Thermal Stability of Titanium Contacts to MoS2

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Surfaces, Interfaces, and Applications

Thermal Stability of Titanium Contacts to MoS2 Keren M Freedy, Huairuo Zhang, Peter M Litwin, Leonid A Bendersky, Albert Davydov, and Stephen J. McDonnell ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08829 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Thermal Stability of Titanium Contacts to MoS2 Keren M. Freedy1, Huairuo Zhang*2,3, Peter M. Litwin1, Leonid A. Bendersky3, Albert V. Davydov3, Stephen McDonnell*1 1Department

of Materials Science and Engineering, University of Virginia, Charlottesville, VA

22904, USA 2Theiss

Research, Inc., La Jolla, CA 92037, USA

3Materials

Science and Engineering Division, National Institute of Standards and Technology

(NIST), Gaithersburg, MD 20899, USA

KEYWORDS: X-ray photoelectron spectroscopy, transmission electron microscopy, interface reactions, contacts, thermal annealing

ABSTRACT Thermal annealing of Ti contacts is commonly implemented in the fabrication of MoS2 devices however its effects on interface chemistry have not been previously reported in the literature. In this work, the thermal stability of titanium contacts deposited on geological bulk single crystals of MoS2 in ultra-high vacuum (UHV) is investigated with X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM). In the as-deposited condition, the reaction of Ti with MoS2 is observed resulting in a diffuse interface between the

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two materials that is comprised of metallic molybdenum and titanium sulfide compounds. Annealing Ti/MoS2 sequentially at 100 °C, 300 °C, and 600 °C for 30 min in UHV results in a gradual increase in the reaction products as measured by XPS. Accordingly, STEM reveals the formation of a new ordered phase and a Mo rich layer at the interface following heating. Due to the high degree of reactivity, the Ti/MoS2 interface is not thermally stable even at transistor operating temperatures of 100 °C, while post-deposition annealing further enhances the interfacial reactions. These findings have important consequences for electrical transport properties, highlighting the importance of interface chemistry in the metal contacts design and fabrication.

INTRODUCTION MoS2 has gained tremendous attention for device applications, offering a high degree of electrostatic control for channel length scaling and the ability to tune electronic and optical properties by simply varying the number of layers.1-3 It is widely known that the metal/MoS2 contact interface is critical in determining device performance, especially at short channel lengths.4-7 Annealing devices after contact deposition is a commonly implemented fabrication step in order to improve performance.1, 8-11 Baugher et al.11 claim that Schottky behavior was eliminated following vacuum annealing of devices with Ti/Au contacts. A decreased contact resistance for Ag contacts to MoS2 was observed by Abraham and Mohney8 who speculated that annealing causes the diffusion of Ag into MoS2 which results in doping under the contact. In some cases, the effects of annealing are reported to be dependent on the contact metal. For example, Radisavljevic et al.1 reported a factor of 10 decrease in resistance after annealing Au


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contacts to MoS2, while Ti/Au contacts were reported to behave better without annealing. Similarly, English et al.10 demonstrated very low resistance for annealed Au contacts while Ti contacts behaved poorly. Changes in device behavior upon annealing are undoubtedly linked to changes in the chemical composition and structure of the metal/MoS2 interface. Interface chemistry is typically overlooked in discussions of device fabircation and electrical properties. Previous studies of the effects of annealing on interface chemistry of UHV-deposited metals on MoS2 have been performed with Cr, Mn, and Fe by Durbin et al. and Lince et al.12-15 These studies demonstrate that thermal annealing has a drastic effect on interface chemistry, particularly for metals that exhibit reactivity with MoS2. Both Cr and Mn react with MoS2 in their as-deposited condition at room temperature. Heating the Cr/MoS2 and Mn/MoS2 interfaces has been found to drive the reaction to completion. In contrast, heating the non-reactive Fe/MoS2 interface was found to decouple the short range Fe-S chemical interactions at the interface.15 In this work, we examine the effects of thermal annealing on the interface chemistry of Ti contacts to MoS2. Ti contacts are commonly used in MoS2 devices and often annealed at temperatures ranging from 200 °C to 400 °C.10, 16-17 McGovern et al.18 and McDonnell et al.19 have demonstrated that a reaction occurs at room temperature when Ti is deposited on MoS2 in UHV, resulting in the formation of metallic Mo and TixSy compounds. Wu et al.20 arrived at a similar conclusion with density functional theory (DFT) and scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS). Nevertheless, the literature currently lacks reports on the effects of annealing on the interface chemistry of Ti/MoS2, which is essential for proper interpretation of device data.


