Atomic Layer Deposition Process Parameters Research Articles

Atomic Layer Deposition of Cobalt Film from Dicobalt-hexacarbonyl-tert-butylacetylene and Hydrogen

Thermal atomic layer deposition (ALD) of Co using the highly volatile liquid precursor, dicobalt-hexacarbonyl-tert-butylacetylene (CCTBA) and H2 was examined for application to liner or cap layers in Cu interconnects. The ALD process parameters, including deposition temperature and precursor supply time, were examined, and film properties such as composition and resistivity were investigated. At a deposition temperature of 100 °C, no incubation period was observed on Cu underlayers, and the Co film growth exhibited a self-limiting behavior with a saturated growth per cycle (GPC) of 0.051 nm cycle−1. Conformal deposition was achieved on trench structures with an aspect ratio of 8.4. However, deposition at temperatures above 125 °C resulted in the thermal decomposition of CCTBA and a transition to chemical vapor deposition (CVD) mode. The as-deposited Co film contained 29% C and exhibited a high resistivity of 104 μΩ∙cm. Post-annealing at 300 °C under a hydrogen atmosphere significantly reduced the C impurities to 3%, and decreased the resistivity to 53 μΩ∙cm at a film thickness of 20 nm. Annealing at higher temperatures increased the surface roughness, leading to film agglomeration. Therefore, a two-step process of thermal ALD using CCTBA at 100 °C followed by post-annealing at 300 °C is a suitable approach for fabricating high-quality Co films.

Multiscale CFD modelling for conformal atomic layer deposition in high aspect ratio nanostructures

The high-quality and conformal thin film is of great importance to performances of functional materials and devices with high aspect ratio nanostructures. Atomic layer deposition (ALD) is known for outstanding step coverage, yet the ideal coatings in high aspect ratio nanostructures requires the process optimization. In this work, we report a computationally efficient multiscale computational fluid dynamics (CFD) model for an accurate study of ALD process in high aspect ratio nanostructures. The continuum model of precursor mass transfer and surface micro-kinetics in the high aspect ratio nanostructure is coupled to the reactor scale CFD model. The dynamic distribution of the precursor in the high aspect ratio nanostructure is captured and the competition of the surface deposition with the precursor transfer has been analyzed. The complete ALD process parameters including pulse, pressure-holding and purging are studied. Increasing the gas flow rate correspondingly reduces the gas phase reaction, ensures the conformability, and shortens the process time. Pressure-holding leads to a better film conformality for the diffusion-limited ALD process, especially for precursors of low reactivity. The experiment of ALD coating in nanopores with an aspect ratio up to 100:1 agrees well with our simulation results, indicating the validity of the multiscale model. Multiscale modelling provides a manner to quantitatively study the comprehensive effects of full-scale parameters for better conformity and process efficiency.

Surface Texture Design of FBR-ALD Pt/C Catalyst to Enhance PEMFC Performance

Proton exchange membrane fuel cell (PEMFC) is an efficient electrochemical energy conversion device that directly generates electricity from the chemical energy of fuels without the emission of greenhouse gases. The most reliable catalyst in PEMFC is Platinum (Pt) metal nanoparticles (NPs) that exhibit excellent electrochemical activity and stability compared to other catalysts. However, using of Pt catalyst is limited due to its very high cost and low abundance on Earth. Therefore, it is important to use Pt catalyst efficiently for making the PEMFC economically viable. In this regard, several synthesis techniques have been developed to reduce the loading and uniform distribution of Pt NPs on carbon support with high electrochemically active surface area (ECSA). One of the most efficient techniques to uniformly deposit Pt NPs with a controllable size on carbon support is to use a fluidized bed reactor (FBR) atomic layer deposition (ALD). Our group recently demonstrated that FBR-ALD Pt/C catalysts can exhibit high fuel cell performance and high endurance even with low Pt NPs loading by optimizing the surface of carbon supports combined with proper ALD process parameters [1]. However, it is still challenging to further improve the fuel cell performance by rational designing the Pt NPs surfaces in order to make FBR-ALD into a viable commercial production.In this study, a unique way to improve the fuel cell performance was suggested to design and optimize atomic scale surface textures of Pt NPs. During the FBR-ALD of Pt NPs, in-situ surface modulation of Pt NPs was applied via a proper protective oxide deposition and etching. A careful surface studies was performed to analyze the surface morphology, distribution and uniformity of Pt NPs. Electrochemical performances were evaluated and optimized by measuring cyclic voltammetry (CV) and oxygen reduction reaction (ORR). Finally, a fuel cell performance was studied through membrane electrode assembly (MEA) characteristics.References W. J. Lee, S. Bera, H. C. Shin, W. P. Hong, S. J. Oh, Z. wan, S. H. Kwon, Adv. Mater. Interfaces 6, 1901210 (2019).

