ALD Surface Research Articles

(Invited) Achieving Extreme High Ion-Current Densities in Tailored Materials, Structures, and Interfaces

Oxide-based solid-state batteries have the potential to be a transformational and intrinsically safe energy storage solution due to their non-flammable ceramic electrolyte that enables the use of high-capacity metal anodes and high voltage cathodes for higher energy density over a much wider operating temperature range. However, their progress has been limited due to electrode/electrolyte interfacial issues. In particular for metal anodes concerns over dendrite formation/propagation and the requirement for elevated temperature and high stack pressure are still prevalent. To eliminate these concerns a rational design of tailored materials, 3D structures, and interfaces in metal anodes will be presented. While focus will be on Li-metal anodes we will demonstrate same approach works for Na-metal anodes.By use of a few nm thin AlOx ALD interfacial layer we were the first to enable Li-metal wetting of LLZO-garnet surface achieving a negligible interfacial ASR of only ~1 Ohm cm2 at room temperature with no applied pressure. Then by fabricating a ~10 mm LLZO dense layer on an extend 3D porous scaffold, with ALD surface modification to enable Li-metal wetting, we were the first to achieve the US DOE VTO Fast Charge goal for Li-metal cycling of 10 mA/cm2 at room temperature with no applied pressure. We subsequently developed a mixed ion and electron conducting (MIEC) garnet composition which when integrated into the 3D porous scaffold structure enabled critical current densities (CCDs) of 100 mA/cm2 and cumulative Li-metal cycling of over 18.5 Ah/cm2 at room temperature with no applied pressure. More recently due to the concern over Li availability we developed higher conductivity NASICON and demonstrated similar stable cycling of Na-metal in 3D structured NASICON achieving CCDs of 30 mA/cm2 at room temperature with no applied pressure.These new solid-state compositions integrated into surface modified 3D structures enable previously unheard-of metal-anode cycling rates. Their development and progress toward full cells using these tailored materials, structures, and interfaces will be presented.

Area‐selective atomic layer deposition on HOPG enabled by writable electron beam functionalization

AbstractArea‐selective atomic layer deposition (AS‐ALD) techniques are an emerging class of bottom‐up nanofabrication techniques that selectively deposit patterned ALD films without the need for conventional top‐down lithography. To achieve this patterning, most reported AS‐ALD techniques use a chemical inhibitor layer to proactively block ALD surface reactions in selected areas. Herein, an AS‐ALD process is demonstrated that uses a focused electron beam (e‐beam) to dissociate ambient water vapor and “write” highly resolved hydroxylated patterns on the surface of highly oriented pyrolytic graphite (HOPG). The patterned hydroxylated regions then support subsequent ALD deposition. The e‐beam functionalization technique facilitates precise pattern placement through control of beam position, dwell time, and current. Spatial resolution of the technique exceeded 42 nm, with a surface selectivity of between 69.9% and 99.7%, depending on selection of background nucleation regions. This work provides a fabrication route for AS‐ALD on graphitic substrates suitable for fabrication of graphene‐based nanoelectronics.

The Growth Mechanisms of Atomic Layer Deposition: An Overview

In this review, the discussion emphasized on the growth mechanisms of atomic layer deposition which consists of a theoretical model and experimentally growth as well as the measurement testing as evidences. The deposition process description with some testing evidences can be used to facilitate in the effort to understand the basic concept of ALD growth mechanisms. Some metal oxides like Al2O3, HfO2, and TiO2 with these employed precursors are typically used for the detailed illustration during the reaction steps. Although the surface chemistry of ALD process has been well understood, systematic description which combine a theoretical and experimentally growth mechanism is still missing. This paper aims to provide a better understanding of ALD growth mechanisms and surface chemistry which eventually able to contribute on the thin film growth processing.

