Atomic Layer Deposition Nucleation Research Articles

(Invited) Atomistic Insights into Continuous and Area-Selective ALD Processes: First-Principles Simulations of the Underpinning Surface Chemistry

Atomic layer deposition (ALD) is a highly advanced thin film deposition technology that has found widespread use in nanoelectronics, photovoltaics, fuel cell and recently all-solid-state battery applications. It is widely regarded as the state-of-the-art technique for producing thin films due to its exceptional conformality on high-aspect ratio structures, atomic level thickness control, and tuneable film composition. In recent years, area-selective atomic layer deposition (AS-ALD) has emerged as a promising technology for high-fidelity nano-patterning, and significant progress has been made in understanding and developing highly selective processes for various material systems. Along these lines, we have recently shown that high-throughput, low defectivity, highly selective SiO2 deposition can be achieved on various substrates by combining atmospheric-pressure spatial ALD with integrated area-deactivation using small molecule inhibitors and plasma-enhanced spatial etching [1].ALD is a surface-controlled technique that relies on the self-limiting reactions of the precursors and co-reactants with the deposition surface in alternating cycles. The initial step in ALD nucleation on the target substrate is a key factor in the deposited film quality, and efficient precursor binding on the surface is critical. Molecular dynamics (MD) simulations and minimum-energy reaction pathway analysis based on ab initio Density Functional Theory (DFT) calculations provide invaluable insights into the surface chemistry, enabling customisation of ALD processes to boost uniformity, conformality, and area-selectivity of the resulting films.In this talk, I will discuss how first-principles simulations can be used synergistically with experimental findings to achieve high-efficiency plasma-enhanced atomic-layer cleaning (ALC) of impurities from various ALD films and substrates, design proper chemical surface modifications that enable continuous and conformal ALD nucleation even on chemically-inert surfaces (like graphene), and unveil the particular surface chemistry governing high-fidelity area-selectivity with an atomic-level precision in ultrathin coatings [1-9]. By combining the insights gained from first-principles simulations with experimental techniques, we can push the frontiers of device manufacturing to new heights, opening up exciting possibilities for further technological advances in nanoelectronics, photovoltaics, fuel cell and energy storage applications.[1] Advanced Materials, Just Accepted, (DOI: 10.1002/adma.202301204); [2]Nanoscale, 2021, 13, 10092-10099; [3] Chem. Mater. 2019, 31 (4), 1250-1257.; [4] Adv. Mater. Interfaces 2018, 5 (13), 1800268.; [5] Chem. Mater. 2017, 29 (3), 921–925.; [6] ACS Nano 2017, 11 (9), 9303–9311.; [7] Sol. Energy Mater. Sol. Cells 2017, 163, 43–50.; [8] Chem. Mater. 2017, 29 (5), 2090–2100.; [9] Nanoscale 2016, 8 (47), 19829–19845. Figure 1

Al2O3 Layers Grown by Atomic Layer Deposition as Gate Insulator in 3C-SiC MOS Devices.

Metal-oxide-semiconductor (MOS) capacitors with Al2O3 as a gate insulator are fabricated on cubic silicon carbide (3C-SiC). Al2O3 is deposited both by thermal and plasma-enhanced Atomic Layer Deposition (ALD) on a thermally grown 5 nm SiO2 interlayer to improve the ALD nucleation and guarantee a better band offset with the SiC. The deposited Al2O3/SiO2 stacks show lower negative shifts of the flat band voltage VFB (in the range of about -3 V) compared with the conventional single SiO2 layer (in the range of -9 V). This lower negative shift is due to the combined effect of the Al2O3 higher permittivity (ε = 8) and to the reduced amount of carbon defects generated during the short thermal oxidation process for the thin SiO2. Moreover, the comparison between thermal and plasma-enhanced ALD suggests that this latter approach produces Al2O3 layers possessing better insulating behavior in terms of distribution of the leakage current breakdown. In fact, despite both possessing a breakdown voltage of 26 V, the T-ALD Al2O3 sample is characterised by a higher current density starting from 15 V. This can be attributable to the slightly inferior quality (in terms of density and defects) of Al2O3 obtained by the thermal approach and, which also explains its non-uniform dC/dV distribution arising by SCM maps.

