ALD THESES

Feasibility of atomic-layer-deposited Al2O3 /SrTiO3 hetero-oxide interfaces on didoe and transistor devices
Author
Taehwan Moon
University
Seoul National University (Seoul, Korea)
Year
2019
2019
Expanding the toolbox of atomic scale processing
Author
Tahsin Faraz
University
Eindhoven University of Technology (Eindhoven, Netherlands)
Year
2019
Abstract

As we enter an era of atomic scale device dimensions, it has become imperative to  utilize deposition and etching techniques that allow for processing materials at the  atomic level. Furthermore, next-generation devices consist of various material layers  across both planar and three-dimensional (3D) layouts which has led to an additional  need for processing materials in a selective manner. As a result, it is now vitally  important to retain proper control over the thickness and properties of materials grown  or removed during fabrication of nanoscale devices with 3D geometries. Plasmaenhanced atomic layer deposition (ALD) has obtained a prominent position in  synthesizing ultra-thin films of functional materials with atomic scale precision. Uniform  and conformal film deposition even on challenging 3D substrate topographies can be  attained by virtue of the sequential and self-limiting precursor and plasma exposure  steps of plasma ALD. Highly reactive plasma radicals are generated during the plasma  step and the contribution of these electrically neutral species toward film growth is a  well-known feature of plasma ALD. However, the ions generated during plasma  exposure can also play a significant role in the deposition process which has been  relatively less explored. Furthermore, the challenges related to current plasma based  dry-etching processes provide a window of opportunity for being potentially tackled by  the etch counterpart of ALD, i.e., atomic layer etching (ALE). This dissertation  investigates plasma-enhanced atomic scale processing of functional materials and the  role of ions during these processes on planar and 3D substrate topographies, relevant  for next-generation device technologies.  In the first part of this work, a new ALD process for SiNx was developed using a  novel organosilane precursor (DSBAS) and N2 plasma. Dense and wet-etch resistant SiNx  films that can be synthesized at low temperatures serve as spacers or encapsulation  layers for protecting sensitive device components; e.g., gate stacks in 3D transistors or  magnetic tunnel junctions in emerging magnetoresistive memories. SiNx films with a  high density and low impurity content were obtained at low substrate temperatures on  planar substrates using the process developed in this work. Deposition were also  performed on high aspect ratio 3D trench nanostructures to investigate SiNx film  conformality and wet-etch resistance. Sources limiting conformality on 3D substrates  were attributed to factors occurring in the N2 plasma step. Identification of factors  associated with plasma processing conditions is a prerequisite for addressing the  challenge of growing conformal SiNx on 3D substrates. Yet, very low wet-etch rates  were observed at different regions throughout the trenches, confirming high quality SiNx could be grown at low substrate temperature on 3D substrates using the developed  process.  Next, the effects substrate biasing during plasma ALD on the properties of  materials (oxides and nitrides of Ti, Hf, and Si) grown on planar and 3D substrate  topographies were investigated. A commercial 200-mm remote plasma ALD system  equipped with RF substrate biasing was used to control the ion energy during the  plasma exposure step. This technique was demonstrated to significantly enhance the  versatility of plasma ALD processes by providing additional knobs for controlling a wide  range of material properties, appropriate for numerous applications. Substrate biasing  during plasma ALD increased the refractive index and mass density of TiOx and HfOx and  enabled control over their crystalline properties. Plasma ALD of these oxides with  substrate biasing formed crystalline films at a low temperature which would otherwise  yield amorphous films without biasing. Substrate biasing drastically reduced the  resistivity of conductive TiNx and HfNx films. Furthermore, biasing enabled the residual  stress of these materials to be altered from tensile to compressive. The properties of  SiOx were slightly improved whereas those of SiNx were degraded as a function of  substrate biasing. Plasma ALD on 3D trench nanostructures with biasing induced  differing film properties at different regions of the 3D substrate which demonstrated  the potential of this technique in enabling new approaches for topographically selective  deposition.  Ion energy characteristics on grounded and biased substrates during plasma  exposure were also measured to investigate their role in tailoring material properties.  Insights from such measurements are essential toward understanding how a given  plasma ALD process at different operating conditions can be influenced by energetic  ions. Ion flux-energy distribution functions (IFEDFs) were measured using a retarding  field energy analyzer for reactive plasmas typically used in plasma ALD (O2, H2, N2)  without and with RF biasing. The properties of materials (TiOx, HfNx, SiNx) grown using  these plasmas were analyzed as a function of the ion energy and flux parameters  derived from IFEDFs. These results have provided more insight on the relation between  energetic ions and the ensuing material properties, e.g., by providing energy maps of  material properties in terms of the ion energy dose during plasma ALD. They  demonstrate how the measurement and control of ion energy characteristics during  plasma ALD provide a platform for synthesizing ultra-thin films with the desired  properties.  In the final part of this work, past research efforts on ALE were reviewed and the  key defining characteristics of ALE identified. These include cyclic step-wise processing,  self-limiting surface chemistry, repeated removal of atomic layers (not necessarily a full monolayer) of the material, and the presence or absence of directional species that lead  to anisotropic or isotropic ALE processes, respectively. Subsequently, further parallels  were drawn with the more mature and mainstream technology of ALD from which  lessons and concepts were extracted that can be beneficial for advancing the field of  ALE.  To conclude, this dissertation elucidates important aspects associated with plasmaenhanced atomic scale processes that provide deeper insight on the fundamental and  technological opportunities afforded by these techniques, relevant for future 3D device  architectures. It serves to exemplify how the properties of functional materials can be  tailored by accurate control and optimization of plasma based processing conditions  