(Invited) Investigating Surface Reaction Thermodynamics: In Situ Calorimetry for Atomic Layer Deposition

Atomic layer deposition (ALD) is comprised of sequential self-limiting surface reactions that are inherently sensitive to the surface chemistry of the substrate and the reactivity of the chosen precursors. In order to better understand ALD processes and direct their future development we must investigate the fundamental mechanisms of ALD reactions. This is commonly done using a combination of computational modeling and experimental investigations using techniques like spectroscopic ellipsometry and IR spectroscopy. We present pyroelectric calorimetry for ALD as an in situ technique to measure the time resolved heat generation from ALD reactions. This provides a new dimension of thermodynamic and kinetic reaction data.As an experimental technique, pyroelectric calorimetry measures ALD reactions on realistic surfaces as opposed to idealized models. We have designed and constructed custom calorimeters that are compatible with typical ALD process parameters. These calorimeters have been calibrated with a thermal resolution down to 0.1 μJ/cm2 and a temporal resolution of 50ns. To put this in perspective for an ALD reaction, this corresponds to about 0.1% of the heat evolved in the trimethylaluminum (TMA) and water ALD process and about 104 faster than our measurement of the TMA half reaction. The temporal resolution of our calorimeters is orders of magnitude faster than most complementary in situ analysis techniques including ellipsometry, IR spectroscopy, quartz crystal microgravimetry, and mass spectrometry. This time resolution provides information on precursor flow dynamics and reaction kinetics.Pyroelectric calorimetry was used to investigate the TMA and water half reactions for ALD Al2O3 along with in situ spectroscopic ellipsometry thickness measurements and Rutherford backscattering spectrometry (RBS) composition analysis. This enabled comparisons of the change in thickness with the amount of heat generated as each half reaction saturates. Atomic growth rates calculated form RBS were used to calculate the heat generated on a per-atom basis. These results were then compared with proposed reaction mechanisms and energy changes from density functional theory (DFT) computational modeling.Measurements of reaction and adsorption heats can also be used to compare and evaluate different precursors and inhibitor molecules for ALD growth inhibition in area-selective ALD. We investigated how the thermodynamics of adsorption can guide our choice of precursor-inhibitor pairs when using small molecule inhibitors that rely on competitive adsorption and chemical passivation to prevent ALD growth. Pyroelectric calorimetry offers new insight into what drives and limits ALD reactions and provides many opportunities for future investigations.

Conductive SnO2-x thin films deposited by thermal ALD with H2O reactant

In this study, SnO2-x films were deposited by a thermal atomic layer deposition (ALD) method with H2O as a reactant. The electrical property of the films was assessed with the intention of applying them as a channel for a back-end-of-line (BEOL) compatible transistor. Under the thermal budget constraint (<450 °C) required for a BEOL transistor, the effect of ALD process parameters and post-annealing conditions on the carrier concentration and mobility of the films was investigated. While the as-deposited films were amorphous, more distinctive recrystallization could be observed with the deposition temperature of 200 °C combined with the annealing ambient of N2. As a result, the mobility of 21.55 cm2/Vs and 3.3 × 1019 cm−3 carrier concentration were achieved with an 18-nm thick n-type SnO2-x film. However, it was also observed the mobility of the N2-annealed SnO2-x decreased by ∼50% of an initial value upon exposure to humidity for 2–3 days. 5-Nm Al2O3 on SnO2-x as a capping layer could prevent the degradation, suggesting the presence of a more conductive path near the surface. This work demonstrates SnO2-x deposited by a thermal ALD can exhibit an excellent electrical property suitable for various electronic devices such as a BEOL compatible transistor.

