Electrothermal Local Annealing via Graphite Joule Heating on Two

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Functional Nanostructured Materials (including low-D carbon)

Electrothermal Local Annealing via Graphite Joule Heating on Two-Dimensional Layered Transistors Yoojoo Yun, Jeongmin Park, Hyun Kim, Jung Jun Bae, Min-Kyu Joo, and Dongseok Suh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06630 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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ACS Applied Materials & Interfaces

Electrothermal Local Annealing via Graphite Joule Heating on Two-Dimensional Layered Transistors

Yoojoo Yun,†,‡ Jeongmin Park,† Hyun Kim,†,‡ Jung Jun Bae,‡ Min-Kyu Joo┴,* and Dongseok Suh†,*

†

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of

Korea. ‡Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea. ┴Department of Applied Physics, Sookmyung Women’s University, Seoul 04310, Republic of Korea.

Keywords: Electrothermal local annealing (ELA), transition-metal dichalcogenides (TMDs), molybdenum disulfide (MoS2), surface adsorbates, coulomb scattering.

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Abstract

A simple but powerful device platform for electrothermal local annealing (ELA), via graphite Joule heating on the surface of transition-metal dichalcogenide, is suggested here to sustainably restore intrinsic electrical properties of atomically thin layered materials. Such two-dimensional materials are easily deteriorated by undesirable surface/interface adsorbates and are screened by a high metal-to-semiconductor contact resistance. The proposed ELA allows one to expect a better electrical performance such as an excess electron doping, an enhanced carrier mobility, and a reduced surface traps in a monolayer molybdenum disulfide (MoS2)/graphite heterostructure. The thermal distribution of local heating measured by an infrared thermal microscope and estimated by a finite element calculation shows that the annealing temperature reaches up to > 400 K at ambient condition and the high efficiency of site-specific annealing is demonstrated unlike the case of conventional global thermal annealing. This ELA platform can be further promoted as a practical gas sensor application, From an O2 cycling test and a low-frequency noise spectroscopy, the graphite on top of the MoS2 continuously recovers its initial condition from surface adsorbates. This ELA technique significantly improves the stability and reliability of its gas sensing capability, which can be expanded in various nanoscale device applications.


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Introduction

Tremendous interest in two-dimensional (2D) layered materials such as graphene and transition metal dichalcogenides (TMDCs), including their heterostructures, has been generated towards the exploration of unique electronic and optoelectronic physics1-13. However, in an extreme case such as an atomically thin monolayer, the intrinsic charge carrier features are easily hampered by the undesirable surface/interface adsorbates (such as moistures, oxygen molecules, polymer residues, etc.), interfacial traps, and high metal-tosemiconductor channel access resistance effects14-16. This is because ambient gases, including oxygen (O2) and water molecules, react with the surface of 2D layered materials, and capture the electrons inside, resulting in a significant degradation in device performance. To resolve these issues, a post thermal annealing treatment at a high temperature regime (200 − 400 °C) in vacuum is commonly used during the device fabrication processes2,15-19. This conventional thermal annealing (CTA) allows 2D layered materials to possess a high crystallinity, a low metal-semiconductor contact resistance, a small interfacial trap density, and a reduced PMMA residual influence. However, once exposed to an ambient condition after the CTA, without a proper passivation layer, surface contamination and oxidation processes would deteriorate the quality of channels made of 2D TMDCs, mainly owing to their 2D nature of an ultimately high surface-to-volume ratio. This further suggests a surface passivation with h-BN, poly(methyl methacrylate) (PMMA), and high-κ oxide layer, which largely suppresses undesired influences14,20-23. However, a CTA often degrades the quality of a TMDC’s crystallinity, and induces leakage current paths inside the gate dielectric materials as well as a passivation layer24. Basically, a CTA is a non-local heat treatment process affecting all parts of the sample, and it is only available in the ex-situ condition.


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Herein, a simple and sustainable curing device architecture is designed, using electrothermal local annealing (ELA) via graphite Joule heating on the surface of molybdenum disulfide (MoS2) to alleviate undesirable surface/interface of adsorbatesand high channel access barrier effects, leading to a negative flat-band (VFB) shift, carrier mobility enhancement, and a reduction in the surface trap density of MoS2 transistors without affecting other factors due to the advantages of local heating characteristics. In addition, the installation of this site-specific annealing function inside the nano device of 2D materials can enhance its performance in sensor applications. Experimental results of a cycling test for O2 exposure, low-frequency (LF) noise spectroscopy, and thermal image analysis allow for the further suggestion of a MoS2/graphite (MG) ELA platform as a non-disposable optimal chemical gas sensor system. Monolayer MoS2 flakes on silicon dioxide (SiO2), synthesized by chemical vapor deposition (CVD) method25, are first transferred to clean SiO2/p+-Si (300 nm) substrate by a standard wet transfer method. A mechanically exfoliated graphite film, ranging from 30 nm to 100 nm on a poly(vinyl alcohol)/PMMA, is then placed over the target MoS2 flake using a dry transfer method. The deposition of the bimetal layer of Cr/Au (5 nm/50 nm) to make the source (S) and drain (D) electrodes on both the MoS2 and graphite is followed by a lift-off process. The final schematic and the representative optical image of the MoS2/graphite (MG #1) ELA heterostructure are presented in Figure 1a and b. The monolayer MoS2 in the sample MG #1 was verified using two prominent Raman signals of (386 cm−1) and (404.3

