Current Enhancement and Bipolar Current Modulation of Top-Gate

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Functional Nanostructured Materials (including low-D carbon)

Current Enhancement and Bipolar Current Modulation of TopGate Transistors Based on Monolayer MoS on Three-layer WMo S 2

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1-x

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Kuan-Chao Chen, Cing-Yu Jian, Yi-Jia Chen, Si-Chen Lee, Shu-Wei Chang, and Shih-Yen Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06327 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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ACS Applied Materials & Interfaces

Current Enhancement and Bipolar Current Modulation of Top-Gate Transistors Based on Monolayer MoS2 on Three-layer WxMo1-xS2 Kuan-Chao Chen †,‡, Cing-Yu Jian ‡,§, Yi-Jia Chen §, Si-Chen Lee†, Shu-Wei Chang*,‡, and ∥

Shih-Yen Lin*,†,‡ †

Graduate Institute of Electronics Engineering, National Taiwan University, No.1, Sec. 4,

Roosevelt Rd., Taipei 10617, Taiwan ‡

Research Center for Applied Sciences, Academia Sinica, No. 128, Sec. 2, Academia Rd., Taipei

11529, Taiwan §

Department of Materials Science and Engineering, National Dong Hwa University, No. 1, Sec. 2,

Da Hsueh Rd., Shoufeng, Hualien 97401, Taiwan ∥

Department of Photonics, National Chiao Tung University, 1001 University Rd., Hsinchu 30010,

Taiwan KEYWORDS: 2D material alloys, Hetero-structures, Activation energy, Hopping probability, Strained 2D materials, Transistors, Top gates

1 ACS Paragon Plus Environment

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ABSTRACT

We demonstrated the top-gate transistors composed of monolayer MoS2 grown on threelayer alloys MoxW1-xS2 prepared by sequential sulfurization of pre-deposited transition metal films. The elemental mapping of the alloy indicates uniform distributions of both cations Mo and W in the grown samples. Surprisingly, we find that the drain current of transistors could be enhanced by two orders of magnitude as the composition of Mo increases while the gate-controlled current modulation turns bipolar and ultimately vanishing. These features might originate from the formation of in-gap defect states with modest activation energy for transport and moderate hopping probability for current conduction, or a reduced electronic bandgap of the conducting channel due to strain.


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Introduction Personal computers and mobile devices have become necessities in our everyday life. The fast development of computing technologies based on bulk semiconductors such as silicon (Si) and gallium arsenide (GaAs) is the main backbone that supports the demand. However, with rapid evolutions of the Si-based technology, further shrinkage of device sizes has gradually run into a bottleneck for the development of next-generation electronics. The major advantages of twodimensional (2D) materials, as compared to those of bulk semiconductors, are that their functional devices can be realized with only one to several atomic layers1-3. This characteristic has made 2D materials a promising candidate for device applications in the nanometer range. However, in views of the matured growth and device fabrication of conventional semiconductors, many key technologies are still to be explored for 2D materials. One of them is the stacking of heterostructures made of 2D materials. The establishment of these hetero-structures makes the applications of individual devices based on 2D materials more versatile. Experimentally, lateral and vertical hetero-structures of 2D materials prepared by the chemical vapor deposition (CVD) and the sulfurization of pre-deposited transition metals have been demonstrated in the last few years4-6. Enhanced device performances were also observed from hetero-structure transistors based on molybdenum disulfide (MoS2) and tungsten disulfide (WS2)7. For the device fabrication of 2D materials and their hetero-structures, atomic-layer etching of MoS2 and equivalent selective etching of WS2/MoS2 hetero-structures are also demonstrated8. An alternative approach to develop analogous hetero-structures without seeking new compounds is the preparation of alloys based on the existing 2D materials. In previous publications, it has been demonstrated that the alloy MoxW1xS2 can be prepared with CVD9, 10. Although tunable bandgaps were observed from the alloy MoxW1xS2 with sound lattice structures, the small