Comparative Study of Titanium Nitride ALD using High Purity Hydrazine vs Ammonia

Emerging devices (Logic and Advanced Memory) require high quality thin (5-20 Å) electrode and barrier films. Difficult thermal budget constraints are now being placed on well-known materials such as TiN and TaN. Deposition temperature limitations have reached 350°C and below while very low resistivity (<100 micro-ohm/cm) must be achieved. Novel Ti precursors are being explored.1 Our approach has focused on hydrazine (N2H4) as an alternative nitrogen source to NH3.Hydrazine is generally difficult to handle from a safety perspective due to its flammability and toxicity. We have developed a novel method to store and generate hydrazine in situ as a high purity gas which is then delivered to ALD process chambers.2 In this study, we examine the effect that hydrazine has on low deposition temperature TiN ALD processes. We experimentally evaluate TiN ALD growth and film characteristics using TiCl4/ N2H4 in comparison to that using TiCl4/NH3.Titanium Nitride films were deposited using the hot wall type tubular furnace and a sequential gas supply system. Deposition temperature was set to predetermined value in the range from 250°C to 400°C. In the case of TiCl4/NH3, GPC was 0.10-0.27 Å/cycle at 250-400°C, where a decrease is noted as the deposition temperature is reduced. With the use of hydrazine, an increase in GPC is observed at each deposition temperature vs NH3. Moreover, the temperature dependence of GPC moved in the opposite direction of TiCl4/NH3. Specifically, GPC of TiCl4/N2H4 ALD was 19% higher at 400°C while an increase of 310% was seen at 250°C versus NH3. In the case of TiCl4/N2H4, the ALD window appears in the range of 250-400°C, where TiCl4 adsorption saturation curves are observed at 250°C and 400°C. Refractive index (R.I.) of TiCl4/N2H4 ALD was about 1.68-1.73 at 250-400°C. These values were approximately equal to R.I. of typical sputtered TiN which was about 1.66. In contrast, R.I. of TiCl4/NH3 ALD was larger than 2.00 in the 250- 300°C range. Because of R.I. of typical titanium oxide is about 2.4-2.5, it is assumed that the TiCl4/NH3 ALD film is contaminated by oxygen. In the ammonia case, background contaminants in the reactor appear to compete with low reactivity NH3 for reaction sites on the ALD surface. In the TiCl4/N2H4 case, higher temperature is not required to form high quality TiN films compared to TiCl4/NH3 ALD.From the above results, the use of high purity hydrazine is beneficial to maintaining high throughput and good film quality under the new constraints for emerging devices. Film composition by XPS and Resistivity measurements consistent with this conclusion will also be presented.

Lithium Metal Anodes: Operando Observation of Nucleation, Dendrite Growth, and Dead Lithium Formation

Lithium (Li) metal anodes have experienced a resurgence of research in recent years, which has been fueled by advances in electrolyte chemistry (both solid and liquid), interfacial engineering, and rational design of electrode architectures1. This has enabled Coulombic efficiency values to push above 99.5%, and cycle life to extend into relevant ranges for transportation applications2. However, while performance metrics are beginning to approach relevant values for consideration of their use in electric vehicles, several fundamental questions remain on how Li metal anodes dynamically evolve during cycling, especially at high current densities. Towards this goal, there is a continued need for new methods to understand the evolving morphology from nucleation, to growth, to irreversible capacity loss.In this talk, operando optical microscopy will be discussed as an enabling platform to study the coupled chemical, electrochemical, mechanical, and morphological evolution of Li metal during plating and stripping. By time synchronization of the morphological evolution of Li metal anodes with electrochemical signatures during cycling, significant insights can be obtained into the mechanistic origins of poor performance3-4. Both cross-sectional and plan-view perspectives on the electrode surface will be described, which allow for a full 3-dimensional understanding of nucleation and growth processes. Video imaging of Li metal propagation in both liquid and solid electrolytes will be presented, and the critical role of mechanical stress evolution in Li metal morphology will be described5-6. A focus will be on the formation of “dead Li”, which form as a result of electronic isolation of metallic Li from the electrode surface3. Finally, strategies to modify surface chemistry and electrode geometry will be described, providing design rules for interfacial engineering of optimized electrodes2.1) Wood, K. N.; Noked, M.; Dasgupta, N. P. Lithium Metal Anodes: Toward an Improved Understanding of Coupled Morphological, Electrochemical, and Mechanical Behavior. ACS Energy Lett. 2017, 2 (3), 664–672.2) Chen, K.-H.; Sanchez, A. J.; Kazyak, E.; Davis, A. L.; Dasgupta, N. P. Synergistic Effect of 3D Current Collectors and ALD Surface Modification for High Coulombic Efficiency Lithium Metal Anodes. Adv. Energy Mater. 2019, 9 (4), 1802534.3) Wood, K. N.; Kazyak, E.; Chadwick, A. F.; Chen, K.-H.; Zhang, J.-G.; Thornton, K.; Dasgupta, N. P. Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy. ACS Cent. Sci. 2016, 2 (11) 790-801.4) Chen, K.-H.; Wood, K. N.; Kazyak, E.; LePage, W. S.; Davis, A. L.; Sanchez, A. J.; Dasgupta, N. P. Dead Lithium: Mass Transport Effects on Voltage, Capacity, and Failure of Lithium Metal Anodes. J. Mater. Chem. A 2017, 5 (23), 11671–11681.5) LePage, W. S.; Chen, Y.; Kazyak, E.; Chen, K.-H.; Sanchez, A. J.; Poli, A.; Arruda, E. M.; Thouless, M. D.; Dasgupta, N. P. Lithium Mechanics: Roles of Strain Rate and Temperature and Implications for Lithium Metal Batteries. J. Electrochem. Soc. 2019, 166 (2), A89–A97.6) Gupta, A.; Kazyak, E.; Craig, N.; Christensen, J.; Dasgupta, N. P.; Sakamoto, J. Evaluating the Effects of Temperature and Pressure on Li/PEO-LiTFSI Interfacial Stability and Kinetics. J. Electrochem. Soc. 2018, 165 (11), A2801–A2806.