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RESULTS AND DISCUSSION MoS2 geological crystals were used for this work as described in Experimental Methods. Before investigating the effects of thermal annealing, we closely examine chemistry of the interface in its as-deposited condition. Fitted XPS data of pristine and as-deposited Ti/MoS2 is shown in Figure 1. Following the deposition of a thin layer of Ti in UHV, the Mo0 chemical state is observed in the Mo 3d spectrum. This state appears at 227.83 eV whereas the Mo-S chemical state is at 229.50 eV. New chemical states in the S 2p spectrum correspond to Ti-S reaction products. This is consistent with previous reports by McGovern et al.18 and McDonnell et al19. Calculations by both McGovern et al.18, 21and Lince et al.24 conclude that the reaction to form titanium monosulfide, TiS, from Ti and MoS2 is thermodynamically favorable at 298 K having a Gibbs free energy change of -24.1 kcal/mol. Reference spectra are only available for TiS2 and TiS3 compounds22-25 but not for the other sulfides that exist in the Ti-S system26. TiS2 has its 2p3/2 component at 160.8 eV.23-24 TiS3 is characterized by two S 2p chemical states, one corresponding to the S2- atom at 161.2 eV and one corresponding to the S22- disulfide cluster at 162.5 eV.23 Our S 2p spectra do not consistently match with either indicating that a different stoichiometry occurs. Our spectra indicate that new chemical states exist with 2p3/2 components at 162.49 eV and 163.36 eV. The S-Mo chemical state is at 162.34 eV. Both the Mo-S and S-Mo states in Mo 3d and S 2p, respectively, exhibit a shift of 0.34 eV to higher binding energy. This is indicative of a shift in the position of the Fermi level which indicates that the Ti overlayer and its reaction products result in n-type doping of the substrate. Ti and Mo metal have been shown to be n-type dopants in MoS2.27-28


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Figure 1. Mo 3d and S 2p core levels before and after Ti deposition.

Atomic-resolution annular dark field STEM (ADF-STEM), X-ray energy dispersive spectroscopy (EDS) and EELS spectrum-imaging (SI) data acquired from a cross section of the as-deposited Ti/MoS2 is shown in Figure 2. The ADF-STEM image reveals the presence of a disordered Mo and S rich layer in contact with MoS2 (Figure 2a). This layer exhibits a diffuse interface with a Ti-rich disordered layer at the surface. The EDS measurements along with EELS line-scans provide unambiguous evidence of Ti diffusion into MoS2 as well as S and Mo outdiffusion into the Ti layer (Figure 2a-2d). Similar STEM/EELS analysis of the Ti/MoS2 interface by Wu et al.20 has shown the presence of Ti atoms in the topmost five layers, while the chemical composition the metal contact was assumed to be pure Ti. This assumption would appear to be inconsistent with our results, which show a substantial presence of Mo and S in the Ti layer (see Figure 2c-d). We note that Wu et al. acknowledged that the density of Ti increased with distance away from the Ti-MoS2 interface and therefore these results may in fact be consistent. Crosssectional HAADF-STEM analysis shows a ~7 nm thick Mo/S-rich amorphous layer which due to the damage of MoS2 during Ti deposition.


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Figure 2. (a) Cross-sectional ADF-STEM image of as deposited Ti/MoS2 showing disordered Ti-rich and Mo/S-rich layers, (b) 90° rotated ADF-STEM image showing the dotted white line where an EDS and EELS line-scan was simultaneously acquired, and (c) the corresponding EDS line profiles of the Ti-K, Mo-K (enlarged 5 times) and the overlapped Mo-L and S-K edges, (d) EELS line profiles of S-L and Ti-L edges. False colors are added in the ADF-STEM images to aid the eye.

The XPS spectra acquired during the in-situ sequential annealing are shown in Figure 3(a) with the intensity ratios shown in Figure 3(b). A substantial increase in reaction products is observed after 100 °C exposure as highlighted in Figure 3(c). While this change is minor in comparison with the other annealing temperatures, it may have important implications for reliability of MoS2 transistors since 100 °C is within the range of transistor operating temperatures.7 At 300 °C, we observe a clear increase in the Mo0 intensity relative to that of the Mo-S state, accompanied by an overall broadening of the S 2p spectra as more Mo-S bonds are broken to form new TixSy states. Figure 3(b) highlights the increase in the total S/MoS2 ratio, indicative of the increased diffusion of S to the surface. At 600 °C, the Mo-S chemical state is nearly completely consumed. We note that additional 100 °C anneals after annealing to higher


