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Effect of AlO Deposition on Performance of TopGated Monolayer MoS based Field Effect Transistor 2

Jeong-Gyu Song, Seok Jin Kim, Whang Je Woo, Youngjun Kim, Il-Kwon Oh, Gyeong Hee Ryu, Zonghoon Lee, Jun-Hyung Lim, Jusang Park, and Hyungjun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07271 • Publication Date (Web): 29 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016

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ACS Applied Materials & Interfaces

Effect of Al2O3 Deposition on Performance of TopGated Monolayer MoS2 based Field Effect Transistor Jeong-Gyu Song,a‡ Seok Jin Kim,a‡ Whang Je Woo,a Youngjun Kim,a Il-Kwon Oh,a Gyeong Hee Ryu,b Zonghoon Lee,b Jun Hyung Lim,c Jusang Park*a and Hyungjun Kim*a a

School of Electrical and Electronics Engineering, Yonsei University, 262 Seongsanno,

Seodaemun-gu, Seoul 120-749, Korea. b

School of Materials Science and Engineering, Ulsan National Institute of Science and

Technology (UNIST), Ulsan 689-798, Korea. c

Display R & D Center, Samsung Display Co., Ltd., Nongseo-dong, Kiheung-gu, Yongin,

Gyeonggi-do 449-902, Korea.

KEYWORDS Molybdenum Disulfide, Atomic Layer Deposition, Two Dimensional Materials, Transition Metal Dichalcogenides, Field Effect Transistor

ACS Paragon Plus Environment

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ABSTRACT

Deposition of high-k dielectrics on two-dimensional (2D) MoS2 is an important process for successful application of the transition-metal dichalcogenides in electronic devices. Here, we show the effect of H2O reactant exposure on monolayer (1L) MoS2 during atomic layer deposition (ALD) of Al2O3. The results showed that the ALD Al2O3 caused degradation of the performance of 1L MoS2 FETs owing to the formation of Mo–O bonding and trapping of H2O molecules at the Al2O3/MoS2 interface. Furthermore, we demonstrated that reduced duration of exposure to H2O reactant and post-deposition annealing were essential to the enhancement of the performance of top-gated 1L MoS2 FETs. The mobility and on/off current ratios were increased by factors of approximately 40 and 103, respectively, with reduced duration of exposure to H2O reactant and with post-deposition annealing.


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INTRODUCTION Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) have been recognized as a promising material for future electronics.1–3 Owing to their atomic-scale thickness and bandgaps in the range of 1–2 eV, 2D semiconducting TMDCs exhibit excellent gate-tunable electronic properties with high on/off current ratio and high field-effect carrier mobility.3–8 To assess whether the materials can be successfully applied to electrical devices, significant efforts have been devoted to explore the properties of monolayer (1L) TMDC based FETs in terms of contact resistance,7,9–11 channel length,12,13 and dielectric environment.3,14–16 In particular, previous studies showed that the performance of 1L TMDC based FETs is significantly affected by the properties of high-k dielectrics on 1L TMDCs, such as dielectric constant and roughness, resulting from dielectric screening, which enhance the carrier mobility, or from the induced short- and long-ranged charge disorder, which reduces the carrier mobility.17–19 Thus, optimization of the deposition of high-k dielectrics on 1L TMDCs is critical to ensure high performance of 1L TMDC-based FETs. Atomic layer deposition (ALD) is the one of the most widely used technique for the deposition of high-k dielectrics owing to its excellent electrical properties, thickness control on an atomic scale, large-area thickness uniformity, and low deposition temperature. Recently, studies on uniform ALD high-k dielectric films (e.g., Al2O3 and HfO2) on exfoliated MoS2 have been reported.14–16,19–21 However, investigations on the effects of ALD of high-k dielectrics on electrical properties of 1L MoS2 have never been reported, despite the important issue that the process uses oxidants (e.g., H2O, O3, and O2 plasma) as reactants, which can degrade the performance of 1L MoS2-based FETs (1L


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MoS2 FETs) by formation of Mo–O bonding22,23 and trapping of H2O molecules between high-k dielectrics/MoS2 interfaces.24–27 Here, we report our investigation on the electrical properties of MoS2 obtained after different H2O reactant exposure durations (tr) during ALD of Al2O3 on 1L MoS2. We observed that the performances of top-gated 1L MoS2 FETs were significantly degraded with longer tr of the Al2O3 ALD process owing to the formation of Mo–O bonding and trapping of H2O molecules at the Al2O3/MoS2 interface. Furthermore, we employed postdeposition annealing to eliminate the trapped H2O molecules at the Al2O3/MoS2 interface. The performances of top-gated 1L MoS2 FETs were considerably increased after postdeposition annealing. Spectroscopic measurements were used to analyze the effect of tr on ALD of Al2O3 on MoS2.

