Publications by authors named "Wilhelmus M M Kessels"

27 Publications

  • Page 1 of 1

Electrochemical Activation of Atomic Layer-Deposited Cobalt Phosphate Electrocatalysts for Water Oxidation.

ACS Catal 2021 Mar 15;11(5):2774-2785. Epub 2021 Feb 15.

Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.

The development of efficient and stable earth-abundant water oxidation catalysts is vital for economically feasible water-splitting systems. Cobalt phosphate (CoPi)-based catalysts belong to the relevant class of nonprecious electrocatalysts studied for the oxygen evolution reaction (OER). In this work, an in-depth investigation of the electrochemical activation of CoPi-based electrocatalysts by cyclic voltammetry (CV) is presented. Atomic layer deposition (ALD) is adopted because it enables the synthesis of CoPi films with cobalt-to-phosphorous ratios between 1.4 and 1.9. It is shown that the pristine chemical composition of the CoPi film strongly influences its OER activity in the early stages of the activation process as well as after prolonged exposure to the electrolyte. The best performing CoPi catalyst, displaying a current density of 3.9 mA cm at 1.8 V versus reversible hydrogen electrode and a Tafel slope of 155 mV/dec at pH 8.0, is selected for an in-depth study of the evolution of its electrochemical properties, chemical composition, and electrochemical active surface area (ECSA) during the activation process. Upon the increase of the number of CV cycles, the OER performance increases, in parallel with the development of a noncatalytic wave in the CV scan, which points out to the reversible oxidation of Co species to Co species. X-ray photoelectron spectroscopy and Rutherford backscattering measurements indicate that phosphorous progressively leaches out the CoPi film bulk upon prolonged exposure to the electrolyte. In parallel, the ECSA of the films increases by up to a factor of 40, depending on the initial stoichiometry. The ECSA of the activated CoPi films shows a universal linear correlation with the OER activity for the whole range of CoPi chemical composition. It can be concluded that the adoption of ALD in CoPi-based electrocatalysis enables, next to the well-established control over film growth and properties, to disclose the mechanisms behind the CoPi electrocatalyst activation.
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http://dx.doi.org/10.1021/acscatal.0c04933DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8025676PMC
March 2021

Reaction Mechanisms during Atomic Layer Deposition of AlF Using Al(CH) and SF Plasma.

J Phys Chem C Nanomater Interfaces 2021 Feb 10;125(7):3913-3923. Epub 2021 Feb 10.

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Metal fluorides generally demonstrate a wide band gap and a low refractive index, and they are commonly employed in optics and optoelectronics. Recently, an SF plasma was introduced as a novel co-reactant for the atomic layer deposition (ALD) of metal fluorides. In this work, the reaction mechanisms underlying the ALD of fluorides using a fluorine-containing plasma are investigated, considering aluminum fluoride (AlF) ALD from Al(CH) and an SF plasma as a model system. Surface infrared spectroscopy studies indicated that Al(CH) reacts with the surface in a ligand-exchange reaction by accepting F from the AlF film and forming CH surface groups. It was found that at low deposition temperatures Al(CH) also reacts with HF surface species. These HF species are formed during the SF plasma exposure and were detected both at the surface and in the gas phase using infrared spectroscopy and quadrupole mass spectrometry (QMS), respectively. Furthermore, QMS and optical emission spectroscopy (OES) measurements showed that CH and CH F ( ≤ 3) species are the main reaction products during the SF plasma exposure. The CH release is explained by the reaction of CH ligands with HF, while CH F species originate from the interaction of the SF plasma with CH ligands. At high temperatures, a transition from AlF deposition to AlO etching was observed using infrared spectroscopy. The obtained insights indicate a reaction pathway where F radicals from the SF plasma eliminate the CH ligands remaining after precursor dosing and where F radicals are simultaneously responsible for the fluorination reaction. The understanding of the reaction mechanisms during AlF growth can help in developing ALD processes for other metal fluorides using a fluorine-containing plasma as the co-reactant as well as atomic layer etching (ALE) processes involving surface fluorination.
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http://dx.doi.org/10.1021/acs.jpcc.0c10695DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8016095PMC
February 2021

Conformal Growth of Nanometer-Thick Transition Metal Dichalcogenide TiS -NbS Heterostructures over 3D Substrates by Atomic Layer Deposition: Implications for Device Fabrication.

ACS Appl Nano Mater 2021 Jan 4;4(1):514-521. Epub 2021 Jan 4.

Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, Eindhoven 5600 MB, The Netherlands.

The scalable and conformal synthesis of two-dimensional (2D) transition metal dichalcogenide (TMDC) heterostructures is a persisting challenge for their implementation in next-generation devices. In this work, we report the synthesis of nanometer-thick 2D TMDC heterostructures consisting of TiS -NbS on both planar and 3D structures using atomic layer deposition (ALD) at low temperatures (200-300 °C). To this end, a process was developed for the growth of 2D NbS by thermal ALD using (-butylimido)-tris-(diethylamino)-niobium (TBTDEN) and HS gas. This process complemented the TiS thermal ALD process for the growth of 2D TiS -NbS heterostructures. Precise thickness control of the individual TMDC material layers was demonstrated by fabricating multilayer (5-layer) TiS -NbS heterostructures with independently varied layer thicknesses. The heterostructures were successfully deposited on large-area planar substrates as well as over a 3D nanowire array for demonstrating the scalability and conformality of the heterostructure growth process. The current study demonstrates the advantages of ALD for the scalable synthesis of 2D heterostructures conformally over a 3D substrate with precise thickness control of the individual material layers at low temperatures. This makes the application of 2D TMDC heterostructures for nanoelectronics promising in both BEOL and FEOL containing high-aspect-ratio 3D structures.
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http://dx.doi.org/10.1021/acsanm.0c02820DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7885689PMC
January 2021

Nanoscale Encapsulation of Perovskite Nanocrystal Luminescent Films via Plasma-Enhanced SiO Atomic Layer Deposition.