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temperatures exhibited no detectable change in interface chemistry. This suggests that the interface could be stabilized to avoid changes at operating temperatures. However, we highlight that earlier work has demonstrated that as deposited Ti/Au contact perform better before annealing.1 The Ti 2p spectra exhibit broadening due to the superposition of the different Ti-S states and show a decrease in intensity with increasing temperature. The reason for the apparent increase in the Ti/MoS2 intensity ratio in Figure 3(b) can be understood as the result of the MoS2 signal decreasing more significantly than the decrease of the Ti signal. The decrease in Ti intensity can be explained as a result of Ti diffusion into the MoS2 substrate and its potential agglomeration. Durbin et al.13 reported that in the case of Cr/MoS2, annealing to 650 °C drives the reaction between Cr and MoS2 while simultaneously causing coalescence of the Crcontaining portion of the film. Additionally, the core-level shift exhibited by the sample increases by another +0.36 eV after the final heating step at 600 °C, amounting to a total shift of 0.70 eV to higher binding energy compared to the pristine MoS2 surface.


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Figure 3. (a) XPS spectra acquired following 30 min anneals at each temperature. These were performed sequentially on the same sample. (b) Intensity ratios based on the data in (a) where (c) highlights the changes that occur at 100 ºC.


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Figure 4. (a) Cross-sectional ADF-STEM image of Ti/MoS2 after 30 min anneal at 400 ºC showing a Mo-rich layer and a partially recrystallized layer grown out from the disordered Mo/Srich layer, (b) and (c) are FFT images of the white dotted-line framed regions in (a), (d) 90° rotated ADF-STEM image showing the dotted white line where an EELS line-scan was acquired, and (e) the corresponding EELS line profiles of the S-L and Ti-L edges, the black dash-dot lines showing the fast dip of S and Ti in the Mo-rich layer. (f) ADF-STEM image showing the white dotted-line framed region where EDS mapping was acquired, and (g) the corresponding Mo-K, (h) Ti-K, and (i) the overlapped Mo-L and S-K edges maps. False colors are added to aid the eye. Cross-sectional ADF-STEM image of a sample annealed at 400 ºC is shown in Figure 4a. The disordered Mo/S-rich layer on top of MoS2 in the as-deposited sample was partially recrystallized. The fast Fourier Transform (FFT) images in Figure 4b and 4c, obtained from locations indicated in Figure 4a, suggest that this recrystallized layer has a structure similar to that of MoS2. In addition, an approximately 2 nm thick layer with enhanced contrast was found


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on top of the recrystallized layer. EELS line-scan in Figure 4d-e shows a fast decrease in S and Ti in this thin layer, while the EDS mapping in Figure 4f-i confirms that it is Mo-rich.

CONCLUSION The results shown here indicate that the Ti/MoS2 interface is highly reactive: a disordered, compositionally graded layer, comprised of Mo0 and TixSy species, forms at the surface of the MoS2 crystal following the deposition of Ti. Thermal annealing in the 100 ºC - 400 ºC temperature range results in further chemical and structural changes at the Ti/MoS2 interface and causes a degradation of Ti metal contact. STEM provides an evidence of the formation of new ordered phases upon heating to 400 ºC that did not exist in the as-deposited sample. Our work strongly suggests that the effects of thermal annealing on the compositional and structural evolution of the Ti/MoS2 interface must be taken into account when developing a post-deposition annealing process in the device fabrication steps and when interpreting changes in devices characteristics after annealing. EXPERIMENTAL SECTION MoS2 geological single crystals (SPI29) were cleaned by mechanical exfoliation, mounted onto Mo plates with Ta foil, and then immediately loaded into a Scienta Omicron UHV30 system described elsewhere with a base pressure 2 x 10-9 mTorr.31 XPS data was acquired at a pass energy of 50 eV with a monochromated 1486.7 eV Al kα X-ray source. XPS data was acquired on the pristine samples prior to metal deposition. A thin layer of metal was deposited by electron


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beam evaporation (Mantis Quad EV-C) in our UHV system and XPS data was acquired in-situ after deposition. Samples were heated in the UHV chamber to 100 °C, 300 °C, and 600 °C for 30 minutes at each temperature with ramp and cool times of approximately 30 minutes each. XPS measurements were performed following each heat treatment. Two samples were prepared for STEM: the first one did not undergo heat treatment following deposition and the second sample was heated directly to 400 °C for 30 min. An FEI Nova NanoLab 600 dual-beam (SEM/FIB) scanning electron microscopy and focused ion beam system was employed to prepare cross sections of the samples. Using electron beam deposition, 10 nm carbon and subsequently 100 nm Pt were deposited on top of the sample to protect its surface, followed by 2 µm Pt capping by ion-beam induced deposition. To reduce Ga-ions damage in the final step of FIB preparation, the STEM samples were thinned with 2 kV Ga-ions using a low beam current of 29 pA and a small incident angle of 3 degrees. An FEI Titan 80-300 STEM/TEM equipped with a monochromator and a probe spherical-aberration corrector was employed to perform atomic-resolution STEM imaging, X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) spectrum-imaging (SI). Atomic-resolution annular dark field (ADF) STEM images were acquired with an operating voltage of 300 kV, probe convergence angle of 14 mrad and collection angle of 34-195 mrad.