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flakes of 2D materials and lateral composition variations are disadvantageous for applications based on vertical hetero-structures of 2D materials. In one of our previous works, we have demonstrated that under the sulfur-sufficient condition, the planar formation of MoS2 rather than oxide segregation of Mo is the dominant growth mode based on sulfurization of pre-deposited Mo films11. Since the co-deposition of W and Mo may provide uniform metal alloys, a thin film composed of Mo-W-alloy oxide with uniform compositions could be produced after the sample is removed from the vacuum chamber. The sulfurization of this alloy oxide with limited oxide segregation then leads to the growth of large-area and uniform alloy of MoxW1xS2. In this work, we present the characterizations of alloys MoxW1xS2 prepared with sulfurization and current modulations of top-gate transistors composed of these alloys and monolayer MoS2 grown on them. The uniform formation of MoxW1xS2 is confirmed through elemental mapping of Mo and W in the alloys, and their chemical compositions, Fermi levels, and band energies are also identified through various spectroscopic measurements. For the transistors fabricated on the monolayer MoS2 grown on these alloys, we find that the drain current is enhanced by two orders of magnitude as the composition of Mo increases from 0 to 0.3. However, the biascontrolled current modulation also becomes bipolar and even vanish at a high composition x of Mo. There could be two possible causes to the phenomena. One is associated with the higher density of in-gap defect states in the alloy with the larger fraction x of Mo. These defect states might have modest activation energy for transport or moderate hopping probability for conduction. The other is related to the reduction of electronic bandgaps and formation of semimetal in these 2D material systems, possibly due to the larger strain induced in the post-grown monolayer MoS2 on MoxW1xS2 at the higher composition x of Mo.12-15


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Results and Discussions Before sulfurization, three samples with the same sputtering power of 40 W for metal W but different power levels of 0, 10 and 20 W for metal Mo, which are referred to as samples A, B and C, were prepared with a two-gun sputtering system. Raman spectra of the three samples after sulfurization are shown in Figure 1a. In sample A, only the characteristic Raman peaks of WS2 are observed. With the increasing sputtering powers of Mo applied to samples B and C, two Raman peaks corresponding to MoS2 gradually emerge in addition to those of WS2. Compared with the Raman peaks corresponding to MoS2 in sample B, the more intense ones in sample C is indicative of the larger Mo composition which follows the higher sputtering power of Mo. Photoluminescence (PL) spectra of the three samples are shown in Figure 1b. The PL spectrum of a monolayer MoS2 prepared by sulfurizing the pre-deposited film of Mo is also shown in the figure for comparison. As can be told from the spectra, the peak wavelengths of PL from samples A, B and C gradually move from 618, 647 to 664 nm. The results are consistent with the trend of Raman spectra, indicating that the composition of Mo is raised with the increasing sputtering power of this element. An interesting feature of the PL spectrum from sample C is its resemblance to the counterpart corresponding to a monolayer MoS2 grown on the sapphire substrate. A much less prominent shoulder corresponding to the emission of WS2 or MoxW1xS2 is also present. It seems that small regions locally abundant in Mo-S bonds (MoS2-like grains) are present in the alloy. Under such circumstances, some electron-hole pairs excited in these regions could emit photons corresponding to the direct bandgap of MoS2.



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Figure 1. (a) The Raman spectra and (b) PL spectra of the three samples with different compositions of Mo. The PL spectrum of monolayer MoS2 prepared by sulfurizing the predeposited Mo film is also shown in (b) for comparison.

To identify the chemical compositions of alloy MoxW1xS2, we performed the x-ray photoelectron spectroscopy (XPS) for the three samples, and their spectra are shown in Figure 2a. The peaks corresponding to W4+ of the three samples are similar in magnitudes, while those related to Mo4+ increase with an enhanced sputtering power of Mo. The outcomes are indicative of the larger composition x of Mo in the thin films of MoxW1xS2 as the sputtering power of Mo got higher. After integrating the areas of W4+ and Mo4+ peaks beneath the XPS curves of various samples and comparing their magnitudes, we set the chemical compositions x of Mo in samples A, B and C to 0, 0.16 and 0.3, respectively16. The cross-sectional high-resolution transmission electron microscopy (HRTEM) images of the three samples were also taken and are shown in Figure 2b. Despite different sputtering powers of Mo, all the samples have three layers of 2D materials. The average thicknesses of these multilayer transition-metal dichalcogenides (TMDCs) are similar, reflecting close lattice constants of WS2 (1.232 nm) and MoS2 (1.228 nm) along the stacking direction.


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Figure 2. (a) The XPS spectra and (b) cross-sectional HRTEM images of the three samples with different compositions of Mo. The HAADF mappings of both W (left) and Mo (right) are also shown in the figure.

