This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 115.145.196.174

This content was downloaded on 22/06/2017 at 06:40

Please note that terms and conditions apply.

Highly uniform wafer-scale synthesis of -MoO3 by plasma enhanced chemical vapor

deposition

View the table of contents for this issue, or go to the journal homepage for more

2017 Nanotechnology 28 175601

(http://iopscience.iop.org/0957-4484/28/17/175601)

Home Search Collections Journals About Contact us My IOPscience

You may also be interested in:

Wafer-scale monolayer MoS2 grown by chemical vapor deposition using a reaction of MoO3 and H2S

Youngchan Kim, Hunyoung Bark, Gyeong Hee Ryu et al.

The important role of water in growth of monolayer transition metal dichalcogenides

Christoph Kastl, Christopher T Chen, Tevye Kuykendall et al.

Controlled MoS2 layer etching using CF4 plasma

Min Hwan Jeon, Chisung Ahn, HyeongU Kim et al.

Wafer-scale production of highly uniform two-dimensional MoS2 by metal-organic chemical vapor

deposition

TaeWan Kim, Jihun Mun, Hyeji Park et al.

Direct synthesis of large-area continuous ReS2 films on a flexible glass at low temperature

Youngchan Kim, Byunggil Kang, Yongsuk Choi et al.

Large-area, high-quality monolayer graphene from polystyrene at atmospheric pressure

Junqi Xu, Can Fu, Haibin Sun et al.

Synthesis of -MoO3 nano-flakes by dry oxidation of RF sputtered Mo thin films and their

application in gas sensing

Priyanka Dwivedi, Saakshi Dhanekar and Samaresh Das

Synthesis of wafer-scale hexagonal boron nitride monolayers free of aminoborane nanoparticles by

chemical vapor deposition

Jaehyun Han, Jun-Young Lee, Heemin Kwon et al.

http://iopscience.iop.org/page/termshttp://iopscience.iop.org/0957-4484/28/17http://iopscience.iop.org/0957-4484http://iopscience.iop.org/http://iopscience.iop.org/searchhttp://iopscience.iop.org/collectionshttp://iopscience.iop.org/journalshttp://iopscience.iop.org/page/aboutioppublishinghttp://iopscience.iop.org/contacthttp://iopscience.iop.org/myiopsciencehttp://iopscience.iop.org/article/10.1088/0953-8984/28/18/184002http://iopscience.iop.org/article/10.1088/2053-1583/aa5f4dhttp://iopscience.iop.org/article/10.1088/0957-4484/26/35/355706http://iopscience.iop.org/article/10.1088/1361-6528/aa6958http://iopscience.iop.org/article/10.1088/1361-6528/aa6958http://iopscience.iop.org/article/10.1088/2053-1583/aa6533http://iopscience.iop.org/article/10.1088/1361-6528/aa6235http://iopscience.iop.org/article/10.1088/0268-1242/31/11/115010http://iopscience.iop.org/article/10.1088/0268-1242/31/11/115010http://iopscience.iop.org/article/10.1088/0957-4484/25/14/145604http://iopscience.iop.org/article/10.1088/0957-4484/25/14/145604

Highly uniform wafer-scale synthesis ofα-MoO3 by plasma enhanced chemical vapordeposition

Hyeong-U Kim1,5, Juhyun Son2,5, Atul Kulkarni2, Chisung Ahn3,Ki Seok Kim4, Dongjoo Shin2, Geun Yong Yeom1,4 and Taesung Kim1,2

1 SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, Republic ofKorea2 School of Mechanical Engineering, Sungkyunkwan University, Suwon, Republic of Korea3 Institute of Advanced Machinery and Technology, Sungkyunkwan University, Suwon, Republic of Korea4 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, Republic ofKorea

E-mail: tkim@skku.edu

Received 4 December 2016, revised 24 February 2017Accepted for publication 15 March 2017Published 3 April 2017

