Tailoring the electrical properties of multilayer MoS₂ transistors using ultraviolet light irradiation

Abstract

Two dimensional layered semiconductor transition-metal dichalcogenides such as molybdenum disulfide (MoS₂) have attracted tremendous interest as a new class of electronic material and generated considerable attention as a promising channel material for field-effect transistors (FETs). Modulating the electronic properties of MoS₂ is essential in order to obtain the best performance of its electronic and optoelectronic devices as well as enabling fabrication of various complex devices. In this paper we report a very straightforward and simple technique to tailor the performance of multilayer (ML) MoS₂ FETs by exposure to nitrogen (N₂) gas under deep-ultraviolet (DUV) light. Threshold voltages of ML MoS₂ FETs shifted towards a negative gate voltage after exposure to N₂ gas in the presence of DUV light. We also observed that drain-to-source current, carrier density and charge-carrier mobility of ML MoS₂ are significantly improved after exposure to N₂ gas under DUV light irradiation.

---

1. Introduction

In recent years, two dimensional (2D) layered transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS₂) have attracted tremendous research attention due to their exciting electrical and optical properties.^1–4 MoS₂ exhibits many other excellent properties such as superior mechanical flexibility, impressive thermal stability, absence of dangling bonds and compatibility with silicon CMOS processes, as similar to its well-known cousin, graphene.^5,6 However, it is totally distinct from graphene in context of band-gap, graphene has linear dispersion characteristic and absence of band-gap in the electronic band structure.^7,8 The absence of energy gap in graphene limits its practical applications in logic circuits applications. MoS₂ is a n-type semiconductor with thickness-dependent band-gap of 1.2 to 1.8 eV (single-layer MoS₂ has a direct band-gap of 1.8 eV and bulk MoS₂ has an indirect band-gap of 1.2 eV).^9,10

Due to presence of band-gap, MoS₂ based fascinating electronic devices including field-effect transistors (FETs), ultrasensitive photodetectors, chemical and bio sensors, nonvolatile memory devices, integrated circuits and supercapacitors have already been demonstrated.^11–17 Among these devices, the FET is the most important and is a basic building block of any logic circuit/device. Nowadays, majority of the efforts have been focused to improve the performance of MoS₂ FETs by integration of MoS₂ devices in the back or dual-gated geometry, using different metal contacts (source and drain electrodes), scaling down the device dimensions, doping of MoS₂ with metal nanoparticles, or with self-assembled monolayers or molecules.^18–23 Doping is the most effective and easy technique to modify the electrical characteristics of any semiconducting material. The controlled doping in MoS₂ is not only essential for tuning the charge density and electronic properties but also enabling fabrication of various complex devices. However, tuning the electrical properties of ML MoS₂ has not yet explored. Few previous reports suggested that ML MoS₂ may be better than single-layer MoS₂ in FET applications because it has a larger density of states.^24,25 The larger density of state helps to create a larger channel carrier density, which boosts the current drive of FETs. The large drive current has better noise immunity in air. In addition, ML MoS₂ is more suitable and easy for practical fabrication process.

In this paper, we report the enhancement as well as tailoring the electrical properties of multilayer MoS₂ by N₂ gas under deep-ultraviolet (DUV) light irradiation. ML MoS₂ were obtained by micromechanical exfoliation and identified by optical microscope and Raman spectroscopy. The fabricated FET of ML MoS₂ flakes are exposed to N₂ gas in the presence of DUV light for different periods. The electrical transport measurements are used to investigate the changes in electrical properties of ML MoS₂ after exposing to N₂ gas in the presence of DUV light for different periods of time. The threshold voltages are shifted towards the negative gate voltage after exposure to N₂ gas in the presence of DUV light, which reveals the n-doping of ML MoS₂ FETs. The exposure of DUV light in the presence of N₂ gas remarkably improves the drain-to-source current, carrier density and charge-carrier mobility of ML MoS₂.

2. Experimental section

2.1. Sample preparation

ML MoS₂ flakes were obtained by micromechanical exfoliation of naturally occurring crystals of molybdenite (SPI supplies, USA) using Scotch tape and transfer on a 300 nm SiO₂ substrate with underlying highly p-doped silicon. The ML MoS₂ films were characterized by optical contrast and Raman spectroscopy. Atomic force microscopy (AFM) was used to measure the thickness of the samples. Raman spectra were collected at room temperature with a Renishaw microspectrometer having a laser wavelength of 514 nm.

