Letter www.acsami.org
Interface Properties of Atomic-Layer-Deposited Al2O3 Thin Films on Ultraviolet/Ozone-Treated Multilayer MoS2 Crystals Seonyoung Park,† Seong Yeoul Kim,† Yura Choi,† Myungjun Kim,‡ Hyunjung Shin,‡ Jiyoung Kim,§ and Woong Choi*,† †
School of Advanced Materials Engineering, Kookmin University, Seoul 02707, Korea Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea § Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, United States ‡
S Supporting Information *
ABSTRACT: We report the interface properties of atomic-layer-deposited Al2O3 thin films on ultraviolet/ozone (UV/O3)-treated multilayer MoS2 crystals. The formation of S−O bonds on MoS2 after low-power UV/O3 treatment increased the surface energy, allowing the subsequent deposition of uniform Al2O3 thin films. The capacitance−voltage measurement of Au− Al2O3−MoS2 metal oxide semiconductor capacitors indicated n-type MoS2 with an electron density of ∼1017 cm−3 and a minimum interface trap density of ∼1011 cm−2 eV−1. These results demonstrate the possibility of forming a highquality Al2O3−MoS2 interface by proper UV/O3 treatment, providing important implications for their integration into field-effect transistors.
KEYWORDS: interface property, atomic layer deposition, Al2O3, ultraviolet/ozone treatment, MoS2, capacitance−voltage
R
O bonds on the MoS2 surface.8,9 Yet, the formation of Mo−O bonds was also observed on UV/O3-treated MoS2 surfaces in some cases.10,11 Moreover, understanding the interface between ALD Al2O3 and UV/O3-treated MoS2 becomes further complicated because of the lack of studies on the electrical properties at the interface. Here, we explore the interface properties of ALD Al2O3 thin films on UV/O3-treated multilayer MoS2 crystals to demonstrate the possibility of forming a high-quality Al2O3−MoS2 interface. We investigate the correlation between UV/O3 treatment and the chemical change of the MoS2 surface as well as the morphology of ALD Al2O3 thin films. We further investigate the electrical properties of the Al2O3−MoS2 interface by measuring the capacitance− voltage (C−V) characteristics of Au−Al2O3−MoS2 metal oxide semiconductor (MOS) capacitors. MoS2 flakes, mechanically exfoliated from a natural single crystal (2D Semiconductors), were transferred to highly doped silicon wafers. Some MoS2 flakes were exposed to UV/O3 (254 nm) under two different UV/O3 powers of 15 and 25 mW m−2. The surface energy of MoS2 before and after UV/O3 treatment was evaluated by measurement of the static advancing contact angle made between the substrate and two different liquids, deionized water and diiodomethane (Sigma-Aldrich). Contact angles were input in the Wu model for calculation of the surface
ecently, there is great interest in two-dimensional (2D) transition-metal dichalcogenides (TMDs) because of their interesting electronic, optical, and chemical properties. Among these TMDs, molybdenum disulfide (MoS2) has been most extensively investigated for the application of field-effect transistors (FETs). Single-layer or multilayer MoS2 FETs exhibit outstanding performance metrics, including high on/off current ratio (∼107), high mobility (∼100 cm2 V−1 s−1), and low subthreshold swing (∼70 mV decade−1).1,2 In the aforementioned demonstration of MoS2 FETs, the integration of high-k gate dielectrics with MoS2 channels by atomic layer deposition (ALD) is essential in the enhancement of the carrier mobility in MoS2. The field-effect mobility of single-layer MoS2 FETs decreases to 1−20 cm2 V−1 s−1 without a high-k dielectric layer on MoS2.1,3 Moreover, high-k layers on MoS2 channels can reduce the hysteresis of FETs by preventing moisture adsorption from ambient air.4 However, the absence of dangling bonds on a MoS2 surface makes it very difficult for MoS2 to react with metal−organic precursors such as trimethylaluminum (TMA) during the ALD process of high-k dielectric layers.5 To enhance the reactivity during the ALD process, the pretreatment of a MoS2 surface with oxygen plasma6,7 or ultraviolet/ozone (UV/O3)8,9 has been suggested. While an oxygen plasma treatment led to the uniform deposition of Al2O3 and HfO2 thin films, the formation of a semiconducting MoO3 layer on the surface may degrade the electrical properties of MoS2.6 Such a problem can be avoided by the UV/O3 treatment of MoS2 because it forms S− © XXXX American Chemical Society
Received: February 5, 2016 Accepted: April 27, 2016
A
DOI: 10.1021/acsami.6b01568 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 1. (a) Surface energy of MoS2 as a function of the UV/O3 treatment time. The inset shows the contact angles of water droplets on MoS2 after 0, 2, and 4 min of treatment. (b) XPS spectra of MoS2 before and after 6 min of UV/O3 treatment at 15 mW m−2. (c) XPS spectra of MoS2 before and after 6 min of UV/O3 treatment at 25 mW m−2. (d) Raman spectra of MoS2 before and after 4 min of UV/O3 treatment.