AbstractMolybdenum oxide (MoO3) has gained immense attention because of its high electron mobility,wide band gap, and excellent optical and catalytic properties. However, the synthesis of uniformand large-area MoO3 is challenging. Here, we report the synthesis of wafer-scale α-MoO3 byplasma oxidation of Mo deposited on Si/SiO2. Mo was oxidized by O2 plasma in a plasmaenhanced chemical vapor deposition (PECVD) system at 150 °C. It was found that thesynthesized α-MoO3 had a highly uniform crystalline structure. For the as-synthesized α-MoO3sensor, we observed a current change when the relative humidity was increased from 11% to95%. The sensor was exposed to different humidity levels with fast recovery time of about 8 s.Hence this feasibility study shows that MoO3 synthesized at low temperature can be utilized forgas sensing applications by adopting flexible device technology.

Supplementary material for this article is available online

Keywords: MoO3, plasma enhanced chemical vapor deposition (PECVD), wafer-scale, Ramanspectroscopy, humidity sensor

(Some figures may appear in colour only in the online journal)

1. Introduction

Two-dimensional (2D) materials have attracted immenseattention owing to their unique chemical and physical prop-erties [1]. For example, graphene, the first 2D atomic crystal,possesses excellent material properties such as mechanicalstiffness, strength, elasticity, high electrical and thermalconductivities [2]. However, graphene lacks semiconductingcharacteristics because of its zero band gap. In the past fewyears, it has been revealed that transition metal dichalcogen-ides can replace graphene owing to their tunable band gap.

Especially, MoS2 has a tunable band gap, which can be tunedfrom an indirect (1.29 eV) to a direct (1.90 eV) band gapdepending on the number of layers. However, it has a lowcarrier mobility despite its tunable band gap [3]. On the otherhand, 2D semiconducting metal oxides (e.g., molybdenumoxide (MoO3)) have high dielectric constant (high-k), highelectron mobility [4], and a wide band gap (3.2 eV). More-over, MoO3 is relatively abundant in nature and shows opticaland catalytic properties.

2D material synthesis methods are classified into physicaland chemical methods. Mechanical exfoliation is a typicalphysical method, which yields 2D materials with high qualityand small size. Chemical vapor deposition (CVD) is a popular

Nanotechnology

Nanotechnology 28 (2017) 175601 (6pp) https://doi.org/10.1088/1361-6528/aa67d1

5 These authors contributed equally.

0957-4484/17/175601+06$33.00 © 2017 IOP Publishing Ltd Printed in the UK1

mailto:tkim@skku.eduhttps://doi.org/10.1088/1361-6528/aa67d1https://doi.org/10.1088/1361-6528/aa67d1http://crossmark.crossref.org/dialog/?doi=10.1088/1361-6528/aa67d1&domain=pdf&date_stamp=2017-04-03http://crossmark.crossref.org/dialog/?doi=10.1088/1361-6528/aa67d1&domain=pdf&date_stamp=2017-04-03

chemical method for the synthesis of uniform and large-areathin films. Graphene can be introduced a band gap bynanoribbons but the required complex synthesis processeswith significant loss in carrier mobility [5]. Therefore, layeredmaterials (such as MoO3 and MoS2) synthesized by varioustypes of CVD using different types of sources and precursorshave been reported to be potential alternatives to graphene[6]. However, the synthesis of these layered materials is atime-consuming process and requires high temperatures(>500 °C). To overcome this problem, the atmosphericpressure CVD technology was reported for the preparation oftransition metal oxide thin films at low temperature [7] andalso metal organic CVD has been developed for the synthesisof MoS2 thin films. In this method, two source gases [8] and aprecursor film with one source gas are used for reducing theprocess temperature [9]. On the other hand, plasma enhancedCVD (PECVD) is used for the synthesis of MoS2 thin films atlow temperatures (

performance of the MoO3-based sensors were determined bymeasuring current using semiconductor characterization sys-tem (KEITHLEY 4200-SCS) in real time. Water vapor wasinjected using an atomizer into the chamber. The RH levelwas controlled with a two channel mass flow controller at1 l min−1, one for water vapor and one for dry air. The lowcapacity vacuum pump was connected to outlet of thechamber at 1 l min−1. The RH level in chamber was measuredby hygrometer, which is placed near the sensor device undertest. When the sensor response reached a saturated value forset RH level, the inlet was closed for removing the humidityin the chamber. All measurements were carried out at roomtemperature (18 °C–21 °C) at different level of RH. Theresponse time and recovery time are defined on the basis ofthe time required for the sensor to achieve 90% of the totalcurrent change.