Modelling atomic layer deposition overcoating formation on a porous heterogeneous catalyst.

Atomic layer deposition (ALD) was used to deposit a protective overcoating (Al2O3) on an industrially relevant Co-based Fischer-Tropsch catalyst. A trimethylaluminium/water (TMA/H2O) ALD process was used to prepare ∼0.7-2.2 nm overcoatings on an incipient wetness impregnated Co-Pt/TiO2 catalyst. A diffusion-reaction differential equation model was used to predict precursor transport and the resulting deposited overcoating surface coverage inside a catalyst particle. The model was validated against transmission electron (TEM) and scanning electron (SEM) microscopy studies. The prepared model utilised catalyst physical properties and ALD process parameters to estimate achieved overcoating thickness for 20 and 30 deposition cycles (1.36 and 2.04 nm respectively). The TEM analysis supported these estimates, with 1.29 ± 0.16 and 2.15 ± 0.29 nm average layer thicknesses. In addition to layer thickness estimation, the model was used to predict overcoating penetration into the porous catalyst. The model estimated a penetration depth of ∼19 μm, and cross-sectional scanning electron microscopy supported the prediction with a deepest penetration of 15-18 μm. The model successfully estimated the deepest penetration, however, the microscopy study showed penetration depth fluctuation between 0-18 μm, having an average of 9.6 μm.

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

Atomic layer deposition (ALD) for environmental protection and whisker mitigation of electronic assemblies

In this study, we demonstrate how metal-oxide thin-film conformal coatings grown by atomic layer deposition (ALD) can be exploited as an effective approach to mitigate tin whisker growth on printed circuit boards. First, we study the effect of different ALD coatings and process parameters on Sn–Cu-electroplated test coupons, by combining optical imaging and scanning electron microscopy and evaluating whisker distribution on the surface. On these samples, we found that one important parameter in mitigating whisker growth is the time interval between electroplating and the ALD coating process (pre-coat time), which should be kept of the order of few days (2, based on our results). Atomic layer-deposited coatings were also found to be effective toward whisker formation in different storage conditions. Furthermore, we show that ALD coating is also effective in limiting the need for outgassing of electronic assemblies (PCBAs), which is an additional stringent requirement for applications in space industry. Our experimental results thus demonstrated that atomic layer deposition is a suitable technique for aerospace applications, both in terms of degassing and whisker mitigation.

Optimization of substrate-selective atomic layer deposition of zirconia on electroplated copper using ethanol as both precursor reactant and surface pre-deposition treatment

Recent developments on the further optimization of the selective atomic layer deposition (ALD) of zirconium oxide (ZrO2) onto silicon over electroplated copper are reported with tris(dimethylamino)cyclopentadienyl zirconium as the zirconium precursor and ethanol as both oxygen source for precursor and pretreatment of copper surface to reduce surface oxides. A comparison between the electroplated (EP) and e-beam deposited (EB) copper before ALD using high-resolution X-ray photoelectron spectroscopy (XPS) showed EP Cu having more oxides on the surface and unexpectedly the EP Cu was found to have higher selectivity of ALD. In addition, a comparison of deposition of zirconia on silicon over EP and EB copper substrates for 70 ALD cycles show experimental conditions can impact the degree of selective deposition with the electroplated copper. With further optimization of the substrate pretreatment and ALD process parameters, both XPS survey analysis and ellipsometry showed ALD selectivity was enhanced up to 100 cycles on electroplated copper samples which is higher than ever previously reported forming an amorphous ZrO2 film of ~ 3 nm thickness on silicon and no deposition on copper. Lack of crystallinity of the ALD ZrO2 films is manifested by X-ray absorption fine structure and grazing incidence X-ray diffraction. Atomic force microscopy and scanning electron microscopy performed on the samples indicated no significant changes of the roughness and topography on the samples before and after ZrO2 ALD. Both thermodynamic calculations and experimental data presented here show that ALD selectivity is mainly dictated by the balance of the total amounts of oxygen and carbon in the reactor along with the ALD conditions and reactor design. These studies show pre-deposition ethanol exposure of the substrate, deposition/adsorption of reduction byproducts formed, and the total oxygen and carbon in the ALD reactor are important contributors to whether ALD selectivity can be significantly enhanced or even achieved at all.