ACS Appl Mater Interfaces 2020 Nov 11;12(47):53519-53527. Epub 2020 Nov 11.

State Key Laboratory of Digital of Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, P. R. China.

Photoluminescence perovskite nanocrystals (NCs) have shown significant potential in optoelectronic applications in view of their narrow band emission with high photoluminescence quantum yields and color tunability. The main obstacle for practical applications is to obtain high durability against an external environment. In this work, a low temperature (50 °C) plasma-enhanced atomic layer deposition (PE-ALD) protection strategy was developed to stabilize CsPbBr NCs. Silica was employed as the encapsulation layer because of its excellent light transmission performance and water corrosion resistance. The growth mechanism of inorganic SiO via PE-ALD was investigated in detail. The Si precursor bis(diethylamino)silane (BDEAS) reacted with the hydroxyl groups (-OH) and thereby initiated the subsequent silica growth while having minimal influence to the organic ligands and did not cause PL quenching. Subsequently, O plasma with high reactivity was used to oxidize the amine ligands of the BDEAS precursor while did not etch the NCs. The obtained CsPbBr NCs/SiO film exhibited exceptional stability in water, light, and heat as compared to the pristine NC film. Based on this method, a white light-emitting diode with improved operational stability was successfully fabricated, which exhibited a wide color gamut (∼126% of the National Television Standard Committee). Our work successfully demonstrates an efficient protection scheme via the PE-ALD method, which extends the applied range of other materials for stabilization of perovskite NCs through this approach.
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http://dx.doi.org/10.1021/acsami.0c16082DOI Listing
November 2020

Atomic Layer Deposition of Al-Doped MoS: Synthesizing a p-type 2D Semiconductor with Tunable Carrier Density.

ACS Appl Nano Mater 2020 Oct 23;3(10):10200-10208. Epub 2020 Sep 23.

Applied Physics, Eindhoven University of Technology, 5600MB Eindhoven, The Netherlands.

Extrinsically doped two-dimensional (2D) semiconductors are essential for the fabrication of high-performance nanoelectronics among many other applications. Herein, we present a facile synthesis method for Al-doped MoS via plasma-enhanced atomic layer deposition (ALD), resulting in a particularly sought-after -type 2D material. Precise and accurate control over the carrier concentration was achieved over a wide range (10 up to 10 cm) while retaining good crystallinity, mobility, and stoichiometry. This ALD-based approach also affords excellent control over the doping profile, as demonstrated by a combined transmission electron microscopy and energy-dispersive X-ray spectroscopy study. Sharp transitions in the Al concentration were realized and both doped and undoped materials had the characteristic 2D-layered nature. The fine control over the doping concentration, combined with the conformality and uniformity, and subnanometer thickness control inherent to ALD should ensure compatibility with large-scale fabrication. This makes Al:MoS ALD of interest not only for nanoelectronics but also for photovoltaics and transition-metal dichalcogenide-based catalysts.
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http://dx.doi.org/10.1021/acsanm.0c02167DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7590523PMC
October 2020

Area-Selective Atomic Layer Deposition of Two-Dimensional WS Nanolayers.

ACS Mater Lett 2020 May 8;2(5):511-518. Epub 2020 Apr 8.

Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.

With downscaling of device dimensions, two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) such as WS are being considered as promising materials for future applications in nanoelectronics. However, at these nanoscale regimes, incorporating TMD layers in the device architecture with precise control of critical features is challenging using current top-down processing techniques. In this contribution, we pioneer the combination of two key avenues in atomic-scale processing: area-selective atomic layer deposition (AS-ALD) and growth of 2D materials, and demonstrate bottom-up processing of 2D WS nanolayers. Area-selective deposition of WS nanolayers is enabled using an ABC-type plasma-enhanced ALD process involving acetylacetone (Hacac) as inhibitor (A), bis(-butylimido)-bis(dimethylamido)-tungsten as precursor (B), and HS plasma as the co-reactant (C) at a low deposition temperature of 250 °C. The developed AS-ALD process results in the immediate growth of WS on SiO while effectively blocking growth on AlO as confirmed by in situ spectroscopic ellipsometry and ex situ X-ray photoelectron spectroscopy measurements. As a proof of concept, the AS-ALD process is demonstrated on patterned AlO/SiO surfaces. The AS-ALD WS films exhibited sharp Raman ( and ) peaks on SiO, a fingerprint of crystalline WS layers, upon annealing at temperatures within the thermal budget of semiconductor back-end-of-line processing (≤450 °C). Our AS-ALD process also allows selective growth on various TMDs and transition metal oxides while blocking growth on HfO and TiO. It is expected that this work will lay the foundation for area-selective ALD of other 2D materials.
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http://dx.doi.org/10.1021/acsmaterialslett.0c00093DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7217612PMC
May 2020

Probing the Origin and Suppression of Vertically Oriented Nanostructures of 2D WS Layers.