2.2. Device fabrication and measurements

The big patterned electrodes Cr/Au thickness of (6/30 nm) for ML MoS₂ films were made by optical lithography. Source and drain electrodes were patterned employing e-beam lithography and evaporation of Cr/Au (6/80 nm). The channel length and width are ∼1.5 μm and ∼3 μm. The devices were annealed in a tube furnace at a temperature of 200 °C, in a flow of 100-sccm Ar and 10-sccm H₂ for 4 hours to remove the residue and minimize the contact resistance of the devices. The fabricated FETs of ML MoS₂ were electrically characterized using a Keithley 2400 source meter and Keithley 6485 picometer at room temperature in a vacuum.

2.3. Ultraviolet-light irradiation and characterizations

ML MoS₂ films were exposed to N₂ gas flow in the presence of DUV light (λ = 220 nm, average intensity of 10 mW cm⁻²) for different periods. After each exposure to DUV and N₂ gas, devices were characterized by Raman spectroscopy and measurements of electrical transport. The sample chamber was vacuum pumped for 30 minutes before each electrical measurement and during each measurement. All electrical measurements were conducted in a vacuum at room temperature.

3. Results and discussion

Fig. 1a displays optical micrograph of ML MoS₂ layers. An optical microscope was used to identify the ML MoS₂, which were further characterized by Raman spectroscopy. On the basis of contrast, we can easily identified ML MoS₂ flakes. It is well know that different layer have different contrast on Si/SiO₂ substrate and yield of ML MoS₂ flakes is much higher than single or bi-layer MoS₂ flakes. Thickness of ML MoS₂ flakes was measured by AFM and thickness of the ML MoS₂ is about 5.5 nm, which indicates 8 layers of MoS₂ as given Fig. S1 in ESI.† Fig. S2 in ESI† displays typical Raman spectra of ML MoS₂ layer on Si/SiO₂ substrate. Raman spectra were collected at room temperature with a Renishaw micro spectrometer having a laser wavelength of argon ion laser (λ = 514 nm). E¹_2g and A_1g are located at 383.5 and 407.8 cm⁻¹, respectively for ML MoS₂, it is consistent with previously reported results.^26–28 The frequency difference between two peaks (E¹_2g and A_1g) is also revealed that MoS₂ flakes are multilayer, frequency difference is about 24.3 cm⁻¹.²⁹ Fig. 1b shows the scanning electron microscopy (SEM) image of fabricated device. First, the big patterned electrodes (Cr/Au ratio of 5/30 nm) around selected ML MoS₂ were made employing photolithography on Si/SiO₂ substrate. The source and drain electrodes of transistors were made using e-beam lithography, consisting of 5 nm of Cr covered by 80 nm of Au. The channel length and width are ∼1.5 μm and ∼3 μm. We have made several multilayer devices of nearly same thickness and tried to maintain same channel length of the all devices. Fabricated devices were annealed at 200 °C for four hours in a flow of 100 standard cubic centimeters per minute (SCCM) Ar and 10 sccm H₂ to remove the residue and contamination, before any electrical characterizations.

Fig. 1 (a) Optical image of mechanically exfoliated ML MoS₂, (b) the SEM image of fabricated ML MoS₂ FET device.

The basic electronic properties of MoS₂ FETs were first investigated using standard back-gate devices on Si/SiO₂ substrate. Fig. 2 shows electrical characterization of our pristine ML MoS₂ transistor and all electrical characterizations of our devices are performed at room temperature in a vacuum chamber. The fabricated FETs of ML MoS₂ were electrically characterized using a Keithley 2400 source meter and Keithley 6485 picometer. The drain current I_DS versus drain–source voltage V_DS at different gate voltages (ranging from −6 to +6 V in steps of 2 V) are shown in Fig. 2a. The linear (I_DS–V_DS) characteristic of our devices shows good ohmic contact with source and drain electrodes. The I_DS as a function of the applied back-gate voltage V_g at a fixed drain–source voltage, V_DS = 10 mV is depicted in Fig. 2b, which clearly revealed the n-type characteristics.