Modeling atomic layer deposition process parameters to achieve dense nanocrystal-based nanocomposites

Atomic layer deposition (ALD) is a technique capable of depositing conformal coatings in highly tortuous 3D nanostructures. One configuration that has attracted attention is nanocrystal (NC) based nanocomposite films, whereby a 3D network of randomly packed nanocrystals is infilled via ALD to yield a dense nanocomposite. In this work, we demonstrate criteria for predicting three important thermal ALD process parameters necessary to completely infill 3D NC networks: cycle number, precursor pulse time, and purge time. A description of representative pore geometry is developed using parameters of the film comprised of nanocrystals before infill, specifically NC diameter, NC volume fraction, and film thickness. This geometric description allowed for prediction of required precursor pulse times to saturate the NC film surface. A finite-difference model of water vapor transport during purging revealed that desorption kinetics can be used to predict purge times required to achieve complete infill. The model predictions show good agreement with experiments carried out by infilling films comprised of GaN NCs with ZnO by the diethylzinc/water process and films comprised of Al2O3 NCs with Al2O3 by the trimethylaluminum/water process.

Characterization of physical and mechanical properties of Al2O3-doped ZnO (AZO) thin films deposited on transparent polyimide supports with various ALD process parameters

Fabrication of Al2O3 doped ZnO (AZO) thin films on transparent polyimide substrates using ex-situ oxygen plasma-assisted atomic layer deposition (ALD) is studied. The alumina composition is controlled by adjusting the ALD cycle ratio in the reaction sequence from 9:1 to 49:1 (diethylzinc/H2O:trimethylaluminum/H2O). Nanoscratch and electromechanical tensile tests are performed to evaluate the mechanical reliability of the AZO thin films fabricated under various processing conditions. Conventionally deposited AZO shows the lowest sheet resistance of 294 Ω/sq at a doping ratio of 19:1 at 150 ℃. However, the resistance of AZO grown on a polymer surface modified by plasma is higher at 429 Ω/sq. Nanoscratch results reveal that the first critical load could be increased by surface hardening and strong adherence as a function of the process temperature and plasma treatment. Under a tensile strain, doping of alumina delayed the strain (%) at crack initiation from 0.58 to 0.82%. The initial elastic modulus of the AZO/transparent polyimide composites showed a transient decrease with an increase in the alumina content from 2.7 to 3.8 at.%. These results demonstrating the mechanical characteristics of thin films as a function of the processing conditions are significant for flexible display manufacturing.

Review Article: Atomic layer deposition of doped ZnO films

This article reviews the process-structure-property relationship in doped ZnO thin films via atomic layer deposition (ALD). ALD is an important manufacturing-scalable, layer-by-layer, thin film deposition process that precisely controls dopant type and concentration at the nanoscale. ZnO is an important technological material, which can be doped to modulate structure and composition to tailor a wide variety of optical and electronic properties. ALD doped ZnO is viewed as a transparent conducting oxide for application in solar cells, flexible transparent electronics, and light-emitting diodes. To date, there are 22 elements that have been reported as dopants in ZnO via ALD. This article studies the underlying trends across dopants and establishes generalized relationships for (1) the role of ALD process parameters, (2) the impact of these parameters on the structure of the ZnO matrix, and (3) the impact of dopants on the optical and electrical properties. The article ends with a brief discussion on the limitations of the ALD-based doping scheme, knowledge gaps in the compositional maps, and a perspective on the future of ALD doped ZnO films.

Atomic Layer Deposition of Photoconductive Cu2O Thin Films.

Herein, we report an atomic layer deposition (ALD) process for Cu2O thin films using copper(II) acetate [Cu(OAc)2] and water vapor as precursors. This precursor combination enables the deposition of phase-pure, polycrystalline, and impurity-free Cu2O thin films at temperatures of 180–220 °C. The deposition of Cu(I) oxide films from a Cu(II) precursor without the use of a reducing agent is explained by the thermally induced reduction of Cu(OAc)2 to the volatile copper(I) acetate, CuOAc. In addition to the optimization of ALD process parameters and characterization of film properties, we studied the Cu2O films in the fabrication of photoconductor devices. Our proof-of-concept devices show that approximately 20 nm thick Cu2O films can be used for photodetection in the visible wavelength range and that the thin film photoconductors exhibit improved device characteristics in comparison to bulk Cu2O crystals.

