ALD/ALE 2025 Session NS-MoP: Nanostructure Synthesis and Fabrication Poster Session

Porous silicon templates have attracted increasing attention because of their controllable geometry, tunable nanoporous structure, large pore volume/high specific surface area, and versatile surface chemistry. Porous templates show significant advantages and application potential in microfluidics, electro-osmotic pumps, biomedical drug delivery, sensing, photonics, integrated opto-electronics, energy conversion, thermoelectrics, thermo-acoustics, electronics and Lab-on-a-chip technology for biomedical, pharmaceutical and environmental monitoring. For this study we have focused on energy conversion with ALD lead chalcogenide PbTe film coatings since PbTe is a useful narrow band gap thermoelectric material that can operate at comparatively higher temperatures in the range of 600~850 K due to its better chemical stability and high melting point. PbTe thin films have been synthesized inside of porous silicon templates with native oxide by Atomic Layer Deposition (ALD) using lead (II)bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C₁₁H₁₉O₂)₂), (trimethylsilyl) telluride ((Me₃Si)₂Te) as ALD precursors for lead, and tellurium. The Si native oxide surface was functionalized before ALD PbTe thin film deposition to ensure reproducible chemisorption of the ALD precursor compounds. The growth temperature during ALD was varied over a range from 135^oC to 170^oC. The Lead precursor was volatilized at a temperature of 140 ^oC and the Tellurium precursor was heated at 40 ^oC. The chamber base pressure was kept at 40 mTorr.Several physical characterization techniques have been employed to determine the ALD PbTe thin film characteristics. The crystal structure and phase purity of samples of PbTe films were analyzed by X-ray diffraction (XRD). The film morphology and structure of the products were determined by field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). The surface roughness was analyzed by atomic force microscopy (AFM). The analysis of the composition and stoichiometry of the ALD coatings were carried out by Energy dispersive X-ray spectroscopy (EDS). The experimental evidence revealed the ALD growth of lead telluride followed the Vollmer-Weber Island growth model. We found a strong dependence of the nucleation process of the polycrystalline grain distribution on the temperature.We report a systematic study of all ALD parameters required to optimize the coating of the interior walls of porous membranes which open front and backside, straight pores of varying diameter and porosity with dead endings in the substrate and pores with spatially modulated ondulating cross sectional shapes.

Over the past decade, metal silicides have been re-discovered for their significant potential across various fields of nanotechnology. [1,2,3] However, integrating them into modern devices continues to pose challenges, primarily due to limited control over phase formation during their synthesis. [4] Additionally, since they are typically grown as continuous layers, their structuring often relies on complex top-down patterning techniques, which significantly limit their practical applications. [5] This study addresses these issues by proposing a method for Area-Selective (AS) silicidation (Fig. 1a SD). Our approach ensures that during material synthesis, the resulting silicide nanostructures acquire a predefined morphology and nucleate exclusively in the targeted regions of the substrate.

The AS silicidation process is demonstrated through the formation of NiSi₂ structures on Si(100) and Si(111) substrates (Fig. 1b SD), and its broader applicability is further illustrated with the successful AS synthesis of Cu₃Si (Fig. 1c SD). To direct growth, the silicon substrates are patterned using ion beams, which create surface defects that promote silicide nucleation in the desired areas. Unlike conventional Solid State (SS) synthesis, which involves annealing a metal thin film on silicon, our method begins with an ALD deposited metal oxide. Silicidation is performed by annealing in the presence of a reductant (H₂), which reduces the metal oxide to its metallic state. The resulting metal then reacts with the underlying silicon, forming a metal silicide. During this two-step reaction, the material re-arranges along surface defects, which act as templates, guiding both the shape and positioning of the forming silicide crystals.

The effectiveness of this multi-step fabrication method, coupled with its adaptability to various metal-silicide systems, underscores its potential to create functional, structured silicides without the need for post-synthesis nanopatterning.

References

1. Z. He, Y. He, Y. Qiu, Q. Zhao, Z. Wang, X. Kang, L. Yu, L. Wu, Y. Jiang, Appl. Catal. B Environ. 2024, 342, 123386.
2. Y. Niu, K. Zhang, X. Cui, X. Wu, J. Yang, Nano Lett. 2023, 23, 2332–2338.
3. R. Reichlova, H. Lopes Seeger, R. González-Hernández, Nat. Commun. 2024, 15, 4961.
4. Y. C. Chou, L. J. Chen, K. N. Tu, Nano Lett. 2022, 22, 6895–6899.
5. N. Breil, C. Lavoie, A. Ozcan, F. Baumann, N. Klymko, K. Nummy, B. Sun, J. Jordan-Sweet, J. Yu, F. Zhu, S. Narasimha, M. Chudzik, Microelectron. Eng. 2015, 137, 79–87.