energy.12 The as-transferred and UV/O3-treated MoS2 flakes were also analyzed by X-ray photoelectron spectroscopy (XPS; ULVAC-PHI X-TOOL) and Raman spectroscopy (Horiba Jobin-Yvon LabRam Aramis) using an Ar + laser. Al2O 3 deposition was carried out using TMA/water precursors in a custom-built thermal ALD system. An Al2O3 deposition cycle consisted of a series of alternating precursor and purging gas injections for different durations in a reliable ALD regime (TMA/N2/H2O/N2 injections for 0.2/10/0.2/10 s). The morphology of ALD Al2O3 was examined by atomic force microscopy (AFM; Park Systems XE-100). To form top gate electrode for MOS capacitors, Au (100 nm)/Ti (10 nm) was deposited by e-beam evaporation and patterned by photolithography and a lift-off technique. C−V characteristics were measured at room temperature using an Agilent 4284A Precision LCR meter. Figure 1a shows the MoS2 surface energy for five different UV/O3 treatment times at a power of 25 mW m−2. As the treatment time increases from 0 to 4 min, the surface energy monotonically increases from 47 mJ m−2 (hydrophobic) to 84 mJ m−2 (hydrophilic). The surface energy saturates when the treatment time is longer than 4 min. The inset shows the contact angles of water droplets on 0, 2, and 4 min UV/O3treated MoS2 surfaces. The actual values of the contact angles used to calculate the surface energy can be found in the Supporting Information. The MoS2 surface energy after UV/O3 treatment at a power of 15 mW m−2 shows a negligible difference from that in Figure 1a. To clarify the origin of the
observed change in the surface energy, the binding energy of the UV/O3-treated MoS2 surface was measured by XPS. Parts b and c of Figure 1 show the binding energies of the MoS2 surface before and after 6 min of UV/O3 treatment at powers of 15 and 25 mW m−2, respectively. Regardless of the UV/O3 power, UV/O3 treatment generates a negligible difference in the Mo4+ (Mo 3d3/2 and Mo 3d5/2) and S2− (S 2s, S 2p1/2, and S 2p3/2) states. However, either S−O (at 15 mW m−2) or Mo− O bonds (at 25 mW m−2) are observed on the UV/O3-treated MoS2 surface. The formation of either S−O8,9 or Mo−O10,11 bonds was observed on the UV/O3-treated MoS2 surface in the literature. (The experimental conditions of UV/O3 treatment are not given.8−11) Our results indicate that the magnitude of the UV/O3 power can make a significant impact on the surface bonding states. To avoid oxidation of MoS2, we use UV/O3 treatment at 15 mW m−2 for further experiments. We also compare the Raman spectra of MoS2 before and after UV/O3 treatment. Figure 1d shows the two characteristic Raman modes (A1g and E12g) of MoS2 before and after 4 min of UV/O3 treatment. UV/O3 treatment changes neither the intensities of the A1g and E12g modes nor the intensity ratio between the A1g and E12g modes. In contrast, an oxygen plasma treatment can change the intensities of the A1g and E12g modes or the ratio of the intensity between the A1g and E12g modes.6,13 Hence, it is clear from the Raman spectra that UV/O3 treatment at low power (15 mW m−2) causes minimal structural damage in MoS2. B
DOI: 10.1021/acsami.6b01568 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 2. AFM images (1 μm × 1 μm) of 10-nm-thick ALD Al2O3 thin films on MoS2 (a) without and (b) with 4 min of UV/O3 treatment.