ACS Appl Mater Interfaces 2020 Jan 9;12(3):3873-3885. Epub 2020 Jan 9.

Department of Applied Physics , Eindhoven University of Technology , 5600 MB Eindhoven , The Netherlands.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) such as WS are promising materials for nanoelectronic applications. However, growth of the desired horizontal basal-plane oriented 2D TMD layers is often accompanied by the growth of vertical nanostructures that can hinder charge transport and, consequently, hamper device application. In this work, we discuss both the formation and suppression of vertical nanostructures during plasma-enhanced atomic layer deposition (PEALD) of WS. Using scanning transmission electron microscopy studies, formation pathways of vertical nanostructures are established for a two-step (AB-type) PEALD process. Grain boundaries are identified as the principal formation centers of vertical nanostructures. Based on the obtained insights, we introduce an approach to suppress the growth of vertical nanostructures, wherein an additional step (C)-a chemically inert Ar plasma or a reactive H plasma-is added to the original two-step (AB-type) PEALD process. This approach reduces the vertical nanostructure density by 80%. It was confirmed that suppression of vertical nanostructures goes hand in hand with grain size enhancement. The vertical nanostructure density reduction consequently lowers film resistivity by an order of magnitude. Insights obtained in this work can contribute toward devising additional pathways, besides plasma treatments, for suppressing the growth of vertical nanostructures and improving the material properties of 2D TMDs that are relevant for nanoelectronic device applications.
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http://dx.doi.org/10.1021/acsami.9b19716DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6978813PMC
January 2020

Low-Temperature Phase-Controlled Synthesis of Titanium Di- and Tri-sulfide by Atomic Layer Deposition.

Chem Mater 2019 Nov 28;31(22):9354-9362. Epub 2019 Oct 28.

Department of Applied Physics and Laboratory for Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Phase-controlled synthesis of two-dimensional (2D) transition-metal chalcogenides (TMCs) at low temperatures with a precise thickness control has to date been rarely reported. Here, we report on a process for the phase-controlled synthesis of TiS (metallic) and TiS (semiconducting) nanolayers by atomic layer deposition (ALD) with precise thickness control. The phase control has been obtained by carefully tuning the deposition temperature and coreactant composition during ALD. In all cases, characteristic self-limiting ALD growth behavior with a growth per cycle (GPC) of ∼0.16 nm per cycle was observed. TiS was prepared at 100 °C using HS gas as coreactant and was also observed using HS plasma as a coreactant at growth temperatures between 150 and 200 °C. TiS was synthesized only at 100 °C using HS plasma as the coreactant. The S species in the HS plasma, as observed by optical emission spectroscopy, has been speculated to lead to the formation of the TiS phase at low temperatures. The control between the synthesis of TiS and TiS was elucidated by Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution electron microscopy, and Rutherford backscattering study. Electrical transport measurements showed the low resistive nature of ALD grown 2D-TiS (1T-phase). Postdeposition annealing of the TiS layers at 400 °C in a sulfur-rich atmosphere improved the crystallinity of the film and yielded photoluminescence at ∼0.9 eV, indicating the semiconducting (direct band gap) nature of TiS. The current study opens up a new ALD-based synthesis route for controlled, scalable growth of transition-metal di- and tri-chalcogenides at low temperatures.
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http://dx.doi.org/10.1021/acs.chemmater.9b02895DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6883357PMC
November 2019

Edge-Site Nanoengineering of WS by Low-Temperature Plasma-Enhanced Atomic Layer Deposition for Electrocatalytic Hydrogen Evolution.

Chem Mater 2019 Jul 25;31(14):5104-5115. Epub 2019 Jun 25.

Department of Applied Physics and Laboratory for Inorganic Materials and Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands.

Edge-enriched transition metal dichalcogenides, such as WS, are promising electrocatalysts for sustainable production of H through the electrochemical hydrogen evolution reaction (HER). The reliable and controlled growth of such edge-enriched electrocatalysts at low temperatures has, however, remained elusive. In this work, we demonstrate how plasma-enhanced atomic layer deposition (PEALD) can be used as a new approach to nanoengineer and enhance the HER performance of WS by maximizing the density of reactive edge sites at a low temperature of 300 °C. By altering the plasma gas composition from HS to H + HS during PEALD, we could precisely control the morphology and composition and, consequently, the edge-site density as well as chemistry in our WS films. The precise control over edge-site density was verified by evaluating the number of exposed edge sites using electrochemical copper underpotential depositions. Subsequently, we demonstrate the HER performance of the edge-enriched WS electrocatalyst, and a clear correlation among plasma conditions, edge-site density, and the HER performance is obtained. Additionally, using density functional theory calculations we provide insights and explain how the addition of H to the HS plasma impacts the PEALD growth behavior and, consequently, the material properties, when compared to only HS plasma.
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http://dx.doi.org/10.1021/acs.chemmater.9b01008DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6662884PMC
July 2019

On the role of micro-porosity in affecting the environmental stability of atomic/molecular layer deposited (ZnO)(Zn-O-CH-O) films.

Dalton Trans 2019 Mar 25;48(10):3496-3505. Epub 2019 Feb 25.

