Interband Transitions in Mono- and Few-Layer WSe2 Probed using

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Interband Transitions in Mono- and Few-Layer WSe Probed using Photo-Excited Charge Collection Spectroscopy Kyunghee Choi, Kimoon Lee, Sanghyuck Yu, Sehoon Oh, Hyoung Joon Choi, Heesun Bae, and Seongil Im ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04056 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Interband Transitions in Mono- and Few-Layer WSe2 Probed using Photo-Excited Charge Collection Spectroscopy Kyunghee Choi∥†, Kimoon Lee∥ ‡, Sanghyuck Yu§, Sehoon Oh§, Hyoung Joon Choi§, Heesun Bae*§, and Seongil Im*§

†

Reality Display Device Research Group, Electronics and Telecommunications Research

Institute (ETRI), 218 Gajeong-ro, Yuseong-gu, Daejeon, 34129, Republic of Korea ‡

Department of Physics, Kunsan National University, Gunsan 54150, Republic of Korea

§Van

der Waals Materials Research Center, Institute of Physics and Applied Physics, Yonsei

University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea ∥

These authors contributed equally to this work.

* E-mail: [email protected] * E-mail: [email protected]; tel.: 82-2-2123-2842; fax: 82-2-392-1592


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Abstract Transition metal dichalcogenides are currently under rigorous investigation because of their distinct layer-dependent physical properties originating from the corresponding evolution of the band structure. Here, we report the highly resolved probing of layer-dependent band structure evolution for WSe2 using photo-excited charge collection spectroscopy (PECCS). Monolayer, a few layer, and multi-layer WSe2 can be probed in top-gate field-effect transistor platforms, and their interband transitions are efficiently observed. Our theoretical calculations show a great coincidence with the PECCS results, proving that the indirect Γ-K and Γ-Λ transitions as well as the direct K-K are clearly resolved in multi-layer WSe2 by PECCS.

Keywords: Transition metal dichalcogenide, WSe2, Field-effect transistor (FET), Interband transition, First-principles calculation


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There has been much demand for novel semiconducting materials to overcome the scaling limit of conventional Si technology.1 Two-dimensional (2D) layered materials with atomic scale thickness have arisen as promising candidates to fulfill these requirements.2 In this context, the transition metal dichalcogenides (TMDs) have drawn enormous attention as a result of their distinct structural, optical and optoelectronic properties.3,4 Based on the sizable bandgaps of semiconducting TMDs, which can be modulated from indirect to direct bandgaps depending on the number of layers, WSe2 allows various applications in electronic and optoelectronic devices.5-8 Among the TMDs, mono- and few-layer tungsten diselenides (WSe2) have reported a new family of 2D crystals not only from the fundamental point of view9-14, but also with regard to their potential applications, such as field-effect transistors (FETs), photo-transistors, gas sensors, solar cells, and complementary logic circuits especially with its bipolarity both on n- and p-type conduction unlike other TMD-based semiconductors.15-19 With these advances in device application, it is essential to understand the layer-dependent optical and optoelectronic properties, which are related to the evolution of WSe2 band structure.11,14 Mono-layer (1L) WSe2 is composed of a single internal tungsten (W) metal layer and two external selenium (Se) layers, forming a Se-W-Se sandwiched structure with strong covalent intra-layer bonds. The structures of few-layer and bulk WSe2 are comprised by stacking such 1L units of WSe2, and a weak van der Waals interaction operates to bond adjacent layers. According to numerous theoretical studies, both the conduction band minimum (CBM) and the valence band maximum (VBM) occur at the same K point which is in the Brillouin zone for 1L WSe2, confirming the direct gap nature.9-11 For few-layer WSe2, it is reported that the VBM occurs as the Γ point while CBM exists at the Λ point, which indicates the indirect gap semiconductor.9-11 Such dramatic changes in the band structure for WSe2 depending on the number of layers has