Uniform distributions of Mo atoms in samples B and C could be observed from the high angle annular dark field (HAADF) mappings of both W and Mo elements shown in Figure 2b. The images also support the inference that there should be no formations of macroscopic domains of MoS2 in most part of the alloys. However, the existence of nanoscale grains with a majority of Mo-S bonding could not be completely ruled out, as suggested by the nearly identical PL spectra of sample C and monolayer MoS2 in Figure 1b. Such small MoS2-like clusters may be stochastically formed during the growth of alloys, and their size distribution likely depends on the conditions of sputtering, oxidation, and sulfurization. Based on this picture, schematic diagrams showing the top view of possible lattice structures in samples B and C are illustrated in Figure 3. As indicated by marked regions in the figure, the probability of finding a large amount of Mo-S



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bonds within a local spot may increase with the composition x of Mo, and these areas effectively play the role of monolayer MoS2. The luminescence from these regions may dominate in sample C and is responsible for its similar PL spectrum to that of monolayer MoS2. Further investigations are required in the future to clarify this point. Still, the gradual red shift of PL peaks and generically uniform distributions of both Mo and W atoms should indicate that thin films of alloy MoxW1xS2 were indeed formed by sulfurizing the co-sputtered films of transition metals.

Figure 3. The lattice structures of alloys MoxW1xS2. The schematic diagrams showing the top views of lattice structures in samples B and C, respectively.

The ultraviolet photoelectron spectroscopy (UPS) and characterizations of absorption spectra at visible wavelengths were performed for samples A, B and C in order to investigate the Fermi levels and various band energies of alloys MoxW1xS2. For comparisons, the same measurements were also applied to a sample of 3-layer MoS2 prepared with the same growth procedures. The tails at low binding-energy side of UPS spectra are shown in Figure 4a. The


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energies Ebind of samples A to C and 3-layer MoS2 which represent energy differences between the corresponding Fermi levels (EF) and valence-band maxima (VBM) and are extrapolated near the sharp take-off the UPS spectra are 1.42, 1.34, 1.00 and 0.93 eV, respectively. The tails above energies Ebind of these four samples mostly correspond to in-gap states due to defects or disorder. A negative bias was then applied to the samples during UPS measurements to obtain their work functions 7, 17-19. The bias would push ejected electrons away from the samples and overcome the work function of the analyzer. The tails extending to the high-binding energy side of UPS spectra are shown in Figure 4b. The related cutoff energies Ecutoff of samples A, B, C and 3-layer MoS2 extracted from the spectral curves are 16.51, 16.40, 15.94 and 15.76 eV, respectively. The work functions are then estimated with the relation  = h Ecutoff, where h = 21.22 eV is the photon energy of incident ultraviolet beams. The work functions obtained in this way are 4.71 (A), 4.82 (B), 5.28 (C) and 5.46 eV (3-layer MoS2) for the four samples, respectively. The absorption spectra of the four samples at visible wavelengths are shown in Figure 4c. The band gaps of the four samples extracted from the absorption spectra are 1.85 (A), 1.81 (B), 1.74 (C) and 1.73 eV (3-layer MoS2), respectively. With the bandgap energies and VBMs determined from the UPS spectra, the conduction band minima (CBM) of the four samples can be determined. With these results, the various energies of the four samples are depicted in Figure 4d.



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Figure 4. (a) The tails at a low binding energy and (b) those at a high binding energy of UPS spectra corresponding to samples A, B and C and 3-layer MoS2. (c) The absorption spectra of the four samples. (d) The band energies of the four samples deduced from the aforementioned spectra.

It has been demonstrated in literature that the type-II band alignment of MoS2/WS2 heterostructures improve the transistor performance due to the electrons transferred from the WS2 layer to MoS2 channels. Therefore, it is speculated that the drain current of a MoS2/MoxW1xS2 transistor would decrease as the composition x of Mo increases owing to the smaller band offset and hence fewer electrons in the conduction channel confined in MoS2. In view of this, we fabricated three top-gate transistors after the deposition of an additional monolayer MoS2 onto the three alloys of 3-layer MoxW1xS2 with a composition of Mo x = 0, 0.16, and 0.3, which are identical to those of samples A, B, and C, respectively. The schematic device structures of the top-gate transistors are shown in Figure 5a. The drain currents ID as a function of gate-source voltage VGS (ID-VGS curves)


