ALD2019 Session NS-TuP: Nanostructures Synthesis and Fabrication Poster Session

The discovery and success of graphene paved the way for developing two-dimensional materials with outstanding properties [1]. In particular, monolayers of two-dimensional transition metal dichalcogenides (2D TMDCs) possess a direct band gap [2] that is crucial for optoelectronic applications. Additionally, the direct band gap can be easily tuned by either chemical composition or external stimuli. In parallel to monolayer TMDCs structures, a high surface area layer of TMDCs flakes shows promising properties for hydrogen evolution [3], photodegradation of organic dyes [4] or as electrodes in Li ion batteries [5].

To date, various top-down (e.g. exfoliation) and bottom-up techniques, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD) have been reported for the preparation of 2D-TMDCs [1]. However, ALD is the only technique enabling sub-nanometer thickness control, which is revealed as a crucial factor in the properties of 2D TMCDs materials. In the last years, it has been reported the possibility to employ ALD as a technique to grow MoS₂, which has been extensively studied for many different applications. In these works (CH₃)₂S₂ [6] or H₂S [7, 8] were used as the S precursor and Mo(CO)₆ [6], MoCl₅ [7] or Mo(thd)₃ [8] as the Mo precursors. Further, important efforts have also been devoted to attain the ALD fabrication of MoSe₂, since it possesses higher electrical conductivity than MoS₂ [9, 10]. Recently, we have shown that ALD deposition of MoSe₂ [11] or Mo-O-Se [12] is feasible using (CH₃)₂S₂ as the Se precursor and the MoCl₅ or Mo(CO)₆, respectively, as the Mo precursors.

The presentation will focus on the synthesis of MoS₂ and MoSe₂ by ALD, their characterization and applications in various fields. Experimental details and some recent photocatalytic, battery and hydrogen evolution results will be presented and discussed.

[1] A. V. Kolobov, J. Tominaga, Two-Dimensional Transition-Metal, Dichalcogenides. Springer Series in Materials Science, Springer International Publishing AG, Switzerland 2016

[2] B. Radisavljevic et al, Nat. Nanotechnol. 6 (2011) 147

[3] L. Wang et al, Adv. Mater. Interfaces. 2 (2015) 1500041

[4] Y. Wu et al, Nanoscale. 8 (2016) 440

[5] D. Ilic et al, J.Power Sources. 14 (1985) 223

[6] Z. Jin et al, Nanoscale. 6 (2014) 14453.

[7] L. K. Tan et al, Nanoscale. 6 (2014) 10584

[8] M. Mattinen et al., Adv. Mater. Interfaces 2017, 4, 1700123.

[9] D. Kong, H. et al, Nano Lett. 13 (2013) 1341.

[10] A. Eftekhari, Appl. Mater. Today 2017, 8.

[11] M. Krbal et al., Phys. Stat. Sol. RRL. 12 (2018) 1800023

[12] S. Ng et al., Adv. Mater. Interfaces. (2017) 1701146.

Molybdenum disulfide has attracted great interest due to its outstanding mechanical, electrical and optical properties. These unique properties make MoS₂ investigated as a promising candidate material for opto-electronic, sensing and catalysis applications. MoS₂ thin films have been achieved by a variety of methods such as sputtering and chemical vapor deposition. Nevertheless, these methods are limited to controlling the number of layer and uniformity over wafer scale. The most crucial problem of these methods is that they demand high growth temperature or additional heat treatment. High temperature process hinders the application of MoS₂ to a wide range such as flexible and transparent devices. In this work, MoS₂ thin films were deposited by H₂S plasma sulfurization of ALD-grown MoO₃ thin films. While ALD-grown MoO₃ provided precise thickness control and uniformity, H₂S plasma process effectively sulfurized MoO₃ into MoS₂ at low temperature. The sulfurization behavior and physical quality of MoS₂ thin films were investigated under various plasma conditions. The film thickness was measured by ellipsometry and Raman analysis. X-ray photoelectron spectroscopy was carried out to analyze the chemical composition of MoO₃ and MoS₂ thin films. The crystallization behavior of the films was characterized by X-ray diffraction. Furthermore, the potential of MoS₂ for electric device component was investigated.

Different transition metal disulfides (TMDs) ultra-thin films with wafer-scale uniformity have been successfully synthesized by ALD-based process. Two growth routes have been developed: sulfurizing transition metal oxides deposited by ALD, and annealing amorphous TMD films deposited by ALD. Molybdenum hexacarbonyl and ozone were used as precursors to deposit MoO₃ film by plasma enhanced ALD followed by sulfurization in a tube furnace to form MoS₂ TMD films. Tungsten hexachloride and hexamethyldisilathiane (HMDST) were used as ALD precursors to deposit WS₂ films which were further annealed at high temperature to improve crystallinity. Both approaches can produce high-quality and thickness-controllable TMD films with wafer-scale uniformity, which have been confirmed by AFM, XPS, Raman and TEM characterizations. Field-effect transistor (FET) device arrays have been further fabricated based on both films showing excellent homogeneous and reproducible electrical performance. The FET on/off ratio was about ~10⁴ with decent mobility over 10 cm²V^-1s^-1. These experimental results demonstrated attractive and promising potentials of the novel two-dimensional TMD films in future micro-/nanoelectronics device integrations and applications by using ALD-based techniques.

Several two-dimensional (2D) atomic-layered transition metal dichalcogenides (TMDs) are of great interest for electronic and optoelectronic applications due to their wide direct band gaps for few layer materials. Beyond mechanical or liquid exfoliation of bulk materials, high quality 2D TMDs have been grown via chemical vapor deposition (CVD) but growth is typically performed at high temperatures. Several studies have reported atomic layer deposition (ALD) of thin TMD films at lower temperatures, but as-deposited films are typically amorphous our nano-crystalline. Complementary to in-situ experimental studies of the growth and nucleation of nanoscale films, we have employed density functional theory (DFT) to understand the nuanced interactions during precursor nucleation. Hydroxyl groups are commonly found on metal oxide surfaces and play an important role during many ALD processes. To develop a deeper understanding of the ALD of MoS₂ from MoF₆ and H₂S, we studied the reactivity of MoF₆ with three substrates: Al₂O₃, MgO, and HfO₂ without hydroxyls and fully saturated with hydroxyls using DFT. We calculated the electronic distribution of the surfaces, how the electronic structures changed with single MoF₆ precursors, and calculated the valence electron transfer. Our calculations support the important role of hydroxyls in the nucleation of MoF6 with oxide surface and provide insight into the formation of precursor bonds at the surface.