Figure 3. (a) Schematic cross section and an optical image (top view) of a fabricated Au−Al2O3−MoS2 MOS capacitor. (b) C−V curves of an MOS capacitor measured at 100 Hz, 1 kHz, and 100 kHz. (c) Schematic energy-band diagrams of an MOS capacitor for accumulation, depletion, and inversion regions.
Al2O3−MoS2 MOS capacitors and measure their C−V characteristics. Figure 3a depicts the schematic cross section and optical image of an MOS capacitor. The C−V measurement of MOS capacitors provides valuable insight into the electrical properties such as the conduction type, carrier density, and interface trap density.14 Figure 3b shows the C− V response measured at 100 Hz, 1 kHz, and 100 kHz. The C−V curves show typical low-frequency (at 100 Hz) and highfrequency (at 1 kHz and 100 kHz) behavior with well-defined regions of accumulation, depletion, and inversion. The appearance of accumulation capacitance at higher voltages indicates n-type MoS2. The energy-band diagrams of an n-type MoS2-based MOS capacitor are schematically shown in Figure
We next investigate the effects of UV/O3 treatment on the growth of ALD Al2O3 on MoS2 by comparing the morphology of ALD Al2O3 thin films. Figure 2 shows AFM images of ALD Al2O3 thin films (10 nm in thickness) on as-transferred and 4 min UV/O3-treated MoS2, respectively. While unevenly located islands of Al2O3 are observed on as-transferred MoS2, the Al2O3 thin films on UV/O3-treated MoS2 show uniform coverage. Because of improved surface morphology, the root-meansquare surface roughness of Al2O3 thin films significantly decreases from 0.55 to 0.18 nm after 4 min of UV/O3 treatment. To explore the electrical properties of the interface between ALD Al2O3 and UV/O3-treated MoS2, we further fabricate Au− C
DOI: 10.1021/acsami.6b01568 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces
Figure 4. (a) Electron density within MoS2 as a function of the distance from the MoS2 surface. (b) Interface trap density as a function of the gate voltage.
cm−2 comparable with that of the Al2O3−Si interface. These results suggest that a high-quality Al2O3−MoS2 interface can be formed with proper processes, providing important implications on the application of MoS2 and other 2D materials into high-performance transistors.
3c. It needs to be mentioned that the measured C−V curves are negatively shifted, suggesting the existence of positive charges in Al2O3. ALD Al2O3 thin films formed at relatively low temperature contain a large amount of fixed charges in the range of 1012−1013 cm−2.15,16 The carrier density near the MoS2 surface can be extracted from the high-frequency C−V curve in Figure 3b. If the interface trap density is low, the carrier density N at a distance W from the surface is given by N(W) = 2/[qεSε0A2d(1/C2)/ d(V)] and W = εSε0A(1/C − 1/Cox), where εS, ε0, and A are the dielectric constant of the semiconductor, permittivity of a vacuum, and capacitor area, respectively.14 Figure 4a shows the profile of the extracted electron density within MoS2 near the Al2O3−MoS2 interface. The electron density is fairly uniform (∼1017 cm−3) in MoS2. The interface trap density Dit at the Al2O3−MoS2 