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also been observed experimentally, indicating the important role of interlayer coupling.13, 14 G. Eda et al. reported the probing of direct and indirect transitions by temperature-dependent photoluminescence (PL) measurement, which was combined with density functional theory (DFT) calculation.14 Although the well-focused signal on the micro-sized 2D crystals enabled the successful de-convolution of PL spectra to provide information of various optical transition routes, well-resolved detection of various transitions with tens of meV order precision, is still limited in respect to the emissive nature of PL that is inherently insensitive for indirect transition.20 Hence novel optical analysis techniques based on the absorption process should be critical for sensitive analysis of detailed optical transitions, which are independent from their momentum differences.5 In the present study, we present a highly resolved probing of layer-dependent band structure evolution for WSe2 using photo-excited charge collection spectroscopy (PECCS), a novel absorption based technique for 2D materials. We previously used the same technique mainly to probe the trap density of state (DOS) at the 2D MoS2/dielectric interfaces,21 however, the present study fundamentally focuses on the band structure of WSe2 in a working FET platform. Experimentally, monolayer (1L), few-layer (4L), and bulk (13L) WSe2 were integrated into topgate field-effect transistors (FETs) as their channels, to be efficiently probed for both direct and indirect interband transitions with tens of meV resolution at room temperature. DFT calculations show a great coincidence with our PECCS results, proving that the indirect Γ-K and Γ-Λ transitions as well as the direct K-K in WSe2 are clearly resolved by PECCS. For the fabrication of our WSe2 top-gate field-effect transistors as shown in Figure 1a, we used a p+-Si wafer with 285 nm-grown SiO2 as a substrate. The mechanically exfoliated mono- (1L), four- (4L) and thirteen-layer (13L) WSe2 flakes from bulk WSe2 crystals (HQ Graphene) were


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dry-transferred to the substrate. The thickness of WSe2 flakes was confirmed using an atomic force microscope (AFM, Nanowizard I, JPK Instrument), as well as Raman spectroscopy and photoluminescence (PL, LabRam Aramis, Horriba Hovin Yvon). The source (S) and drain (D) electrodes (Pt/Ti/Pt with 25/25/50 nm) were patterned by conventional photolithography and liftoff processes. Then, the device was heated at 523K for 10 min in air ambient to make ohmic contact. Bilayer gate dielectric comprised of a high- and low-k layer was used for fabricating the p-type WSe2 top-gate FETs: a 20-nm thick conventional fluoropolymer Cytop (Asahiglass, CTX809M) and 30 nm-thick Al2O3 layer were sequentially deposited by spin-coating and atomic layer deposition (ALD), respectively. Finally, we deposited 50 nm-thin indium-tin-oxide (ITO) on bilayer gate dielectric as an optically transparent gate electrode by DC magnetron sputtering system. PECCS measurements were carried out as conducted previously.21 After the experimental implementation, DFT calculations were performed for the electronic band structures 1L, 4L and bulk WSe2. The experimental lattice parameters and atomic positions22 are used for calculating the band structure of bulk WSe2. Atomic structures of 1L and 4L WSe2 are constructed using the experimental lattice parameters and atomic positions of bulk WSe2. The vacuum region for a slab is ~100 Å thick along the out-of-plane direction in order to avoid fictitious interactions between layers generated by the periodic boundary condition. Firstprinciples DFT calculations were performed using generalized gradient approximation (GGA) implemented in the SIESTA code, and spin-orbital coupling was used to calculate the electronic structure as reported previously.23-28 We used 128×128×1 Monkhorst-Pack k-point mesh for 4L WSe2 and 64×64×16 mesh for bulk WSe2 with a real-space mesh cut-off of 2000 Ry for all of our calculations.