2019
Development of mechanical characterization methods for thin films and interfaces
Author
Jussi Aav
University
Aalto University School of Chemical Engineering (Espoo, Finland)
Year
2019
Abstract

Probably the most important thin film characteristic is its' adhesion to the used substrate. It is  very important to understand the fundamental mechanics of adhesion-related failures, and by  having suitable characterization methods to detect any problems as early as possible. Tailored or  correctly conducted quantitative analysis of adhesion is required for building reliable devices. In  many conventional test methods significant loading to the substrate is applied which can result in  problems especially with brittle substrates where the substrate can break before the film is  delaminated. The characterization of interfacial mechanical properties of increasingly thinner films  is challenging with many practical shortcomings and thus method development is needed.  In this thesis, three measurement methods where the loading to the substrate is minimized are  presented and demonstrated for the interfacial and mechanical testing of especially atomic layer  deposited (ALD) thin films. (i) Microelectromechanical system (MEMS) test chip assisted shaftloaded  blister testing through a hole in the substrate to the backside of the thin film, (ii)  microrobotic manipulation of embedded microspheres using lateral loading mostly to the thin film  and to the interface and (iii) a combination of nanoscratch and scanning nanowear for minimized  interaction volume of loading to the substrate. The relationship between adhesion and cohesion  as competing processes during film/ substrate failure is shown. When the film-interface-substrate  system is under loading the energy will dissipate through the path of least resistance. This will  happen either (i) through plastic deformation of the coating/ substrate, (ii) film fracture (decohesion) or (iii) delamination (de-adhesion) of the film. Usually, however the energy is  dissipated as a combination of these three different mechanisms, unless some of the mechanisms  is dominant in the energy release.  The presented characterization methods are mostly generic, and can be applied for the evaluation  of mechanical and interfacial properties, such as adhesion, between practically any materials of  choice with some limitations. Compared to some of the existing methods, the quantitative nature  of these characterization methods enables a more in-depth possibility for the analysis, understanding, tuning and improvement of the properties of the thin films and processes aiding  in maintaining and improving product and process quality. The main outcome of this thesis is that  the authors have demonstrated the potential and versatility of especially the MEMS test structures  and microrobotic testing systems, either on their own or as a combination as a solution to  developing new tailored interfacial and mechanical characterization methods for current and future  needs of research and the industry. 

2019
Atomic layer deposition of ruthenium and silver
Author
matthias minjauw
University
Ghent University (Ghent, Belgium)
Year
2019
Abstract

This PhD thesis presents a study on the atomic layer deposition (ALD) of silver and ruthenium. The research was started in August 2013, and results  up to September 2018 are included. The lion’s share of the experimental  work was done within the Department of Solid State Sciences at Ghent  University.  The thesis is paper-based and contains four original research articles, published in peer-reviewed journals. The structure of this document is as  follows: First an introductory chapter is given, which provides the research  context and motivation to the reader; this is followed by the four articles  which were reformatted and included as stand-alone chapters; while in the  final chapter conclusions are made. A description of the used experimental  techniques is given in the appendix.  I hope that I have managed to present my contributions to the field of ALD  in a clear and interesting way for both the expert and the layman.

