CVD Growth of Monolayer MoS2 on Sapphire Substrates by using MoO3 Thin Films as a Precursor for Co-Evaporation

Sajeevi S Withanage1,2 and Saiful I Khondaker1,2,3

1 Department of Physics, University of Central Florida, Orlando, FL 32816, United States

2 NanoScience Technology Center, University of Central Florida, Orlando, FL 32816, United States

3 Department of Electrical & Computer Engineering, University of Central Florida, Orlando, FL 32816, United States

ABSTRACT

Scalable synthesis of two-dimensional molybdenum disulfide (MoS2) via chemical vapor deposition (CVD) is of considerable interests for many applications in electronics and optoelectronics. Here, we investigate the CVD growth of MoS2 single crystals on sapphire substrates by using thermally evaporated molybdenum trioxide (MoO3) thin films as molybdenum (Mo) source instead of conventionally used MoO3 powder for co-evaporation synthesis. The MoO3 thin film source provides uniform Mo vapor pressure in the growth chamber resulting in clean and reproducible MoS2 triangles without any oxide or oxy-sulfide species. Scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy and atomic force microscopy characterization were performed to characterize the growth results. Very high photoluminescence (PL) response was observed at 1.85 eV which is a good implication of high optical quality of these crystals directly grown on sapphire substrate.

INTRODUCTION

Molybdenum disulphide (MoS2) is an atomically thin semiconductor with direct bandgap in single layer (SL) which shows a great potential to wide range of applications in modern 2D electronics and optoelectronics including solar cells[1], photodectors[2, 3] and light emitting devices[4]. Chemical vapor deposition (CVD) based co-evaporation of molybdenum (Mo) and sulfur (S) precursors is becoming prevalent for the synthesis of SL MoS2 since[5] conventional exfoliation methods cannot produce large scale samples for scalable device fabrication[6]. Even though molybdenum trioxide (MoO3) powder is the commonly used Mo precursor for co-evaporation synthesis, it has been noted that the control of uniform Mo vapor pressure can be very challenging resulting in a growth of oxide/oxysulfide (MoO2/MoOS2) species along with MoS2[7-9]. Recently, we found that by using MoO3 thin films as a precursor rather than MoO3 powder, it is possible to grow monolayer MoS2 on Si/SiO2 substrates with high reproducibility since the uniform evaporation rate of these thin films allow to maintain uniform Mo vapor pressure at the growth phase[7].

For certain optoelectronic applications, choice of transparent growth substrate become critical as it can avoid transfer methods that involve chemicals which can degrade optical and electrical quality of as-grown monolayer crystals. Sapphire being an excellent candidate to serve this purpose has the additional benefit of good control over the crystal orientation of MoS2 grains avoiding high density of grain boundaries in large area growth[10, 11]. Here, we investigate the effect of uniform Mo vapor pressure using MoO3 thin films as Mo precursor on the CVD growth of MoS2 single crystals on sapphire substrate. We show that this uniform Mo vapor pressure base technique also produce MoS2 crystals with high reproducibility on sapphire substrates without any special substrate treatments. As grown crystals were characterized to be highly uniform monolayer MoS2 by SEM, Raman, PL spectroscopy and atomic force microscopy.

EXPERIMENTAL DETAILS

Growth process

Monolayer MoS2 single crystals were grown in a home-built CVD system. A schematic representation of our experimental setup is shown in figure 1. Sapphire substrates cut in to 1.5 cm x 1.5 cm were used as growth substrates and was cleaned via 5 min sonication in acetone followed by 5 min sonication in IPA, DI rinse and 10 min mild oxygen plasma treatment. Ceramic crucible containing 600-750 mg of S powder (99.5%, Sigma Aldrich) was loaded in to a 1 inch quartz tube placed in the tube furnace (Barnstead International F79300 Tube Furnace) upstream at the edge of the furnace (low temperature

 

Figure 1. Experimental setup. a) Schematic representation of the experimental setup with the relative S, and substrate

positioning. b) The temperature profile at the central zone of the furnace during the growth.

zone). Thermally evaporated MoO3 (99.5%, Sigma Aldrich) thin films on Si/SiO2 substrates with thickness of 10 – 20 nm were cut in to 1cm x 1cm size and placed in another ceramic boat. Bare growth substrates were placed on the same boat facing down toward MoO3 and placed at the center of the furnace. Distance between S and MoO3 was maintained at 23.5cm for the presented results. Argon gas with 99.995% purity was used as the carrier gas. Temperature of the furnace was raised to the growth temperature of 7500 C at the rate of 200 C/min and kept for 10 min. Furnace was allowed to cool down naturally after this dwell time until the temperature dropped to 5000 C, then open the furnace hood for rapid cooling. Prior to heating up the furnace, 200 sccm Ar flow was maintained for 10 min to minimize the effect of other reactive gasses such as oxygen after which it was kept constant at 10 sccm. When temperature of the furnace dropped to 3500C it was purged again with 200 sccm Ar to flush off excess reactants.

