ALD2018 Session NS-MoA: 2D Materials

Many efforts have been dedicated to 2-D metal chalcogenides because of their characteristic properties which can hardly be expected from bulk materials. A challenging task for implementation of 2-D metal chalcogenides in emerging devices is to synthesize the well-crystallized layer on large area substrates at low temperatures which are compatible with the current fabrication processes for electronic devices are. SnS, a p-type layered semiconductor with high hole mobility, is a promising candidate for the realization of the large-area growth at low temperature because of its low melting point (882^oC). Several techniques such as spray pyrolysis, chemical vapor transport, sulfurization and e-beam evaporation have been introduced to synthesize 2-D SnS thin films. However, There are difficulties in synthesizing phase-pure SnS thin films because tin sulfides exist in various phases such as SnS, Sn₃S₄, Sn₂S₃, and SnS₂.

Here, we demonstrate a successful synthesis of single phase and impurity-free p-type SnS thin films using an ALD technique at low temperatures (< 240^oC). The use of a Sn precursor with an oxidation state of +2, bis(1-dimethylamino-2-methyl-2propoxy)tin(II), enabled the synthesis of single phase SnS(II) thin films at temperatures ranging from 90ºC to 240ºC, which is an exceptionally wide window for an ALD process. The SnS grain size increased with increasing the growth temperature. It was also found out that the SnS van der Waals interlayers were well aligned in parallel to the substrate at 240ºC. Impurities such as carbon, oxygen, and nitrogen were negligibly detected in the SnS(II) films and other phases such as Sn₂S₃ and SnS₂ are not incorporated. Furthermore, we investigated the feasibility of the SnS(II) thin films as a functional material in emerging devices such as thin film transistors and gas sensors.

Two-dimensional (2D) materials have attracted broad interest due to their unique properties and wide range of applications stemming from their layered crystal structures. In particular, semiconducting 2D materials, such as MoS₂, have been extensively studied for applications including field-effect transistors (FETs), photodetectors, catalysis, energy storage, and sensing. Tin disulfide (SnS₂) has recently emerged as a promising alternative for MoS₂ as a 2D semiconductor. It has an indirect band gap ranging from 2.2 eV in bulk¹ to approximately 2.6 eV in monolayer² and it has shown performance comparable to MoS₂ in FET and photodetector applications.³ Furthermore, SnS₂ appears to have potential for low-temperature processing compared to the dichalcogenides of refractory metals, such as MoS₂.

We have developed a new process for atomic layer deposition of 2D SnS₂ films using tin(IV) acetate and H₂S at 150 °C combined with mild post-deposition annealing at 250 °C in H₂S/N₂ atmosphere. Deposition of continuous, uniform, and conformal ultrathin (2 to 11 monolayers) SnS₂ films is demonstrated for the first time (Supplementary Figure 1). The annealed films exhibit good crystallinity with the expected, layered SnS₂ structure and are very smooth, stoichiometric, and show n-type semiconducting behavior (Supplementary Figure 2). Using the present method, SnS₂ films can be deposited on a range of different substrates.

[1]Burton et al., J. Mater. Chem. A, 2016, 4, 1312–1318

[2]Ye et al., Nano Res., 2017, 10, 2386-2394

[3]Huang et al., ACS Nano, 2014, 8, 10743–10755

Two-dimensional (2-D) metal chalcogenides have received great attention because of their unique characteristics which are not expected from bulk materials. In order to implement the 2-D materials in nanoelecronic devices, it is imperative to develop a facile route for large-area synthesis with precise thickness controllability and excellent uniformity in a temperature range compatible to common fabrication processes for electronics. SnS₂ is great a great candidate material for meeting the above conditions. Because the melting point(T_m) of SnS₂ is 860^oC, lower than that of other 2-D mateirals such as MoS­₂ and WS₂ (>1000 ^oC). The low meliting point of SnS₂ renders the high crystallized growth below 400^oC that satisfies the device process temperature. In addition, this material has a few hundreds of cm²V^-1s^-1 and shows a indirect band gap ~ 2.4 eV, lager than that MoS₂ and WS₂. This larger band gap of SnS₂ allows that higher on/off current ratio and lower leakage currents of off state. Also SnS₂ materials is environmental friendly, abundant materials and nontoxic. However, tin sulfides exist various crystal phases such as SnS₂, SnS, and Sn₂S₃. The important point is that the electronic properties of tin sulfides are strongly dependent on the crystal phase. Therfore, it has been challenging to synthesize the pure SnS₂ single phase without the other crystal phase.