Nucleation dependence of atomic layer deposition on diamond surface termination

Surface termination and interfacial interactions are critical for advanced solid-state quantum applications. In this paper, we demonstrate that atomic layer deposition (ALD) can both provide valuable insight on the chemical environment of the surface, having sufficient sensitivity to distinguish between the common diamond (001) surface termination types and passivate these interfaces as desired. We selected diamond substrates exhibiting both smooth and anomalously rough surfaces to probe the effect of morphology on ALD nucleation. We use high resolution in situ spectroscopic ellipsometry to monitor the surface reaction with sub-angstrom resolution, to evaluate the nucleation of an ALD Al2O3 process as a function of different ex and in situ treatments to the diamond surface. In situ water dosing and high vacuum annealing provided the most favorable environment for nucleation of dimethylaluminum isopropoxide and water ALD. Hydrogen termination passivated both smooth and rough surfaces while triacid cleaning passivated the smooth surface only, with striking effectiveness.

Plasma-enhanced atomic layer deposition of WO3-SiO2 films using a heteronuclear precursor

Tungsten oxide–silicon dioxide (WOx–SiOy) composite thin films were deposited for the first time via the remote oxygen plasma-enhanced atomic layer deposition (ALD) process using a novel metal-organic heteronuclear and heteroleptic precursor, bis(tert-butylimido)bis(trimethylsilylmethyl)tungsten. Self-limiting ALD growth was demonstrated over a wide temperature window of 203–328 °C with growth per cycle decreasing with increasing temperature from 0.75 to 0.4 Å/cycle, respectively. Residual gas analysis revealed ligand competition and showed that ligand reaction during ALD nucleation and growth was a function of deposition temperature, thereby affecting the film composition. As the temperature increased from 203 to 328 °C, the film composition [W/(Si + W)] ranged from 0.45 to 0.53. In addition, the carbon impurity content was reduced and the refractive index increased from 1.73 to 1.96, the density increased from 4.63 to 5.6 g/cm3, and the optical bandgap decreased from 3.45 to 3.27 eV. Grazing angle x-ray diffraction indicated that as-deposited films were amorphous. Upon annealing in O2 at 500 °C or higher, depending on deposition temperature, films are crystalized into the triclinic WO3 phase. At the same time, WO3 is sublimed from the surface and films are reduced in thickness.

Tailoring the Interfacial Interactions of Porous Polymer Membranes to Accelerate Atomic Layer Deposition: The Latent Path to Antifouling Membranes

Atomic layer deposition (ALD) is a powerful strategy to engineer hybrid organic-inorganic membranes with emergent functionalities. The combination of atomic-level thickness control, wide materials palette, and unprecedented conformality allow the physiochemical properties (e.g., hydrophilicity) of mesoporous polymer membranes to be precisely tuned. The nucleation of ALD materials growth on polymer surfaces relies on chemical interactions between the ALD metalorganic precursor and functional groups in the polymer structure and these interactions dictate the number of ALD cycles required to achieve a continuous coating. Strategies to enhance these interactions could enable desirable properties such as anti-fouling behavior to be imparted on inert polymer surfaces that lack the necessary functional groups for ALD nucleation. In this study, we demonstrate that the reactivity of polyacrylonitrile (PAN) membranes towards ALD metal oxide (MO) precursors with Lewis acid characteristics is enhanced by introducing Lewis base functional groups (amidoxime: Am) on the PAN backbone. The resulting Lewis acid-base interactions accelerates the MO nucleation in Am-PAN and reduce the number of deposition cycles required to achieve hydrophilicity compared to the untreated PAN membrane. Unveiling the reaction mechanism, the in-situ FTIR intensity changes established enhanced interaction dynamics between the ALD MO precursors and the Am-PAN membrane, unlike the PAN membrane. For similar MO cycles, through both spectroscopic and thermogravimetric analysis, we observe enhanced MO loading in the Am-PAN membrane compared to the PAN membrane. Here we have verified that strong Lewis acid-base interactions led to enhanced loading for a range of ALD MO materials including Al2O3, TiO2, SnO2, and ZnO. Most importantly, the Al2O3-Am-PAN hybrid membrane showed 23.3% higher antifouling capability compared to the pristine PAN membrane. Our approach expands the scope of design options for fouling-resistant porous hybrid inorganic-organic membranes and may reduce manufacturing costs of water treatment membranes.