Photoassisted atomic layer deposition of oxides employing alkoxides as single-source precursors

Photoassisted atomic layer deposition (photo-ALD) is a variant of an ALD process where photons of ultraviolet or visible range are utilized to supply energy to, and to modify, the ALD surface reactions. In this paper, the authors report photo-ALD processes for titanium, zirconium, hafnium, niobium, and tantalum oxides by employing the corresponding liquid, volatile metal alkoxides as precursors in a single-source approach, i.e., without any additional reactant. The ALD reactor was equipped with a light source delivering photons over a continuous spectrum between 190 and 800 nm in wavelength. The deposition sequence consisted of a precursor pulse, a purge, a photon exposure, and another purge. The process characteristics and film properties were explored. Nb2O5 and Ta2O5 films were amorphous, whereas TiO2, ZrO2, and HfO2 showed an amorphous and polycrystalline structure, depending on the deposition conditions. With photo-ALD, area-selective deposition is realized by shadow masking. The character of the growth process, i.e., whether the chemistry is driven by photolytic or photothermal mechanism, is discussed based on deposition experiments with patterned substrates and optical filtering. Electrical characterization of photo-ALD HfO2 shows promising dielectric properties.

Synergistic Effect of 3D Current Collectors and ALD Surface Modification for High Coulombic Efficiency Lithium Metal Anodes

Improving the performance of Li metal anodes is a critical bottleneck to enable next‐generation battery systems beyond Li‐ion. However, stability issues originating from undesirable electrode/electrolyte interactions and Li dendrite formation have impaired long‐term cycling of Li metal anodes. In this work, we have developed a bottom‐up fabrication process of vertically aligned and highly uniform Cu pillar arrays on a Cu foil via templated electrodeposition for use as a 3D current collector.[1] By rationally controlling geometric parameters of the 3D current collector architecture, including pillar diameter, spacing, and length, we have demonstrated the morphology of Li plating/stripping upon cycling can be controlled and optimal cycling performance can be achieved. In addition, we showed that deposition of an ultrathin layer of ZnO by atomic layer deposition on the current collector surface can facilitate the initial Li nucleation, which dictates the morphology and reversibility of subsequent cycling. This core–shell pillar architecture allows for the effects of geometry and surface chemistry to be decoupled and individually controlled to optimize the electrode performance in a synergistic manner. Using this model platform, we have demonstrated Li metal anodes with Coulombic efficiency up to 99.5% at 0.5 mA/cm2 and 99.4% at 1 mA/cm2, which are among the highest reported values to date. The results gained from this work can thus provide a pathway toward high‐efficiency and long‐cycle life Li metal batteries with reduced excess Li loading. [1] K.-H. Chen, A.J. Sanchez, E. Kazyak, A.L. Davis, N.P. Dasgupta, Adv. Energy Mater. 9, 1802534 (2019).