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Atomic/molecular layer deposited (ALD/MLD) inorganic-organic thin films form a novel class of materials with tunable properties. In selected cases, hybrid materials are reported to show environmental instability, specifically towards moisture. In this article, we focus on zinc oxide/zincone multi-layers with the theoretical formula of (ZnO)(Zn-O-CH-O). We show by means of ellipsometric porosimetry that micro-porosity in the range of 0.42 and 2 nm in the pristine zincone layer is responsible for its environmental degradation. During degradation, it is found that a relative micro-porosity content of 1.2 ± 0.1 vol% in the pristine zincone films evolves into micro-mesoporosity with a relative content of 39 ± 1 vol%. We also show that the micro-porosity in the zincone layer can be gradually suppressed when few cycles (a = 1-10) of ZnO are introduced. The resulting (ZnO)(Zn-O-CH-O) periodic multilayer is an environmentally stable film with a = 10. It is found that the suppressed micro-porosity is due to the development of continuous ZnO layers with a≥ 10.
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http://dx.doi.org/10.1039/c9dt00189aDOI Listing
March 2019

From the Bottom-Up: Toward Area-Selective Atomic Layer Deposition with High Selectivity.

Chem Mater 2019 Jan 19;31(1):2-12. Epub 2018 Dec 19.

Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands.

Bottom-up nanofabrication by area-selective atomic layer deposition (ALD) is currently gaining momentum in semiconductor processing, because of the increasing need for eliminating the edge placement errors of top-down processing. Moreover, area-selective ALD offers new opportunities in many other areas such as the synthesis of catalysts with atomic-level control. This Perspective provides an overview of the current developments in the field of area-selective ALD, discusses the challenge of achieving a high selectivity, and provides a vision for how area-selective ALD processes can be improved. A general cause for the loss of selectivity during deposition is that the character of surfaces on which no deposition should take place changes when it is exposed to the ALD chemistry. A solution is to implement correction steps during ALD involving for example surface functionalization or selective etching. This leads to the development of advanced ALD cycles by combining conventional two-step ALD cycles with correction steps in multistep cycle and/or supercycle recipes.
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http://dx.doi.org/10.1021/acs.chemmater.8b03454DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6369656PMC
January 2019

Chemical Analysis of the Interface between Hybrid Organic-Inorganic Perovskite and Atomic Layer Deposited AlO.

ACS Appl Mater Interfaces 2019 Feb 24;11(5):5526-5535. Epub 2019 Jan 24.

Department of Applied Physics , Eindhoven University of Technology (TU/e) , P.O. Box 513, 5600 MB Eindhoven , The Netherlands.

Ultrathin metal oxides prepared by atomic layer deposition (ALD) have gained utmost attention as moisture and thermal stress barrier layers in perovskite solar cells (PSCs). We have recently shown that 10 cycles of ALD AlO deposited directly on top of the CHNHPbICl perovskite material, are effective in delivering a superior PSC performance with 18% efficiency (compared to 15% of the AlO-free cell) with a long-term humidity-stability of more than 60 days. Motivated by these results, the present contribution focuses on the chemical modification which the CHNHPbICl perovskite undergoes upon growth of ALD AlO. Specifically, we combine in situ Infrared (IR) spectroscopy studies during film growth, together with X-ray photoelectron spectroscopy (XPS) analysis of the ALD AlO/perovskite interface. The IR-active signature of the NH stretching mode of the perovskite undergoes minimal changes upon exposure to ALD cycles, suggesting no diffusion of ALD precursor and co-reactant (Al(CH) and HO) into the bulk of the perovskite. However, by analyzing the difference between the IR spectra associated with the AlO coated perovskite and the pristine perovskite, respectively, changes occurring at the surface of perovskite are monitored. The abstraction of either NH or CHNH from the perovskite surface is observed as deduced by the development of negative N-H bands associated with its stretching and bending modes. The IR investigations are corroborated by XPS study, confirming the abstraction of CHNH from the perovskite surface, whereas no oxidation of its inorganic framework is observed within the ALD window process investigated in this work. In parallel, the growth of ALD AlO on perovskite is witnessed by the appearance of characteristic IR-active Al-O-Al phonon and (OH)-Al═O stretching modes. Based on the IR and XPS investigations, a plausible growth mechanism of ALD AlO on top of perovskite is presented.
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http://dx.doi.org/10.1021/acsami.8b18307DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6369720PMC
February 2019

Low-Temperature Plasma-Assisted Atomic-Layer-Deposited SnO as an Electron Transport Layer in Planar Perovskite Solar Cells.

ACS Appl Mater Interfaces 2018 Sep 28;10(36):30367-30378. Epub 2018 Aug 28.

Department of Applied Physics , Eindhoven University of Technology (TU/e) , 5600 MB Eindhoven , The Netherlands.

In this work, we present an extensive characterization of plasma-assisted atomic-layer-deposited SnO layers, with the aim of identifying key material properties of SnO to serve as an efficient electron transport layer in perovskite solar cells (PSCs). Electrically resistive SnO films are fabricated at 50 °C, while a SnO film with a low electrical resistivity of 1.8 × 10 Ω cm, a carrier density of 9.6 × 10 cm, and a high mobility of 36.0 cm/V s is deposited at 200 °C. Ultraviolet photoelectron spectroscopy indicates a conduction band offset of ∼0.69 eV at the 50 °C SnO/Cs(MAFA)Pb(IBr) interface. In contrast, a negligible conduction band offset is found between the 200 °C SnO and the perovskite. Surprisingly, comparable initial power conversion efficiencies (PCEs) of 17.5 and 17.8% are demonstrated for the champion cells using 15 nm thick SnO deposited at 50 and 200 °C, respectively. The latter gains in fill factor but loses in open-circuit voltage. Markedly, PSCs using the 200 °C compact SnO retain their initial performance at the maximum power point over 16 h under continuous one-sun illumination in inert atmosphere. Instead, the cell with the 50 °C SnO shows a decrease in PCE of approximately 50%.
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http://dx.doi.org/10.1021/acsami.8b09515DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6137428PMC
September 2018

Tuning Material Properties of Oxides and Nitrides by Substrate Biasing during Plasma-Enhanced Atomic Layer Deposition on Planar and 3D Substrate Topographies.