Conformal coating of high aspect ratio tortuous 3-dimensional nanostructures has been shown to require careful consideration of dose and purge times to achieve uniform ALD coatings of high quality.^1,2 Specifically, through static-dosing of ALD precursors cycle times for complete surface saturation are often in excess of 10s of minutes for macroscopic substrates.² Recently our group has demonstrated an order of magnitude reduction in saturation dose times by forcing precursor flow through the compact via a pressure gradient of 50-100 Torr across the 3D nanocomposite. In this work, diethylzinc (DEZ) and water were pulsed sequentially to deposit conformal films of ZnO completely through the ~2mm thickness of nanocomposite compacts comprised of 200 nm SiO₂ nanoparticles. Due to the nonuniform pressure across the nanocomposite compact, precursors undergo a transition from convective to diffusive transport. Preliminary modeling of the internal pressure gradients of the compacts was utilized to determine the relative contributions of these two transport modes in addition to structural nonuniformities (i.e. cracks, large pores, etc.). It was found that uniformity in the internal pore structure and structural integrity of the initial, uncoated, nanoparticle compact was crucial in achieving uniform coatings on the entire surface. Additionally, the effect of precursor partial pressure during doses on saturation dose times and the extent to which the diffusive transport could be enhanced was studied. Saturation dose times were measured via in-situ quadrupole mass spectrometry of the effluent gas and film uniformity and conformality were studied via ex-situ­ cross-sectional scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), and X-ray diffraction (XRD).

1. Gayle, A. J.; Berquist, Z. J.; Chen, Y.; Hill, A. J.; Hoffman, J. Y.; Bielinski, A. R.; Lenert, A.; Dasgupta, N. P. Tunable Atomic Layer Deposition into Ultra-High-Aspect-Ratio (>60000:1) Aerogel Monoliths Enabled by Transport Modeling. Chem. Mater. 2021, 33 (14), 5572–5583.

2. Greenberg, B. L.; Anderson, K. P.; Jacobs, A. G.; Cendejas, A. J.; Hajzus, J. R.; Patterson, E. A.; Wollmershauser, J. A.; Feigelson, B. N. Conformal Coating of Macroscopic Nanoparticle Compacts with ZnO via Atomic Layer Deposition. J. Vac. Sci. & Technol. A 2023, 42 (1), 012402.

Scaling grains down to nanometer size enhances many properties of polycrystalline solids due to an increase in the grain boundary interfacial volume fraction. Sintering of nanoparticles is the most versatile way to produce nanocrystalline solids of various materials. A crucial prerequisite for achieving advanced, size-dependent properties in these nanocrystalline solids is removing porosity by sintering while preserving nanoscale morphology and grain size. Using our environmentally controlled pressure-assisted sintering (EC-PAS) technique, we have successfully produced fully dense ceramics with grain sizes down to 4 nm [1,2].

By engineering nanoparticle building blocks with designed core/shell structures using particle atomic layer deposition (ALD) [3], we can fabricate novel nanocomposite solids with properties controlled by rationally designed core/shell geometries, constituent properties, and the resultant vast network of interfaces. We refer to such nanocomposites as artificial interfacial solids (AIS). For instance, when the core/shell nanoparticle structure is preserved during EC-PAS, the resulting nanocomposite comprises isolated nanoparticles (cores of initial core/shells) embedded in a host material (a continuous 3D percolated network of shells from adjacent core/shells merged together).

We have further developed the EC-PAS process to produce percolated AIS (PAIS) nanocomposite solids with two separate and interconnected 3D paths for electron and/or heat transport. The process involves creating a porous compact from nanopowder, followed by ALD infiltration and conformal deposition of a second material on all available surfaces within the porous compact [4], and finally, a sintering stage. The nanopowder in the sintered nanocomposite forms the first 3D percolated network, which becomes surrounded by the deposited and merged 3D network of the second material. Both networks are strongly bonded to each other through sintering.

Examples of AIS and PAIS nanocomposites for various applications will be presented and discussed, highlighting their potential properties, applications, and versatility.

1. Ryou, H., J. W. Drazin, K. J. Wahl, S. B. Qadri, E. P. Gorzkowski, B. N. Feigelson and J. A. Wollmershauser, ACS Nano 12(4): 3083-3094 (2018).
2. Anderson, K. P., J. A. Wollmershauser, H. Ryou, R. Goswami, S. Rommel, M. Aindow, E. P. Gorzkowski and B. N. Feigelson, Acta Materialia 275: 120004 (2024).
3. Manandhar, K., J. A. Wollmershauser and B. N. Feigelson, J. Vac. Sci. Technol. A 35(4): (2017).
4. Greenberg, B. L., K. P. Anderson, A. G. Jacobs, A. J. Cendejas, J. R. Hajzus, E. A. Patterson, J. A. Wollmershauser and B. N. Feigelson, J. Vac. Sci. Technol. A 42(1) (2023).

Over the past decade, environmentally controlled pressure-assisted sintering (EC-PAS) has been established as a reliable technique for producing dense, nanocrystalline ceramics with exceptional mechanical properties due to their low porosity (less than 1%) and small grain size (as small as 4 nm). Examples include transparent Mg₂AlO₄ with hardness up to 22 GPa¹ and WC with hardness up to 39 GPa². Recently, our group has demonstrated that EC-PAS can also be combined with ALD infiltration to produce nanocomposites with unique combinations of optical, electrical, and thermal properties, which are tunable via the sintering conditions and the number of ALD cycles.