RESULTS AND DISCUSSION Figure 1a shows an optical microscope image of MoS2 synthesized by CVD. The intentionally scratched region revealed the underlying SiO2 substrate. In contrast to the SiO2 substrate region, MoS2 showed a violet color. The MoS2 layer was characterized using Raman spectroscopy, photoluminescence (PL) spectroscopy, and high-resolution transmission electron microscopy (HRTEM), and the results are shown in Figure 1b–d. The Raman spectrum (λexc = 532 nm) for MoS2 in Figure 1b exhibits in-plane and out-ofplane vibrational modes at 385.7 and 405.9 cm-1 (E12g and A1g). From the Raman spectrum, we calculated the distance between the peaks for modes E12g and A1g, which is a criterion of layer numbering of MoS2 based on the softening and stiffening in the inplane and out-of-plane mode frequencies, respectively, with increasing layer number.28


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The calculated relative peak distance is 20.2 and is in good agreement with previously reported value for synthesized 1L MoS2.28–30 The PL spectrum of 1L MoS2 in Figure 1c shows a peak at 1.86 eV, which corresponds to the direct excitonic transition of MoS2. These Raman and PL results confirm the layer number of CVD MoS2. Figure 1d is an HRTEM image of the synthesized 1L MoS2, which had a honeycomb-like structure with lattice spacing of 0.27 and 0.16 nm for the (100) and (110) planes, respectively. In addition, six-fold coordination symmetry was observed in the fast-Fourier-transformed (FFT) image (Inset of Figure 1d). The approximate grain size was 30–40 µm (see Figure S1). To investigate the effect of tr in the ALD-Al2O3 process on the performance of topgated 1L MoS2 FETs, we deposited a 40-nm-thick Al2O3 layer by ALD, using different values of tr for the initial 3-nm-thick Al2O3 layer on the as-synthesized 1L MoS2; the process is illustrated in Figure 2a (detailed process is in experimental section). We varied tr as tr = 0.05, 1, and 4 s for deposition of the first 3-nm layer and kept tr constant at 1 s to deposit the remaining 37-nm layer of Al2O3. To investigate the surface morphology and the effect of tr on the initial growth of Al2O3 on MoS2, we analyzed the Al2O3(3 nm)/1L MoS2 using atomic force microscopy (AFM); AFM images of the samples are shown in Figure 2b–d. We observed that pin-holes are on the surface of Al2O3(3 nm)/1L MoS2 independent on the tr with 3 nm depth as plotted in Figure 2e. This indicates that the initial growth of the Al2O3 is not affected by tr. These pin-holes are removed with increasing thickness of Al2O3 on MoS2, and continuous and flat Al2O3 films with 40 nm thick were deposited on 1L MoS2 with extracted root-mean-square (rms) values of approximately 0.5–0.6 nm, independent of tr for the initial 3-nm Al2O3 layer, as shown in


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Figure S3. Hereafter, the Al2O3(40 nm)/1L MoS2 samples with tr = 0.05, 1, and 4 s for the initial 3-nm Al2O3 layer are referred to as 0.05t, 1t, and 4t, respectively. We fabricated top-gated 1L MoS2 FETs on samples 0.05t, 1t, and 4t using Au(40 nm)/Ti(1 nm) electrodes, as shown in the diagram in Figure 3a (OM image for top-gated FET is in Figure S4). The as-fabricated top-gated 1L MoS2 FETs were measured at room temperature in air. Figure 3b shows transfer curves of top-gated 1L MoS2 FETs fabricated on samples 0.05t, 1t, and 4t. The top-gated 1L MoS2 FETs show decreased on-current level with increasing tr for initial 3 nm of Al2O3. The field effect electron mobility (µe) was extracted from the linear regime of the transfer curve using the equation µe = (∆Id/∆Vg) × L /(WCoxVd), where Id is the drain current; Vg is the gate voltage; and L, W, and Cox are the channel length, channel width, and gate capacitance per unit area, respectively. The extracted µe and ratio between the on-current and off-current (Ion/Ioff) versus tr for initial 3 nm of Al2O3 are plotted in Figure 3c. We averaged µe from values measured at 10 FETs on each sample. The average µe significantly decreased with increasing tr for the initial 3-nm Al2O3 layer: 0.6 cm2 V–1 s–1 for tr = 0.05 s, 0.07 cm2 V–1 s–1 for tr = 1 s, and 0.05 cm2 V–1 s–1 for tr = 4 s. In addition, the value of Ion/Ioff showed similar dependence on tr for the initial 3-nm Al2O3 layer; it decreased with increasing tr: Ion/Ioff had a value of 2 × 105 for tr = 0.05 s, 4 × 104 for tr = 1 s, and 2 × 102 for tr = 4 s. It should be noted that the top-gated 1L MoS2 FETs show very large negative threshold voltage (Vth) regardless of tr for the initial 3-nm Al2O3 layer. This could be attributed to the large amount of positive fixed charges in the Al2O3 gate insulator.16 The degradation of performance of the top-gated 1L MoS2 FETs with increasing tr for the initial 3-nm Al2O3 layer can be explained by induced charge disorder due to Mo–O