2019
Atomic Layer Deposition of Late First-Row Transition Metals: Precursors and Processes
Author
Katja Väyrynen
University
University of Helsinki (Helsinki, Finland)
Year
2019
Abstract

Late first-row transition metals, namely copper, nickel, and cobalt, are pivotal materials in many modern and future applications. Because of its low resistivity, Cu has for long been the metal of choice for interconnects in microelectronic devices. Co is needed in the smallest features of the 10 nm technology node interconnects, as it is more robust than Cu toward electromigration, a phenomenon causing damage to the interconnects. Being ferromagnetic, Co and Ni are in the focal point of developing faster and more durable magnetic memories capable of handling the exponentially increasing amounts of data being generated annually. The development of faster yet smaller electronic devices requires a constant increase in computational power. To improve the performance without increasing device size, the components on integrated circuits should be shrunk and packed more closely. The shrinking is achieved by using thin films with nanoscale thicknesses preferably arranged in threedimensional forms. For downscaling to continue, accurate thin film deposition methods are needed. Atomic layer deposition (ALD) provides atomic level accuracy and is thus the number one thin film deposition technique for modern and future devices. ALD is based on a cyclically repeated alternate supply of gaseous precursors that react on a substrate and form a uniform layer of material, atom by atom, even on complex three-dimensional structures. ALD is based solely on chemistry; to benefit from the many advantages the method has to offer, suitable precursors must first be found for each of the desired materials. ALD has been employed to deposit a myriad of materials ranging from pure elements to, for example, oxides, nitrides, and chalcogenides, but the deposition of metals has been hindered by a lack of reactive precursors and reducing agents. Thermal ALD processes exist mostly for noble metals, but mere thermal activation has often proven insufficient for the reduction of the late first-row transition metals. The aim of this thesis was to find and develop new precursors and processes for the ALD of high-quality Cu, Ni, and Co thin films, thus promoting the development of better microelectronics. Within the scope of this thesis, several new metal precursors for the ALD of the late firstrow transition metals were developed and tested. Out of all of them, the diamine adducts of Co(II) and Ni(II) chlorides showed the best performance in the ALD experiments. In addition to the new metal precursors, the focus of this thesis was also on finding more efficient alternatives for the conventional reducing agents, H2 and NH3. Tert-butylhydrazine showed high reactivity to produce Cu and Ni3N by ALD, providing significant improvement on film purity and resistivity over the existing processes. Tributyltin hydride, another powerful reducing agent, was studied for the ALD of Co and Ni. Instead of producing metallic Co or Ni, intermetallic Co3Sn2 and Ni3Sn2 were deposited unveiling a new field of ALD: the ALD of intermetallics. The same approach was also applied to the ALD of Ni2Ge thin films. Postdeposition reduction of the corresponding metal oxides and nitrides was also explored as an alternative route for the preparation of metal thin films. 

2019
Atomic layer deposition of catalytic materials for environmental protection
Author
Tatiana Ivanova
University
Lappeenranta University of Technology (Lappeenranta, Finland)
Year
2019
Abstract

The reduction of toxic pollutants emitted by human activities to ambient air is an important issue nowadays. The technological approach to this problem is the development of different oxidation techniques together with catalytic materials, which can convert toxic emission products to safe compounds. Current methods for the preparation of heterogeneous catalysts which fully control the structure, size and composition are limited. The atomic layer deposition (ALD) technique can create catalytic thin films with precise thickness and structure control even on complex substrates. The present work describes the development of TiO2, CeO2 and Ag-doped CeO2 catalytic thin films deposited by ALD in order to find their capacity for the decomposition of toluene and soot. TiO2 catalytic films with different thicknesses were grown to investigate their nucleation delay and changes in their polycrystalline structure and the impact of these on their photocatalytic properties. It was shown that porous glass filters coated by TiO2 in combination with a dielectric barrier discharge (DBD) reactor could decompose toluene at a concentration of 2450 ppm with the specific input energy (SIE) of 336 J/l. In CeO2 studies it was found that a deposition temperature of 300 °C changes the structural properties of the catalytic thin films. The combination of small crystallites, larger clusters and the existence of Ce3+ in CeO2 catalytic films showed 100% soot decomposition at 450°C under loose contact mode. The doping of CeO2 with Ag in the ratio of CeO2:Ag = 10:1 by ALD reduced the soot decomposition temperature to 390°C. It was proposed that Ag+ sites could promote oxygen species and reduce the Ce ions in stoichiometric CeO2 from Ce4+ to Ce3+. Most catalytic thin films prepared by ALD showed good durability after repetitive tests of soot decomposition. 