Unveiling the Role of Al2O3 in Preventing Surface Reconstruction During High-Voltage Cycling of Lithium-Ion Batteries

Recent achievements in high-energy batteries have been made by using Ni-rich NMC cathodes (LiNixMnyCo1–x–yO 2with x > 0.5) in conjunction with higher cell voltages. However, these gains have come at a cost of fast capacity fade and poor rate performace. In our previous study, we showed that Al2O3 ALD coatings on LiNi0.8Mn0.1Co0.1O2 (NMC811) and LiNi0.8Co0.15Al0.05O2 (NCA) cathodes prevented surface phase transitions, reduced impedance, and extended cycle life in high voltage cells. Here, neutron diffraction (ND), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) are used to fully investigate the mechanism by which ALD surface coatings mitigate NMC811 cathode degredation. Refinement of ND patterns indicated no changes in the bulk crystal structure of cycled cathodes with or without the Al2O3 coating. Rather, the improved performance of ALD-coated cathodes is clearly due to surface stabilization. EELS established that all three transition metal oxidation states were reduce...

Emerging applications of atomic layer deposition for the rational design of novel nanostructures for surface-enhanced Raman scattering

The review highlights ALD surface chemistry, and the reaction mechanisms of various functional materials with special emphasis on their SERS applications.

Synergistic Effect of 3D Current Collectors and ALD Surface Modification for High Coulombic Efficiency Lithium Metal Anodes

AbstractImproving the performance of Li metal anodes is a critical bottleneck to enable next‐generation battery systems beyond Li‐ion. However, stability issues originating from undesirable electrode/electrolyte interactions and Li dendrite formation have impaired long‐term cycling of Li metal anodes. Herein, a bottom‐up fabrication process is demonstrated for a current collector for Li metal electrodeposition and dissolution composed of highly uniform vertically aligned Cu pillars. By rationally controlling geometric parameters of the 3D current collector architecture, including pillar diameter, spacing, and length, the morphology of Li plating/stripping upon cycling can be controlled and optimal cycling performance can be achieved. In addition, it is demonstrated that deposition of an ultrathin layer of ZnO by atomic layer deposition on the current collector surface can facilitate the initial Li nucleation, which dictates the morphology and reversibility of subsequent cycling. This core–shell pillar architecture allows for the effects of geometry and surface chemistry to be decoupled and individually controlled to optimize the electrode performance in a synergistic manner. Using this platform, Li metal anodes are demonstrated with Coulombic efficiency up to 99.5%, providing a pathway toward high‐efficiency and long‐cycle life Li metal batteries with reduced excess Li loading.

Application of AlGaN/GaN Heterostructures for Ultra-Low Power Nitrogen Dioxide Sensing

Ultra-low power room temperature NO2 sensors are demonstrated using AlGaN/GaN. The chemically stable semiconductor was sensitized to increase the sensitivity to enable ultra-low power, low ppb level detection without additional heaters. Sensors were sensitized by two methods, ultra-thin ALD SnO2 and surface enhancement by ICP-RIE in BCl3 gas. Both sensitization techniques demonstrate room temperature response, while the unsensitized sensors did not respond. At room temperature, surface enhanced sensors show a significant increase in sensitivity compared to SnO2 sensitized sensors. Sensitized sensors have fast response times and ultra-low power consumption to enable wearable monitoring systems with high spatial resolution of NO2.

Probing initial-stages of ALD growth with dynamic in situ spectroscopic ellipsometry

The initial stages of ALD surface reactions are probed using dynamic in situ spectroscopic ellipsometry (d-iSE) technique during plasma-enhanced ALD of zirconium nitride (ZrN) thin films in spectral range of 0.73–6.4eV. The measured change in the ellipsometry parameter Δ, with every precursor (TDMAZr) and reactant (forming gas plasma) exposure is interpreted as the combined effect of film growth and change in surface chemistry during ALD. We present application of Bruggeman's effective-medium approximation (B-EMA) in the analysis of d-iSE data to determine fractional surface coverage (θ) of ALD grown film at the end of every deposition cycle. During the deposition of first few ZrN monolayers, d-iSE datasets are analyzed on the basis of surface diffusion enhanced ALD growth, where the surface adsorbed precursor molecules can diffuse over substrate surface to occupy energetically favorable surface sites. The determined surface coverage of ZrN films highlights the effects of substrate enhanced ALD growth.