Atomic Layer Deposition of Thin Films onto 3D Nanostructures: The Effect of Wall Tilt Angle and Aspect Ratio of Trenches

A theoretical model predicting the spatial profile of a film grown on walls by atomic layer deposition (ALD) is developed. The possible initial nonverticality of trench walls and the dynamic variation of the aspect ratio of the structure in the process of film growth in nanosized trenches are considered in this model. The dependence of the resulting film thickness and conformity on ALD process parameters is studied theoretically.

Atomically dispersed vanadium oxides on multiwalled carbon nanotubes via atomic layer deposition: A multiparameter optimization

The focus of the present work is to investigate the bonding characteristics of vanadium oxide species to different oxygen functional groups on multiwalled carbon nanotubes (MWCNT). Atomic layer deposition (ALD) was used to deposit atomically dispersed vanadium oxide species on MWCNT. To generate atomically dispersed vanadium, only one ALD cycle was applied for the deposition of vanadium. The MWCNT functional groups that are involved in the deposition process were identified by thermal analysis and grafting experiments. A variety of ALD process parameters were tested, and revealed that purging times between dosing of vanadium precursor and dosing of water as coreactant had a strong influence on the ratio of vanadium species that are physisorbed or chemisorbed to the MWCNT. The ALD process parameters were optimized to focus on the immobilization of the vanadium due to a chemical bond between vanadium species and MWCNT. Because of the direct correlation between catalytic stability and immobility of the vanadium species, the importance of knowledge about the influence of the ALD parameter onto the bond formation is essential. Raman spectroscopy and high resolution scanning transmission electron microscopy images were used to prove the single site structure of the vanadium oxide.

Influence of ALD process parameters on the physical and chemical properties of the surface of vascular stents

Studies of haemocompatibility of AISI 316LVM steel point to the need for nickel elimination from the surface and replacing it with other elements showing higher biotolerance. Such layers include titanium, carbon or silicon coatings. Therefore, the authors attempted to evaluate some selected physicochemical properties of TiO2 layers, grown by atomic layer deposition (ALD) method, on the surface of 316LVM steel at variable process temperature. ALD temperature has a major role in the final quality of the surface layer grown with the use of such method, regardless of the type of the base. It was observed that the growth of temperature had an adverse influence on corrosive resistance in the artificial plasma environment and contributed to formation of a double (porous) layer showing decreased tightness. Further on, assessment of the coating adhesion to the base showed that too low process temperature T=100°C had an adverse effect on mechanical properties, resulting in substantially reduced critical force. On the other hand, the performed surface wettability tests showed no influence of ALD temperature in the obtained angle values.

Plasma-Assisted Atomic Layer Deposition of Thin Film Li2CO3 for All-Solid-State Li-Ion Batteries