Area‐Selective Atomic Layer Deposition on Functionalized Graphene Prepared by Reversible Laser Oxidation

AbstractArea‐selective atomic layer deposition (ALD) is a promising “bottom‐up” alternative to current nanopatterning techniques. While it has been successfully implemented in traditional microelectronic processes, selective nucleation of ALD on 2D materials has so far remained an unsolved challenge. In this article, a precise control of the selective deposition of ZnO on graphene at low temperatures (<250 °C) is demonstrated. Maskless femtosecond laser writing is used to locally activate predefined surface areas (down to 300 nm) by functionalizing graphene to achieve excellent ALD selectivity (up to 100%) in these regions for 6‐nm‐thick ZnO films. The intrinsic conductive properties of graphene can be restored by thermal annealing at low temperature (300 °C) without destroying the deposited ZnO patterns. As the graphene layer can be transferred onto other material surfaces, the present patterning technique opens new attractive ways for various applications in which the functionalized graphene is utilized as a template layer for selective deposition of desired materials.

A toolbox for easy entry low wavenumber in situ atomic layer deposition transmission FTIR spectroscopy studies.

A detailed description of a flexible and portable atomic layer deposition (ALD) system is presented for conducting in situ Fourier transform infrared (FTIR) absorption spectroscopy studies during the evolution and growth of ALD films. The system is directly integrated with a commercial FTIR spectrometer (Bruker Vertex 80V) to avoid the necessity of an external optical path to the instrument, thereby mitigating complexity and optical losses. In this work, we use potassium bromide (KBr) with a 5nm layer of sputtered Si as a substrate due to higher infrared transmittance when compared to a single-side polished Si wafer. The FTIR absorption study is conducted at normal incidence in transmission mode using a deuterated L-alanine doped triglycine sulfate (DTGS) detector owing to its potential applicability for reliable measurements at wavenumbers below ∼700cm-1. We demonstrate this by measuring ex situ the transverse optical phonon of bulk Al2O3 centered at 680cm-1. The integrity and functionality of the system to track the nucleation stage are validated by conducting in situ FTIR absorption measurements of Al2O3 using tri-methyl aluminum (TMA) and H2O. The measured IR absorption spectra for the Al2O3 growth after each cycle of TMA and H2O show the formation and removal of CH3 (2800-3000cm-1) groups on the substrate surface and CH4 (3016 and 1306cm-1) molecules in the reactor, thus confirming the successful tracking of ligand exchange. Thus, this instrument, together with the choice of KBr as substrate, can enable straightforward ALD nucleation studies using a DTGS detector having sufficient signal without additional optical setup and modifications to off-the-shelf FTIR systems that allow low wavenumber experiments.

Selective Hydration of Rutile TiO2 as a Strategy for Site-Selective Atomic Layer Deposition.

The feasibility of a site-selective hydration strategy that enables site-selective atomic layer deposition (ALD) is investigated among four rutile TiO2 facets [(110), (100), (101) and (001)] and their most prevalent step edges. First-principles simulations of asymmetric slab models were utilized to create accurate representations of pristine terrace and step edge sites. The adsorption free energies for molecular and dissociative adsorption of H2O were calculated to evaluate this strategy as a viable route to step edge selectivity. We predict that selective hydroxylation is possible on the 110 and 001 step edges and further computationally evaluate three metalorganic ALD precursors for their compatibility with the selective hydration strategy. Experimental evidence for delayed nucleation of ALD on rutile (001), (110), and (100) TiO2 single crystals corroborates predictions of the dehydration of the surface and suggests the possibility of site-selective ALD.

Substrate-Driven Atomic Layer Deposition of High-κ Dielectrics on 2D Materials

Atomic layer deposition (ALD) of high-κ dielectrics on two-dimensional (2D) materials (including graphene and transition metal dichalcogenides) still represents a challenge due to the lack of out-of-plane bonds on the pristine surfaces of 2D materials, thus making the nucleation process highly disadvantaged. The typical methods to promote the nucleation (i.e., the predeposition of seed layers or the surface activation via chemical treatments) certainly improve the ALD growth but can affect, to some extent, the electronic properties of 2D materials and the interface with high-κ dielectrics. Hence, direct ALD on 2D materials without seed and functionalization layers remains highly desirable. In this context, a crucial role can be played by the interaction with the substrate supporting the 2D membrane. In particular, metallic substrates such as copper or gold have been found to enhance the ALD nucleation of Al2O3 and HfO2 both on monolayer (1 L) graphene and MoS2. Similarly, uniform ALD growth of Al2O3 on the surface of 1 L epitaxial graphene (EG) on SiC (0001) has been ascribed to the peculiar EG/SiC interface properties. This review provides a detailed discussion of the substrate-driven ALD growth of high-κ dielectrics on 2D materials, mainly on graphene and MoS2. The nucleation mechanism and the influence of the ALD parameters (namely the ALD temperature and cycle number) on the coverage as well as the structural and electrical properties of the deposited high-κ thin films are described. Finally, the open challenges for applications are discussed.