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bonding23,31,32 and trapped H2O molecules at the Al2O3/MoS2 interface,27,33 which occurred during ALD of Al2O3 on 1L MoS2.17 To clarify this, we characterized samples 0.05t, 1t, and 4t using X-ray photoelectron spectroscopy (XPS) and PL analysis as presented in Figure 3d–f. Figure 3d shows the Mo 3d core level of XPS spectra of samples 0.05t, 1t, and 4t after calibration against the C 1s peak at 285 eV. The Mo 3d core level had three strong peaks at 232.5, 229.4, and 226.6 eV, which correspond to the Mo 3d3/2 and Mo 3d5/2 levels for Mo4+ and the S 2s state, respectively. More importantly, Mo–O-related small peaks were observed at 235.6 eV for samples 1t and 4t, indicating the presence of Mo–O bonding14, 20, 34 when tr values of 1 and 4 s were used for the initial 3-nm Al2O3 layer. The PL spectra of samples 0.05t, 1t, and 4t are shown in Figure 3e. Each PL spectrum could be deconvolved into two PL peaks from trion recombination (X-) and exciton recombination (X) at 1.86 and 1.89 eV, respectively.32,35,36 A plot of the integrated PL intensities (X- and X) and ratios (X/X-) versus tr for initial 3-nm Al2O3 layer is shown in Figure 3f. The overall integrated PL intensities X- and X decreased with increasing tr owing to the oxidation of 1L MoS2.32,36 Meanwhile, X/X- increased from 0.2 to 1.1, indicating the switch in dominant PL process from trion recombination to exciton recombination with increasing tr for the initial 3-nm Al2O3 layer. This change in dominant PL process can be explained by the decrease in excessive electrons (p-doping) of MoS2 resulting from the increase in amount of trapped H2O molecules at the interface between Al2O3 and MoS2 with increasing tr.3,27,32,35 As a result, we conclude that H2O exposure during ALD of Al2O3 on 1L MoS2 significantly degraded the performance of top-gated


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1L MoS2 FETs by formation of Mo–O bonding and trapping of H2O molecules at the Al2O3/MoS2 interface. The above results demonstrate that effective elimination of Mo–O bonding and trapped H2O molecules at the Al2O3/MoS2 interface was important to performance enhancement of top-gated 1L MoS2 FETs. The M–O bonding could have been prevented by decreasing tr for the initial 3-nm Al2O3 layer on 1L MoS2 to 0.05 s. In contrast, small amounts of trapped H2O molecules were present at the Al2O3/MoS2 interface even when tr = 0.05 s. Therefore, post-deposition annealing was necessary to remove the trapped H2O molecules from the Al2O3/MoS2 interface.27 Figure 4a shows transfer curves of top-gated 1L MoS2 FETs fabricated on samples 0.05t, 1t, and 4t samples after they were annealed at 120 °C for 24 h under a pressure of 10-6 Torr without any gas injection. The top-gated 1L MoS2 FETs showed enhanced on-current levels after annealing. A plot of the extracted µe and Ion/Ioff before (dashed line) and after annealing (solid line) for different tr for the initial 3nm Al2O3 layer is shown in Figure 4b. After annealing, the value of µe was significantly increased to 2.0 cm2 V–1 s–1 for tr = 0.05 s, 1.6 cm2 V–1 s–1 for tr = 1 s, and 1.6 cm2 V–1 s–1 for tr = 4 s. Furthermore, Ion/Ioff was also observed to increase after annealing to 1.3 × 106 for tr = 0.05 s, 6 × 105 for tr = 1 s, and 6 × 105 for tr = 4 s. It should be noted that despite the enhanced performance of top-gated 1L MoS2 after annealing, the values of µe and Ion/Ioff were still higher for tr = 0.05 s than for tr = 1 and 4 s. To investigate the effect of annealing on samples 0.05t, 1t, and 4t, they were characterized by XPS and PL analysis after their post-deposition annealing; the results are shown in Figure 4c–e. The Mo 3d core level of the annealed samples 0.05t, 1t, and 4t are shown in Figure 4c; just like the XPS spectra of as-deposited samples 0.05t, 1t, and 4t,