3. Result and discussion

The wafer-scale MoO3 obtained by PECVD of the Modeposited Si/SiO2 wafer was characterized by Ramanspectroscopy, XPS, HR-TEM, and EDS.

Figure 2(a) shows the Raman spectrum of the synthe-sized wafer-scale MoO3 at an excitation wavelength of532 nm and in the 100–1100 cm−1 range. The obtained peakscorrespond to the vibration of the ordered structure of MoO3.The peaks observed at 300, 671, 821, and 999 cm−1 corre-spond to the characteristic peaks of MoO3. The band at300 cm−1 (B2g, B3g) is attributed to the Mo3–O bendingmode. The band at 671 cm−1 (B2g, B1g) is attributed to theasymmetric stretching of the triply connected bridge-oxygenMo3–O bridge entity along the c-axis. This bridge-oxygenMo3–O bridge entity was formed by edge sharing of oxygen(Mo3–O) with the three adjacent octahedra. The 821 cm

−1

(Ag, B1g) band was observed only under the 30 nm conditionand corresponded to the symmetric stretching of the terminaloxygen atoms or the doubly connected bridge-oxygen Mo–O–Mo entity. Mo–O–Mo was formed by corner sharing ofoxygen with two octahedra. The 999 cm−1 (Ag, B1g) bandcorresponds to the symmetric stretching of the terminaloxygen atoms (Mo6+=O), which are responsible for thelayered structure of α-MoO3 [12, 13]. β-MoO3 shows twomajor vibrational bands at 775 and 850 cm−1 [14]. However,these peaks were not observed in the PECVD-synthesizedMoO3, as seen from figure 2(a). Further, the wafer-scaleuniformity of the synthesized MoO3 was investigated byobtaining the Raman spectra at three different locations(figure 2(b)). Moreover, for Raman mapping three peaks wereselected 671 cm−1 (Mo3–O), 821 cm

−1 (Mo2–O) and999 cm−1 (Mo=O) as shown in figure S4. It is observed thatall the three peaks show uniformity in the 20×20 μm2

mapping area. The Raman spectra and mapping clearly revealthat MoO3 was deposited uniformly on the wafer. The che-mical bonding state and surface composition of the synthe-sized MoO3 were investigated by XPS. The XPS scans forMo and O binding energies of the MoO3 layer are shown infigure 2(c). The Mo 3d5/2 and 3d3/2 peaks were observed at

234.0 and 237.2 eV, respectively. The energy differencebetween the 3d5/2 and 3d3/2 peaks was 3.2 eV. The lowbinding energy component located 533.2 eV (O 1s)(figure 2(d)) originated from the lattice oxygen of MoO3. Theatomic ratio of Mo to O was 1:3.10 and it is very similar withstoichio-metrically expected ratio value of 3. Additionally,Mo and O binding energy were compared for their actualintensity as depicted in online supplementary figure S5. Thus,the XPS results confirmed that MoO3 was successfully syn-thesized [15].

The structure of the synthesized MoO3 was furtherinvestigated by HR-TEM. Figure 2(e) shows the cross-sectionimage obtained by exposing the MoO3 sample to a focusedion beam (FIB). The inset shows the EDS spectrum of MoO3(30 nm Mo condition). The EDS analysis confirmed the pre-sence of O and Mo atoms (denoted by yellow circles). Thestoichiometric ratio (Mo:O) estimated from the EDS spectrumwas close to 3. The result further confirmed the formation ofMoO3. The preferential (100) orientations of MoO3 shown infigure 2(f) were used to determine the lattice spacing of thesynthesized MoO3 with bright field mode of the scanningTEM (STEM) mode. The lattice spacing was similar to thatreported for MoO3 in previous reports. A lattice spacing of0.38 nm was obtained, corresponding to the (100) plane of theorthorhombic phase of MoO3 [12].