ACS Appl Mater Interfaces 2018 Apr 9;10(15):13158-13180. Epub 2018 Apr 9.

Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven , The Netherlands.

Oxide and nitride thin-films of Ti, Hf, and Si serve numerous applications owing to the diverse range of their material properties. It is therefore imperative to have proper control over these properties during materials processing. Ion-surface interactions during plasma processing techniques can influence the properties of a growing film. In this work, we investigated the effects of controlling ion characteristics (energy, dose) on the properties of the aforementioned materials during plasma-enhanced atomic layer deposition (PEALD) on planar and 3D substrate topographies. We used a 200 mm remote PEALD system equipped with substrate biasing to control the energy and dose of ions by varying the magnitude and duration of the applied bias, respectively, during plasma exposure. Implementing substrate biasing in these forms enhanced PEALD process capability by providing two additional parameters for tuning a wide range of material properties. Below the regimes of ion-induced degradation, enhancing ion energies with substrate biasing during PEALD increased the refractive index and mass density of TiO and HfO and enabled control over their crystalline properties. PEALD of these oxides with substrate biasing at 150 °C led to the formation of crystalline material at the low temperature, which would otherwise yield amorphous films for deposition without biasing. Enhanced ion energies drastically reduced the resistivity of conductive TiN and HfN films. Furthermore, biasing during PEALD enabled the residual stress of these materials to be altered from tensile to compressive. The properties of SiO were slightly improved whereas those of SiN were degraded as a function of substrate biasing. PEALD on 3D trench nanostructures with biasing induced differing film properties at different regions of the 3D substrate. On the basis of the results presented herein, prospects afforded by the implementation of this technique during PEALD, such as enabling new routes for topographically selective deposition on 3D substrates, are discussed.
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http://dx.doi.org/10.1021/acsami.8b00183DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5909180PMC
April 2018

Dopant Distribution in Atomic Layer Deposited ZnO:Al Films Visualized by Transmission Electron Microscopy and Atom Probe Tomography.

Chem Mater 2018 Feb 5;30(4):1209-1217. Epub 2018 Feb 5.

Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

The maximum conductivity achievable in Al-doped ZnO thin films prepared by atomic layer deposition (ALD) is limited by the low doping efficiency of Al. To better understand the limiting factors for the doping efficiency, the three-dimensional distribution of Al atoms in the ZnO host material matrix has been examined on the atomic scale using a combination of high-resolution transmission electron microscopy (TEM) and atom probe tomography (APT). Although the Al distribution in ZnO films prepared by so-called "ALD supercycles" is often presented as atomically flat δ-doped layers, in reality a broadening of the Al-dopant layers is observed with a full-width-half-maximum of ∼2 nm. In addition, an enrichment of the Al at grain boundaries is observed. The low doping efficiency for local Al densities > ∼1 nm can be ascribed to the Al solubility limit in ZnO and to the suppression of the ionization of Al dopants from adjacent Al donors.
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http://dx.doi.org/10.1021/acs.chemmater.7b03501DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5833938PMC
February 2018

Area-Selective Atomic Layer Deposition of Metal Oxides on Noble Metals through Catalytic Oxygen Activation.

Chem Mater 2018 Feb 1;30(3):663-670. Epub 2017 Dec 1.

Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands.

Area-selective atomic layer deposition (ALD) is envisioned to play a key role in next-generation semiconductor processing and can also provide new opportunities in the field of catalysis. In this work, we developed an approach for the area-selective deposition of metal oxides on noble metals. Using O gas as co-reactant, area-selective ALD has been achieved by relying on the catalytic dissociation of the oxygen molecules on the noble metal surface, while no deposition takes place on inert surfaces that do not dissociate oxygen (i.e., SiO, AlO, Au). The process is demonstrated for selective deposition of iron oxide and nickel oxide on platinum and iridium substrates. Characterization by spectroscopic ellipsometry, transmission electron microscopy, scanning Auger electron spectroscopy, and X-ray photoelectron spectroscopy confirms a very high degree of selectivity, with a constant ALD growth rate on the catalytic metal substrates and no deposition on inert substrates, even after 300 ALD cycles. We demonstrate the area-selective ALD approach on planar and patterned substrates and use it to prepare Pt/FeO core/shell nanoparticles. Finally, the approach is proposed to be extendable beyond the materials presented here, specifically to other metal oxide ALD processes for which the precursor requires a strong oxidizing agent for growth.
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http://dx.doi.org/10.1021/acs.chemmater.7b03818DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5828705PMC
February 2018

Simultaneous scanning tunneling microscopy and synchrotron X-ray measurements in a gas environment.

Ultramicroscopy 2017 11 14;182:233-242. Epub 2017 Jul 14.

Huygens-Kamerlingh Onnes Laboratory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands; Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands.