Keywords: atomic layer deposition, titanium dioxide, cerium dioxide, silver, photocatalytic activity, soot oxidation, toluene.

2019
Atomic Layer Deposited 3D Nanostructured Materials for Efficient Energy Storage
Author
Arpan Dhara
University
IIT Bombay (Mumbai, India )
Year
2019
Abstract

The rapid advancement in the field of nanotechnology in the past several years has promised great potential for controlling materials at the nanoscale and stimulated vast opportunities to architect materials with desirable properties. This evolution has also contributed significantly to the development in the area of energy storage, which is a crucial technology in the present time. Electrode architecture always plays an important role in the domain of energy storage. In specific, engineering at nanoscale offers exclusive properties resulting in an improvement in the performance of electrodes and electrolytes in different energy storage technologies. Accordingly, significant efforts have been put forward in recent years to accomplish the present demands of energy storage using these advanced nanostructured materials. Various nanostructured materials with larger surface area and low bulk volume are presently being studied to improve the energy and power densities together for next-generation storage devices. The main objective of this thesis is to develop three-dimensional (3D) nanostructured electrodes with superior energy density and power density for electrochemical energy storage. Controlling the thickness of the active material in a few nanometers ensures the short diffusion length and full utilization of the active material. To get the nano level control, the ALD technique is used for active material deposition. Highly conducting templates like CNTs, graphenes and metal nanostructures are utilized to anchor the active material ensuring the effective electron transportation. Three-dimensional electrodes represent a unique way to improve the storage density and rate capability without the complex process of material development. This thesis presents the synthesis of 3D hybrid nanostructured electrodes with superior energy density and power density for electrochemical energy storage using template assisted methods. Carbon nanotubes (CNT) and Cu-NWs are used as template layer because of their high surface area and electrical conductivity. The active material is being deposited on these template layers by atomic layer deposition (ALD) process because of its extreme conformal and sub-nanometer thickness control.
Due to its high theoretical capacity, energy density and excellent reversibility with Li/Li+, molybdenum oxides are one of the vastly studied electrode material in lithium-ion batteries. However, like most of the oxides, it also suffers from poor cyclic stability because of their low electrical conductivity. In 3D core-shell structure prepared by ALD coating provide superiority in nanoscale decoration because of its extreme conformality and precise thickness control on high aspect ratio surfaces. The first part of this thesis focuses on the fabrication and electrochemical activities of 3D CNT/MoOx electrodes. These electrodes exhibit much higher areal and overall cell capacity than its planer 2D counterpart. An optimal thickness of MoOx on CNT is also found out in order to attain the most stable cyclic performance of this nanostructure. A stable reversible areal capacity of 645 μAh cm-2 with a specific capacity of 915 mAh g-1 is achieved from optimized MoOx/CNT assembly.


The second part of the thesis focuses on increasing the conductivity of the MoOx layer by N-incorporation for better electrochemical performance. It is well studied that the transition metal oxynitrides have better conductivity than their pure oxide phase. This particular work highlighted three important factors, (i) synthesis, (ii) electron transportation and (iii) electrochemical performances of N-incorporated MoOx films. It is found that N-atoms are homogeneously distributed throughout the films at the deposition temperature, no post-annealing is required for diffusion. The conductivity of those films increases with increasing nitrogen concentration. The electrochemical study reveals the superior performance of N-incorporated films against Li/Li+ than that of the pristine materials. The highest discharge capacity of 1287 mAh g-1 was achieved in the first cycle at a current rate of 0.1 A g-1 and a stable capacity of 974 mAh g-1 and 610 mAh g-1 achieved when discharged at 0.1 and 2 A g-1 from the core-shell 3D CNT/MoOx:N electrodes.
In the final part, Cu-nw scaffold is used as template layer by replacing CNTs. The high abundance, low-cost and high electronic conductivity make the choice simpler to use Cu as an affordable scaffold layer. Cu/ MoOx electrode showed a stable capacity of 993 mAh g-1 at the rate of 0.1 A g-1 and retained 45% of its initial capacity with 40 times higher current rate. This kind of state-of-the-art fabrication method helps to deposit more amount of active material with less Li+ diffusion length and ease the electron/ion transportation during the charge-discharge process
 