Previous studies, PECVD are used to deposit by theadsorption and reaction of dissociated gases with precursorson the substrate [16]. In this study, a Mo film was depositedon the Si/SiO2 substrate and the resulting Mo-depositedSi/SiO2 was used as the precursor. The precursor was thenoxidized by O2 for the synthesis of MoO3. We have used thistechnique in our previous studies for the synthesis of MoS2thin films [10, 17]. The mechanism for the synthesis of wafer-scale MoO3 is illustrated in figure 3. During the plasmaactivation, the Ar gas present in the chamber was ionized.Herein, the specific reason to use Ar is to produce moreoxygen ions in the sheath area of plasma, this can further helpto penetrate O2 plasma into the 30 nm thick Mo film toachieve MoO3.

This ionization of Ar gas led to a charge transfer reactionbetween the resulting Ar ions and O2 because of collision.This collision resulted in the conversion of O2 to

+O ,2 and asheath layer was formed above the Mo-deposited wafer. AsO2+ carried a positive charge and the Mo wafer was chargeless

(i.e. it was grounded), an electric field was generated and +O2reacted with Mo on the Si/SiO2 substrate. Because of thisreaction, it was possible to oxidize Mo at a low temperature(i.e. 150 °C). Further, this reaction continued for approxi-mately 90 min to finally yield MoO3 on the Si/SiO2wafer [18].

The dependence of current versus RH is shown in figure 4.The sensing characteristics of the optimized MoO3 film devicewere measured current change by increasing or decreasing RHat the room temperature. It is reported that, the water moleculeacts as an electron donor (n-type doping), hence with theexposure of RH, the Fermi level of MoO3 shifted toward theconduction band by decreasing the resistance of MoO3,resulting in increase in the current [19]. It is obvious that the

3

Nanotechnology 28 (2017) 175601 H-U Kim et al

Figure 2. (a) Raman spectra of MoO3 (for 30 nm Mo condition). (b) Raman spectra of three points on the synthesized MoO3 (for 30 nm Mocondition). (c) XPS spectra of Mo 3d. (d) O 1s spectrum (for 30 nm Mo condition). (e) TEM cross-section image of the sample obtained byexposing the sample to a FIB; the inset shows the EDS spectrum of MoO3. (f) STEM mode image of MoO3.

4

Nanotechnology 28 (2017) 175601 H-U Kim et al

response and recovery behavior is one of the most importantcharacteristics for evaluating the performance of humiditysensors. The stability of the MoO3 gas sensor was checkedwith two cycles. The stability of the sensor was performed inconjugative cycles between two RH levels, by increasing theRH from 11% to 95%, as shown in figure 4(a). It is observedthat, for RH 95%, the response and recovery time are ∼195 and∼15 s, respectively. Figure 4(b) presents the current–time plotobtained by exposing the MoO3 sensor to the RH level such as95%, 71%, and 45% and returned to low humidity level of11%. At low humidity (11% RH), only a few water moleculeswill be present. At 71% RH, the response time and recoverytime were ∼185 s and ∼11 s, respectively. At a lower RH of45%, the response time and recovery time were ∼175 s and∼8 s, respectively. As observed, the sample had a quickrecovery to the RH change. Table 1 shows the comparativeresults for various kinds of metal oxide nanostructures withdifferent morphology, in terms of response time and recoverytime, for comparing the performance of the humidity sensorfabricated in this study [20–22]. For the MoO3 sensor fabricatedin this study, the recovery time was very low because theadsorbed water molecules desorbed quickly during humiditydecrease from high to low levels. Hence, low temperature

Figure 3. Schematic of the synthesis of MoO3 on Si/SiO2, showing the film formation mechanism.

Figure 4. A typical current–time plot of the MoO3-based RH sensor.(a) Response of the sensor during the RH switching between 11%and 95% for repeated cycles. (b) RH sensing response at differenthumidity levels at room temperature.

Table 1. The comparative data of various kinds of nanomaterialswith MoO3 film related to response/recovery time.