cm−1) (Figure S1a)26. The thickness of the graphite (∼43 nm) is then determined from a height profile obtained using atomic force microscopy (AFM) (Figure S1b). Figure 1c illustrates a schematic for the ELA operation via graphite Joule heating on the surface of the MoS2. The top graphite layer in the MG #1 plays a crucial role, not only as a heat source but also as a passivation layer to protect the MoS2 from O2 or water molecules. In


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the same panel (Figure 1d), the representative transfer curves (drain current−gate voltage; ID−VG) of MG #2 from S (or Graphite) to D, measured at a 0.5 V drain voltage (VD), are compared after each treatment of O2 exposure, CTA, and ELA, respectively. Since O2 molecules on the MoS2 surface capture abundant free electrons inside of the MoS2, the O2 exposure leads to a relative p-doping effects (see inset of Figure 1d). To verify the pristine condition of the MG #2, a reference transfer curve of as-fabricated device was first obtained and CTA was then carried out for 4 h at 150 °C. After the CTA, O2 exposure was once again conducted to recover the original status, following which an ELA treatment was performed for 2 h using a power consumption of 109 − 115 mW for a direct comparison. Figure 1d clearly illustrates the large shift in turn-on voltage (approximately 40 V compared to that of a CTA), indicating a powerful ability to restore the pristine doping concentration of MoS2 in MG #2. It indicates that the ELA is an efficient annealing method better than CTA. Figure 2a presents the linear-scaled output characteristics (ID−VD) of MG #1 before and after the ELA for 110 min, which uses a power consumption of 200 mW, as a function of back-gate voltage (VG) in a high vacuum chamber (10−6 torr). The same graph in a logarithmic scale is displayed in Figure S2. The ELA heat treatment is easily accomplished by applying an electrical current from the drain (DG) to the source (SG) electrodes connected to the MG #1 graphite. This current flowing in the graphite induces Joule heating and effectively anneals the MoS2 channel near the graphite film, ideally up to 700 − 800 K in a vacuum condition27. After ELA, an ohmic-like behavior is clearly observed and the oncurrent (Ion) at VD = 200 mV is enhanced by a factor of 5. Figure 2b shows the corresponding ID−VG transfer curves at VD = 200 mV, with respect to a 10-min span of ELA from 0 to 110 min. The on-current of the MG #1 is gradually increased with the ELA time, resulting in a significant enhancement in the (= ( /)( ) , where , /, and denote


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the transconductance, geometrical ratio of the MoS2 channel, and oxide capacitance per unit area, respectively) at a VG of 50 V (Figure 2c). This further substantiates the beneficial effects of ELA on the MoS2 channel and contact resistance independently from MG #3 via 4-probe measurements (Figure S3). In addition to Ion, the VFB that was determined at ID = 100 pA is shifted negatively and saturated with ELA time (Figure 2d). The total voltage difference in VFB is found to be 14 V from the initial VFB. This directly corresponds to the relative electron excess doping concentration of the MoS2 from the initial VFB condition, which can be denoted by ∆ = ( − )/ (where is the saturated ). According to this expression, a ELA

time-dependent ∆ is plotted, because the obtained ∆ can be primarily attributed to the number of defects or vacancy sites in the MoS2, providing the opportunity for reaction with the undesirable surface/interface adsorbates16,28,29. In addition to the time-resolved ∆ evolution, an almost identical tendency between Figure 2d and the first-order chemical $%& adsorption model30,31 (which can be described by ln!∆ (")# = ∆ ∙ ($ with the

reaction rate coefficient ) (= 0.0013) and the incident time ") further implies that this ndoping process is governed predominantly by one specific origin. From this study, one tends to believe that the most plausible candidate can be the O2 among various other undesired surface/interface adsorbates on MoS2 as studied previously28. To investigate the heat spread by ELA and its corresponding temperature (T), the current bias-(through graphite (IG)) dependent thermal distribution images for MG #4 are shown in Figure 3a-d. The background temperature for this experiment, kept at 343.15 K, serves as a reference temperature. When IG increases to 50 mA (Figure 3c), the temperature of graphite (TG) reaches 403.07 K under ambient conditions, which would be much higher in case of vacuum conditions. Also, the spread of heat in the width direction is quite narrow,


