ALD/ALE 2024 Session NS-TuA: 2D Nanomaterials by ALD (Including Transition Metal Dichalcogenides)

Electrically conductive nanocomposites are critical components of electrocatalytic devices, thermoelectric generators, flexible electronics, and many other existing or envisioned technologies. In a disordered conductor/insulator composite with nanoscale domains, high and stable conductivity is achievable provided that (1) the conductive phase does not oxidize or otherwise degrade, (2) the conductive domains percolate—i.e., form continuous networks that traverse the material, and (3) the conductive channels within the network have sufficient cross-sectional area.

In this work, we explore a nascent nanocomposite fabrication strategy based on ALD infiltration and pressure-assisted sintering of nanoparticle compacts. Specifically, we use ALD with long static precursor doses (>10 min) to deposit Al-doped ZnO inside ~2-mm-thick porous compacts of ceramic (e.g., ZrO₂, SiO₂) nanoparticles with ~100 nm diameter, and then we use environmentally controlled pressure-assisted sintering (EC-PAS)¹ at ~2 GPa and 700–1000 °C to remove residual porosity and form fully dense composites. Each ZnO:Al ALD supercycle consists of 15 diethylzinc/water cycles and 1 trimethylaluminum/water cycle, and the deposition temperature is 160 °C except during diethylzinc doses, during which it is lowered to 120 °C to prevent metallic Zn deposition. In principle, this ALD/sintering approach (1) prevents degradation of the ZnO:Al by sealing off the bulk of the composite from the atmosphere, (2) forms percolating ZnO:Al via complete and conformal ALD infiltration, and (3) allows control over the conductive channel cross-sectional area via the number of ALD cycles. In practice, we indeed observe complete infiltration: scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS) of a representative sample reveals Zn throughout the depth of the composite. Electrical resistivity, however, is surprisingly high: despite deposition of enough ZnO:Al to yield channels with ~7 nm thickness—predicted to enable metallic (band-like) conductivity²—the composite resistivity is >10⁵ Ω-cm. Employing SEM-EDS, x-ray diffraction (XRD), electrical measurements, and other characterization techniques, we investigate potential causes of this high resistivity, which include dopant deactivation and disruption of conductive channels during sintering. We also explore several routes toward achieving metallic conductivity, including in situ post-ALD ozone cleaning and optimization of sintering parameters to maintain the doping level and continuity of the ZnO:Al.

1. H. Ryou et al., ACS Nano 12, 3083 (2018).
2. T. Chen et al., Nat. Mater. 15, 299 (2016).

Through its course of technology development, atomic layer deposition (ALD) has mostly been applied to flat substrates, typically for applications in the semiconductor industry. Albeit, the inherent processing and coating characteristics of ALD, namely its ability to conformally deposit materials in high aspect ratio geometries with sub-nanometer thickness control, are also attractive for the coating of particulate matter. Application areas requiring substances with large specific surface area, such as heterogeneous catalysis, adsorbents and separations, or highly precise thickness control, such as energy conversion and storage (batteries) or pharmaceuticals are strong drivers for research activities in the realm of ALD on particulate materials. Several scientific review articles on aspects of this field have been published, particularly over the last decade; however, the scope of them is mostly limited to either a processing or an applications point of view.

In this contribution, we present a comprehensive review of the developments in the field of ALD on particles, covering approximately 700 articles in a data-driven rather than anecdotal manner. We target an audience of researchers from the conventional ALD domain who wish to extend their scope of activities to particular media, as well as researchers involved in particle technology who consider ALD as a tool to enhance their technical applications (see supplementary PDF for applications already using ALD on particles). Challenges related to the processibility of powders/particles are addressed by the presentation of an overview of the different reactor engineering approaches implemented. Furthermore, the substrate materials (supports) used are systematically categorized and evaluated with respect to their geometric characteristics (shape and size) but also their surface functionalities. Finally, we cover the range of coating materials attainable and the respective processing conditions; in this way, we also address the similarities and deviations of conventional ALD and ALD on particles in terms of growth characteristics. Through exploring the current state of the art, this review aims to provide both inspiration for new applications of ALD, and a starting point for researchers in particle-based technologies in search of ALD processes for any given application.

Crystalline 2D MoS₂ is a semiconductor material with potential application in the next generation of logic and memory devices. The adoption of MoS₂ for future high-volume manufacturing requires the synthesis of high quality and large area 2D material on substrates of interest. The control of MoS₂ film properties such as film uniformity, thickness, and crystallinity are critical for this objective. In the literature, reports of large area deposition of 2D materials include deposition techniques such as chemical vapor deposition (CVD) and atomic layer deposition (ALD). [1] Among the proposed chemical systems for ALD, the MoCl₅-based process has produced crystalline 2D MoS₂ [2,3].

In this work, we investigate the MoCl₅ and H₂S system to deposit MoS₂ by thermal ALD. The effect of the MoCl₅ etch component will be discussed. It was found that self-etching of MoCl₅ can be tuned by process parameters such as precursor pulse time and deposition temperature. The deposition window for 300mm SiO₂/Si wafer substrates will be presented. X-ray fluorescence, Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were used to characterize the ALD-grown MoS₂. The resulting 2D MoS₂ films exhibit low surface roughness, high uniformity across the wafer, and high level of crystallinity [Fig. 1]. The demonstration of large area and high quality 2D MoS₂ produced by the MoCl₅-based process provides a path toward the implementation of ALD processes for deposition of large area 2D MoS₂.

[1] Y. Liu and F. Gu, Nanoscale Adv., 2021, 3, 2117.

[2] Y. Huang, et al., Thin Solid Films, 2017, 624, 101–105.