In Situ IR Spectroscopic Investigation of Alumina ALD on Porous Silica Films: Thermal versus Plasma-Enhanced ALD

A novel in situ infrared (IR) approach is demonstrated for investigating and identifying ALD surface reactions during both the steady state and the initial growth regime. The unique combination of reflection–absorption IR spectroscopy in grazing incidence mode with a high surface area reflecting substrate allows for ALD process monitoring with an acceptable acquisition time and a high sensitivity in the entire mid-IR spectral region. Using a mesoporous silica film deposited on a reflecting platinum layer as substrate, the thermal and plasma-enhanced ALD processes of alumina with use of trimethylaluminum (TMA) are compared. Due to the high sensitivity of the method, the relative amount of surface hydroxyl groups added or removed during the process could be determined versus the number of ALD half-cycles. These data reveal substrate-inhibited growth on the silica surface for the thermal process with use of TMA and water, as compared to direct growth for the plasma-based ALD process with use of TMA and O2 plasma. This different behavior could be linked to the formation of Si–CH3 surface groups after the first precursor pulse, as evidenced by the raw IR spectra. It is found that the oxygen radicals in the plasma can remove these surface groups during the next few ALD cycles, while the H2O molecules cannot, thus explaining the initial slower growth for the thermal process.

Multiscale Simulations of ALD in Cross Flow Reactors

We have developed a multiscale simulation code that allows us to study the impact of surface chemistry on the coating of large area substrates with high surface area/high aspect-ratio features. Our code, based on open-source libraries, takes advantage of the ALD surface chemistry to achieve an extremely efficient two-way coupling between reactor and feature length scales, and it can provide simulated quartz crystal microbalance and mass spectrometry data at any point of the reactor. By combining experimental surface characterization with simple analysis of growth profiles in a tubular cross flow reactor, we are able to extract a minimal set of reactions to effectively model the surface chemistry, including the presence of spurious CVD, to evaluate the impact of surface chemistry on the coating of large, high surface area substrates.

Multiscale Simulations of ALD in Cross Flow Reactors

Two of the biggest challenges in reactor-scale simulations of ALD processes are our lack of knowledge on the mechanistic details of surface chemistry and the need to integrate reactor scale and feature scales in the simulations. In order to assist our process development and scale up we have developed a multiscale simulation code that allows us to study the impact of surface chemistry on the coating of large area substrates with high surface area/high aspect-ratio features. Our code, based on open-source libraries, takes advantage of the ALD surface chemistry to achieve an extremely efficient two-way coupling between reactor and feature length scales, and it can provide simulated quartz crystal microbalance and mass spectrometry data at any point of the reactor. By combining experimental surface characterization with simple analysis of growth profiles in a tubular cross flow reactor, we are able to extract a minimal set of reactions to effectively model the surface chemistry, including the presence of spurious CVD. These reactions are then used as an input for the large-scale 3D simulations. In this work, we will focus on two aspects of the problem: the impact of non-ideal surface chemistry and the role of reactor geometry in the coating of both planar and high surface area substrates. Finally, we will discuss the impact of moving substrates characteristic of roll-to-roll processes.

Impact of Q-Time on the Passivation of Al2O3/p-In0.53Ga0.47As Interfaces Using Various Surface Treatments