Investigating the Nucleation of AlOx and HfOx ALD on Polyimide: Influence of Plasma Activation

There is an increasing interest in atomic layer deposition (ALD) on polymers for the development of membranes, electronics, (3D) nanostructures and specially for the development of hermetic packaging of the new generation of flexible implantable micro-devices. This evolution demands a better understanding of the ALD nucleation process on polymers, which has not been reported in a visual way. Herein, a visual study of ALD nucleation on polymers is presented, based on the different dry etching speeds between polymers (fast) and metal oxides (slow). An etching process removes the polyimide with the nucleating ALD acting as a mask, making the nucleation features visible through secondary electron microscopy analyses. The nucleation of both Al2O3 and HfO2 on polyimide was investigated. Both materials followed an island-coalescence nucleation. First, local islands formed, progressively coalescing into filaments, which connected and formed meshes. These meshes evolved into porous layers that eventually grew to a full layer, marking the end of the nucleation. Cross-sections were analyzed, observing no sub-surface growth. This approach was used to evaluate the influence of plasma-activating polyimide on the nucleation. Plasma-induced oxygen functionalities provided additional surface reactive sites for the ALD precursors to adsorb and start the nucleation. The presented nucleation study proved to be a straightforward and simple way to evaluate ALD nucleation on polymers.

In Situ and Ex Situ diagnostics of ALD-Ultrathin Lithium-Ion Films

All solid-state ion layers has an ever-growing interest in order to accomplish on-chip energy storage devices. As such, lithium-ion synaptic transistor to build artificial neural networks, integrated area-enhanced capacitors and lithium-ion microbatteries for energy supply are suitable candidates fulfilling energy-efficient devices. Atomic layer deposition (ALD) is one of the most promising techniques for manufacturing ultrathin layers for lithium-ion devices with exceptional capabilities to enable conformal growth at atomic scale. In this area, in situ diagnostics has become of vital importance for the growth process understanding especially on view of the air-sensitivity character of lithium ultra-thin layers.The purpose of this work is to present an in situ spectroscopic ellipsometry (SE) study of ALD thin films used on the fabrication of lithium-ion devices. We will consider several aspects of ALD ultrathin films growth, such as film thickness, saturation curves and initial films nucleation. We shall compare in situ SE results with those of ex situ diagnostics such as XRR (x-ray reflectometry) and XPS (x-ray photoelectron spectroscopy).We focus on the characterization of 10-100 nm thin lithium phosphate electrolyte with and without nitrogen doping deposited at ALD temperature range 300-350°C using LiHMDS-TMPO and LiHMDS-DEPA precursors, respectively. On the other hand, the realization of silicon integrated lithium ionic devices requires the implementation of essential barrier films to prevent Li-ion into the Si substrate. We will explore here the ability of titanium nitride depositions to act as lithium-ion diffusion barriers. TiN deposition (10-20 nm thin) by ALD process (350°C-400°C) uses TiCl4 and NH3 precursors.First, we investigate the growth process of LiPO, LiPON and TiN intrinsic layers deposited using thermal ALD. Secondly, we apply in situ and ex situ diagnostics on exemplary LiPO/LiPON deposited on top of the 10 nm ALD TiN film (Figure 1: Monitoring of LiPON thickness vs deposition time using in-situ Ellipsometry). The film thickness deduced from in situ SE measurements was corroborated by thickness measurements with XRR technique (Figure 2: Evolution of LiPON thickness vs ALD cycles using Ellipsometry and SE characterization techniques). Ex situ SE was used most often for WiW uniformity evaluating (Figure 3: 8” Wafer thickness mapping of ALD LiPON layers (a: 10nm TiN film; b: 17nm LiPON film). The preliminary results underline the fact that ALD growth rate and nucleation of lithium electrolyte and titanium nitride obviously depends on the ALD process parameters (temperature, pulse and purge times, number of cycles) but on the substrate nature (silicon, thermal oxide, and metallic). In situ and ex situ diagnostics highlight that initial film growth of lithium electrolytes and TiN layers strongly depends on the precursors kind used in the ALD process and the substrate material employed. Figure 1

Low-Power and Ultra-Thin MoS2 Photodetectors on Glass.