2019
ATOMIC FORСE MICROSCOPY OF COMPOSITE POLYMERIC AND SILICATE MATERIALS SYNTHESIZED BY THE METHOD OF MOLECULAR LAYERING
Author
A.S. Kochetkova
University
Petersburg State Institute of Technology (Technical University) (Saint Petersburg, Russia)
Year
2019
Abstract

NOTES: full Russian thesis available on “Link to external PDF”. English + Russian summary available here by clicking the title or “Read Thesis” link. 

1. Systematic studies using AFM were carried out for the first time surfaces of polymeric (LDPE, PVC films filled with Al nanoparticles2O3) and inorganic silicate matrices of various geometric shapes (glass microspheres, quartz fibers, plates of borosilicate and quartz glass) at different stages of the formation of element oxide compositions of various compositions and structures on them during the MN process, associated with the previously identified structural and size effects in the products obtained the specified method. eighteen 2. Proposed methodological approaches to the study by the AFM method materials of various geometric shapes and developed methods for calculating the effective diffusion coefficient of water vapor in PVC films containing 2 wt. % nanodispersed Al2O3, qualitative assessment of adhesion to the substrate surface of nanosized coatings, based on a combination of contact and semi-contact AFM modes, and calculation of the pressure in the “probe-sample” contact zone. 3. It has been established that the process of MN of titanium oxide nanocoatings on the surface hollow soda borosilicate glass microspheres using TiClfour accompanied by side reactions with the formation of NaCl crystals and TiO particles2. 4. Using AFM, differences were established in the formation by the method MN and the occurrence of recrystallization processes during the subsequent heat treatment (900°С) of the aluminum oxide nanocoating (after 400 cycles of treatment with Al(CH3)3and H2O) on the surface of optical fibers and quartz plates. It is shown that coatings on a flat surface crack when heated, but without a significant change in the size of aluminum oxide structures (size 40–50 nm), and large crystallites (up to lateral sizes of 200–210 nm) form on fibers during recrystallization without disturbing the continuity of the coating. 5. The AFM method was used to study the change in the morphology of titanium oxide coating formed on the surface of borosilicate glass by conducting 25, 50, 100, 200 and 300 MN cycles, and studied the effect of simultaneous exposure to heat treatment at 480°C and X-rays (irradiation dose of 10-3C/kg (~ 4 R)) in vacuum (10-3 Pa) on the structure and properties of titanium oxide coatings of various thicknesses (100, 200 and 300 MN cycles) formed on the inner surface of the glass cases of X-ray tubes. It is shown that the combined action of temperature and Xray radiation in vacuum intensifies the recrystallization process in the titanium oxide layer, with the smallest transformations occurring in the composition of the coating formed as a result of 300 cycles of glass treatment with TiCl vapor.fourand H2Oh 6. On the surface of high-pressure polyethylene (LDPE) films by processing in a different specified sequence with pairs of titanium, phosphorus and water chlorides, two-component nanostructures with different mutual arrangement of phosphorus and titanium oxide groups were synthesized. According to the AFM data, it was found that during the modification of LDPE films, the change in the polymer surface morphology is significantly affected by the sequence in which reagents are fed into the reaction chamber. The strongest amorphization of the LDPE surface occurs during the formation of phosphorus- and phosphorus-titanium oxide groups, in comparison with the polymer film samples, where TiCl was used in the first MT cycle.four. At the same time, according to the RMSCA data, the concentration of phosphorus in such samples significantly exceeds the concentration of titanium (0.22 and 0.01 mmol/g, respectively). 7. It has been established that two-component energy traps in the form of titanium phosphorus oxide and phosphorus-titanium oxide surface nanostructures provide, in comparison with single-component compositions, an increase in the thermal stability of the surface potential of electrets made on their basis. The residual potential of the electret, with a value of 100 - 150 V, is maintained up to 19 temperature of the beginning of melting of the polymer film (200ºС). A mechanism is proposed for the effect of physically sorbed water on the electret properties of modified HDPE films, which is based on the redistribution of electron density in the COPO-Ti-OH chain. 8. Using AFM data, an assessment of the effectiveness of hydrophobization of an LDPE film with phosphorus oxide centers subjected to additional treatment with Si(CH3)2Cl2. A regular increase in the size of structures corresponding to hydrated regions of the polymer surface containing grafted functional groups was revealed. List of major papers published on the topic of the dissertation Articles: 1. Kochetkova A.S. Study of nanocomposites based on polyvinyl chloride using atomic force microscopy / A.S. Kochetkova, N.Yu. Efimov, E.A. Sosnov // Scientific and technical. Bulletin of St. Petersburg State Polytechnical University, Fiz.-Mat. sciences.- 2013.- № 1 (165).- p.114-119. 2. Kochetkova A.S. Influence of chemical modification of the filler surface on the structure and permeability of a composite film based on polyvinyl chloride / A.S. Kochetkova, N.Yu. Efimov, E.A. Sosnov, A.A. Malygin // Zhurn. appl. Chemistry. - 2015. - T.88, No. 1. - P.116-124. 3. Kochetkova, A.S. Evaluation of the wear resistance of the surface of modified PVC films using scanning probe microscopy /A.S. Kochetkova, P.N. Gorbushin, E.A. Sosnov, K. Kolert, A.A. Malygin // Deformation and destruction of materials. - 2016. - No. 8. - P.36 - 43. 4. Malygin, A.A. Synthesis by molecular layering and functional properties of metal oxide nanocoatings on the surface of quartz optical fibers / A.A. Malygin, V.V. Antipov, A.S. Kochetkova, G.Ya. Buimistryuk // Zhurn. appl. chemistry. - 2018. - V.91, No. 1. - P.17-27 