Fig. 2 (a) I_D–V_DS output characteristics at various gate voltages V_bg (ranging from −6 to +6 V with steps of 2 V) for ML MoS₂ transistor. (b) The I_D–V_bg transfer characteristics of ML MoS₂ transistor at V_DS = 0.01 V.

After characterizations of our pristine devices, ML MoS₂ films were exposed to N₂ gas flow in the presence of DUV light (λ = 220 nm, average intensity of 10 mW cm⁻²) for different periods (5, 10, 20, and 30 min). The schematic illustration of exposure of device under DUV light is shown in Fig. 3a. The effect of exposure of N₂ gas in the presence of DUV light irradiation for different period of time is investigated by electrical charge transport measurements and Raman spectroscopy. The sample chamber was vacuum pumped for 30 minutes before each electrical measurement and vacuum pump was also continuously running during each measurement. Fig. 3b shows I_DS as a function of V_g before and after N₂ exposure under constant DUV light treatment for different periods. The shifting of the threshold voltage towards a negative gate voltage with N₂ gas in the presence of DUV light for different periods are shown in Fig. 3b. The shifting of threshold voltage towards a negative gate voltage revealed n-doping in ML MoS₂. We have measured the performance of several multilayer devices and observed that effect of DUV light with N₂ gas on performance of transistors are nearly same. Of course performance of MoS₂ transistor depends on numbers of layers. However, in this manuscript we have focused on the performance of multilayer MoS₂ transistor and performance of multilayer devices did not significantly changed with 5 layers to 10–12 layers. The electrical performance of one more ML MoS₂ device is shown in Fig. S3 of ESI.†

Fig. 3 (a) Schematic diagram of exposure of device under DUV light, (b) the I_D–V_bg characteristics with different exposure time of N₂ gas under DUV light for ML MoS₂ transistors. (c) The threshold voltage as a function of the time of exposure of N₂ gas in the presence of DUV light for ML MoS₂ transistors.

The shifting of threshold voltage toward negative gate voltage (n-doping) of ML MoS₂ by exposure of N₂ gas under irradiation of DUV light can be understood in terms of the removal of oxygen or an oxygen-derived group from MoS₂ nanosheets. The energy dispersive X-ray spectroscopy was used for confirmation of removal of oxygen after exposure of N₂ gas under DUV light irradiation. The X-rays were scanned on the area of 100 × 100 nm² on the different locations of ML MoS₂ nanosheets. Table 1 represents the results of the analysis of energy dispersive X-ray spectroscopy of pristine and after 30 minutes N₂ gas exposure under DUV light irradiation for ML MoS₂ FET. The significant amount of oxygen reduced after 30 minutes exposure of N₂ gas under DUV light irradiation as revealed by the result of energy dispersive X-ray spectroscopy.

Table 1 Energy-dispersive X-ray spectroscopy of ML MoS₂ flakes

Samples	Element	Pristine	After DUV + N2
ML MoS2	O	23.46% ± 0.3%	22.93% ± 0.2%

---

Fig. 3c shows the shifting of the threshold voltage as a function of time of exposure of N₂ gas in the presence of DUV irradiation for ML MoS₂. The threshold voltage of the device is obtained by extrapolating the linear portion of the curve of I_DS versus V_g to zero I_DS, the intercept on the V_g axis is the threshold voltage. The threshold voltage of pristine ML MoS₂ is found to be 7.5 V, which is shifted to −20.5 V after 30 minutes exposure of N₂ gas under DUV light irradiation. ML MoS₂ was also characterized by Raman spectroscopy after 30 minutes DUV light irradiation in N₂ gas atmosphere. The A_1g peak position shifted toward a lower wave number, which also revealed n-doping in ML MoS₂ (Fig. S4 ESI†). The shifting of the peaks toward lower wave number position attributed n-doping in ML MoS₂. It has already been reported for other systems.^13,21