ASSOCIATED CONTENT Supporting Information is a PDF document including XPS spectra of the samples prepared for STEM analysis.


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AUTHOR INFORMATION Corresponding Author Stephen McDonnell, [email protected]; Huairuo Zhang, [email protected] Author Contributions The manuscript was written through contributions from all the authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT A.V.D. acknowledges the support of Material Genome Initiative funding allocated to NIST. H.Z. acknowledges support from the U.S. Department of Commerce, NIST under the financial assistance award 70NANB17H249. Research Innovation Funding from the UVa School of Engineering and Applied Science is acknowledged.

REFERENCES 1. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-Layer MoS2 Transistors. Nat Nano 2011, 6 (3), 147-150. 2. Amani, M.; Lien, D.-H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; Madhvapathy, S. R.; Addou, R.; KC, S.; Dubey, M.; Cho, K.; Wallace, R. M.; Lee, S.-C.; He, J.-H.; Ager, J. W.; Zhang, X.; Yablonovitch, E.; Javey, A., Near-Unity Photoluminescence Quantum Yield in MoS2. Science 2015, 350 (6264), 1065-1068. 3. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F., Atomically Thin MoS2: A New Direct-Gap Semiconductor. Physical Review Letters 2010, 105 (13), 136805. 4. Allain, A.; Kang, J.; Banerjee, K.; Kis, A., Electrical Contacts to Two-Dimensional Semiconductors. Nat Mater 2015, 14 (12), 1195-1205. 5. Kang, J.; Liu, W.; Banerjee, K., High-Performance MoS2 Transistors with LowResistance Molybdenum Contacts. Applied Physics Letters 2014, 104 (9), 093106.


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6. Liu, H.; Neal, A. T.; Ye, P. D., Channel Length Scaling of MoS2 MOSFETs. ACS Nano 2012, 6 (10), 8563-8569. 7. Giannazzo, F.; Fisichella, G.; Piazza, A.; Di Franco, S.; Greco, G.; Agnello, S.; Roccaforte, F., Impact of Contact Resistance on the Electrical Properties of MoS2 Transistors at Practical Operating Temperatures. Beilstein Journal of Nanotechnology 2017, 8, 254-263. 8. Abraham, M.; Mohney, S. E., Annealed Ag Contacts to MoS2 Field-Effect Transistors. Journal of Applied Physics 2017, 122 (11), 115306. 9. Han, J.; Lee, J.; Lee, J.; Woo, H.; Kim, J.; Jo, Y.; Cho, S.; Kim, H.; Kim, H.; Pawar, S. M.; Inamdar, A. I.; Jung, W.; Im, H., Electrical Properties of N2- and H2 -Annealed Bulk MoS2/Metal Junctions. Journal of the Korean Physical Society 2015, 67 (7), 1228-1231. 10. English, C. D.; Shine, G.; Dorgan, V. E.; Saraswat, K. C.; Pop, E., Improved Contacts to MoS2 Transistors by Ultra-High Vacuum Metal Deposition. Nano Letters 2016, 16 (6), 3824-3830. 11. Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P., Intrinsic Electronic Transport Properties of High-Quality Monolayer and Bilayer MoS2. Nano Letters 2013, 13 (9), 4212-4216. 12. Durbin, T. D.; Lince, J. R.; Didziulis, S. V.; Shuh, D. K.; Yarmoff, J. A., Soft X-ray Photoelectron Spectroscopy Study of the Interaction of Cr with MoS2(0001). Surface Science 1994, 302 (3), 314-328. 13. Durbin, T. D.; Lince, J. R.; Yarmoff, J. A., Chemical Interaction of Thin Cr Films with the MoS2(0001) Surface Studied by X‐Ray Photoelectron Spectroscopy and Scanning Auger Microscopy. Journal of Vacuum Science & Technology A 1992, 10 (4), 25292534. 14. Lince, J. R.; Stewart, T. B.; Fleischauer, P. D.; Yarmoff, J. A.; Taleb‐Ibrahimi, A., The Chemical Interaction of Mn with the MoS2(0001) Surface Studied by High‐Resolution Photoelectron Spectroscopy. Journal of Vacuum Science & Technology A 1989, 7 (3), 2469-2474. 15. Lince, J. R.; Stewart, T. B.; Hills, M. M.; Fleischauer, P. D.; Yarmoff, J. A.; TalebIbrahimi, A., Photoelectron Spectroscopic Study of the Interaction of Thin Fe Films with the MoS2(0001) Surface. Surface Science 1989, 223 (1), 65-81. 16. Qiu, H.; Pan, L.; Yao, Z.; Li, J.; Shi, Y.; Wang, X., Electrical Characterization of Back-Gated Bi-Layer MoS2 Field-Effect Transistors and the Effect of Ambient on Their Performances. Applied Physics Letters 2012, 100 (12), 123104. 17. Li, H.-M.; Lee, D.-Y.; Choi, M. S.; Qu, D.; Liu, X.; Ra, C.-H.; Yoo, W. J., MetalSemiconductor Barrier Modulation for High Photoresponse in Transition Metal Dichalcogenide Field Effect Transistors. Scientific Reports 2014, 4, 4041.