A combined X-ray and scanning tunneling microscopy (STM) instrument is presented that enables the local detection of X-ray absorption on surfaces in a gas environment. To suppress the collection of ion currents generated in the gas phase, coaxially shielded STM tips were used. The conductive outer shield of the coaxial tips can be biased to deflect ions away from the tip core. When tunneling, the X-ray-induced current is separated from the regular, 'topographic' tunneling current using a novel high-speed separation scheme. We demonstrate the capabilities of the instrument by measuring the local X-ray-induced current on Au(1 1 1) in 800 mbar Ar.
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http://dx.doi.org/10.1016/j.ultramic.2017.07.011DOI Listing
November 2017

Area-Selective Atomic Layer Deposition of InO:H Using a μ-Plasma Printer for Local Area Activation.

Chem Mater 2017 Feb 23;29(3):921-925. Epub 2017 Jan 23.

Department of Applied Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands; Department Thin Film Technology, TNO, High Tech Campus 21, 5656 AE Eindhoven, The Netherlands.

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http://dx.doi.org/10.1021/acs.chemmater.6b04469DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5384477PMC
February 2017

Atomic Layer Deposition of Wet-Etch Resistant Silicon Nitride Using Di(sec-butylamino)silane and N Plasma on Planar and 3D Substrate Topographies.

ACS Appl Mater Interfaces 2017 Jan 6;9(2):1858-1869. Epub 2017 Jan 6.

Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

The advent of three-dimensional (3D) finFET transistors and emergence of novel memory technologies place stringent requirements on the processing of silicon nitride (SiN) films used for a variety of applications in device manufacturing. In many cases, a low temperature (<400 °C) deposition process is desired that yields high quality SiN films that are etch resistant and also conformal when grown on 3D substrate topographies. In this work, we developed a novel plasma-enhanced atomic layer deposition (PEALD) process for SiN using a mono-aminosilane precursor, di(sec-butylamino)silane (DSBAS, SiHN(Bu)), and N plasma. Material properties have been analyzed over a wide stage temperature range (100-500 °C) and compared with those obtained in our previous work for SiN deposited using a bis-aminosilane precursor, bis(tert-butylamino)silane (BTBAS, SiH(NHBu)), and N plasma. Dense films (∼3.1 g/cm) with low C, O, and H contents at low substrate temperatures (<400 °C) were obtained on planar substrates for this process when compared to other processes reported in the literature. The developed process was also used for depositing SiN films on high aspect ratio (4.5:1) 3D trench nanostructures to investigate film conformality and wet-etch resistance (in dilute hydrofluoric acid, HF/HO = 1:100) relevant for state-of-the-art device architectures. Film conformality was below the desired levels of >95% and attributed to the combined role played by nitrogen plasma soft saturation, radical species recombination, and ion directionality during SiN deposition on 3D substrates. Yet, very low wet-etch rates (WER ≤ 2 nm/min) were observed at the top, sidewall, and bottom trench regions of the most conformal film deposited at low substrate temperature (<400 °C), which confirmed that the process is applicable for depositing high quality SiN films on both planar and 3D substrate topographies.
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http://dx.doi.org/10.1021/acsami.6b12267DOI Listing
January 2017

Atomic Layer Deposition of InO:H from InCp and HO/O: Microstructure and Isotope Labeling Studies.

ACS Appl Mater Interfaces 2017 Jan 27;9(1):592-601. Epub 2016 Dec 27.

Eindhoven University of Technology , PO Box 513, 5600 MB Eindhoven, The Netherlands.

The atomic layer deposition (ALD) process of hydrogen-doped indium oxide (InO:H) using indium cyclopentadienyl (InCp) and both O and HO as precursors is highly promising for the preparation of transparent conductive oxides. It yields a high growth per cycle (>0.1 nm), is viable at temperatures as low as 100 °C, and provides a record optoelectronic quality after postdeposition crystallization of the films ( ACS Appl. Mat. Interfaces , 2015 , 7 , 16723 - 16729 , DOI: 10.1021/acsami.5b04420 ) . Since both the dopant incorporation and the film microstructure play a key role in determining the optoelectronic properties, both the crystal growth and the incorporation of the hydrogen dopant during this ALD process are studied in this work. This has been done using transmission electron microscopy (TEM) and atom probe tomography (APT) in combination with deuterium isotope labeling. TEM studies show that an amorphous-to-crystalline phase transition occurs in the low-temperature regime (100-150 °C), which is accompanied by a strong decrease in carrier density and an increase in carrier mobility. At higher deposition temperatures (>200 °C), enhanced nucleation of crystals and the incorporation of carbon impurities lead to a reduced grain size and even an amorphous phase, respectively, resulting in a strong reduction in carrier mobility. APT studies on films grown with deuterated water show that the incorporated hydrogen mainly originates from the coreactant and not from the InCp precursor. In addition, it was established that the incorporation of hydrogen decreased from ∼4 atom % for amorphous growth to ∼2 atom % after the transition to crystalline film growth.
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http://dx.doi.org/10.1021/acsami.6b13560DOI Listing
January 2017

Continuous and ultrathin platinum films on graphene using atomic layer deposition: a combined computational and experimental study.