Material TypeResponsetime

Recoverytime References

CuO NW 120 120 [19]SnO2 NW 120–170 20–60 [20]TiO2 Nanotube 100 190 [21]MoO3 Film 175 8 Present work

5

Nanotechnology 28 (2017) 175601 H-U Kim et al

synthesized MoO3 can be suitable candidate for the sensitivehumidity/gas sensor application and can be applied flexibledevice technology.

4. Conclusions

In the present study, wafer-scale α-MoO3 was successfullysynthesized on a Si/SiO2 wafer using a PECVD system at150 °C. To the best of our knowledge, this is the first attemptmade till date to synthesize wafer-scale MoO3 at such a lowtemperature. The synthesized α-MoO3 characterization byRaman spectroscopy, HR-TEM, XPS, and EDS confirms thesuccessful synthesis which are comparable to other synthesismethods. The Raman spectroscopy results revealed that the α-MoO3 was deposited uniformly on the Si/SiO2 wafer. Thesynthesized α-MoO3 responded well for the RH from 11%–95%. Hence this feasibility study shows that MoO3 synthe-sized at low temperature can be utilized for the gas sensingapplications by adopting flexible device technology.

Acknowledgments

This work was supported by the International S&T CooperationProgram of China (ISTCP) (2014DFG52760) and NRF-2013K1A3A1A20046951. This work was also supported by aNational Research Foundation of Korea (NRF) grant funded bythe Korean government (NRF-2015R1D1A1A01057861).

References

[1] Bonaccorso F et al 2010 Graphene photonics andoptoelectronics Nat. Photon. 4 611–22

[2] Geim A K and Grigorieva I V 2013 Van der Waalsheterostructures Nature 499 419–25

[3] Lee C et al 2010 Anomalous lattice vibrations of single-andfew-layer MoS2 ACS Nano 4 2695–700

[4] Balendhran S et al 2013 Enhanced charge carrier mobility intwo‐dimensional high dielectric molybdenum oxide Adv.Mater. 25 109–14

[5] Jiao L et al 2010 Facile synthesis of high-quality graphenenanoribbons Nat. Nanotechnol. 5 321–5

[6] Jeon J et al 2015 Layer-controlled CVD growth of large-areatwo-dimensional MoS2 films Nanoscale 7 1688–95

[7] Gesheva K et al 2014 APCVD transition metal oxides—functional layers in ‘smart windows’ J. Phys.: Conf. Ser.559 012002

[8] Mun J et al 2016 Low-temperature growth of layeredmolybdenum disulphide with controlled clusters Sci. Rep. 621854

[9] Lee Y et al 2014 Synthesis of wafer-scale uniformmolybdenum disulfide films with control over the layernumber using a gas phase sulfur precursor Nanoscale 62821–6

[10] Ahn C et al 2015 Low‐temperature synthesis of large‐scalemolybdenum disulfide thin films directly on a plasticsubstrate using plasma‐enhanced chemical vapor depositionAdv. Mater. 27 5223–9

[11] Galatsis K et al 2001 Semiconductor MoO3–TiO2 thin film gassensors Sensors Actuators B 77 472–7

[12] Kalantar-Zadeh K et al 2010 Synthesis of nanometre-thickMoO3 sheets Nanoscale 2 429–33

[13] Lupan O et al 2014 Versatile growth of freestandingorthorhombic α-molybdenum trioxide nano-andmicrostructures by rapid thermal processing for gasnanosensors J. Phys. Chem. C 118 15068–78

[14] Haro-Poniatowski E et al 1998 Laser-induced structuraltransformations in MoO3 investigated by Ramanspectroscopy J. Mater. Res. 13 1033–7

[15] Lee Y J et al 2009 Chemical vapour transport synthesis andoptical characterization of MoO3 thin films J. Phys. D: Appl.Phys. 42 115419

[16] Kim H et al 2013 Synthesis of MoS2 atomic layer usingPECVD ECS Trans. 58 47–50

[17] Kim H-U et al 2015 In situ synthesis of MoS2 on a polymerbased gold electrode platform and its application inelectrochemical biosensing RSC Adv. 5 10134–8