Fig. 4a shows the carrier density of ML MoS₂ as a function of exposure time to N₂ and in the presence of DUV light. The charge-carrier densities (n) of the devices were estimated using the relation where V_T is the threshold voltage of the device, V_g = 30 V and e is the electronic charge. The carrier densities of ML MoS₂ remarkably increase with exposure of N₂ gas under irradiation of DUV light for different period of time. We have plotted I_DS with function of exposure time to N₂ gas in the presence of DUV light at V_g = +4 V and V_DS = 0.5 V. The drain current (I_DS) drastically increased with exposure time to N₂ gas in the presence of DUV light. The considerable improvement in I_DS current is due to the removal of oxygen absorbates from MoS₂ nanosheets and increase in electron concentrations in ML MoS₂ nanosheets. It is well know that oxygen absorbates trap the electrons and decrease the electron density in 2D nanomaterials.^30,31 The removal of oxygen is also confirmed by EDX data. In the presence of DUV light, oxygen molecules may be dissociated and removed from flow of N₂ gas. However, the exact mechanism is not clear at present. Another possibility is a shifting of Fermi level of MoS₂ at the metal–semiconductor interface or lowering of the barrier at interface and increase the drain current. N₂ gas alone does not affect the electrical properties of MoS₂ nanosheets.^30,32 However, N₂ gas in the presence of DUV light remarkably affected the electrical properties of MoS₂ nanosheets, as reflected in our case.

Fig. 4 (a) The carrier density and drain current I_D as a function of the time of exposure of N₂ gas in the presence of DUV light for ML MoS₂ transistors. (b) The mobility as a function of the time of exposure of N₂ gas in the presence of DUV light for ML MoS₂ transistors.

Fig. 4b shows the mobility of ML MoS₂ as a function of the time of exposure to N₂ gas under DUV light illumination. The field effect mobility of the samples was obtained using the relation where W is the channel width, L is the channel length, is the slope of I_DS–V_g of the device, V_DS = 0.01 V and C_g is the gate capacitance of ∼115 aF μm⁻² for our Si/SiO₂ substrate. The value of is obtained by fitting the linear regime of the transfer characteristic curves of ML MoS₂ FETs. The field effect mobility of our pristine ML MoS₂ is comparable to previously reported values. The field effect mobility of ML MoS₂ is significantly enhanced with exposure of N₂ gas under illumination of DUV light.

4. Conclusion

In conclusion, we have presented a very simple and easy technique to enhance the performance of multilayer (ML) MoS₂ FETs by exposure of N₂ gas under deep-ultraviolet (DUV) light illumination. Electrical transport measurement confirmed that the ML MoS₂ FETs showed enhancements in charge carrier mobility, carrier density, and drain current after exposure of N₂ gas under DUV light irradiation. Threshold voltages of ML MoS₂ FETs shifted towards a negative gate voltage after exposure of N₂ gas in the presence of DUV light which revealed n doping effect. We believe that this work provides important scientific insights for improving the electrical properties of emerging layered two-dimensional materials.

Acknowledgements

This research was supported by Nano-Material Technology Development Program (2012M3A7B4049888) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning. This research was also supported by Priority Research Center Program (2010-0020207) and the Basic Science Research Program (2013R1A1A2061396) through NRF funded by the Ministry of Education. Dr A. K. Singh also acknowledges to Department of Science and Technology (DST), India for support.