Integration of low-power consumer electronics on glass can revolutionize the automotive and transport sectors, packaging industry, smart building and interior design, healthcare, life science engineering, display technologies, and many other applications. However, direct growth of high-performance, scalable, and reliable electronic materials on glass is difficult owing to low thermal budget. Similarly, development of energy-efficient electronic and optoelectronic devices on glass requires manufacturing innovations. Here, we accomplish both by relatively low-temperature (<600 °C) metal-organic chemical vapor deposition growth of atomically thin MoS2 on multicomponent glass and fabrication of low-power phototransistors using atomic layer deposition (ALD)-grown, high-k, and ultra-thin (∼20 nm) Al2O3 as the top-gate dielectric, circumventing the challenges associated with the ALD nucleation of oxides on inert basal planes of van der Waals materials. The MoS2 photodetectors demonstrate the ability to detect low-intensity visible light at high speed and low energy expenditure of ∼100 pico Joules. Furthermore, low device-to-device performance variation across the entire 1 cm2 substrate and aggressive channel length scalability confirm the technology readiness level of ultra-thin MoS2 photodetectors on glass.

Recent Advances in Seeded and Seed-Layer-Free Atomic Layer Deposition of High-K Dielectrics on Graphene for Electronics

Graphene (Gr) with its distinctive features is the most studied two-dimensional (2D) material for the new generation of high frequency and optoelectronic devices. In this context, the Atomic Layer Deposition (ALD) of ultra-thin high-k insulators on Gr is essential for the implementation of many electronic devices. However, the lack of out-of-plane bonds in the sp2 lattice of Gr typically hinders the direct ALD growth on its surface. To date, several pre-functionalization and/or seed-layer deposition processes have been explored, to promote the ALD nucleation on Gr. The main challenge of these approaches is achieving ultra-thin insulators with nearly ideal dielectric properties (permittivity, breakdown field), while preserving the structural and electronic properties of Gr. This paper will review recent developments of ALD of high k-dielectrics, in particular Al2O3, on Gr with “in-situ” seed-layer approaches. Furthermore, recent reports on seed-layer-free ALD onto epitaxial Gr on SiC and onto Gr grown by chemical vapor deposition (CVD) on metals will be presented, discussing the role played by Gr interaction with the underlying substrates.

Platinum Nanoparticles on Plasma-Pretreated Carbon Nanotubes Prepared By Atomic Layer Deposition As Polymer Electrolyte Membrane Fuel Cell Catalysts

Polymer electrolyte fuel cells (PEMFC) are attracting attention as the next generation power supply for electric vehicles, unmanned aerial vehicles and submarines. However, the high price of fuel cell catalysts i.e. platinum (Pt) or platinum alloys is considered a major hurdle to the development of economical fuel cell electric vehicles. Another problem with Pt catalysts is the low stability of long term operation due to separation and aggregation of the catalyst from the carbon support resulting from repeated on-off switching. Recently, atomic layer deposition (ALD) has been proposed as a technique for synthesizing metal nanoparticles. ALD is considered to be the most advanced deposition technology that can control material size and composition in the atomic scale. In addition, ALD has demonstrated large scale conformal deposition of uniform films and particles over high aspect ratio trenches and complex three dimensional contours. In this study, Pt nanoparticles were successfully deposited on carbon nanotubes (CNTs) by ALD. The key to the success of high quality ALD Pt production has been found to be a plasma pretreatment that introduces functional groups that make the CNT surface ready to initiate the ALD nucleation. ALD Pt-CNT catalysts were evaluated as cathode catalysts for PEMFC membrane-electrode-assemblies (MEAs). As a result, even though the Pt load is significantly reduced, the performance is significantly improved compared to commercial PEMFC MEAs in terms of power and stability.

