Similar results for O2 and H2O adsorption were calculated for the S vacancy in defective MoS2.47,48 Finally, Figure 4d illustrates the band structure for a dissociated O2 molecule where one O atom chemisorbs or isoelectronically fills the Se vacancy, and the other O atom adsorbs on a neighboring Se. This process has an about ~ 0.58 eV activation barrier,44 but when overcome, 1.99 e- are transferred from WSe2 and again the gap state is eliminated. Both the chemically adsorbed molecular O2 and atomic O at the Se vacancy effectively eliminate the donor level associated with the Se vacancy and thus for both mechanisms in an extended material one would expect a progressive shift in the Fermi level towards mid-gap. Interestingly, our calculations also show a downshift in both the conduction band and valence band energy for the isoelectronic O substitution relative to the pristine WSe2. Thus for this mechanism, a further increase in the work function is realized. Unfortunately, both of these mechanisms are difficult to confirm because the doping level concentrations ( 0.1 nA are plotted. Devices were measured initially in air and vacuum immediately after fabrication, then aged in atmosphere and periodically measured in air and vacuum (and in one instance in an N2 ambient) over a period of 900 hours. One device in the set was measured in air and vacuum after 3600 hours of ambient exposure to observe extended aging. X-ray photoelectron spectroscopy (XPS) data was collected using monochromatic Al Kα x-rays (1486.7 eV) at a pass energy of 26 eV in a PHI VersaProbe III UHV system, equipped with an auto dual-charge neutralizer. A spot size of 9 µm was used. Spectral analysis was carried out using kolXPD software53 to fit the features present in the W 4f and Se 3d spectra, and accurately determine peak positions. All peaks were fit with Voigt 17



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lineshapes. Electronic structure calculation Plane-wave DFT calculations were carried out via the VASP package54 equipped with the projector augmented-wave (PAW) pseudopotentials. The exchange-correlation interactions were considered in the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional.55 Single-layer WSe2 was modeled by a periodic slab geometry with 18 Å vacuum separation in the out-of-plane direction to avoid spurious interactions with periodic images. For the hexagonal unit cell of pristine WSe2, its optimized in-plane lattice constant is 3.32 Å and 24×24×1 k-point samplings were used. A 5×5 supercell was then constructed to introduce a point Se vacancy to study the chemisorption of an O2 molecule near the vacancy site. For the supercell, 6×6×1 k-point samplings were used. For all studied systems, all atoms were relaxed until the residual forces below 0.01 eV/Å, and the cut-off energy was set at 400 eV.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional electrical and Raman results. (PDF) AUTHOR INFORMATION Corresponding Author 18


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*Email: [email protected] Author Contributions PDR conceived of the experimental plan and managed the project. ANH performed the device fabrication and electrical and Raman characterization. MGS, CZ, INI, ADO and KX facilitated device fabrication training and helped with the electrical and Raman measurements; MGS and SJM performed XPS measurements; LL and BGS did the DFT measurements; MRK and DGM grew the WSe2 bulk crystals. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan Notes The authors declare no conflict of interests. Acknowledgments

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PDR, ANH, LL, ADO, KX, and BGS acknowledge support from the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. L.L. was also supported by Eugene P. Wigner Fellowship at the Oak Ridge National Laboratory (ORNL). MGS, AH, and CZ acknowledge support by U.S. Department of Energy (DOE) under Grant No. DESC0002136. MK and DM acknowledge support from the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4416. MGS acknowledges support through the UVA Engineering Distinguished Fellowship. The XPS used for this work was provided through the NSF-MRI Award# 1626201. Device synthesis, characterization, Raman mapping, and electronic structure calculations were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility (http://energy.gov/downloads/doe-public-access-plan).

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Figures 29



Cr/Au

S

D WSe2

SiO2

VSD VGS

Silicon

As-Fab Air As-Fab Vacuum 900hr Aged Air 900hr Aged Vacuum

10 -5

I SD /w (A/um)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 39

10 -7 10 -9

10 -11 10 -13 -60

-30

0

30

60

V GS (V)

Abstract Graphic b. a.

c.

Selenium Tungsten Oxygen Hydrogen

d.

e.

30


Page 31 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. WSe2 transfer characteristics are sensitive to ambient and aging. (a) IDS-VGS curves of a 7-layer WSe2 FET measured in air and vacuum for as-fabricated, 300 hours, and 900 hours of atmospheric aging times. Additional measurement in inert N2 environment at 300 hours aged. (be) Device schematic illustrating the as-fabricated device in (b) air (c) and vacuum illustrating that physisorbed H2O on pristine WSe2 dominates the physisorbed atmospheric gas effects on the electrical properties. (d) Device in vacuum at modest atmospheric aging times, H2O and O2 physiosorb to Se vacancies depicting initial phase of atmospheric aging. (e) Device in vacuum at long-term aging times, molecular O2 and atomic O chemisorb at Se vacancies.

31



a.

10

-8

10

-10

10

-12

b.

Air 2m Vac 3m Vac 4m Vac 5m Vac 10m Vac 20m Vac

-6

Vacuum 2min 3min 4min 5min 10min 20min

10 -6

ISD /w (A/um)

10

I SD /w (A/um)

10 -8

10 -10

-60 -30

0

30

10 -12

60

-60 -30

c.