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The inset of Figure 1b displays the optical microscopy (OM) image of the patterned electrodes and the WSe2 flake prior to the ITO top-gate electrode deposition. From the OM images, we could exploit the number of WSe2 layers to select flakes which have uniform thickness as well as a large enough area because of their strong image contrast depending on the layer. (See Figure 1b inset and Figure S1.) With AFM and Raman spectra measurement, the number of WSe2 layers was more precisely identified. As shown in Figure 1c, vibrational peaks at ~ 248.4 cm-1 are observed, which correspond to the in-plane (E ) modes of the WSe2 flakes, while those at 260.0,

257.5 and 256.3 cm-1 are attributed to the out-of-plane A modes of the 1L, 4L and 13L WSe2 flakes respectively. Moreover, we could not observe any peak around 308 cm-1 assigned as B mode for the 1L WSe2 flake, indicating the absence of additional interlayer interaction as shown in the inset of Figure 1c.12 Thus, we consider Raman spectroscopy to be another tool which clearly pinpoints the number of layers for WSe2.11,12 Figure 1d shows PL spectra from mono-(1L), four- (4L) and thirteen-layer (13L) WSe2 flakes. The most notable point is that monolayer WSe2 flakes exhibit strong emission at 1.64 eV, whereas the peak intensity of emission is dramatically suppressed for few-layer WSe2 flakes since they show indirect interband transition. The PL peak for 4L (13L) flake can be deconvoluted with two peaks located at 1.60 eV and 1.5 eV (1.38 eV) which originate from direct and indirect transition respectively, which consistent with previous reports.11 While red-shift in the direct interband transition as well as splitting of the direct and indirect transition is well observed as layer number increases, it is hard to distinctively resolve the detailed indirect interband transitions such as I1 and I2 that has been theoretically predicted. 14


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Figure 2 displays the output (drain current-drain voltage, ID-VD) and transfer (drain currentgate voltage, ID-VG) curves of the WSe2 FETs with increasing layers from 1L (~0.7 nm) to 13L (~10 nm). With the increase of the layer number present in a WSe2 channel, the drain current increased, clearly exhibiting strong thickness-dependent carrier transport in WSe2 FETs. (Figure 2a) As shown in Figure 2b, the on/off current ratio is also improved from ~104 to ~106 with the increase of the WSe2 layer number, while gate leakage current (IG) was as negligible as a few pA for all FETs. We plotted linear field-effect mobility (μ ) as a function of VG in the inset of Figure 2b based on the following well-known equation (1) 29, μ =

(/)

(1)

where Cox is the dielectric capacitance per unit area and the maximum values of µlin are respectively estimated to be 0.04, 0.2 and 8 cm2/Vs for 1L, 4L, and 13L WSe2 channel. Such an enhanced linear mobility and ID current with regard to layer numbers has been reported in other 2D FETs of MoS2.30 It is thought that contact resistance between channel and electrode should be higher in fewer layers of WSe2 which have a larger band gap and Schottky barrier. High contact resistance in FET can be one of the essential reasons for low field effect mobility. Since the mobility value of FETs does not have much relationship with their direct and indirect transitions, we could carry out photoelectric probing (or PECCS) experiments on our FETs with 1L, 4L and 13L WSe2. Figure 3a shows the representative (ID-VG) transfer curves of the 13L WSe2 FET at a drain voltage of VD=-0.5 V without and with exposure to several monochromatic photons. (The transfer curves of the other FETs are shown in Figure S2.) The 1L WSe2 FET only responds the photon energy above 1.77 eV, while the other FETs start to respond to photon energy above 1.59 eV. This result indicates that the required energy for interband