Nanoscale 2016 Dec;8(47):19829-19845

Eindhoven University of Technology, Department of Applied Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Integrating metals and metal oxides with graphene is key in utilizing its extraordinary material properties that are ideal for nanoelectronic and catalyst applications. Atomic layer deposition (ALD) has become a key technique for depositing ultrathin, conformal metal(oxide) films. ALD of metal(oxide) films on graphene, however, remains a genuine challenge due to the chemical inertness of graphene. In this study we address this issue by combining first-principles density functional theory (DFT) simulations with ALD experiments. The focus is on the Pt ALD on graphene, as this hybrid system is very promising for solar and fuel cells, hydrogen technologies, microreactors, and sensors. Here we elucidate the surface reactions underpinning the nucleation stage of Pt ALD on pristine, defective and functionalized graphenes. The employed reaction mechanism clearly depends on (a) the available surface groups on graphene, and (b) the ligands accompanying the metal centre in the precursor. DFT calculations also indicate that graphene oxide (GO) can afford a stronger adsorption of MeCpPtMe, unlike Pt(acac), as compared to bare (non-functionalized) graphene, suggesting that GO monolayers are effective Pt ALD seed layers. Confirming the latter, we evince that wafer-scale, continuous Pt films can indeed be grown on GO monolayers using a thermal ALD process with MeCpPtMe and O gas. Besides, the current in-depth atomistic insights are of practical use for understanding similar ALD processes of other metals and metal oxides on graphene.
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http://dx.doi.org/10.1039/c6nr07483aDOI Listing
December 2016

Dynamic Ellipsometric Porosimetry Investigation of Permeation Pathways in Moisture Barrier Layers on Polymers.

ACS Appl Mater Interfaces 2016 Sep 13;8(38):25005-9. Epub 2016 Sep 13.

Department of Applied Physics, Eindhoven University of Technology , 5600 MB Eindhoven, The Netherlands.

The quality assessment of moisture permeation barrier layers needs to include both water permeation pathways, namely through bulk nanoporosity and local macroscale defects. Ellipsometric porosimetry (EP) has been already demonstrated a valuable tool for the identification of nanoporosity in inorganic thin film barriers, but the intrinsic lack of sensitivity toward the detection of macroscale defects prevents the overall barrier characterization. In this contribution, dynamic EP measurements are reported and shown to be sensitive to the detection of macroscale defects in SiO2 layers on polyethylene naphthalate substrate. In detail, the infiltration of probe molecules, leading to changes in optical properties of the polymeric substrate, is followed in time and related to permeation through macroscale defects.
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http://dx.doi.org/10.1021/acsami.6b08520DOI Listing
September 2016

Role of Surface Termination in Atomic Layer Deposition of Silicon Nitride.

J Phys Chem Lett 2015 Sep 2;6(18):3610-4. Epub 2015 Sep 2.

Department of Applied Physics, Eindhoven University of Technology , Den Dolech 2, 5600 MB Eindhoven, The Netherlands.

There is an urgent need to deposit uniform, high-quality, conformal SiN(x) thin films at a low-temperature. Conforming to these constraints, we recently developed a plasma enhanced atomic layer deposition (ALD) process with bis(tertiary-butyl-amino)silane (BTBAS) as the silicon precursor. However, deposition of high quality SiNx thin films at reasonable growth rates occurs only when N2 plasma is used as the coreactant; strongly reduced growth rates are observed when other coreactants like NH3 plasma, or N2-H2 plasma are used. Experiments reported in this Letter reveal that NH(x)- or H- containing plasmas suppress film deposition by terminating reactive surface sites with H and NH(x) groups and inhibiting precursor adsorption. To understand the role of these surface groups on precursor adsorption, we carried out first-principles calculations of precursor adsorption on the β-Si3N4(0001) surface with different surface terminations. They show that adsorption of the precursor is strong on surfaces with undercoordinated surface sites. In contrast, on surfaces with H, NH2 groups, or both, steric hindrance leads to weak precursor adsorption. Experimental and first-principles results together show that using an N2 plasma to generate reactive undercoordinated surface sites allows strong adsorption of the silicon precursor and, hence, is key to successful deposition of silicon nitride by ALD.
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http://dx.doi.org/10.1021/acs.jpclett.5b01596DOI Listing
September 2015

Atomic layer deposition of Pd and Pt nanoparticles for catalysis: on the mechanisms of nanoparticle formation.

Nanotechnology 2016 Jan 4;27(3):034001. Epub 2015 Dec 4.

Department of Applied Physics, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands.

The deposition of Pd and Pt nanoparticles by atomic layer deposition (ALD) has been studied extensively in recent years for the synthesis of nanoparticles for catalysis. For these applications, it is essential to synthesize nanoparticles with well-defined sizes and a high density on large-surface-area supports. Although the potential of ALD for synthesizing active nanocatalysts for various chemical reactions has been demonstrated, insight into how to control the nanoparticle properties (i.e. size, composition) by choosing suitable processing conditions is lacking. Furthermore, there is little understanding of the reaction mechanisms during the nucleation stage of metal ALD. In this work, nanoparticles synthesized with four different ALD processes (two for Pd and two for Pt) were extensively studied by transmission electron spectroscopy. Using these datasets as a starting point, the growth characteristics and reaction mechanisms of Pd and Pt ALD relevant for the synthesis of nanoparticles are discussed. The results reveal that ALD allows for the preparation of particles with control of the particle size, although it is also shown that the particle size distribution is strongly dependent on the processing conditions. Moreover, this paper discusses the opportunities and limitations of the use of ALD in the synthesis of nanocatalysts.
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http://dx.doi.org/10.1088/0957-4484/27/3/034001DOI Listing
January 2016

Low-Temperature Plasma-Assisted Atomic Layer Deposition of Silicon Nitride Moisture Permeation Barrier Layers.