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taken from the three devices at a drain-source voltage VDS = 2 V are shown in Figure 5b. In contrast to the expectation that the drain current decreases with the increasing compositions x of Mo from 0, 0.16 to 0.3, the drain current is surprisingly elevated from 1.2  107, 4.0  106 to 5.1  105 A at VGS = 10 V, respectively, which is an enhancement of about two orders of magnitudes. Accompanied by the enhancement of ID is the disappearance of channel modulation controlled by the gate bias VGS. For transistors based on WS2 and alloy Mo0.16W0.84S2 (small composition x of Mo), the channel modulation exhibits a bipolar trend which is the characteristic of graphene transistors rather than that of typical MoS2 n-channel transistors7. In addition, the modulation capability of the transistor based on the alloy at a Mo fraction of x = 0.3 is virtually absent, as can be told from the similar drain currents at VGS = 0 and  10 V. Such behaviors of enhanced drain current and loss of current modulation are in fact absent in the transistors made of only MoS2 or multilayer MoxW1xS2 alloy. We note that the major difference between the devices here and MoS2/WS2 hetero-structure transistor reported previously with clear current modulations7 is the insertion of monolayer rather than multilayer MoS2 between the gate and host (alloyed) material. To examine the effect of layer number of MoS2, we also fabricated another top-gate heterostructure transistor composed of 5-layer MoS2 and 3-layer Mo0.3W0.7S2. The ID-VGS curve of the device at VDS = 2 V is shown in Figure 5c. As can be told by the trend with gate bias, the current modulation of n channels analogous to that of standalone MoS2 transistors is recovered. On the other hand, a significant drop of the drain current to 2.3  108 A at VGS = 10 V also took place in the device.



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Figure 5. (a) The schematic of the top-gate transistor based on the alloy of TMDC. The gate electrode is separated from the conducting channel by an insulating layer of Al2O3. The panel indicates the layer structure of channel material which is composed of a monolayer MoS2 grown on alloy MoxW1xS2. (b) The ID-VGS curves of the three devices with monolayer MoS2 on 3-layer WS2, Mo0.16W0.84S2 and Mo0.3W0.7S2, respectively, and (c) ID-VGS curves of the device with 5-layer MoS2 on 3-layer Mo0.3W0.7S2. The device structure corresponding to (c) is identical to that of (a) except for the layer number of MoS2. The voltage VDS was set to 2 V for all measurements.

There are two possible causes to the bipolar and vanishing current modulations of the transistors based on the monolayer MoS2 grown on 3-layer alloy MoxW1xS2 as the composition x of Mo increases. One possibility may be related to the leakage current arising from the in-gap defect states. The thermalization (de-trapping) of electrons/holes in the in-gap defect states to the


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conduction band of MoS2 channel or valence band of MoxW1xS2 alloy may stochastically provide carriers for current conductions. Effectively, those in-gap defect states unintentionally dope the material system. Such processes would be efficient if the difference between main distribution energy of defects and mobility edge, namely, characteristic activation energy, is comparable to or smaller than the thermal energy. In addition, if the density of in-gap defect levels is large enough, the hopping probability of carriers among the in-gap defect states may become significant. This additional channel of carrier migration could also increase the leakage current. Since those in-gap states are induced by structural defects and disorder, they might be more likely present at the higher faction x of Mo, leading to a significant drain current which is not easily modulated by the gate bias. The other is the shrinkage or absence of electric bandgaps between conduction and valence subbands confined near the interface of monolayer MoS2 and alloy MoxW1xS2 as x increases. This would bring about a reduced or vanishing bias range within which the channel is turned off. The new conducting states introduced above and below the shrunk bandgap may also significantly boost up the magnitudes of drain currents as the gate bias increases. In the limit that the bandgap is completely absent, namely, various conduction and valence bands overlap with each other in energy, the conducting channel turns into a semimetal. Under such circumstances, the semiconductor-metal transition occurs in the system of 2D material, and the bias control over current modulations becomes ineffective. In fact, the band structure of TMDC is sensitive to the interlayer distance and in-plane lattice constants, namely, strain . It has been shown that a few percents of biaxial tensile or normal compressive strain can effectively reduce the electronic bandgap of MoS2-WS2 bilayers12-15, as illustrated by the schematic band structures shown in Figure 6a and 6b. With the even larger strain, the CBM can become lower than VBM, as in indicated in



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Figure 6c. Under such circumstances, the conduction and valence subbands overlap with each other and potentially make the MoS2-WS2 bilayer a semimetal, and the variation of the Fermi level EF in the energy range of overlapped bands can no longer switch on or off the conducting channel, indicating the loss of current modulations.