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optical transition is larger for 1L WSe2 than those for few layered ones, which agrees well with our PL results in Figure 1d. The photo-response of transfer curves can be quantitatively characterized by the photo-induced threshold voltage shift ∆Vth. (In the present study, we defined the threshold voltage, Vth of FETs as 1x10-10 A in transfer curves.) As illustrated in Figure 3b, photon illumination beyond band gap energy (Eg) induces electron-hole pairs in the 2D WSe2 semiconducting channel along the allowed interband transition route, thus resulting in quasiFermi levels for electrons and holes as denoted by Efs,n and Efs,p, respectively.29 Because all of our 2D WSe2 FETs exhibit p-channel conduction, the flat-band voltage (VFB) should be determined by the difference between Fermi levels of the gate electrode (Efm) and the Efs,p of the WSe2 channel (work function difference) as seen below in equation (2),29 =

( !,# ) $

&

(

(

− − ', ( )(*)+*

(2)

where, Qf is the fixed trap charge per unit area near the WSe2/dielectric interface, ρ(x) is the bulk charge trap density in the gate dielectric, and xox is the dielectric thickness. While previous PECCS measurements mainly focused on the trap density of states in Qf as a function of photon energy lower than Eg21, the present study investigates the band density of states (DOS) values utilizing a more energetic photon than Eg which would cause a shift of VFB (∆VFB) due to Efs,n and Efs,p. Such a ∆VFB results in the same amount of ∆Vth as a function of the photon energy, ∆VFB (ε). Considering that the lowering of Efs,p level logarithmically depends on the optical

transition rate (-./,0 ~ ln( ), where f is the number of optically excited electron-hole pairs per . unit time) which is proportional to the joint density of states (JDOS) for interband transition 20, the photo-induced ∆Vth should be abruptly observed in a certain moment when the allowed interband transition occurs in association with the amount of JDOS.


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By tracking the photo-induced transfer characteristics, we measured a series of photo-induced threshold voltage shift ∆Vth and its derivative (∂Vth/∂ε) as a function of the photon energy, ε in Figure 3c. A quite abrupt voltage shift ∆Vth was observed at the onset of the interband transition due to band-to-band light absorption, so the direct transition was found to be 1.63 eV for monolayer WSe2 as consistent with 1.64 eV from the PL measurement. While it is only observed as a single peak at 1.63 eV originating from the direct interband transition (as denoted A) for 1L WSe2 FET, both 4L and 13L WSe2 FETs clearly show red-shifted A peak and secondary peaks as well at lower energies (denoted as I1 and I2, which were weakly resolved by PL in fact). From the fact that our PECCS is based on the absorption process that allows both direct and indirect transitions, I1 and I2 should originate from indirect transitions which were difficult to observe using emission based PL measurement. To further clarify the detailed transition origins for each peak, as the next step we performed the first-principle calculations for the electronic band structure of 1L, 4L and bulk WSe2. Figures 4a – c display the band structure evolution of WSe2 upon increasing the number of layers from 1L to 4L and bulk WSe2. As consistent with our PL and PECCS results, the monolayer WSe2 shows a direct interband transition as the minimum energetic route corresponding to A peak from VBM to CBM at the same K point. (Figure 4a) In 4L WSe2, the VBM position changes from K to Γ point due to the raised electronic energy at Γ, and the CBM position simultaneously shifts from K to Λ point as this result induces additional indirect routes of I1 and I2 as marked in Figure 4b. When we compare with the PECCS result of 4L WSe2, only two peaks (A and I1) were observed in the PECCS due to the negligible difference (theoretically calculated as ~68.6 meV) between VBM at K and Γ point resulting in the almost identical transition energy for A and I2.