In this work, we examine the nanostructure and electrical properties of SiO₂/ZnO:Al nanocomposites fabricated by this combination of EC-PAS and ALD with the aim of understanding how to maximize electrical conductivity. In an air-free process chain, porous compacts of SiO₂ nanoparticles are infiltrated with ZnO:Al using diethylzinc, trimethylaluminum, and water (15:1 ZnO:Al₂O₃ cycle ratio) prior to sintering at 450 °C and 2 GPa to produce monolithic nanocomposites with ~1 mm thickness and ~100 nm SiO₂ domain size. 16 or 32 ALD cycles yields conductive ZnO:Al channels comprising 16% or 27% of the nanocomposite's volume with nominal channel width of ~6 or ~12 nm and average zincite crystallite size of ~4 or ~6 nm, respectively. Based on theoretical predictions and experimental measurements of the metal-insulator transition criterion for networks of ZnO nanocrystals,³ these channel widths are expected to be sufficient for metallic/band-like charge transport. Temperature-dependent resistance measurements down to as low as 0.4 K, however, reveal negative temperature coefficients characteristic of non-metallic charge transport. Specifically, for 16 cycles of ZnO:Al ALD, the charge transport mechanism appears to be Efros-Shklovskii variable-range hopping, and for 32 cycles, the temperature dependence is weaker but still clearly negative. To investigate whether this lack of metallic conduction is due to deviations from the ideal ZnO:Al geometry (percolating channels with uniform thickness), we characterize the nanocomposites' micro- and nanostructure via scanning electron microscopy (SEM) and scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy (STEM-EDS). We also compare the nanocomposites' electrical properties to those of ALD ZnO:Al films on SiO₂ wafers and explore the influence of the SiO₂/ZnO:Al interface on conduction.

Improving thermoelectric material performance is essential for energy harvesting and solid-state cooling applications. This study demonstrated a novel structure of Bi₂Te₃-based thermoelectric materials with ZnO nanowire-bundled grain boundaries, realized via atomic layer deposition (ALD) and subsequent spark plasma sintering (SPS). The ZnO nanowires formed at the interfaces due to the rearrangement of the ALD-grown ZnO ultrathin layer over Bi_0.4Sb_1.6Te₃ powder, driven by localized heating during the SPS process and the anisotropic nature of ZnO. The nanowire-bundled interfaces enhanced phonon scattering, thereby reducing lattice thermal conductivity while maintaining excellent electronic transport. This structural innovation achieved a high figure-of-merit, zT_max = 1.69 ± 0.09 at 373 K and an average zT of 1.55 over the range of 300–473 K. A thermoelectric module fabricated with 127 p–n pairs achieved a record-high conversion efficiency of 6.57% at a temperature difference of 163 K. These findings highlight the potential of nanowire-bundled interfaces to enhance the thermoelectric material performance and pave the way for scalable next-generation energy conversion technologies.

Two-dimensional transition metal dichalcogenides (2D TMDCs) have emerged as promising semiconductor materials for next-generation electronic devices due to their high mobility within atomic-scale thickness. To preserve the superior performance of 2D semiconductors in field-effect-transistor (FET) applications, gate dielectrics should be deposited via physisorption to prevent chemical reactions between the atomic-scaled 2D surface and ALD precursors.^[1] A straightforward approach to achieving this is reducing the deposition temperature; however, this is often accompanied by the formation of lower-quality gate dielectrics. It is thus essential to establish alternative strategies for gate dielectric deposition, such as exploring precursors and/or employing advanced ALD techniques.

This study will present a promising approach utilizing H₂O₂ as an oxidant source for gate dielectric deposition, along with a strategy to achieve uniform deposition without damaging 2D materials. Comparative studies were conducted using H₂O, O₃, and H₂O₂ for high-k HfO₂ growth on 2D MoS₂. Each oxidant exhibited distinct growth behaviors. While O₃ facilitated uniform HfO₂ deposition, its strong oxidation effect led to Mo-S bond conversion into Mo-O bonds, inducing damage to the MoS₂ surface. The resulting surface damage led to degraded FET device performance, indicating that O₃ cannot be a viable candidate for high-k dielectric deposition on 2D semiconductors. For H₂O and H₂O₂, achieving fully conformal coverage remains a significant challenge. To address this limitation, stop-valve techniques were employed to extend oxidant exposure time and dosage, effectively enhancing dielectric coverage while maintaining interface integrity. The interface properties were further analyzed using a top-gate FET structure,^[2] providing insights into the interface trap density associated with different oxidants.

This presentation will cover material characterization, ALD techniques, and electrical performance, offering a comprehensive evaluation of oxidant effects on 2D semiconductor integration.

This work was supported by Samsung Electronics through GRO program (IO240612-10229-01). We gratefully acknowledge TMEIC for ozone generator (OP-250H) and RASIRC for BRUTE^® Peroxide. S. E. Yu acknowledges KIAT/MOTIE (RS-2024-00435406).

[1] S. Yang et al., Adv. Mater. 35, 2207901 (2023). [2] Y. C. Lin et al., IEEE Trans. Electron Devices 70, 1598-1613 (2023).