interface can also be extracted from the C−V curves in Figure 3b. Dit is given by Dit = (Cox/q){(CLF/Cox)/[1 − (CLF/Cox)] − (CHF/Cox)/[1 − (CHF/Cox)]} in the depletion region, where CLF and CHF are low- and high-frequency capacitances, respectively.14 Figure 4b shows Dit as a function of the gate voltage. The minimum Dit is estimated to be ∼1011 eV−1cm−2, which is comparable with that in the literature. Dit of the MoS2−Al2O3 interface estimated from the subthreshold swing of MoS2 thin-film transistors (TFTs)2,17,18 is in the range of 2.6 × 1011−7.7 × 1012 eV−1 cm−2, and that from the carrier number fluctuation model19 is ∼1011−1012 eV−1 cm−2. Although the minimum Dit is an order of magnitude higher than that of a state-of-the-art thermal SiO2−Si interface, it is comparable with that of the Al2O3−Si interface.14,20 We also fabricate Al2O3-encapsulated bottom-gated MoS2 TFTs and measure their current−voltage characteristics (Supporting Information). We expect Dit at the top MoS2−Al2O3 interface in our bottom-gated MoS2 TFTs to be ∼1012 eV−1 cm−2. Although this is higher than the minimum Dit that we estimated from the C−V measurement of MOS capacitors, it is still comparable with Dit of the MoS2−Al2O3 interface in the literature.2,17−19 In summary, we investigated the interface properties of ALD Al2O3 on UV/O3-treated multilayer MoS2 crystals. As the UV/ O3 treatment time increased, the surface energy also increased monotonically because of the formation of either S−O (at low UV/O3 power) or Mo−O (at high UV/O3 power) bonds on the MoS2 surface. The C−V measurement of Au−Al2O3−MoS2 MOS capacitors indicated n-type MoS2 with an electron density of ∼1017 cm−3. Minimum Dit was estimated to be ∼1011 eV−1
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01568. Calculation of the surface energy, electron density, and interface trap density and Al2O3-encapsulated bottomgated MoS2 TFTs (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (Grants NRF-2013R1A1A2008191 and NRF2013K1A4A3055679) and the Industrial Strategic Technology Development Program (Grant 10045145).
■
REFERENCES
(1) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (2) Kim, S.; Konar, A.; Hwang, W.-S.; Lee, J. H.; Lee, J.; Yang, J.; Jung, C.; Kim, H.; Yoo, J.-B.; Choi, J.-Y.; Jin, Y. W.; Lee, S. Y.; Jena, D.; Choi, W.; Kim, K. High-Mobility and Low-Power Thin-film Transistors Based on Multilayer MoS2 Crystals. Nat. Commun. 2012, 3, 1011. (3) Ghatak, S.; Pal, A. N.; Ghosh, A. Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors. ACS Nano 2011, 5, 7707−7712. (4) Late, D. J.; Liu, B.; Matte, H. S. S. R.; Dravid, V. P.; Rao, C. N. R. Hysteresis in Single-Layer MoS2 Field Effect Transistors. ACS Nano 2012, 6, 5635−5641. (5) Liu, H.; Xu, K.; Zhang, X.; Ye, P. D. The Integration of High-k Dielectric on Two-Dimensional Crystals by Atomic Layer Deposition. Appl. Phys. Lett. 2012, 100, 152115. (6) Yang, J. Y.; Kim, S. K.; Choi, W.; Park, S. H.; Jung, Y. K.; Cho, M. H.; Kim, H. S. Improved Growth Behavior of Atomic-Layer-Deposited D
DOI: 10.1021/acsami.6b01568 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Letter