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approximately 34 µm (Figure 3d). To further strengthen the temperature profile across the graphite, a finite-element method was employed, taking into account IG (20 mA), power consumption (156 mW), thermal conductivities of graphite (600 W/mK)32 and MoS2 (35 W/mK)33, and the thermal interfacial conductance of graphene/MoS2 (13.8 MW/m2K)34, graphene/SiO2 (50 MW/m2K)35, MoS2/SiO2 (0.01 MW/m2K)36, and Au/Ti/SiO2 (100 MW/m2K)37, respectively (Figure 3e-h). The found highest temperature is 434 K, which is higher than the TG by ELA obtained from an infrared thermal-imaging microscope system. The temperature gap between TG values from experiment (403.07 K in an ambient condition) and simulation (434 K in a vacuum) can be ascribed mainly to the difference in the surrounding air conditions. Based on the experimental and simulation temperature profile, the effective thermal width can be regarded as twice the graphite width, enabling the exclusive ‘on-site’ MoS2 annealing of the MG heterostructure. As a clear indicator, O2 gas was chosen as an optimal gas sensor platform for ELA in this study. Figure 4a displays the sustainable ELA effect on the ID−VG transfer curve of the MG #5 heterostructure at VD = 1 V. The ELA and O2 exposure process include a 1-h annealing with a power consumption of 110.44 mW, and a 2-h O2 exposure with a 600-torr condition, respectively. After O2 exposure, VFB was shifted positively as compared to the fabricated sample, indicating a relative p-doping effect. Meanwhile, the negative shift in VFB was apparently observed after ELA, suggesting strong n-doping effects. The similar Ion level as the fabricated sample after O2-exposure implies a lack of available defects and/or vacant sites for the reaction of O2 on the surface of MoS2. However, ELA effectively desorbs any O2 on the surface of MG #5, giving rise to the restoration of intrinsic electrical properties. This again demonstrates the effectiveness of the ELA platform as an optimal gas sensor application.


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To estimate the minimum ac noise level and the surface trap density, a low-frequency (LF) noise measurement at VD = 1 V was performed, with frequencies (f) ranging from 5 Hz to 10 kHz. Figure 4b shows the power spectrum density (PSD) of MG #5 at ID ∼ 2 µA for the same cases of Figure 4a: 1) fabricated, 2) post O2 exposure, and 3) post ELA. All the PSD curves follow a 1/f trend, indicating the quasi-equilibrium state of the carriers38,39. The corresponding transient data plots are displayed in Figure 4c. As discussed previously, the acLF noise level for the case 1) and 2) is almost similar, revealing insufficient sites for the reaction of O2 on the surface of MG #5. However, the ac-LF noise is suppressed approximately by a factor of 10 after ELA. To determine the surface trap density (NST) and Coulomb scattering parameter (αSC) of MG #5 before and after ELA, the LF noise measurements were carried out as a function of several VG. The corresponding LF noise data was then fitted to a carrier number fluctuation and correlated mobility fluctuation model (CNF-CMF)38,39 (Figure S4). As expected, NST increases slightly after exposure to O2, but largely decreases (by a factor of 50) after ELA. In addition, for each case, the 105 order of

αSC is also extracted, whose value is similar to that of bulk Si metal–oxide–semiconductor transistors38.

Conclusion

In summary, a site-specific ELA method was evaluated as a versatile device platform for 2D materials. With a graphite Joule heating on the surface of MoS2, a sustainable restoration through the removal of surface/interface adsorbates and the suppression of high metal-tosemiconductor channel access barrier could be achieved, which results in electron excess doping, carrier mobility enhancement, and surface trap density reduction. The comparison between the thermal simulation and the infrared microscope measurement confirms the local heating phenomena in this technique, whose effects were again examined using the low-


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frequency noise analysis. Its in-situ controllability inside a 2D-material device has a great potential in various applications, especially in the field of gas/chemical sensing as demonstrated above.