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For bulk WSe2, distinct A, I1, and I2 transitions are shown in both the PECCS and the calculation results as shown in Figure 4c, because the VBM at the K and Γ point became separated as shown in Figure 4c. Moreover, the energy difference between I1 and I2 become increased as the layer number increases, which is consistent with both our PECCS and previously reported temperature dependent PL results.14 It is thus concluded that our PECCS clearly resolves the I1 and I2 transition which was difficult to identify using PL and our results perfectly match the theoretical values. In summary, we have presented a highly resolved probing of layer-dependent band structure evolution for WSe2 by PECCS. For PECCS measurements, 1L, 4L and 13L WSe2 were integrated into top-gate field-effect transistors (FETs) to be efficiently probed for both direct and indirect interband transitions at room temperature. As a result, we could clearly observe the direct K-K transition from 1L WSe2, its red-shift and splitting from 4L, and indirect Γ-K and Γ-Λ transitions upon increasing the layer number of WSe2. Our PECCS results turned out to be consistent with the theoretical expectation according to the first-principles calculations based on DFT. Our PECCS clearly resolves the interband transitions which were difficult to identify using emissive PL even at room temperature. We thus conclude that PECCS is a highly prominent technique for investigating the optoelectronic properties of WSe2 and can be broadly applicable to the study of other 2D semiconductor materials.

ASSOCIATED CONTENT


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Supporting Information. Optical microscopy (OM) images and the photo-induced transfer characteristics of the 1L and 4L WSe2 FETs for PECCS measurement; AFM image and thickness profile of 4L WSe2 flake

AUTHOR INFORMATION Corresponding Authors * (Heesun Bae) E-mail: [email protected] * (Seongil Im) E-mail: [email protected]. Phone: 82-2-2123-2842. Fax: 82-2-392-1592. Address: Electron Device Laboratory, Science Building, Room 240, Yonsei University, Seoul, 03722, Korea.

AUTHOR CONTRIBUTIONS ∥

These authors contributed equally. H. B. and S. I. conceived the study. K. C. and K. L.

conducted all the experiments and analysis. S. Y. designed and S. O. and H. C. carried out most of the calculations. All the authors discussed the results and commented on the manuscript.

ACKNOWLEDGMENT This research was supported by SRC program: vdWMRC center through the National Research Foundation

(NRF)

of

Korea

funded

by

the

Ministry

of

Education

(NRF-Grant

No.2017R1A5A1014862). Kimoon Lee and Heesun Bae acknowledge Basic Science Research


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Program through the NRF (2016R1D1A3B03933785 and 2017R1D1A1B03028257). Hyoung Joon Choi acknowledges the supporting from NRF (2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2017-C3-0079).


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FIGURE

Figure 1. (a) 3D schematic image of the top-gate FET with the WSe2 flakes, ITO gate electrode and the bilayer Cytop/Al2O3 gate dielectric upon exposure to monochromatic light for the PECCS measurements. (b) AFM line profile showing the height measurement of the 13L WSe2 flake as FET channel along with the OM and AFM 3D topographic images in the inset. (c) Raman spectra with the inset of the magnified peak near 308 cm-1 and (d) PL spectra of the 1L, 4L and 13L WSe2 flakes. Dotted lines show the Gaussian deconvolution in the PL spectrum of 4L WSe2.


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Figure 2. (a) Output (ID-VD) and (b) transfer (ID-VG) curves of top-gated FETs with 1L, 4L and 13L WSe2 channels. The gate leakage current (IG) and the linear mobility curves (inset) of the same FETs at VD=-0.5 V.


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Figure 3. (a) Transfer curves of the 13L WSe2 FET without and with exposure to monochromatic light of various wavelengths for PECCS measurement. (b) Schematic of photoinduced VFB shift mechanism which results in ∆Vth as a function of photon energy. (c) The plot of ∆Vth and its derivatives (∂Vth/∂ε) with respect to ε. The dashed lines denote the onset of the direct (A) and indirect (I1 and I2) interband transitions for 1L, 4L and 13L WSe2 FETs.


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Figure 4. Theoretical calculation results of the energy band structures of (a) 1L, (b) 4L and (c) bulk WSe2. Arrows are used to represent the excitations, where A, I1, and I2 are the interband transitions from K to K, from Γ to Λ, and from Γ to K, respectively.. In each plot, the energy of valence band maximum state is set to zero energy and marked with the horizontal dashed line.


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Interband Transitions in Mono- and Few-Layer WSe2 Probed using

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