[18] Floquet N et al 1992 Structural and morphological studies ofthe growth of MoO3 scales during high-temperatureoxidation of molybdenum Oxid. Met. 37 253–80

[19] Anderson J H Jr and Parks G A 1968 Electrical conductivity ofsilica gel in the presence of adsorbed water J. Phys. Chem.72 3662–8

[20] Wang S-B et al 2012 CuO nanowire-based humidity sensorIEEE Sens. J. 12 1884–8

[21] Kuang Q et al 2007 High-sensitivity humidity sensor based ona single SnO2 nanowire J. Am. Chem. Soc. 129 6070–1

[22] Zhang Y et al 2008 Synthesis and characterization of TiO2nanotubes for humidity sensing Appl. Surf. Sci. 254 5545–7

6

Nanotechnology 28 (2017) 175601 H-U Kim et al

https://doi.org/10.1038/nphoton.2010.186https://doi.org/10.1038/nphoton.2010.186https://doi.org/10.1038/nphoton.2010.186https://doi.org/10.1038/nature12385https://doi.org/10.1038/nature12385https://doi.org/10.1038/nature12385https://doi.org/10.1021/nn1003937https://doi.org/10.1021/nn1003937https://doi.org/10.1021/nn1003937https://doi.org/10.1002/adma.201203346https://doi.org/10.1002/adma.201203346https://doi.org/10.1002/adma.201203346https://doi.org/10.1038/nnano.2010.54https://doi.org/10.1038/nnano.2010.54https://doi.org/10.1038/nnano.2010.54https://doi.org/10.1039/C4NR04532Ghttps://doi.org/10.1039/C4NR04532Ghttps://doi.org/10.1039/C4NR04532Ghttps://doi.org/10.1088/1742-6596/559/1/012002https://doi.org/10.1038/srep21854https://doi.org/10.1038/srep21854https://doi.org/10.1039/c3nr05993fhttps://doi.org/10.1039/c3nr05993fhttps://doi.org/10.1039/c3nr05993fhttps://doi.org/10.1039/c3nr05993fhttps://doi.org/10.1002/adma.201501678https://doi.org/10.1002/adma.201501678https://doi.org/10.1002/adma.201501678https://doi.org/10.1016/S0925-4005(01)00737-7https://doi.org/10.1016/S0925-4005(01)00737-7https://doi.org/10.1016/S0925-4005(01)00737-7https://doi.org/10.1039/B9NR00320Ghttps://doi.org/10.1039/B9NR00320Ghttps://doi.org/10.1039/B9NR00320Ghttps://doi.org/10.1021/jp5038415https://doi.org/10.1021/jp5038415https://doi.org/10.1021/jp5038415https://doi.org/10.1557/JMR.1998.0144https://doi.org/10.1557/JMR.1998.0144https://doi.org/10.1557/JMR.1998.0144https://doi.org/10.1088/0022-3727/42/11/115419https://doi.org/10.1149/05808.0047ecsthttps://doi.org/10.1149/05808.0047ecsthttps://doi.org/10.1149/05808.0047ecsthttps://doi.org/10.1039/C4RA14839Hhttps://doi.org/10.1039/C4RA14839Hhttps://doi.org/10.1039/C4RA14839Hhttps://doi.org/10.1007/BF00665191https://doi.org/10.1007/BF00665191https://doi.org/10.1007/BF00665191https://doi.org/10.1021/j100856a051https://doi.org/10.1021/j100856a051https://doi.org/10.1021/j100856a051https://doi.org/10.1109/JSEN.2011.2180375https://doi.org/10.1109/JSEN.2011.2180375https://doi.org/10.1109/JSEN.2011.2180375https://doi.org/10.1021/ja070788mhttps://doi.org/10.1021/ja070788mhttps://doi.org/10.1021/ja070788mhttps://doi.org/10.1016/j.apsusc.2008.02.106https://doi.org/10.1016/j.apsusc.2008.02.106https://doi.org/10.1016/j.apsusc.2008.02.106

1. Introduction2. Experimental3. Result and discussion4. ConclusionsAcknowledgmentsReferences