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18. McGovern, I. T.; Dietz, E.; Rotermund, H. H.; Bradshaw, A. M.; Braun, W.; Radlik, W.; McGilp, J. F., Soft X-ray Photoemission Spectroscopy of Metal-Molybdenum Bisulphide Interfaces. Surface Science 1985, 152-153, 1203-1212. 19. McDonnell, S.; Smyth, C.; Hinkle, C. L.; Wallace, R. M., MoS2–Titanium Contact Interface Reactions. ACS Applied Materials & Interfaces 2016, 8 (12), 8289-8294. 20. Wu, R. J.; Udyavara, S.; Ma, R.; Wang, Y.; Chhowalla, M.; Koester, S. J.; Neurock, M.; Mkhoyan, K. A., An Inside Look at the Ti-MoS2 Contact in Ultra-thin Field Effect Transistor with Atomic Resolution. arXiv preprint arXiv:1807.01377 2018. 21. Lince, J. R.; Carré, D. J.; Fleischauer, P. D., Schottky-Barrier Formation on a Covalent Semiconductor without Fermi-Level Pinning: The Metal-MoS2 Interface. Physical Review B 1987, 36 (3), 1647-1656. 22. Endo, K.; Ihara, H.; Watanabe, K.; Gonda, S.-I., XPS Study of One-Dimensional Compounds: TiS3. Journal of Solid State Chemistry 1982, 44 (2), 268-272. 23. Hawkins, C. G.; Whittaker-Brooks, L., Controlling Sulfur Vacancies in TiS2–x Cathode Insertion Hosts via the Conversion of TiS3 Nanobelts for Energy-Storage Applications. ACS Applied Nano Materials 2018, 1 (2), 851-859. 24. Gonbeau, D.; Guimon, C.; Pfister-Guillouzo, G.; Levasseur, A.; Meunier, G.; Dormoy, R., XPS Study of Thin Films of Titanium Oxysulfides. Surface Science 1991, 254 (1–3), 81-89. 25. Fleet, M. E.; Harmer, S. L.; Liu, X.; Nesbitt, H. W., Polarized X-Ray Absorption Spectroscopy and XPS of TiS3: S K- and Ti L-edge XANES and S and Ti 2p XPS. Surface Science 2005, 584 (2), 133-145. 26. Murray, J. L., The S−Ti (Sulfur-Titanium) System. Bulletin of Alloy Phase Diagrams 1986, 7 (2), 156-163. 27. Tsai, Y.; Li, Y., Impact of Doping Concentration on Electronic Properties of Transition Metal-Doped Monolayer Molybdenum Disulfide. IEEE Transactions on Electron Devices 2018, 65 (2), 733-738. 28. Feng, L.-p.; Su, J.; Liu, Z.-t., Effect of Vacancies in Monolayer MoS2 on Electronic Properties of Mo–MoS2 Contacts. RSC Advances 2015, 5 (26), 20538-20544. 29. SPI, https://www.2spi.com/. 30. Certain commercial equipment instruments or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology nor does it imply that the materials or equipment identified are necessarily the best available for the purpose 31. Freedy, K. M.; Litwin, P. M.; McDonnell, S. J., (Invited) In-Vacuo Studies of Transition Metal Dichalcogenide Synthesis and Layered Material Integration. ECS Transactions 2017, 77 (8), 11-25.


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Thermal Stability of Titanium Contacts to MoS2

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