The downscaling of complementary metal-oxide-semiconductor (CMOS)mayneedtheintegrationofIII-Vsemiconductorsandrelated high-k materials, since Si-based devices have reached their physical limitations. Unfortunately, the inherent poor interfacial quality of IIIV substrates and high dielectric constant gate oxides lead to the large defect state densities (Dit) which results in Fermi level pinning and the loss of channel controlling. 1,2 This issue is still a major challenge that needs to be addressed in spite of many achievements in interface passivation treatments. 3‐12 Recently, either ex-situ chemical and in-situ ALD surface treatments of the In0.53Ga0.47As substrates exhibited the strong minority carrier response at inversion region, which means Fermi level can be freely swept through the bandgap of III/V semiconductor. 9‐11 Similarly, O’Connor et al. 12 achieved the strong inversion behavior for n, p-In0.53Ga0.47As substrates with sulfur treatment and ALD-Al2O3 deposition. They also emphasized the impact of the delay time between the ex-situ chemical surface treatment and the ALD chamber loading (Q-time) on the quality of the high-k/III-V interface. The Q-time should be limited as low as possible; otherwise, the interfacial quality of high-k/III-V will be degraded. In this study, p-In0.53Ga0.47As substrates were treated by several chemical solutions with different Q-times prior to ALD chamber loading. Yet the nice C-V response was still observed even after 24 hours of Q-time for the MOSCAP structures studied. In terms of Q-time property, our results have supported the conclusion which might stand in contrast with previous studies, 7,8,12 i.e., reduced transfer time (less than few minutes) is not necessarily a prerequisite for an efficient chemical passivation of Al2O3/In0.53Ga0.47As interfaces. The nature of interface trap states discussed in this work would facilitate further understanding and passivating of high-k/III-V interfaces.

Molybdenum Atomic Layer Deposition Using MoF6 and Si2H6 as the Reactants

Mo ALD has been demonstrated by fluorosilane elimination chemistry using MoF6 and Si2H6 as the reactants. The nucleation and growth characteristics of Mo ALD were investigated using a variety of in situ and ex situ techniques in both high vacuum and viscous flow reactors. Quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) investigations showed that Mo ALD has significant growth rate of 500−600 ng/cm2 per cycle or 6−7 Å per cycle for temperatures between 90 and 150 °C. The large growth rates could result from extra Mo deposition by MoF6 → Mo + 3F2 that may be facilitated by the very exothermic reaction of MoF6 with silicon-containing surface species. The QCM studies revealed that the Mo ALD surface chemistry is self-limiting. The QCM and Auger electron spectroscopy (AES) studies indicated that Mo ALD nucleates very rapidly on Al2O3 ALD surfaces and reaches the linear growth regime after only 4−5 ALD cycles. Oscillatory behavior for the total mass gain and individual mass gains was observed versus ALD cycle number during the nucleation region. The AES studies revealed that Mo films grown in a high vacuum reactor do not contain silicon impurities. In contrast to the AES results, Rutherford backscattering spectroscopy (RBS) analysis showed that Mo ALD films grown in a viscous flow reactor contain ∼16 at % Si impurities. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of silicon and showed that varying temperature, precursor dose and purge parameters did not lower the Si impurities significantly. Glancing incidence X-ray diffraction (GIXRD) studies indicated that Mo ALD films were nanocrystalline. The Si impurities may exist at grain boundaries or amorphous Mo silicides as a result of Si2H6 decomposition during the highly exothermic fluorosilane elimination reaction. Fourier transform infrared (FTIR) analysis revealed that MoFx surface species are reduced to metallic Mo during the Si2H6 exposure. Because of its rapid nucleation rate, Mo ALD films could serve as ultrathin continuous conducting films or as adhesion layers for other metal ALD systems on oxide surfaces.

Ballistic transport and reaction modeling of atomic layer deposition manufacturing processes

AbstractIn this paper we develop a model describing the ballistic transport of chemical precursor species for an atomic layer deposition process (ALD). Because of the large Knudsen number corresponding to ALD over nanoporous materials, the gas-phase transport inside the nanostructures takes place in a purely ballistic manner. Precursor transmission probability functions describing the fluxes between the pore surface features are developed and then are coupled to ALD surface reaction models, spatially discretized, and integrated over each precursor exposure period to determine the pore spatial surface reaction extent profile. Predictions from our dynamic model then are compared to a previously published study of ALD in nanopores to validate our simulator. The utility of physically based models of the type we develop can be exploited to determine optimal precursor exposure levels for ALD based nanomanufacturing operations.

ALD and MLD Surface Coatings for Performance and Safety Enhancement of Li-Ion Batteries

not Available.

ALD and MLD Surface Coatings for Performance and Safety Enhancement of Li-Ion Batteries

not Available.