2019
Window Layer Structures for Chalcopyrite Thin-Film Solar Cells
Author
Fredrik Larsson
University
Uppsala University (Uppsala, Sweden)
Year
2020
Abstract

This thesis aims to contribute to the development of improved window layer structures for chalcopyrite thin-film solar cells, with an emphasis on the buffer layer, to assist future reductions of the levelized cost of energy. This is realized by exploring the potential of existing materials and deposition processes, as well as developing new buffer layer processes based on atomic layer deposition (ALD).
Ternary compound ALD processes are more complicated to control than when depositing binary compounds and the composition can be significantly different at the absorber interface as compared to the bulk. A method based on in-situ quartz crystal microbalance that can measure these compositional variations is demonstrated in the thesis. Furthermore, the addition of alkali-metal fluoride post-deposition treatments (PDTs) can further complicate ALD of buffer layers, due to residual salts that are formed on the absorber surface during a PDT process. When applying ALD ZnO1-xSx to KF-treated CIGS absorbers, competitive solar cell efficiencies could only be obtained after performing additional wet-chemical treatments prior to ALD processing.
It is shown that the performance of wide-bandgap solar cells can be greatly enhanced by improving the conduction band alignment between the absorber and buffer layers. By applying ALD Zn1-xSnxOy buffer layers in CuGaSe2 solar cells, record efficiency (η = 11.9%) and open-circuit voltage (Voc = 1017 mV) values are demonstrated.
In search of a new buffer layer suitable for a wide range of absorber materials (and surface bandgaps), amorphous tin-gallium oxide grown by ALD is evaluated as a new buffer layer material. This material exhibits a highly variable bandgap (and electron affinity) the absorber/buffer conduction band alignment can be controlled by adjusting the cation composition and deposition temperature. The potential of Sn1-xGaxOy as a buffer layer was studied in combination with low-bandgap (Ag,Cu)(In,Ga)Se2 absorbers (Eg,surface ≈ 1.1 eV). A best cell efficiency of 17.0% was achieved, which was lower than the efficiency of 18.6% obtained for the corresponding CdS reference due to slightly lower Voc and higher series resistance. However, the full potential of Sn1-xGaxOy as a buffer layer remains to be revealed.

