ASD 2026 Session ASD2-TuM: ASD of 2D and Al₂O₃

Two-dimensional (2D) semiconductors offer a compelling pathway beyond the scaling and energy-efficiency limits of silicon nanoelectronics, yet their manufacturable and heterogeneous integration remains a central challenge. Here we report a wafer-scale, conformal growth strategy for continuous mono- to few-layer transition-metal dichalcogenides (TMDs) based on spin-coating single-source dithioacid-derived organosulfur precursors. This chemistry provides an intrinsically high chalcogen-to-metal ratio that enables direct growth and integration without external chalcogen delivery. Upon mild annealing, the precursor undergoes conversion through metastable sulfur-rich intermediates, enabling uniform nucleation, thickness control, and continuous layer formation on both planar substrates and high-aspect-ratio topographies. In situ spectroscopy, molecular dynamics simulations, and density functional theory collectively elucidate the reaction pathway linking precursor chemistry to crystallization kinetics, grain size evolution, and defect formation. The resulting TMD films exhibit optical quality comparable to exfoliated monolayers, as evidenced by strong photoluminescence and well-resolved Raman modes, and enable selective-area growth as well as sub-5 nm high-k dielectric integration without seed layers. Devices fabricated from these films demonstrate robust field-effect transistor performance and vertically integrated memristors exhibiting nanosecond-scale spike-timing-dependent plasticity. By decoupling TMD growth from gas-phase chalcogen chemistry, this intrinsically high-chalcogen single-source approach provides a general and scalable route to the direct heterogeneous integration of layered metal sulfides, establishing a practical foundation for manufacturable 2D electronic and neuromorphic systems.

High contact resistance is one of the main factors holding back the performance of TMD based FETs. One proposed solution to lowering contact resistance is through TMD edge contacts. Edge contacts are theorized to outperform top contacts due to more efficient charge carrier injection, aided by the conductivity anisotropy in TMDs. ALD is an ideal method to deposit these contacts, as it is based on self-limiting surface reactions and has the potential for area selective deposition. Area selectivity would allow for preferential deposition on the edge of the TMD channel rather than the basal plane to form self-aligned lateral heterostructures, allowing for precise contact placement and removing the need for excess lithography steps.

A thermal ALD process for TiS₂ was developed using tetrakis(dimethylamido) titanium(IV) and Ar/H₂S as the reactants. This process is shown to be inherently area selective, as we realize the formation of lateral heterostructures of CVD synthesized MoS₂ and ALD synthesized TiS₂. Beginning with an unclosed monolayer of MoS₂, we are able to selectively deposit on the exposed areas of substrate while avoiding depositions on the basal plane. This selectivity is shown through spectroscopic ellipsometry, AFM, Auger spectroscopy, and SEM.

We also demonstrate the ability to tune the resistivity, stoichiometry, and crystallinity of TiS₂ through altering deposition parameters. At a deposition temperature of 100°C, the material has a stoichiometry of Ti_0.96S₂ and a resistivity of . When increasing the deposition temperature to 150°C the film becomes more metal rich with a Ti:S ratio of Ti_1.07S₂. The more Ti-rich film has a resistivity of , lower than that of bulk TiS₂. We also show that the stoichiometry of the films has a direct impact on the crystallinity. By undersaturating the Ti precursor the excess metal can be modulated, resulting in more crystalline material. The impact of oxidation on the resistivity of TiS₂ is also explored. It was found that films with excess Ti maintain a low resistivity after exposure to ambient conditions, while films with little to no excess Ti have large increases in resistivity, showing an alternate mode of conductance based on stoichiometry.

Ultrathin dielectric integration on 2D semiconductors is essential for continued device scaling in beyond-Si nanoelectronics. However, the chemical inertness of van der Waals basal planes remains a fundamental obstacle to uniform atomic layer deposition (ALD) nucleation and growth. Here, we present an acetic acid (HAc)-modulated ALD strategy implemented through an ABC-type sequence (HAc/TMA/H₂O) to regulate Al₂O₃ nucleation on monolayer (1L) MoS₂. In situ quartz crystal microbalance (QCM) measurements reveal robust HAc adsorption on Al₂O₃ and suppressed subsequent Al₂O₃ growth on passivated surfaces. Applied to 1L MoS₂, this inhibitory pathway mitigates 3D island coarsening and redirects precursor adsorption toward uncovered basal planes, enabling nearly continuous ultrathin (~1.5 nm) Al₂O₃ films with improved uniformity, as confirmed by atomic force microscopy (AFM) and cross-sectional scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDS). Density functional theory calculations further provide atomistic insight, showing the energetic preference of HAc for Al₂O₃ over 1L MoS₂ and attenuated TMA adsorption on HAc-passivated sites. Overall, this small molecule inhibitor-modulated pathway establishes a generalizable route to controlled ALD nucleation for reliable dielectric integration across diverse ALD chemistries and 2D materials, offering opportunities for next-generation nanoelectronics device fabrication.