Atomic Layer Deposition Nucleation on Isolated Self-Assembled Monolayer Functional Groups: A Combined DFT and Experimental Study

We computationally design and experimentally corroborate the atomic layer deposition (ALD) of metal oxides with tailorable nucleation on selected self-assembled monolayers (SAMs) decorated with terminal functional groups. The low-temperature ALD processes for Al2O3, MnO, and especially ZnO are inhibited on nonpolar alkyl-terminated SAMs with high efficacy, in agreement with the thermodynamics calculated for reactivity with the alkyl-terminated SAMs as well as for potentially physisorbed precursors. Functional-SAM surfaces are also considered for the uniform nucleation of ALD oxides, while mixed alkyl-/functional-group-terminated SAMs are considered for the site-directed synthesis of metal oxide clusters. Long alkyl-chain SAMs with functional group termination including OH, COOH, and SH are predicted to react exergonically over a variable energy barrier which depends strongly on ALD precursor chemistry as well as functional group density. Experiments confirm that more efficient nucleation may be catalyzed via a single ALD precursor cycle of a more reactive precursor (e.g., trimethylaluminum) followed by growth of targeted metal oxides (ZnOx and MnOx). The computational strategy and experimental approach may be applicable to the selective deposition of technologically important hybrid organic–inorganic barrier films as well as catalytically relevant metal oxide and mixed-metal oxide precision few-atom clusters.

(Invited) Area Selective Atomic Layer Deposition As an Emerging Process for Advanced Nanofabrication

The rapid growth of technologies such as artificial intelligence and autonomous vehicles is fueling a need for more powerful electronic chips, which the semiconductor industry is working to meet with ever-more complex nanoscale device structures. With this growing need for nanostructure fabrication, selective deposition is gaining much attention as an important process to achieve pattern features at the ~10 nm length scale. Both two-dimensional planar geometries and three-dimensional structures will benefit from selective deposition processes, with bottom-up growth strategies providing advantages in reduced process complexity and improved pattern fidelity. In this presentation, we will describe how control over surface chemistry using molecular monolayers can achieve area selective atomic layer deposition (ALD). ALD is a good choice for selective deposition because its chemical specificity provides a means to achieve selectivity on a spatially patterned substrate. In particular, inhibitory layers such as self-assembled monolayers (SAMs) can alter the native surface reactivity, allowing ALD selectivity to be achieved. We will show that this process provides good selectivity for thin deposited films, and demonstrate selective deposition on different materials, including dielectrics and metals such as Cu, Co, W and Ru. Because area selective ALD processes often break down after more than a few nanometers of material is deposited, strategies to overcome the undesired ALD nucleation on the inhibitory layer and achieve significantly higher selectivity in area selective ALD are also important. Two such strategies that improve the final selectivity will be described, including SAM repair between ALD cycles and a self-correcting process in which selective deposition is combined with selective etching. With both approaches, selective ALD of more than 60 nm of metal oxide dielectric material can be achieved. Directions toward new area selective deposition processes will also be described.

Improve the Stability of Hybrid Halide Perovskite via Atomic Layer Deposition on Activated Phenyl-C61 Butyric Acid Methyl Ester.

Atomic layer deposition (ALD) of oxide film on [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) shows a great promise to dramatically improve the ambient stability of hybrid halide perovskite. The nucleation of an ALD oxide on PCBM is critical to reliably apply this strategy. In this paper, we present the first study of the nucleation behavior of ALD oxides, including Al2O3 and ZnO, on PCBM. We find that PCBM film acts a gas diffusion barrier blocking the ALD reactants (diethyl zinc) from etching the underlying CH3NH3PbI3. However, ZnO is not able to nucleate on PCBM. We further identify that trimethyl aluminum, a strongly Lewis acid, reacts readily with C═O on PCBM to generate a seeding layer for nucleating ZnO ALD. This new chemical route is highly reliable and can be used to synthesize ALD ZnO coatings over PCBM. The synthesized PCBM/Al2O3-ZnO dramatically improves the stability of CH3NH3PbI3 against the ambience and even against liquid water. The result signifies the importance of understanding of nucleation of ALD in enabling reliable barrier coatings for hybrid halide perovskites.