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strong Mo 3d3/2, Mo 3d5/2, and S 2s peaks were observed at 232.5, 229.4, and 226.6 eV, were observed in the spectra of the annealed samples. Furthermore, small Mo–O-related peaks were also observed at 235.6 eV for annealed samples 1t and 4t, indicating that Mo– O bonding was not eliminated by post-deposition annealing. Figure 4d shows PL spectra with deconvolved PL peaks of samples 0.05t, 1t, and 4t after post-deposition annealing. Similar to the PL spectra of the as-deposited samples, decreasing integrated X- and X were observed (Figure 4e) with increasing tr for the initial 3-nm Al2O3 layer, which are attributed to the oxidation of 1L MoS2.32,36 However, the integrated intensity ratios X/X- of annealed samples 0.05t, 1t, and 4t were approximately 0.05 regardless of the value of tr. In other words, the dominant PL process (trion recombination) did not change with increasing tr because annealing eliminated H2O molecules that were trapped at the Al2O3/MoS2 interface. A schematic illustration of the effect of the post-deposition annealing on the asdeposited samples 0.05t, 1t, and 4t is represented in Figure 4f. As-deposited sample 0.05t (left upper panel) had trapped H2O molecules at the Al2O3/MoS2 interface without Mo–O bonding. In contrast, as-deposited samples 1t and 4t (left bottom panel) had a larger amount of trapped H2O molecules at the Al2O3/MoS2 interface with Mo–O bonding. The H2O molecules that were trapped at the interface between the Al2O3 and MoS2 layers were eliminated by annealing from all samples. However, the Mo–O bonding in samples 1t and 4t remained, resulting in lower µe and Ion/Ioff for tr = 1 and 4 s than the values for tr = 0.05 s. Thus, we conclude that the short tr (0.05 s in our experimental setup) for initial growth of ALD-Al2O3 and post-deposition annealing were essential to performance


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enhancement of 1L MoS2 FETs based on the elimination of Mo–O bonding and trapped H2O molecules.

CONCLUSION We investigated the effect of reactant-exposure duration tr for deposition of the initial 3-nm Al2O3 layer on the performance of top-gated 1L MoS2 FET. We observed that the ALD process for deposition of Al2O3 on 1L MoS2 degraded the performance of 1L MoS2 FETs owing to the formation of Mo–O bonding and trapping of H2O molecules at the Al2O3/MoS2 interface as a result of exposure of 1L MoS2 to H2O. Furthermore, we demonstrated that reduced H2O-exposure time for initial growth of ALD-Al2O3 on 1L MoS2 and post-deposition annealing were essential to ensure the enhanced performance of top-gated 1L MoS2 FETs. The values of µe and Ion/Ioff were increased by factors of approximately 40 and 103 with reduced tr (from 4 to 0.05 s) and with post-deposition annealing (detailed in Table S1). We expect our findings to promote the importance of atomic layer deposition of high-k dielectrics in the fabrication of top-gated 2D FETs; our results should also offer insight into optimized fabrication of 2D TMDC-based electronic devices.

EXPERIMENTAL SECTION CVD MoS2. A tube furnace reactor (IH-ALD, I tech U, Korea) was used to synthesize MoS2 directly on SiO2 (300 nm)/Si substrates, using molybdenum(V) chloride (MoCl5) and dimethyl


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sulfide (CH3)2S) (Sigma Aldrich, Korea) as the precursor and reactant, respectively. A bubbler containing the precursor was heated to 90 ºC to ensure an adequate vapor pressure for the precursor molecules to be carried into the tube by pure argon carrier gas (99.999%). An inorganic catalyst was employed to increase the grain size.