2020
Molybdenum Sulfide Prepared by Atomic Layer Deposition: Synthesis and Characterization
Author
Steven Letourneau
University
Boise State University (Boise, USA)
Year
2020
Abstract

Molybdenum disulfide (MoS2) is the prototypical two-dimensional (2D) semiconductor. Like graphite, it has a layered structure containing weak van der Waals bonding between layers, while exhibiting strong covalent bonding within layers. The weak secondary bonding allows for isolation of these 2D materials to single layers, like graphene. While bulk MoS2 is an indirect band gap semiconductor with a band gap of ~1.3 eV, monolayer MoS2 exhibits a direct band gap of ~1.8 eV, which is an attractive property for many opto-electronic applications. Atomic layer deposition (ALD) has been used to grow amorphous films of MoS2 using molybdenum chlorides and carbonates, however many of these molybdenum chemistries require high temperature vapor transport as they are solids at room temperature. We demonstrate the first ALD of MoS2 at 200 ℃ using molybdenum hexafluoride (MoF6), a liquid at room temperature, and hydrogen sulfide (H2S). in situ quartz crystal microbalance measurements were used to demonstrate self-limiting chemistry for both precursors, which is the hallmark of ALD. The deposited films were amorphous, and after annealing in hydrogen, crystalline MoS2 was discernable. The nucleation and early stages of MoS2 ALD on metal oxide surfaces were investigated using in situ Fourier transform infrared (FTIR) spectroscopy. The formation of Al-F and MoOF4 seem to initially form, but after H2S is introduced sulfate species begin to appear. This competition for oxygen seems to inhibit growth initially, until the oxygen at the surface is consumed and steady state growth occurs. To understand the structure of the amorphous films, X-ray absorption spectroscopy (XAS) vii and high-energy X-ray diffraction (HE-XRD) experiments were performed at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). Contrary to previous findings, the MoS2 structure was found to be sulfur rich; however, the atomic coordinations of Mo and S atoms bond distances matched standards. Interestingly, the Mo-Mo coordinations were much lower than reference structures, which could explain the lack of or very weak Raman vibrational modes seen in many as-deposited ALD MoS2 films. Experimental data were consistent with films containing clusters of a sulfur rich [Mo3S(S6)2] 2- phase, but after annealing in H2 and H2S, these clusters decompose forming a layered MoS2 structure. Understanding these complex surface interactions of nucleation, growth, and phase transformations is necessary to enable synthesis of high quality MoS2 for use in future microelectronics. 

2020
Functionalization of particles by atomic layer deposition for energy storage applications
Author
Sarai García García
University
CIC nanoGUNE (Donostia, Spain)
Year
2020
Abstract