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VBM

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Figure 6. The schematic band structures of strained MoS2/MoxW1xS2 bilayer under different biaxial tensile or normal compressive strain: (a) little strain, (b) moderate strain, and (c) considerable strain. An increasing magnitude of strain may reduce the electronic bandgap and ultimately turn the 2D material into a semi-metal.

Ideally, the lattice constants of MoS2 and WS2 are close to each other, and hence a significant internal strain is unexpected in the alloy or hetero-structures. However, small fluctuations of the thicknesses and non-ideal lattice structures do exist in our alloy samples, and they are perhaps more significant in samples with the higher composition x of Mo. This might introduce an internal strain to the post-grown monolayer MoS2 and alloyed layer immediately beneath it. The strain then effectively reduces the electronic bandgap of this hetero-structure or


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even turns it into a semimetal, reflecting the less effective current modulation but giant current enhancement of the conducting channel at a high composition x of Mo in the experiment. The two aforementioned scenarios are also consistent with the relatively clear current modulation of the transistor made of 5-layer MoS2 grown on the 3-layer alloy Mo0.3W0.7S2. In the case of multilayer MoS2, the field-induced conducting channel is confined near the insulator gate and top MoS2 layer which is away from the alloy Mo0.3W0.7S2. The effect of in-gap defects or strain could not reach the conducting channel. Therefore, the conducting channel still acts effectively as a 2D semiconductor with observable current modulations. At this stage, our measurements could not definitely tell which mechanism is responsible for the enhanced drain current and loss of current modulation in the experiment. Further studies on this phenomenon in the future would be carried out to clarify the underlying process so as to improve the device characteristics of transistors based on the alloy MoxW1xS2.

Conclusions In conclusion, we have fabricated top-gate transistors based on the hetero-structures of monolayer MoS2 and 3-layer alloys MoxW1xS2 which were prepared with co-sputtering, oxidation, and sulfurization. The chemical compositions, Fermi levels, and band energies of the alloys have been characterized with various spectroscopic characterizations, and uniform formation of the alloys has been verified with elemental mappings of Mo and W. For the transistors, we observe a current enhancement of about two orders of magnitude and the bipolar or vanishing bias-controlled current modulation as the composition of Mo increases. The behavior might be attributed to the modest activation energy for transport and enhanced hopping probability of the in-gap defect states,



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or reduction of bandgap and transition to semimetals due to the induced strain of monolayer MoS2 and alloy.

Experiments Before sulfurization, Mo/W films were deposited on sapphire substrates using a RF cosputtering system. During the metal deposition, the sputtering power of W was kept at 40 W, and the corresponding power of Mo was varied from 0 to 40 W (20 W/step). The background pressure was kept at 5×10-3 torr with a 40 sccm flow of Ar gas. After metal deposition, the samples were placed in the center of a hot furnace for sulfurization. During the sulfurization, a 130 sccm flow of Ar gas was used as carrier gas, and the furnace pressure was kept at 0.7 torr. The growth temperature of the samples was kept at 800 °C with 1.5 g of S powder placed at the upstream of gas flow. The evaporation temperature of the S powder was kept at 120 °C. For the PL and UPS measurements, two reference samples with monolayer and 3-layer MoS2 were also prepared. The XPS and UPS measurement were carried out with PHI VersaProbe, and absorption at visible wavelengths

was

taken

with

JASCO

V-670

spectrophotometer.

The

Raman

and

photoluminescence (PL) spectra were performed using a HORIBA Jobin Yvon HR800UV Raman spectroscopy system equipped with 488 nm laser. The cross-sectional HRTEM images were taken with a FEI Tecnai G2 F20 transmission electron microscopy system operated at 200 kV.

Corresponding Author *E-mail: [email protected]; [email protected]


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ACKNOWLEDGMENT This work was supported in part by projects MOST 105-2221-E-001-011-MY3 and MOST 106-2622-8-002-001 funded by the Ministry of Science and Technology, Taiwan, and in part by the iMATE project funded by Academia Sinica, Taiwan.



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Current Enhancement and Bipolar Current Modulation of Top-Gate

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