Here we demostrated a useful technique to form single phase SnS₂ thin films by Plasma Enhanced Atomic Layer Deposition (PEALD). The SnS₂ thin films were grown by PEALD using unstable 2⁺ Sn(dmamp)₂ source and H₂S plasma. All the processes were performed below such a low temperature of 300^oC, which is compatible to current fabrication processes for electronic devices. Despite of low temperature, our SnS₂ has van der waals interlayer structure and these films have single phase of n-type SnS₂ without no oher phase. In addition, this method achieved great uniformity over large area (4-inch Si wafer) and excellent step coverage on the 3D structure. Moreover, field effct transtor devices using single phase SnS₂ channel layers were investigated. We expect that this SnS₂ process would provide decisive opportunities for realizing next genenration nano electronic devices.

Tin disulfide (SnS₂) and tin monosulfide (SnS), two representative tin sulfide materials, are emerging as two-dimensional (2D) materials after appearances of graphene and black phosphorus. SnS₂, an n-type semiconductor, has hexagonal structure that individual layers consisting of three atomic planes, such as CdI₂. It has been studied, due to the same crystal structure and similar semiconductor characteristics with transition metal dichalcogenides(TMDCs) . In the case of SnS, it has been attracting attention as an absorber layer of solar cell due to its appropriate band gap and absorption coefficient. Recently, SnS is also studied as a 2D material due to orthorhombic double layer structure like black phosphorus. However, very few studies have been done to obtain tin sulfide thin films with current thin film fabrication methods such as chemical vapor deposition (CVD) and atomic layer deposition (ALD).

In this study, we will present about results of high crystalline tin sulfide thin films deposited by ALD method. High crystalline SnS₂ thin films were obtained by phase transition from the high crystalline SnS thin films. SnS thin films were deposited on Si/SiO₂ substrate with tetrakis(dimethylamino)tin (Sn[N(CH₃)₂]₄, TDMASn) and hydrogen sulfide (H₂S) at 170°C, and subsequent H₂S annealing was performed in the tube furnace at 450°C. We then characterized the crystal structures, chemical bonding states, and optical band structures with XRD, XPS, TEM and UPS. In addition, we fabricated the field effect transistors (FETs) with SnS and SnS₂ thin films, and compared their switching device characteristics. Consequently, changes in the crystal structure and chemical state from SnS to SnS₂ were examined by XRD and XPS, respectively. After transition from SnS to SnS₂, the optical bandgap was measured and increased from 1.35 to 2.70 eV, but absorption coefficient decreased from ~10⁵ to ~10⁴ cm^-1 at visible region. Transition of electrical characteristic from p-type to n-type were also observed, and highly crystalline orthorhombic and hexagonal layer structures of two tin sulfide thin films were directly shown in the images of high resolution-transmission electron microscopy (HR-TEM). The electrical characteristics of FETs of SnS and SnS₂ thin films showed on-off current ratios of 8.8 and 2.1ⅹ10³ and mobilities of 0.21 and 0.014 cm²/Vs, respectively. These deference of switching device characteristics will be discussed based on the major carrier concentrations of the SnS and SnS₂ thin films.

The effective synthesis two-dimensional molybdenum disulfides (2D MoS₂) with atomically controlled thickness is essential for their use in electronic devices. In this work, 2D MoS₂ was grown directly on SiO₂ and Graphene using Atomic Layer Deposition (ALD) with MoF₆ and H₂S. We investigated the growth characteristics of the Molybdenum precursor (MoF₆) by Raman spectroscopy and microscopy. By using MoF₆ precursor, 2D MoS₂ synthesized also on the graphene forming MoS₂/graphene heterostructure. The optical microscopy, Raman spectroscopy, photoluminescence (PL), X-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM) measurements indicate that the ALD MoS₂ is layered structure with good uniformity, stoichiometry and controlled layer number. Furthermore, we demonstrated the electrical properties of 2D MoS₂ and optical characteristics of MoS₂/Graphene 2D heterostructure. This fabrication process could also provide an opportunity for the production of burgeoning MoS­₂ and other Transition Metal Dichalcogenides (TMD) for nanoelectronics and optoelectronics.