[3] Kim, Y., Song, JG., Park, Y. et al., Scientific Reports, Vol. 6, 2016, 18754.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are currently on the roadmap of major semiconductor companies as beyond-Si channel material in field-effect transistors (FETs).[1] While 2D TMDCs are typically grown by chemical vapor deposition (CVD), the addition of post-processing steps such as the transfer to specific substrates limits and complicates the overall fabrication process. Accordingly, atomic layer deposition (ALD) is considered a promising method for the direct growth of TMDCs on planar substrates and 3D structures at low temperatures suitable for back-end-of-line (BEOL) processes and its potential for area-selective deposition.[2] However, the mobility of ALD-grown 2D TMDCs substantially falls behind that of CVD-grown 2D TMDCs often due to smaller grain sizes.

In this work, we present that tuning our developed plasma-enhanced ALD (PE-ALD) process which utilizes an inductively coupled Ar plasma (ICP) as an additional C step (AB → ABC) [3],[4] can modify the material properties of WS₂ during film growth. The comparison between WS₂ films grown using the conventional AB and ABC process showcases a significant decrease of the resistivity by four orders of magnitude from ~10⁸ to ~10⁴ µΩ-cm due to the improvement of the grain morphology (Fig. 1, SI). While the AB WS₂ film was too resistive for Hall analysis, the ABC WS₂ film exhibited a strong p-type character with a Hall mobility µ_H of 0.78 cm²/Vs, which is only one order of magnitude lower than CVD-grown TMDCs. Our results demonstrate effectively improving the electrical properties of WS₂ using the ABC PE-ALD process. Finally, preliminary results on the ion energy E_i,flux, and ion flux Г_i as function of various plasma processing conditions such as the Ar plasma pressure p and plasma exposure time t_p during PE-ALD (Fig. 2 and 3, SI) provide insight into how the material properties can be affected and further tailor them to our specific needs by the use of plasma.

[1] Chung, C.C. et al. International Electron Devices Meeting, IEDM, 3451–3454. (2022)
[2] Balasubramanyam, S. et al. ACS Materials, 2(5), 511-518. (2020)
[3] Balasubramanyam, S. et al. ACS Appl. Mater. and Interfaces, 12(3), 3873-3885. (2020)
[4] Mattinen, M. et al. ACS Appl. Mater. and Interfaces, 15, 35565-35579. (2023)

2D semiconducting materials have received attention for their potential to facilitate further downscaling of microelectronic devices by overcoming the physical shortcomings of silicon. Due to their inherent atomic thinness and lack of surface dangling bonds, 2D materials can be well-controlled by a gate while preserving the carrier mobility, leading to excellent current switching characteristics, even at the scaling limit. However, the lack of dangling bonds and reactive sites on the surface makes it difficult to deposit a gate oxide by ALD, often leading to sparse nucleation and the growth of films that are low density, defective, rough, and filled with pinholes.

In this work, we employ physisorption-assisted ALD processes and study the nucleation and quality of the deposited dielectrics. We deposit Al₂O₃ on monolayer MoS₂ using a series of ALD precursors in order to investigate the impact of the ALD precursor on nucleation. The precursors utilized in this study are dimethyl aluminum isopropoxide (DMAI), trimethylaluminum (TMA), triethylaluminum (TEA), and a novel Al₂O₃ precursor, triisobutylaluminum (TIBA). Using scanning electron microscopy (SEM), we study film nucleation and continuity as a function of ALD precursor, cycle number, and temperature. The results show that the precursor used in the ALD process dramatically impacts coverage and growth. With the best performing precursor, we observe full coverage of MoS₂ after just 3 nm of material is deposited. After optimizing the temperature and purge time of each process to maximize nucleation, we fabricate transistor devices in which the deposited Al₂O₃ serves as a seed layer for high-k HfO₂ deposition to create the gate stack. Using x-ray photoelectron spectroscopy (XPS) and electrical testing, we investigate the character of the dielectric/MoS₂ interface. While all three alkyl precursors lead to seed layers and dielectrics that produce functioning devices, the devices with the best performance are obtained by the precursor that achieves a more uniform, smoother, and denser film, which we attribute to improved nucleation and growth properties. This improved deposition also permits the most thinning of the seed layer. The devices fabricated using the best precursor and improved dielectric show excellent performance such as good on/off ratios (10⁶), small device-to-device variation (ΔV_T < 1 V), and low effective oxide thickness (~1 nm).This work provides useful insights into how ALD precursors can be designed to improve the quality of dielectrics on 2D materials, potentially improving the viability of 2D materials for wide ranging applications.

Due to their large surface-to-volume ratio and stability in aqueous environments, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have recently emerged as promising candidates for biosensing materials.¹ Among various biomolecules, amino acids (AAs) have been proposed as one of key biomarkers of human body status and diseases.² The changes in the physiological levels of AAs in biological fluids (e.g., blood, urine) can be used to determine not only nutritional status, but also specific diseases including cancer and diabetes. Therefore, monitoring and detection of AA profile can be of significant importance in potential early diagnostics. Recently, the possibility of detecting AA molecules through direct charge transfer after adsorption on the TMDC surface has been theoretically investigated. Particularly, noble metal (NM) nanoparticles (NPs), such as Au and Pt, functionalization has been proposed to improve the adsorption of biomolecules including AAs, thereby significantly enhancing the sensing properties.^3,4 However, experimental demonstration of NM NP-functionalized 2D TMDCs for AA sensing is still lacking.

Reference

[1] Materials Science and Engineering: C 70 (2017): 1095-1106.

[2] Amino Acids 48 (2016): 1339-1345.

[3] Computational and Theoretical Chemistry 1118 (2017): 115-122.

[4] ACS Applied Electronic Materials (2023).