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Materials and Methods Device fabrication: The monolayer MoS2 flakes prepared through a chemical vapor deposition technique were first transferred onto a 300 nm SiO2/Si wafer using a wet transfer method, after the removal of a water-soluble promoter underlying the MoS2. A properly chosen graphite film whose thickness ranged from 30 nm to 100 nm, as measured using an optical microscope (Axio Imager 2, CARL ZEISS), on a poly(vinyl alcohol)/PMMA, was then transferred to the target MoS2 flake by a dry transfer method. Metal electrode areas were defined by selective electron-beam lithography pattering and the Cr/Au (5 nm/50 nm) metals were finally deposited on the MoS2 and graphite, respectively. The height profile of graphite film was obtained using AFM (E-sweep W/NanoNavistation). Optical, Electrical, and Thermal Measurement: The monolayer MoS2 was optically characterized by Raman spectroscopy (XperRam 200, Nano Base) with a 100 µW excitation laser at a wavelength of 532 nm. The electrical measurements were carried out under a high vacuum (10−5 − 10−6 torr) using a commercial vacuum probe station, at room temperature, with a semiconductor analyzer (4200-SCS, Keithley Instruments). The LF noise data was acquired using a customized LF noise measurement system, combined with a low noise current-to-voltage pre-amplifier (SR570, Stanford Research Systems) and a data acquisition system (DAQ-4431, National Instruments)40. During the O2 cycling test, the vacuum pump was turned off at a high vacuum state, and high purity O2 (99.999%) was then injected into the chamber at 600 torr. The thermal distribution images were captured using the temperature measurement microscope systems (InfraScope III, Quantum Focus Instruments Corporation) with an electrical current source (Keithley 6221).


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FIGURES

Figure 1. Local electrothermal annealing (ELA) via graphite Joule heating. (a) Schematic of the ELA platform on a monolayer MoS2 (not-to-scale). (b) Representative optical microscopy image of the MoS2/graphite heterostructure (MG #1) (scale bar: 10 µm). (c) Illustration of the ELA operation via graphite Joule heating on the surface of the MoS2. (d) Comparison of the MG #2 transfer curves (at VD = 0.5 V) of as-fabricated device, CTA and ELA treatments after O2 exposure. Experimental sequences: (1) Initial measurement, (2) O2 exposure, (3) CTA for 4 h at 150 °C, (4) O2 exposure, and (5) ELA for 2 h with 109–115 mW power consumption. (Inset: Brief energy band diagrams for the O2 exposure (electron depletion) and post ELA (electron accumulation)).


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Figure 2. ELA effects on electron excess doping. (a) ID-VD output characteristic curves as a function of VG, from −50 V to 50 V, for MG #1 before (red lines) and after 110 min of ELA (blue lines). (b) Various ID-VG transfer curves, at VD = 0.2 V, for MG #1 at 10 min ELA intervals from 0 to 110 min. The time-resolved ELA evolution in MG #1 for (c) µFE, (d) VFB, and (e) ∆n2D.


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Figure 3. ELA effects on the MG heterostructure. (a-d) Experimental results. The IGdependent temperature contour images of MG #4 for IG = (a) 0 mA, (b) 30 mA, and (c) 50 mA, respectively. (d) Temperature profiles with respect to the length and width position of graphite. (e-h) Simulation results. ELA temperature contour images obtained from an infrared simulator at IG = 20 mA with a 156-mW power consumption: (e) top view, (f) threedimensional view, and temperature profiles along (g) X-X′ (length direction) and (h) Y-Y′ (width direction), respectively. The background temperature for the infrared thermal-imaging microscope system is set to 343.15 K as a reference temperature. .


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Figure 4. Cycling test upon oxygen exposure and low-frequency (LF) noise spectroscopy. (a) ID-VG transfer curves, at VD = 1 V, for a fabricated sample (red lines), upon O2 exposure (green lines), and after the ELA (blue lines), respectively. (b) Power spectrum density (PSD) and (c) corresponding transient curves at ID ∼ 2 µA for each case. (d) Estimated NST (violet symbols) and αSC (orange symbols) for each case.


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ASSOCIATED CONTENT Supporting Information. (i) Raman spectrum of the MoS2 monolayer on SiO2 and the height profile of graphite (∼43 nm) of MG #1. (ii) Output characteristic curves as a function of back-gate bias for MG #1 before and after ELA process. (iii) Device performances before and after ELA process for MG #3: ID-VD output characteristic curves, ID-VG transfer curves, VG-dependent contact resistance curves, and field-effect mobility (µFE) results in the 2-probe and 4-probe configuration. (iv) Comparison of power spectrum densities of MG #5 measured before the ELA, during the O2 exposure, and after the ELA. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.-K.J.) and [email protected] (D.S.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF2016R1A2B2012336), the Institute for Basic Science (IBS-R011-D1), and the Sookmyung Women's University Research Grants (Project Number: 1-1803-2012), Republic of Korea.


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BRIEFS One-sentence synopsis The in-situ site-specific electrothermal local annealing, via graphite Joule heating, allows for the sustainable restoration of the undesired surface/interface of adsorbates and high metalto-semiconductor energy barrier, resulting in the enhancement of carrier mobility and surface trap density reduction in 2D materials.

Figure for the Table of Contents


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Electrothermal Local Annealing via Graphite Joule Heating on Two

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