Photon Factory Activity Report 2016 #34 (2017)

BL-27A/2016G583

Modification of local electronic state of MoS2 under applied electric fieldstudied by in-situ near edge X-ray absorption fine structure
Shiro Entani1,* Mitsunori Honda2 Songtian Li1 Hiroshi Naramoto1 and Seiji Sakai1
1National Institutes for Quantum and Radiological Science and Technology,
2-4 Shirakata, Tokai, 319-1195, Japan
2
Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, 319-1195, Japan
1 Introduction
Two-dimensional transition metal dichalcogenides haveattracted considerable attention as one of the mostpromising materials for realizing nanoelectronics andspintronics devices in recent years, due to the novel
electronic and electric properties [1-5]. MoS2 shows highelectron transport properties with an indirect band gap of
1.2 eV (single-layer MoS2 has a direct band gap of 1.8eV), high carrier mobility over 500 cm2/Vs and highon/off-current ratios of high as 108 [2]. In order to obtainfurther high electron transport properties, the elucidationof the basic physical properties, such as an electronicstructure is indispensable. Here studies were conducted
for the electronic state of MoS2 under the applied electricfield by employing in-situ near edge X-ray absorption
fine structure (NEXAFS). We have successfully
demonstrated that the local electronic state was modified
by the electric field-effect.
2 Experiment
Figure 1 (a) shows a schematic drawing of theexperimental setup of the in-situ NEXAFS measurement.
We fabricated a metal-insulator-semiconductor (MIS)
capacitor using MoS2 as a semiconductor layer. Using thisdevice structure, the modification of the electronic state
under the applied electric field was investigated in-situ byfluorescence NEXAFS. P-type Si(100) with 300 nm
oxide layer was used as a substrate. A MoS2 film wassynthesized on the SiO2 substrate by the solutionprocessed method using N-methylpyrrolidone asdispersing agent [6]. The layer number of MoS2 wasapproximately 20 layers. The specimen was thenintroduced into a vacuum chamber and a Ru electrodewas deposited by RF magnetron sputtering through ashadowing mask. The deposition rate was 0.5 nm/min andthe film thickness was 30 nm. By employing Ru as anelectrode, the synchrotron X-ray beam can be irradiatedselectively on the MoS2 device region by tracing the RuL fluorescence X-ray emission from the Ru electrode(Fig.1 (b)). This enables us to improve signal-tobackground ratio. NEXAFS measurements wereconducted at BL-27A of the soft X-ray synchrotron
radiation beamline of Photon Factory, KEK [7,8].
NEXAFS spectra of the device were measured byfluorescence yield and a region of interest (ROI) at arespective fluorescence peak was used for the detection ofthe characteristic X-ray from the element of interest.

Fig. 1: (a) A schematic drawing of the experimental setupof the in-situ NEXAFS measurement. (b) Multi-channel
analyzer (MCA) spectra taken from the Ru/MoS2/SiO2
region (upper) and the MoS2/SiO2 region (lower) in theMoS2 device.
3 Results and Discussion
Figure 2 shows S K-edge and Mo L3-edge fluorescence
NEXAFS spectra taken under applied electric field. The
conduction band (CB) minimum in MoS2 consists of 3pand 3s-like states in addition to Mo 4d-like states. An
intense peak in the Mo L3-edge NEXAFS (around 2524eV) is assigned to Mo 2p3/2 to unoccupied Mo 4d band at
the CB minimum. On the other hand, an intense peak in
the S K-edge NEXAFS (around 2471 eV) is attributed toS 1s to S 3p-like states mixed into the Mo 4d band.
Another peak in the S K-edge spectrum (around 2481 eV)corresponds to transition of S 1s electrons to anti-bonding
S 3p-like state, and a similar peak in the Mo L3-edge
spectrum (around 2533 eV) corresponds to transition ofMo 2p3/2 electron to unoccupied Mo 5s-like states [9].
When electric fields are applied to the MoS2 device, thespectral changes are clearly seen in the S K-edge spectra(Fig.2 (a)). These significant changes arise only at aroundPhoton Factory Activity Report 2016 #34 (2017)2471 eV. This indicates that the modification of theelectronic state by the applied electric field arises only in
the CB minimum. It is also found that the intensity of thepeak increases and decreases according to the polarity ofthe electric field; the S K-edge spectra taken at +10 V and-10 V applied electric field cause the decrease andincrease of the peak intensity. This is presumably related
to accumulation and depletion of electrons by the electricfield-effect. In the Mo L3-edge NEXAFS spectra, on theother hand, neither the intensity change nor the peak shift
is observed under the applied electric field. This suggeststhat the modification of the electronic state by the field
effect might be only around S site. Although themechanism of the modification of the electronic statescaused by the field effect has not been elucidated
completely, the present study provides the clue for theessential understanding of the electric transport propertiesof two-dimensional transition metal chalcogenides.

Fig. 2: (a) S K-edge and (b) Mo L3-edge fluorescence
NEXAFS spectra of the Ru/MoS2/SiO2 device under theapplied electric field of 0 V (black square), -10 V (redsquare) and +10 V (green square). (lower) Difference
spectra between the spectrum taken at -10 V and at 0 V(red open circle) and between the spectrum taken at +10
V and at 0 V (green open circle).
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[2] B. Radisavljevic et al., Nature Nanotechnol. 6, 147(2011).
[3] W. S. Hwang et al., Appl. Phys. Lett. 101, 013107(2012).
[4] H. Fang et al., Nano Lett. 12, 3788 (2012).
[5] H. Wang et al., Nano Lett. 12, 4674 (2012).
[6] G. L. Frey et al., Adv. Mater. 14, 265 (2002).
[7] M. Honda et al., Rev. Sci. Instrum. 86, 035103 (2015).
[8] M. Honda et al., J. Phys. Chem. C 120, 5534 (2016).
[9] D. Li et al., Phys. Chem. Minerals 20, 489 (1994).
* entani.shiro@qst.go.jp