0

30

60

V GS (V)

V GS (V)

ION, Vacuum / ION, Air

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

1000 N

100

P

10 1 0

3

4

7 Layers

9

10

Figure 2. FET conduction shifts during pump down to vacuum and venting to air in as-fabricated device. (a) ISD-VGS curves for 3-layer WSe2 FET during pump down. (b) ISD-VGS curves for 3layer WSe2 FET during venting after two-day vacuum hold. (c) Ratio for various thickness asfabricated devices of ION in vacuum after two-day vacuum hold versus ION in ambient.

32


Page 33 of 39

10

-5

10

-6

10 -6

10

-7

10 -7

b.

Ion/w (A/um)

Ion/w (A/um)

a.

10 -8

10

10

-10

10

-11

10 -5

10 -8

10 -9

-9

10L 9L 7L 4L 3L

0

10L 9L 7L 4L 3L

10 -10

500

1000

10 -11

1500

Time (hr)

c. ION, Air Aged / ION, Air As-Fab

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

d.

500

1000

1500

Time (hr)

N P

1000 100 10 1 0.1 0.01 3

4

7 Layers

9

Figure 3. Long-term ambient aging effects. ION (ISD @ +/- 70VGS) of (a) n-type and (b) p-type conduction in air for various layer WSe2 FETs as a function of atmospheric aging time. (c) Ratio of Ion aged to as-fabricated for various layer thickness devices in air after 900 hours of atmospheric aging. (d) Raman spectra of monolayer WSe2 flake as-fabricated compared to the same monolayer flake after 700 hours of atmospheric aging.

33



Page 34 of 39

Figure 4. DFT calculated electronic band structures of the 5×5 supercell of WSe2 monolayer with (a) no defect, (b) a single selenium vacancy, (c) molecular chemisorption of an O2 molecule at the vacancy site, and (d) dissociative chemisorption of the O2 molecule around the vacancy site. All band energies are aligned to the vacuum potential for direct comparison. The corresponding atomic structure (top and/or side view) is on top of the band structure.

34


Page 35 of 39

-6

10

a.

10

-8

10

-10

10

-12

-60

-30

0

-6

b.

Air 2min 3min 4min 5min 10min 20min

Vacuum 2min 3min 4min 5min 10min 20min

10 -8

I SD/w (A/um)

10

ISD/w (A/um)

30

10

-10

10

-12

-60

60

-30

c.

0

30

60

Vg (V)

Vg (V)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000 N

100

P

10 1 0

3

4

7

9

Layers

Figure 5. FET conduction shifts during pump down to vacuum and venting to air in 900-hour aged device. (a) ISD-VGS curves for 3-layer 900 hour aged WSe2 FET during pump down. (b) ISDVGS curves for 3-layer 900-hour aged WSe2 FET during venting after two-day vacuum hold. (c) Ratio for various thickness 900-hour aged devices of ION in vacuum after two-day hold to ION in ambient.

35



10 -6

c. 1000 100 10 1 0.1 0.01

-8

10

-10

10

-12

10

-14

-60

-30

30

0

I SD /w (A/um)

I SD/w (A/um)

10

b.

Air 2min 3min 4min 5min 10min 20min

60

As-Fab

900hr

3600hr N P

Time

10

-6

10

-8

10

-10

10

-12

10

-14

Vac 2min 3min 4min 5min 10min 20min

-60

-30

d.

Vg (V)

0

30

60

Vg (V)

ION, 2 day Vacuum / ION, 20min Vacuum

a.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 39

As-Fab 300hr 900hr 3600hr 10 1 0.1

N

0.01

P

Time

Figure 6. FET conduction shifts during pump down to vacuum and venting to air in 3600-hour aged device. (a) ISD-VGS curves for 3-layer 3600-hour aged WSe2 FET during pump down. (b) ISD-VGS curves for 3-layer 3600-hour aged sample WSe2 FET during venting after two-day vacuum hold. Ratio of (c) ION in vacuum after two-day hold to ION in air and (d) ION in vacuum after two-day hold to after 20-minute hold for 3L WSe2 FET device at as-fabricated, 300hr atmospheric aged, 900-hour atmospheric aged, and 3600-hour atmospheric aging time.

36


Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Se 3d and W 4f spectra of WSe2 flakes on SiO2 after mechanical exfoliation (pristine), and after atmospheric aging for approximately 500-hour and 2900-hour. The 2900-hour aged sample was also pumped overnight at 10-7 Pa and re-measured. Note the 0.85 eV and 1.09 eV shift in the Fermi level towards the valence band for the 500 and 2900-hour aged sample, respectively, consistent with the electrical results.

37



Page 38 of 39

Table 1. Mobility values (cm2/V-s) for as-fabricated and aged WSe2 FETs Device Thickness

Carrier Type

Age and Environment As-Fab Air

As-Fab Vac

900hr Vac

9L

p n

0.0348 12.7526

0.0296 56.1052

0.9456 15.6556

3L

p n

0.0374 0.0143

0.0278 7.1335

0.4426 0.3765

TOC graphic-

38


Page 39 of 39

Cr/Au

S

D WSe2

SiO2

VSD VGS

Silicon

As-Fab Air As-Fab Vacuum 900hr Aged Air 900hr Aged Vacuum

10 -5

I SD /w (A/um)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 -7 10 -9

10 -11 10 -13 -60

-30

0

30

60

V GS (V)

39


Atmospheric and Long-term Aging Effects on the Electrical Properties

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