Selective deposition of a dielectric film, such as aluminum oxide (Al₂O₃) or hafnium oxide (HfO₂), on metal or metal-oxide surfaces (also known as DoM), without growth on silicon dielectric surfaces, has numerous applications in semiconductor manufacturing. These include the selective deposition of hard masks to enhance dry etch performance. Typical growth surfaces include W, Mo, and Co, with or without their native oxides, whereas inhibited surfaces are usually doped (C, N)- silicon oxides.Typically, this is achieved using small-molecule inhibitors (SMI), such as dimethylaminotrimethylsilane (DMATMS), which selectively chemisorb onto silicon oxide surfaces but not onto metal/metal oxide surfaces. The DMATMS-modified SiO₂ surface exhibits a significant (~2 nm) nucleation delay relative to the uninhibited surface from AlO_x deposition from DMAI/H₂O. The goal of this work is to evaluate alternate aluminum precursors and determine their impact on selectivity. Methods for assessing the selectivity failure modes are also examined in the context of screening alternative precursors.

Evaluation of selectivity performance is primarily conducted using microscopy techniques such as TEM and SEM, spectroscopic methods such as FTIR and ellipsometry, and physical techniques such as water contact angle (WCA) measurement. These methods typically do not provide information about the surface state of the inhibited surface prior to continuous film formation. We use vapor-phase decomposition mass spectrometry (VPD-MS), which can determine effective film thicknesses of <0.0001 nm. Compared with conventional methods, which require>0.1 nm of continuous film for detection, VPD-MS enables the determination and quantification of events during the initial nucleation period. The selectivity and nucleation behavior of several aluminum precursors are evaluated using ellipsometry, WCA, and VPD-MS, in comparison to the baseline process with DMAI/H₂O. In addition, the failure mode mechanisms revealed by this metrology are presented.

Trimethylaluminum (TMA) loses its selectivity within a short time, which is problematic because it exhibits excessively high reactivity toward H2O and surface sites, an extremely small molecular size, and low thermal stability even if a passivation layer is used. To achieve high selectivity, precise control of the reaction between the precursor and the substrate surface and minimization of unwanted reactivity are essential. Therefore, to achieve excellent area-selective atomic layer deposition (AS-ALD), we thoroughly investigated novel precursors and found that AlMe2(iPr-AMD) is an outstanding candidate that exhibits good thermal properties and high volatility. We carried out AS-ALD of an Al2O3 film on Cu and native oxide Si substrates using AlMe2(iPr-AMD) with H2O as a coreactant. For the dielectric-on-dielectric selective growth process, 1-dodecanethiol (DDT) was used as a self-assembled monolayer (SAM) passivant on the Cu surface. The Al2O3 film growth was inhibited on the Cu surface as a result of passivation by the SAM during deposition of a >100 Å-thick Al2O3 film onto the native oxide layer of a Si surface at 150 °C using AlMe2(iPr-AMD). This result substantially exceed thtat achieved with Dimethylaluminum isopropoxide (DMAI) under identical conditions. For the dielectric-on-metal selective growth process, n-octadecyltrichlorosilane (ODTS) was used as a SAM passivant on the dielectric surface. In contrast to the DDT SAM, the ODTS SAM showed restrained Al2O3 film growth on the Si surface and achieved Al2O3 film growth only on the Cu surface. These results indicate that the AlMe2(iPr-AMD) precursor adsorption is strongly inhibited by SAM passivation as a result of its bulky structure, reduced Lewis acidity, and lower level of reactivity compared to TMA. In addition, results from AS-ALD processes employing not only DDT and ODTS but also additional passivation molecules will also be discussed.