ACS Applied Materials & Interfaces High-k Dielectrics on Multilayer MoS2 by Oxygen Plasma Pretreatment. ACS Appl. Mater. Interfaces 2013, 5, 4739−4744. (7) Yang, W.; Sun, Q.-Q.; Geng, Y.; Chen, L.; Zhou, P.; Ding, S.-J.; Zhang, D. W. The Integration of Sub-10 nm Gate Oxide on MoS2 with Ultra Low Leakage and Enhanced Mobility. Sci. Rep. 2015, 5, 11921. (8) Azcatl, A.; McDonnell, S.; K. C, S.; Peng, X.; Dong, H.; Qin, X.; Addou, R.; Mordi, G. I.; Lu, N.; Kim, J.; Kim, M. J.; Cho, K.; Wallace, R. M. MoS2 Functionalization for Ultra-Thin Atomic Layer Deposited Dielectrics. Appl. Phys. Lett. 2014, 104, 111601. (9) Su, W.; Kumar, N.; Spencer, S. J.; Dai, N.; Roy, D. Transforming Bilayer MoS2 into Single-Layer with Strong Photoluminescence Using UV-Ozone Oxidation. Nano Res. 2015, 8, 3878−3886. (10) Le, Q. V.; Nguyen, T. P.; Jang, H. W.; Kim, S. Y. The Use of UV/Ozone-Treated MoS2 Nanosheets for Extended Air Stability in Organic Photovoltaic Cells. Phys. Chem. Chem. Phys. 2014, 16, 13123− 13128. (11) Yang, X.; Fu, W.; Liu, W.; Hong, J.; Cai, Y.; Jin, C.; Xu, M.; Wang, H.; Yang, D.; Chen, H. Engineering Crystalline Structures of Two-Dimensional MoS2 Sheets for High-Performance Organic Solar Cells. J. Mater. Chem. A 2014, 2, 7727−7733. (12) Wu, S. Calculation of Interfacial Tension in Polymer Systems. J. Polym. Sci., Part C: Polym. Symp. 1971, 34, 19−30. (13) Islam, M. R.; Kang, N.; Bhanu, U.; Paudel, H. P.; Erementchouk, M.; Tetard, L.; Leuenberger, M. N.; Khondaker, S. I. Tuning the Electrical Property via Defect Engineering of Single Layer MoS2 by Oxygen Plasma. Nanoscale 2014, 6, 10033−10039. (14) Schroder, D. K. Semiconductor Material and Device Characterization, 3rd ed.; Wiley: Hoboken, NJ, 2005. (15) Esposto, M.; Krishnamoorthy, S.; Nath, D. N.; Bajaj, S.; Hung, T.-H.; Rajan, S. Electrical Properties of Atomic Layer Deposited Aluminum Oxide on Gallium Nitride. Appl. Phys. Lett. 2011, 99, 133503. (16) Simon, D. K.; Jordan, P. M.; Mikolajick, T.; Dirnstorfer, I. On the Control of the Fixed Charge Densities in Al2O3-Based Silicon Surface Passivation Schemes. ACS Appl. Mater. Interfaces 2015, 7, 28215−28222. (17) Liu, H.; Ye, P. D. MoS2 Dual-Gate MOSFET with AtomicLayer-Deposited Al2O3 as Top-Gate Dielectric. IEEE Electron Device Lett. 2012, 33, 546−548. (18) Choi, K.; Raza, S. R. A.; Lee, H. S.; Jeon, P. J.; Pezeshki, A.; Min, S. W.; Kim, J. S.; Yoon, W.; Ju, S.-Y.; Lee, K.; Im, S. Trap Density Probing on Top-Gate MoS2 Nanosheet Field-Effect Transistors by Photo-Excited Charge Collection Spectroscopy. Nanoscale 2015, 7, 5617−5623. (19) Na, J.; Joo, M.-K.; Shin, M.; Huh, J.; Kim, J.-S.; Piao, M.; Jin, J.E.; Jang, H.-K.; Choi, H. J.; Shim, J. H.; Kim, G.-T. Low-Frequency Noise in Multilayer MoS2 Field-Effect Transistors: the Effect of High-k Passivation. Nanoscale 2014, 6, 433−441. (20) Park, D.-G.; Cho, H.-J.; Lim, K.-Y.; Lim, C.; Yeo, I.-S.; Roh, J.-S.; Park, J. W. Characteristics of n+ Polycrystalline-Si/Al2O3/Si Metal− Oxide− Semiconductor Structures Prepared by Atomic Layer Chemical Vapor Deposition Using Al(CH3)3 and H2O Vapor. J. Appl. Phys. 2001, 89, 6275−6280.
E
DOI: 10.1021/acsami.6b01568 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