ALD Al2O3. A tube furnace reactor (IH-ALD, I tech U, Korea) was used to deposit Al2O3 on CVD MoS2, using trimethylaluminum (TMA) and H2O at 150 ºC. An ALD cycle consists of four steps: exposure to TMA precursor (duration: ts), first Ar purge (tp1), exposure to H2O reactant (duration: tr), and second Ar purge (duration: tp2). For the ALD Al2O3 process, ts, tp1, and tp2 were fixed at 0.1, 12, and 60 s, respectively, while tr was varied from 0.05 to 1 and then 4 s for deposition of the initial 3 nm and was fixed at 1 s for deposition of the remaining 37 nm of Al2O3 (total thickness: 40 nm). It should be noted that tp2 was set to be longer than tp1 to purge excess H2O molecules and by-products.

Fabrication of Top-Gated Field-Effect Transistor. The top-gated 1L MoS2 FET was fabricated from as-synthesized 1L MoS2 on a SiO2 (300 nm)/Si substrate by evaporating Au(40 nm)/Ti(1 nm) source and drain electrodes, Au(40 nm) gate electrode and an ALD-Al2O3 (40nm) gate insulator through conventional photolithography and reactive-ion (O2 plasma with flow of 50 sccm and 50 W for 15 s) etching. The reactant exposure time (tr) was varied from 0.05 to 1 and 4 s for the deposition of the initial 3-nm layer of ALD-Al2O3 on 1L MoS2.

Characterization. Optical microscopy (OM; ECLIPSE LV100ND, Nikon, Japan), Raman spectroscopy and PL spectroscopy (Lab Ram ARAMIS, HORIBA, Japan; laser excitation


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wavelength: 532 nm), AFM (Multimode, Veeco Instruments, USA, and Bruker Corporation, USA; tapping mode with Al coated Si cantilever), XPS (Thermo Scientific™ K-alpha™+ XPS System, Thermo Fisher Scientific, USA; monochromated AI Kα source), TEM (Titan G2 Cube 60-300, FEI, USA; accelerating voltage: 80 kV), and a voltage/current meter (Keithley 4200, Keithley Instruments, USA; measured at room temperature in air) were used.


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FIGURES

Figure 1. (a) OM image of 1L MoS2 on SiO2/Si substrate. (b) Raman spectrum and (c) PL spectrum of 1L MoS2 on SiO2/Si substrate. (d) HRTEM image of selected region of 1L MoS2 ; the inset shows the corresponding FFT pattern.


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Figure 2. (a) Schematic illustration of ALD of Al2O3 on 1L MoS2. (b–d) AFM images of Al2O3(3 nm)/1L MoS2 depending on tr for deposition of Al2O3. (e) Corresponding height profiles to the pin-hole in AFM image of (b-d)


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Figure 3. (a) Schematic structure of a top-gated 1L MoS2 FET. (b) Transfer characteristics of top-gated 1L MoS2 FETs. (c) Extracted field-effect electron mobility and ratio between on- and off-current (Ion/Ioff) of top-gated 1L MoS2 FETs. (d) XPS measurements of Mo 3d core levels in samples 0.05t, 1t, and 4t. (e) PL spectra and (f) integrated PL intensities of samples 0.05t, 1t, and 4t.


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Figure 4. (a) Transfer characteristics of top-gated 1L MoS2 FETs after annealing. (b) Extracted field-effect electron mobility and ratio between on- and off-current of top-gated 1L MoS2 FETs after annealing. (c) XPS measurements of Mo 3d core levels in samples 0.05t, 1t, and 4t after annealing. (d) PL spectra and (e) integrated PL intensities of samples 0.05t, 1t, and 4t after annealing. (f) Schematic illustration of ALD of Al2O3 on 1L MoS2 before and after annealing.


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ASSOCIATED CONTENT Supporting Information. Optical microscope image, growth of ALD Al2O3 and PL of 1L MoS2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] * E-mail: [email protected] 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.

ACKNOWLEDGMENT This work was supported by Korea Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry and Energy (MOTIE) (Project No. 10050296, Large scale (Over 8“) synthesis and evaluation technology of 2-dimensional chalcogenides for next generation electronic devices); This work was supported by the Center for Integrated Smart Sensors funded by the Ministry of Science, ICT & Future Planning as Global Frontier Project. (CISS-2011-0031848); This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea


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government(MSIP) (No. NRF-2014R1A2A1A11052588); This work was supported by Samsung Display CO., LTD.

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