ACS Appl Mater Interfaces 2015 Oct 30;7(40):22525-32. Epub 2015 Sep 30.

Department of Applied Physics, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Encapsulation of organic (opto-)electronic devices, such as organic light-emitting diodes (OLEDs), photovoltaic cells, and field-effect transistors, is required to minimize device degradation induced by moisture and oxygen ingress. SiNx moisture permeation barriers have been fabricated using a very recently developed low-temperature plasma-assisted atomic layer deposition (ALD) approach, consisting of half-reactions of the substrate with the precursor SiH2(NH(t)Bu)2 and with N2-fed plasma. The deposited films have been characterized in terms of their refractive index and chemical composition by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). The SiNx thin-film refractive index ranges from 1.80 to 1.90 for films deposited at 80 °C up to 200 °C, respectively, and the C, O, and H impurity levels decrease when the deposition temperature increases. The relative open porosity content of the layers has been studied by means of multisolvent ellipsometric porosimetry (EP), adopting three solvents with different kinetic diameters: water (∼0.3 nm), ethanol (∼0.4 nm), and toluene (∼0.6 nm). Irrespective of the deposition temperature, and hence the impurity content in the SiNx films, no uptake of any adsorptive has been observed, pointing to the absence of open pores larger than 0.3 nm in diameter. Instead, multilayer development has been observed, leading to type II isotherms that, according to the IUPAC classification, are characteristic of nonporous layers. The calcium test has been performed in a climate chamber at 20 °C and 50% relative humidity to determine the intrinsic water vapor transmission rate (WVTR) of SiNx barriers deposited at 120 °C. Intrinsic WVTR values in the range of 10(-6) g/m2/day indicate excellent barrier properties for ALD SiNx layers as thin as 10 nm, competing with that of state-of-the-art plasma-enhanced chemical vapor-deposited SiNx layers of a few hundred nanometers in thickness.
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http://dx.doi.org/10.1021/acsami.5b06801DOI Listing
October 2015

Atomic Layer Deposition of Silicon Nitride from Bis(tert-butylamino)silane and N2 Plasma.

ACS Appl Mater Interfaces 2015 Sep 28;7(35):19857-62. Epub 2015 Aug 28.

Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, Netherlands.

Atomic layer deposition (ALD) of silicon nitride (SiNx) is deemed essential for a variety of applications in nanoelectronics, such as gate spacer layers in transistors. In this work an ALD process using bis(tert-butylamino)silane (BTBAS) and N2 plasma was developed and studied. The process exhibited a wide temperature window starting from room temperature up to 500 °C. The material properties and wet-etch rates were investigated as a function of plasma exposure time, plasma pressure, and substrate table temperature. Table temperatures of 300-500 °C yielded a high material quality and a composition close to Si3N4 was obtained at 500 °C (N/Si=1.4±0.1, mass density=2.9±0.1 g/cm3, refractive index=1.96±0.03). Low wet-etch rates of ∼1 nm/min were obtained for films deposited at table temperatures of 400 °C and higher, similar to that achieved in the literature using low-pressure chemical vapor deposition of SiNx at >700 °C. For novel applications requiring significantly lower temperatures, the temperature window from room temperature to 200 °C can be a solution, where relatively high material quality was obtained when operating at low plasma pressures or long plasma exposure times.
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http://dx.doi.org/10.1021/acsami.5b06833DOI Listing
September 2015

Electron Scattering and Doping Mechanisms in Solid-Phase-Crystallized In2O3:H Prepared by Atomic Layer Deposition.

ACS Appl Mater Interfaces 2015 Aug 23;7(30):16723-9. Epub 2015 Jul 23.

†Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Hydrogen-doped indium oxide (In2O3:H) has recently emerged as an enabling transparent conductive oxide for solar cells, in particular for silicon heterojunction solar cells because its high electron mobility (>100 cm(2)/(V s)) allows for a simultaneously high electrical conductivity and optical transparency. Here, we report on high-quality In2O3:H prepared by a low-temperature atomic layer deposition (ALD) process and present insights into the doping mechanism and the electron scattering processes that limit the carrier mobility in such films. The process consists of ALD of amorphous In2O3:H at 100 °C and subsequent solid-phase crystallization at 150-200 °C to obtain large-grained polycrystalline In2O3:H films. The changes in optoelectronic properties upon crystallization have been monitored both electrically by Hall measurements and optically by analysis of the Drude response. After crystallization, an excellent carrier mobility of 128 ± 4 cm(2)/(V s) can be obtained at a carrier density of 1.8 × 10(20) cm(-3), irrespective of the annealing temperature. Temperature-dependent Hall measurements have revealed that electron scattering is dominated by unavoidable phonon and ionized impurity scattering from singly charged H-donors. Extrinsic defect scattering related to material quality such as grain boundary and neutral impurity scattering was found to be negligible in crystallized films indicating that the carrier mobility is maximized. Furthermore, by comparison of the absolute H-concentration and the carrier density in crystallized films, it is deduced that <4% of the incorporated H is an active dopant in crystallized films. Therefore, it can be concluded that inactive H atoms do not (significantly) contribute to defect scattering, which potentially explains why In2O3:H films are capable of achieving a much higher carrier mobility than conventional In2O3:Sn (ITO).
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http://dx.doi.org/10.1021/acsami.5b04420DOI Listing
August 2015