All-solid-state Li-ion batteries offer the advantage of a higher power density, a wider temperature window and an improved safety with respect to batteries containing liquid electrolytes. However, the lack of solid electrolytes with sufficient ion conductivity and wide electrochemical window is currently inhibiting the implementation of solid state batteries. Although the limited ionic conductivity can be overcome by employing very thin solid-state electrolyte films which shorten the ionic diffusion distance, the capacity will still be insufficient. This makes the use of non-planar devices of particular high interest as the application of 3D-(nano)structured materials will increase the total effective battery area. Moreover, the area enhancement will result in an ever higher energy density, faster (dis)charging, better stability, etc. Conventional deposition techniques, such as PVD and PE-CVD, do not generally result in conformal thin layers on 3D structures. Therefore, novel deposition techniques leading to enhanced control in thin film properties and conformality are being introduced in the field of battery research. Atomic layer deposition (ALD), which is based on sequential and self-limiting reactions, has emerged as a powerful tool since it offers exceptional conformality on high-aspect ratio structures, thickness control at sub-nm level, and tunable film properties. In this work, thin film lithium carbonate (Li₂CO₃) is being deposited by ALD. Li₂CO₃ is widely used as a building block for the fabrication of Li-ion battery active materials, such as lithium cobalt oxide (LiCoO₂), which is used for most lithium-ion battery cathodes [1]. It has also been reported that Li₂CO₃, without combining it with other materials, has a pure ion conductive behavior and a good electrochemical stability [2]. The high electrochemical stability of Li₂CO₃ makes it useful as an artificial solid-electrolyte interface (SEI) or buffer layer [3]. The SEI and buffer layer need to be conformal and very thin, for which the use of ALD would be beneficial. In addition, ultra-thin Li₂CO₃ can also improve the stability of known solid electrolyte layers, such as lithium lanthanum titanate or lithium phosphate. More specifically, we focus on the optimization of the plasma-assisted Li₂CO₃ ALD process by (in situ) growth studies and we investigate the influence of the process parameters on the electrical and electrochemical properties. Using plasmas in combination with ALD opens up the possibility to tune the properties of the layer by varying the plasma exposure time and also to deposit at lower temperatures. Although the plasma-assisted ALD process for Li₂CO₃ has been reported in literature before [4], to our knowledge, the optimization of the process was not reported, and also an electrochemical analysis of Li₂CO₃ layers fabricated with this method is still lacking. The ALD process employed, consists of a combination of LiOtBu as lithium precursor and an O₂ plasma as oxidant source, alternated by Ar purges. A growth rate of ± 0.8 Å/cycle was obtained at a process table temperature of 150 °C on TiN substrates in the linear growth regime (figure). TiN substrates are used as an electrical contact for electrical and electrochemical testing of the layers. The film stoichiometry was investigated with Elastic Recoil Detection (ERD) and X-ray Photoelectron Spectroscopy (XPS). The results are in good agreement with the stoichiometry and density of Li₂CO₃. Excellent electronic insulation properties were confirmed by current-voltage and DC-polarization measurements. This observation is in agreement with impedance measurements at open circuit potential (figure), which showed no apparent electronic conductivity of the film. The dependence of ALD process parameters such as temperature and O₂ plasma exposure time on the electrochemical properties is investigated. Moreover, half cells employing Li₂CO₃ with TiO₂, LiMn₂O₄ and Si electrodes were studied for cyclability and rate performance. These and other results will be presented in this contribution. Acknowledgements: This project is financially supported by the Dutch program “A green Deal in Energy Materials” ADEM Innovation Lab. M. E. Donders et al., Journal of The Electrochemical Society, 160 (5), 2013J. Mizusaki et al., Solid State Ionics, 53-56, 1992P. Verma et al., Electrochimica Acta, 55, 2010A. C. Kozen et al., The Journal of Physical Chemistry C, 118, 2014 Figure caption: On the left: growth curve for the plasma-assited ALD process of Li₂CO₃ measured by in-situ spectroscopic ellipsometry on TiN at 150 °C and the corresponding process steps (insert). On the right: Electrochemical impedance spectroscopy measured at room temperature showing the presence of ionic conductivity. Figure 1

Non-Stoichiometric Titanium Dioxide Incorporation Onto Reduced Graphene Oxide By Atomic Layer Deposition for Oxygen Reduction Catalysis

Renewable energy devices such as fuel cells and metal air batteries harness their energy via lethargic oxygen reduction reactions (ORR) whose catalysis is largely relying on expensive and scarce noble metals. To address this issue, in particular, non-precious metal incorporated in carbon nanostructures have been widely explored. In this study, we focus on synthesizing inexpensive and efficient electrode material for ORR catalysis and energy storage using reduced graphene oxide (rGO) incorporated with non-stoichoimetric titanium dioxide by atomic layer deposition (ALD). Although stoichiometric TiO2 has been rarely considered as an effective ORR electrocatalyst, oxygen-deficient TiO2-x recently showed surprisingly competitive ORR activity and excellent durability[1]. The work was performed on a single crystalline TiO2. Motivated by the recent report and the advantages of TiO2 due to its abundance, safety and cost effectiveness, we probed the feasibility of applying this oxygen-deficient TiO2-x to the widely employed oxide nanoparticle-incorporated carbon nanostructures. This is to leverage extremely high surface area and excellent electronic conductivity of carbon-based 3D structures, which is highly advantageous for maximized catalytically active sites per mass. We used electrophoretically deposited rGO as the matrix structure. Graphene oxides prepared by the modified Hummer’s method are first deposited onto a stainless steel by electrophoresis. The resulting structure was deposited by highly dispersed nanoparticles or ultrathin films of TiO2-x via ALD. The degree of non-stoichiometry is controlled by either changes in ALD temperature or a separate post-thermal process. ALD is a newer variant of chemical vapor deposition in which materials are deposited on a surface by a multiple sequence of ‘self-limiting’ chemical reactions. This unique sequential process enables a highly uniform deposition of dots/films and accurate control over not only the size/thickness at the atomic scale but also chemical composition and location of deposition. In this presentation, we will present 1) the feasibility of using TiO2-x incorporated rGO as ORR electrode in terms of catalytic activity and cyclability and 2) the correlation among the ALD process parameters, resulting chemical composition/structure and ORR catalytic activity. This project is funded by the NASA MUREP Institutional Research Opportunity (MIRO) Program (Grant No. NNX15AQ01A). S.G. is supported by NASA Advanced STEM Training and Research (ASTAR) Fellowship. Figure 1