Layered two dimensional (2D) transition-metal dichalcogenides (TMDs) are finding use in nanoelectronic and optoelectronic applications due to their thickness dependent optical and electrical properties. Scalable fabrication of TMD-based devices requires deposition routes that can produce continuous and uniform films with sub-nanometer thickness control. Atomic layer deposition (ALD) is a highly promising route for the synthesis of 2D TMDs and heterostructures due to digital thickness control achieved by sequential self-limiting chemistry. Another advantage of ALD is that thickness control and the structural development of the films can be decoupled by separating the deposition and crystallization steps. We take advantage of these process attributes to prepare ALD MoS₂/WS₂ nanolaminates that would be otherwise difficult to grow by conventional chemical vapor deposition approaches. We have recently characterized the self-limiting deposition of MoS₂-containing amorphous films from bis(tert-butylimido)-bis(dialkylamido)Mo and 1-propanethiol. Using the homologous W precursor, we now report on the development of MoS₂/WS₂ nanolaminates, which offer a convenient means to tune the optical and electronic properties of few-layer TMD systems.

We deposited thin films using (N^tBu)₂(NMe₂)₂M and 1-propanethiol, where M=(Mo,W), at wafer temperatures of 300°C to 400°C on SiO₂/Si substrates. As-deposited and sulfur annealed films were evaluated using spectroscopic ellipsometry, grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and cross-sectional transmission electron microscopy (TEM). As-grown films were an amorphous matrix of metal sulfides mixed with unreacted ligands, evident as carbon and nitrogen impurities as measured by XPS. As-deposited films were annealed to produce 2H-MoS₂ and 2H-WS₂ under a sulfur atmosphere, which also removed residual ligands. Nanolaminates were prepared by alternating the injections of the two metal precursors at a fixed Mo:W ratio to achieve different film compositions. The overall Mo:W ratio in the film was found to follow the pulsing ratio of the two metal precursors. Upon annealing at 850°C, we obtained films that exhibit Raman modes associated with 2H-MoS₂ and 2H-WS₂, suggesting that distinct phases, rather than a solid solution, were favored. Relative changes in the overall Mo:W composition were also maintained in the annealed films, indicating that final material properties in TMDs could be controlled by implementing ALD nanolaminates. We will also discuss the implications of film incubation during laminate cycling and subsequent effects on film chemistry.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have gained great attention due to its potential applications in electronic and optical devices. Among the 2D-TMDCs family, MoS₂ monolayer has been extensively studied due to high carrier mobility over 200 cm²v^-1s^-1and flexibility. The electronic band structure is transformed from indirect (~1.2 eV) to direct (~1.9 eV) with decreasing the thickness of MoS₂ from bulk to mono layer (0.6 nm), that is suitable for electronic and optical applications. However, most of studies on 2D MoS₂ have utilized small flakes prepared by mechanical exfoliation and chemical vapor deposition with a transfer technique onto SiO₂ substrate due to the difficulty of direct growth of uniform MoS₂ monolayer on SiO₂ substrate.

In this study, an uniform MoS₂ monolayer was grown directly on 4-in. SiO₂/Si substrate by modified atomic layer deposition technique with extremely-precise thickness controllability and uniformity, which were confirmed by various tools such as Raman shift, photoluminance (PL) mapping and atomic force microscopy. The detailed experimental results will be presented.

Atomic layer deposition of molybdenum disulfide has recently been demonstrated using various chemistries. Many of these ALD processes, when performed at low temperatures, yield amorphous films and require annealing to obtain layered MoS₂. Raman spectroscopy is one of the most widely used characterization techniques for 2D materials, but these amorphous structures typically do not show the in-plane and out-of-plane vibrational modes of a layered structure. Characterizing these amorphous films is difficult, but crucial if ALD is to succeed in making monolayer or few layer MoS₂. In this work, we use X-ray absorption spectroscopy (XAS) and pair distribution function (PDF) measurements to investigate the as-deposited structure of ALD MoS₂ films prepared using MoF₆ and H₂S at 200°C. Model fits of the extended X-ray absorption fine structure (EXAFS) regime were used to infer the local coordination around the Mo atom centers. While the Mo-S coordination in ALD MoS₂ films seems to fit well with MoS₂ reference structures, the Mo-Mo coordination number was much lower than expected. In addition, PDF measurements performed to examine the bonding pairs suggested small clusters of trigonally prismatic MoS₂ with polysulfides forming between the sulfur atoms. Annealing these films at 400°C in H₂S yielded layered MoS₂, which was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy, suggesting that thin, layered MoS₂ films are possible using MoF₆ and H₂S.