Characterization of the materials

Optical micrographs of the growth substrates were taken by an Olympus BX51M microscope equipped with Jenoptic Progres Gryphax camera and the SEM images were taken with a Zeiss ULTRA-55 FEG SEM. Height of the grown MoS2 crystals was measured using a tapping mode AFM (Anasys Instruments NanoIR2) topography. Raman and PL spectroscopy measurements were carried out with confocal Raman microscope (Witec alpha 300 RA) at an excitation wavelength of 532 nm and with laser power of 0.293 mW at ambient conditions.

RESULTS AND DISCUSSIONS

Figure 2 shows optical micrographs of MoS2 crystals grown with different precursor amounts on sapphire substrates. We observed clean triangular domains of MoS2 crystals throughout the growth substrate with the largest grain size of ~25 µm and the average size of 4-5 µm. Unlike the growth on Si/SiO2 under the same growth conditions[7], there is high affinity of single crystal growth on sapphire substrates since sapphire act as

 Figure 2. Optical micrographs of the MoS2 crystals grown on sapphire with a) S: 750 mg, MoO3: 10 nm film. b) S: 750

mg, MoO3: 20 nm film. c) S: 600 mg, MoO3: 10 nm film. d) S: 600 mg, MoO3: 20 nm film. the central zone of the

furnace during the growth.

an epitaxial layer for MoS2 growth[10, 11]. It is important to note here that MoO3 thin film precursor always resulted in clean MoS2 crystals unlike MoO3 powder based synthesis where oxysulfide growth along with the MoS2 is common[8, 9]. In addition, all the crystals are triangular in size suggesting that uniform evaporation of MoO3 thin film allowed to maintain uniform Mo vapor throughout the growth region. We repeated the growth over 20 times on sapphire with the thin film precursor and could grow clean MoS2 crystals every time. While same size crystals are grown throughout the center region of the substrate, larger crystals are grown near the edge of the boat as a result of slight changes to the flow due to substrate placement geometry. With 600 mg of S, smaller crystals are grown (figure 2b, 2c) and we were able to increase the crystal size by increasing the initial loading of S to 750 mg. Growth with 10 nm MoO3 film (figure 3a) shows less density of nucleation compared to the 20nm film (figure 3b) resulting much uniform and cleaner growth.

SEM and AFM chracteriation results for a representative sample grown with 750 mg of S and 20 nm thick MoO3 film is shown in figure 3. The SEM micrograph of the growth substrate shown in figure 3a) shows very clean growth of MoS2 crystals. Similar triangular shape of the crystals throughout the substrate suggests the ability of maintaining uniform Mo vapor throughout the growth region with the MoO3 thin film precursor since shape variation is attributed to Mo concentration gradient12. Higher mgnifiction image of a single crystal shown in figure 3b) represents a highly uniform growth of MoS2 with self-seeded nucleation. We also observed very high density of nucleation on sapphire substrate as exposed from figure 3c). This also associated with the reduced grain sizes reported for the MoS2 growth on sapphire[10, 13-15]. AFM image shown in figure 3d) and measured height profile in 3e) also shows the high uniformity of the grown monolayer with the

thickness of 6.5 nm. Atomically smooth surface of sapphire substrate facilitated high uniformity of MoS2 crystal growth which can be realized through very clear step height in

 Figure 3. a), b), c) SEM images of the growth substrate. d) AFM image of the MoS2 crystals. e) AFM height

profile taken along the red line marked in d).

the AFM height profile which we hardly observe in Si/SiO2[7] due to amorphous nature of the substrate.

Raman and PL spectroscopic analysis results for the MoS2 crystals is shown in figure 4. Raman single spectra shows MoS2 signature peaks of E1

2g and A1g which corresponds to in plane and out of plane vibrations of Mo and S atoms with respective wave numbers of 390.76 cm-1 and 409.26 cm-1 (Figure 4a). Wavenumber difference between these two peaks (Δ) is a good classification of the layer number of MoS2[16]. We found Δ = 18.5 cm-1 for our CVD grown samples which confirms monolayer nature of

these MoS2 crystals[16, 17]. High PL response due to direct bandgap transitions (peak A) at 1.85 eV is observed for as-grown crystals. We measured full width at half-maximum (FWHM) for this peak to be ~85 meV which is narrower compared to the exfoliated MoS2 monolayers on sapphire[10] is another indication of greater optical quality of this CVD grown material. Second signature peak (peak B) with reduced intensity is located at 1.99 eV as a result of valence band splitting due to strong spin-orbit coupling[18].