In situ spectroscopic ellipsometry during atomic layer deposition of Pt, Ru and Pd

The preparation of ultra-thin platinum-group metal films, such as Pt, Ru and Pd, by atomic layer deposition (ALD) was monitored in situ using spectroscopic ellipsometry in the photon energy range of 0.75–5 eV. The metals’ dielectric function was parametrized using a ‘flexible’ Kramers–Kronig consistent dielectric function because it was able to provide accurate curve shape control over the optical response of the metals. From this dielectric function, it was possible to extract the film thickness values during the ALD process. The important ALD process parameters, such as the nucleation period and growth per cycle of Pt, Ru and Pd could be determined from the thickness evolution. In addition to process parameters, the film resistivity in particular could be extracted from the modeled dielectric function. Spectroscopic ellipsometry thereby revealed itself as a feasible and valuable technique to be used in research and development applications, as well as for process monitoring during ALD.

Atomic Layer Deposition of Environmentally Benign Tin-Titanate for Ferroelectric Application

Perovskite-based oxides that possess ferroelectric properties, such as BaTiO3 and PbTiO3, have long been a research interest because of their wide applications in devices such as actuators and sensors.1 Due to the toxicity of lead, emphasis has been put on the replacement of Pb2+ by isoelectronic, environment friendly Sn2+, in hope of maintaining similar ferroelectric performance.Theoretical studies using first principle calculations have predicted that perovskite SnTiO3 (P-SNO) would exhibit excellent ferroelectric properties,2-6 yet the actual synthesis of P-SNO by conventional methods remains unsuccessful due to the less stable Sn2+ at high temperatures.2,3 No success was achieved in depositing P-SNO thin films, either. Several studies suggest that P-SNO is metastable,2,7,8 which leads to the proposition of alternative synthetic routes that can stabilize the metastable structure,9 such as atomic layer deposition (ALD)3/chemical vapor deposition (CVD), sol-gel, etc.2 Different from the predicted perovskite structure, nonpolar, ilmentile-type thin film of SnTiO3 has been achieved on sapphire and perovskite substrates using pulsed laser deposition (PLD) techniques from ceramic SnO2 and TiO2 targets.10 This result further motivates a more thorough investigation as to whether or not thin film deposition techniques could stabilize the metastable P-SNO structure which is crucial for high ferroelectric performance.In this work, we report the result of atomic layer deposition of tin-titanate using tin(II)acetylacetonate (Sn(acac)2) and tetrakis(diethylamino)titanium (TDEAT). The ALD reaction is carried out on Si substrates using a custom-built ALD reactor (Fig. 1).11 ALD process parameters were optimized for SnOx and TiO2 deposition. Linear growth behavior was observed for both the films, indicating self-limited ALD growth. The ALD temperature windows of both ALD processes were found to overlap, which facilitates co-deposition of SnOx and TiO2 for tin-titanate deposition. The resulting film is subsequently analyzed by several characterization techniques to determine its stoichiometry, composition, impurities and structural and surface characteristics. The results of this work provide more insights into achieving lead-free, perovskite SnTiO3 structure to facilitate future research on this subject.