References

1. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6, 147–150.
2. Q. H. Wang, K. Kalantar-zadeh, A. Kis, J. N. Coleman and M. S. Strano, Nat. Nanotechnol., 2012, 7, 699–712.
3. W. Choi, M. Y. Cho, A. Konar, J. H. Lee, G. Cha, S. C. Hong, S. Kim, J. Kim, D. Jena, J. Joo and S. Kim, Adv. Mater., 2012, 24, 5832–5836.
4. D. Jariwala, V. K. Sangwan, D. J. Late, J. E. Johns, V. P. Dravid, T. J. Marks, L. J. Lauhon and M. C. Hersam, Appl. Phys. Lett., 2013, 102, 173107.
5. H. Y. Chang, S. Yang, J. Lee, L. Tao, W. S. Hwang, D. Jena, N. Lu and D. Akinwande, ACS Nano, 2013, 7, 5446–5452.
6. B. Simone, B. Jacopo and K. Andras, ACS Nano, 2011, 5, 9703–9709.
7. A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183–191.
8. A. K. Singh, M. Ahmad, V. K. Singh, K. Shin, Y. Seo and J. Eom, ACS Appl. Mater. Interfaces, 2013, 5, 5276–5281.
9. K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Phys. Rev. Lett., 2010, 105, 136805.
10. D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks and M. C. Hersam, ACS Nano, 2014, 8, 1102–1120.
11. D. Krasnozhon, D. Lembke, C. Nyffeler, Y. Leblebici and A. Kis, Nano Lett., 2014, 14, 5905–5911.
12. J. Lin, H. Li, H. Zhang and W. Chen, Appl. Phys. Lett., 2013, 102, 203109.
13. D. J. Late, Y. K. Huang, B. Liu, J. Acharya, S. N. Shirodkar, J. Luo, A. Yan, D. Charles, U. V. Waghmare, V. P. Dravid and C. N. R. Rao, ACS Nano, 2013, 7, 4879–4891.
14. D. Sarkar, W. Liu, X. Xie, A. C. Anselmo, S. Mitragotri and K. Banerjee, ACS Nano, 2014, 8, 3992–4003.
15. M. S. Choi, G. Lee, Y. Yu, D. Lee, S. H. Lee, P. Kim, J. Hone and W. J. Yoo, Nat. Commun., 2014, 4, 1624.
16. R. Branimir, B. W. Michael and K. Andras, ACS Nano, 2011, 5, 9934–9938.
17. L. Cao, S. Yang, W. Gao, Z. Liu, Y. Gong, L. Ma, G. Shi, S. Lei, Y. Zhang, S. Zhang, R. Vajtai and P. M. Ajayan, Small, 2013, 9, 2905–2910.
18. H. Liu, A. T. Neal and P. D. Ye, ACS Nano, 2012, 6, 8563–8569.
19. H. Liu, K. Xu, X. J. Zhang and P. D. Ye, Appl. Phys. Lett., 2012, 100, 152115.
20. H. Fang, M. Tosun, G. Seol, T. C. Chang, K. Takei, J. Guo and A. Javey, Nano Lett., 2014, 13, 1991–1995.
21. T. S. Sreeprasad, P. Nguyen, N. Kim and V. Berry, Nano Lett., 2013, 13, 4434–4441.
22. Y. Shi, J. K. Huang, L. Jin, Y. Hsu, S. F. Yu, L. J. Li and H. Y. Yang, Sci. Rep., 2013, 3, 1839.
23. D. Kiriya, M. Tosun, P. Zhao, J. S. Kang and A. Javey, J. Am. Chem. Soc., 2014, 136, 7853–7856.
24. H. J. Kwon, J. Jang, K. Sunkook, S. Vivek and C. P. Grigoropoulos, Appl. Phys. Lett., 2014, 105, 152105.
25. S. Kim, A. Konar, W. S. Hwang, J. H. Lee, J. Lee, J. Yang, C. Jung, H. Kim, J. B. Yoo, J. Y. Choi, Y. W. Jin, S. Y. Lee, D. Jena, W. Choi and K. Kim, Nat. Commun., 2012, 3, 1011.
26. H. Li, Q. Zhang, C. C. R. Yap, B. K. Tay, T. H. T. Edwin, A. Olivier and D. Baillargeat, Adv. Funct. Mater., 2012, 22, 1385–1390.
27. H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang and H. Zhang, Small, 2012, 8, 63–67.
28. H. Zeng, B. Zhu, K. Liu, J. Fan, X. Cui and Q. M. Zhang, Phys. Rev. B: Condens. Matter Mater. Phys., 2012, 86, 241301.
29. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone and S. Ryu, ACS Nano, 2010, 4, 2695–2700.
30. W. Park, J. Park, J. Jang, H. Lee, H. Jeong, K. Cho, S. Hong and T. Lee, Nanotechnology, 2013, 24, 095202.
31. Z. Luo, N. J. Pinto, Y. Davilla and A. T. C. Johnson, Appl. Phys. Lett., 2012, 101, 253108.
32. Y. C. Cheng, T. P. Kaloni, Z. Y. Zhu and U. Schwingenschlog, Appl. Phys. Lett., 2012, 101, 073110.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14509k

---