CONCLUSIONS

Highly uniform monolayer MoS2 crystals were grown on sapphire substrates by using MoO3 thin film precursor in co-evaporation synthesis. Ability to maintain uniform Mo vapor throughout the growth allowed to grow clean MoS2 crystals without any trace of molybdenum oxysulfides. We optimized the growth by varying MoO3 and S amounts to grow larger, uniform coverage of MoS2 on sapphire. Exceptional optical quality of the as-grown crystals is evidenced via Raman and PL spectroscopy. Direct growth of these clean, high quality MoS2 crystals on transparent sapphire substrates evolve their applications in transfer-free optoelectronic applications.

ACKNOWLEDGMENTS

This work was supported by U.S. National Science Foundation (NSF) under grants No. 1728309. We also acknowledge Dr. Tetard for Raman/PL spectroscopy support.

Figure 4. a) Raman and b) PL single spectrum for a MoS2 crystal taken at the position specified by the red circle in the optical image shown in inset of b).

REFERENCES

1. M. L. Tsai, S. H. Su, J. K. Chang, D. S. Tsai, C. H. Chen, C. I. Wu, L. J. Li, L. J. Chen and J. H. He, Acs Nano 8 (8), 8317-8322 (2014).

2. O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic and A. Kis, Nature Nanotechnology 8 (7), 497-501 (2013).

3. M. Li, J.-S. Chen, P. K. Routh, P. Zahl, C.-Y. Nam and M. Cotlet, Adv Funct Mater 28 (29), 1707558 (2018).

4. O. Salehzadeh, N. H. Tran, X. Liu, I. Shih and Z. Mi, Nano Lett 14 (7), 4125-4130 (2014).

5. A. E. Yore, K. K. H. Smithe, W. Crumrine, A. Miller, J. A. Tuck, B. Redd, E. Pop, B. Wang and A. K. M. Newaz, The Journal of Physical Chemistry C 120 (42), 24080-24087 (2016).

6. Y.-C. Lin, W. Zhang, J.-K. Huang, K.-K. Liu, Y.-H. Lee, C.-T. Liang, C.-W. Chu and L.-J. Li, Nanoscale 4 (20), 6637-6641 (2012).

7. S. S. Withanage, H. Kalita, H. Chung, T. Roy, Y. Jung and S. I. Khondaker, Cond-mat arXiv 1811.06119, 2018.

8. J. V. Pondick, J. M. Woods, J. Xing, Y. Zhou and J. J. Cha, ACS Applied Nano Materials 1 (10), 5655-5661 (2018).

9. J. D. Cain, F. Y. Shi, J. S. Wu and V. P. Dravid, Acs Nano 10 (5), 5440-5445 (2016). 10. D. Dumcenco, D. Ovchinnikov, K. Marinov, P. Lazic, M. Gibertini, N. Marzari, O.

Lopez Sanchez, Y. C. Kung, D. Krasnozhon, M. W. Chen, S. Bertolazzi, P. Gillet, A. Fontcuberta i Morral, A. Radenovic and A. Kis, Acs Nano 9 (4), 4611-4620 (2015).

11. H. Yu, M. Z. Liao, W. J. Zhao, G. D. Liu, X. J. Zhou, Z. Wei, X. Z. Xu, K. H. Liu, Z. H. Hu, K. Deng, S. Y. Zhou, J. A. Shi, L. Gu, C. Shen, T. T. Zhang, L. J. Du, L. Xie, J. Q. Zhu, W. Chen, R. Yang, D. X. Shi and G. Y. Zhang, Acs Nano 11 (12), 12001-12007 (2017).

12. S. S. Wang, Y. M. Rong, Y. Fan, M. Pacios, H. Bhaskaran, K. He and J. H. Warner, Chem Mater 26 (22), 6371-6379 (2014).

13. S. F. Wu, C. M. Huang, G. Aivazian, J. S. Ross, D. H. Cobden and X. D. Xu, Acs Nano 7 (3), 2768-2772 (2013).

14. F. Ghasemi, R. Frisenda, D. Dumcenco, A. Kis, D. P. de Lara and A. Castellanos-Gomez, Electronics-Switz 6 (2) (2017).

15. Q. Ji, M. Kan, Y. Zhang, Y. Guo, D. Ma, J. Shi, Q. Sun, Q. Chen, Y. Zhang and Z. Liu, Nano Lett 15 (1), 198-205 (2015).

16. C. Lee, H. Yan, L. E. Brus, T. F. Heinz, J. Hone and S. Ryu, Acs Nano 4 (5), 2695-2700 (2010).

17. A. Sanne, R. Ghosh, A. Rai, M. N. Yogeesh, S. H. Shin, A. Sharma, K. Jarvis, L. Mathew, R. Rao, D. Akinwande and S. Banerjee, Nano Lett 15 (8), 5039-5045 (2015).

18. J. Zhang, H. Yu, W. Chen, X. Z. Tian, D. H. Liu, M. Cheng, G. B. Xie, W. Yang, R. Yang, X. D. Bai, D. X. Shi and G. Y. Zhang, Acs Nano 8 (6), 6024-6030 (2014).