Methodology for Studying Surface Chemistry and Evolution during the Nucleation Phase of Atomic Layer Deposition Using Scanning Tunneling Microscopy

We study the nucleation stage and growth of atomic layer deposition (ALD) on hydrogen terminated silicon (Si:H) by in situ and ex situ scanning tunneling microscopy (STM). STM allows the in-depth study of surface chemistry and evolution during the ALD nucleation phase. Here, the ALD systems studied to demonstrate this technique are ZnO via diethyl zinc (DEZ) and TiO2 via titanium tetrachloride (TiCl4). In-situ STM revealed that DEZ does not discriminate between different surface sites, in contrast to TiCl4 which shows a strong preference toward dangling or OH bonds. Continued deposition showed distinct island growth for TiO2 deposition on Si:H, versus homogeneous growth for DEZ. ZnO ALD exhibited a delay of approximately 5 ALD cycles in transitioning from lateral to vertical growth and nominal physical film closure occurred after approximately 12–15 cycles. STM observations of these ALD chemistries demonstrated the strength of this technique in quantifying film closure and the effects of surface termination and defects on ALD growth mode. This technique can be applied to the study of a broad variety of ALD systems.

Area-selective atomic layer deposition of Ru on electron-beam-written Pt(C) patterns versus SiO2 substratum

Area selectivity is an emerging sub-topic in the field of atomic layer deposition (ALD), which employs opposite nucleation phenomena to distinct heterogeneous starting materials on a surface. In this paper, we intend to grow Ru exclusively on locally pre-defined Pt patterns, while keeping a SiO2 substratum free from any deposition. In a first step, we study in detail the Ru ALD nucleation on SiO2 and clarify the impact of the set-point temperature. An initial incubation period with actually no growth was revealed before a formation of minor, isolated RuOx islands; clearly no continuous Ru layer formed on SiO2. A lower temperature was beneficial in facilitating a longer incubation and consequently a wider window for (inherent) selectivity. In a second step, we write C-rich Pt micro-patterns on SiO2 by focused electron-beam-induced deposition (FEBID), varying the number of FEBID scans at two electron beam acceleration voltages. Subsequently, the localized Pt(C) deposits are pre-cleaned in O2 and overgrown by Ru ALD. Already sub-nanometer-thin Pt(C) patterns, which were supposedly purified into some form of Pt(Ox), acted as very effective activation for the locally restricted, thus area-selective ALD growth of a pure, continuous Ru covering, whereas the SiO2 substratum sufficiently inhibited towards no growth. FEBID at lower electron energy reduced unwanted stray deposition and achieved well-resolved pattern features. We access the nucleation phenomena by utilizing a hybrid metrology approach, which uniquely combines in-situ real-time spectroscopic ellipsometry, in-vacuo x-ray photoelectron spectroscopy, ex-situ high-resolution scanning electron microscopy, and mapping energy-dispersive x-ray spectroscopy.

Dielectric-MoS2 Interfaces Grown By Atomic Layer Deposition

Molybdenum disulfide (MoS2) has become a prototypical transition metal dichalcogenide (TMDC) atomic-layered material because of its unique materials properties. For example, bulk MoS2 exhibits an indirect band-gap of 1.3 eV, while a single monolayer has a direct band-gap of 1.8 eV. Recently, nanometer scale transistor devices have been made with MoS2 and graphene, yet much of this work relies on layered materials prepared using chemical vapor deposition (CVD) and mechanically exfoliation. Multiple reports have demonstrated the growth of MoS2 via CVD, but only a few studies have reported MoS2 growth using atomic layer deposition (ALD), which offers potential advantages for high volume semiconductor manufacturing. Here, we report the ALD of few-layer MoS2 films using MoF6 and various sulfur precursors on various ALD-grown dielectric surfaces and the ALD of metal oxides on the ALD MoS2 films at temperatures between 100-300 °C. In particular, we used in-situ quartz crystal microbalance measurements to investigate the nucleation of MoS2 ALD on oxide surfaces and vice versa. In general, self-limiting growth of MoS2 was observed on a wide range of dielectric surfaces including alumina, magnesia, and hafnia. The ALD MoS2 films were amorphous as-deposited, but crystallized into a layered atomic structure upon annealing at 800 °C. Using in-situ spectroscopy and electrical measurements, growth inhibition of MoS2 was seen depending on the growth substrate. These studies provide insight into the low-temperature ALD of MoS2 and provide guidance for the ALD of additional TMDC films.