Powders are essential ingredients for many processes and applications. They are usually classified with relation to their particle sizes and functionalities. While particles in the millimeter size range are intensely used in alimentary, pharmaceutical, cleaning and construction sectors, micrometer and nanometer sized particles are commonly used in energy storage applications, catalysis and electronics. Recent research has focused its attention on micrometer and nanometer sized particles due to the special properties arising from their high surface area to particle size ratio. Functionalization of those particles can greatly improve their performance. In this way, providing added value, like protecting and activating them, or changing their performance. Among the most promising ways of functionalization is the generation of core-shell particles through coating, or the deposition of islands or clusters on the surface of the particles. Nowadays, a wide variety of coating technologies are applied for this purpose. Among those coating technologies, chemical vapor deposition (CVD) became attractive in the recent years thanks to its great thickness control over the deposited coating. However, more recently, atomic layer deposition (ALD) was developed, allowing for ultimate thickness and compositional control of the deposited film in a large variety of geometries. The application of ALD in different fields, including microelectronics, energy storage systems or bioapplications, pushed the application of this technology to materials with diverse geometries, among those being particles. The need for coating powders resulted in the modification of reactors for carrying out ALD processes on such materials. The various reactors are distinct in the way of handling particles; namely, static particle reactors and fluidized bed reactors. While static particle reactors are used to coat small amounts of particles, fluidized bed reactors (FBR) can be used to coat large amounts of particles, allowing the scale-up of the technology for its use in industrial applications. The application of ALD to fabricate or coat materials and components for energy storage systems is intensely investigated and it is beginning to deliver breakthroughs. Batteries belong to the most important energy storage systems thanks to their excellent energy density and energy release rate. Lithium-ion batteries (LIB) are currently the most common battery types for a large variety of applications. In fact, they offer a theoretical energy density of around 300 Wh: ke ' However, their limited specific capacity and the precious cathode materials made researchers looking into other kinds of battery systems as alternatives. Lithium-sulfur (Li-S) batteries became a promising alternative due to their better handling and extraordinary properties of sulfur as cathodic material. Namely, it shows a theoretical energy density of 2600 Wh: kg', higher than that of Li-ion batteries. Moreover, sulfur is environmentally friendly and one of the most abundant elements in the world. However, Li-S batteries suffer from several drawbacks that affect their application and have driven researchers to develop solutions to enable the practical use of lithium-sulfur batteries and in this was increase the energy density and long-term stability. The application of ALD in energy storage systems has shown many improvements by now. In fact, the deposition of certain materials at the nanometric scale has many unique benefits for improving the behavior of Li-S batteries. The objective of this thesis is the study and understanding of ALD coatings on powders, paying special attention to materials that can be used for energy storage devices. Micrometer and nanometer sized particles have been coated with metal oxides, which showed promising alterations and functionalities of powders that have not been observed before. In the first part of the thesis, an initial study of coating particles by ALD is done. For this aim, iron oxide nanoparticles (y-Fe:O3) are coated with titanium dioxide (TiO2), generating FeO,-TiO» core-shell nanoparticles. This study shows that the application of ALD not only coats the particles, but also, depending on the reactants (precursors) used, can also reduce them to form core-shell nanoparticles of Fe;0.-TiO2. This study demonstrates that choosing an appropriate ligand of the metal source can unveil a novel approach to concertedly coat and reduce y-Fe.O3 nanoparticles. Moreover, it is found that the more electronegative the cation of the precursor is, the more energy is necessary to release the ligands, which is conditional for their recombination. Thus, the appropriate design of precursors and selection of substrates will pave the way for numerous new compositions with more and improved functionalities. In the second part of the thesis, the study of ALD on energy storage devices, specifically on lithium-sulfur batteries, is carried out. The modification of the cathode material of lithium-sulfur batteries is done by ALD in a conventional static ALD reactor. The optimal parameters for the ALD application to sulfurbased electrodes are evaluated. Aluminum oxide (AlL,O3) is deposited on prefabricated cathodes, improving the capacity of the systems. In fact, applying only 2 ALD cycles at 85 °C increases the capacity of a lithium-sulfur battery by 13 % for low current densities and by 50 % for high current densities. Finally, a promising powder coating technology is applied in order to coat sulfurcarbon composite powders of cathodes of lithium-sulfur batteries by ALD and in this way considerably improving the performance of those batteries. For this aim, a fluidized bed reactor was constructed. The best results are obtained after applying 5 ALD cycles of Al,Os, sufficient to increase the capacity of the system by 30% at low current densities and by 50% at high current densities, with respect to a standard battery system. Besides, the sulfur loading in the cathodes can be doubled thanks to the morphological improvement provided by the aluminum oxide. After coating, uniform and crack-free electrodes can be fabricated, which significantly exceed the performance of standard electrodes increasing the capacity of lithium-sulfur batteries by 60 %. 

2020
Exploring Dye-Sensitized Mesoporous NiO Photocathodes: from Mechanism to Applications
Author
Lei Tian
University
Uppsala University (Uppsala, Sweden)
Year
2020
Abstract

Increasing attention has been paid on solar energy conversion since the abundant solar energy possesses the potential to solve the problems on energy crisis and climate change. Dye-sensitized mesoporous NiO film was developed as one of the attractive photocathodes to fabricate p-type dye-sensitized solar cells (p-DSCs) and dye-sensitized photoelectrosynthetic cells (p-DSPECs) for electricity and chemical fuels generation, respectively. In this thesis, we designed a well-structured NiO-dye-TiO2 configuration by an atomic layer deposition (ALD) technique, with an organic dye PB6 as the photosensitizer. From kinetic studies of charge separation, ultrafast hole injection (< 200 fs) was observed from the excited state of PB6 dye into the valence band of NiO; dye regeneration (electron injection) was in t1/2 ≤ 500 fs, which is the fastest reported in any DSCs. On the basis of NiO-dye-TiO2 configuration, we successfully fabricated solid-state p-type DSCs (p-ssDSCs). Insertion of an Al2O3 layer was adopted to reduce charge recombination, i.e. NiO-dye-Al2O3-TiO2. Theoretically, such a configuration is possible to maintain efficient charge separation and depressed charge recombination. Based on NiO-dye-Al2O3-TiO2 configuration, the open-circuit voltage was improved to 0.48 V. Replacing electron conductor TiO2 with ZnO, short-circuit current density was increased to 680 μA·cm-2. The photocatalytic current density for H2 evolution was improve to 100 μA·cm-2 with a near unity of Faraday efficiency in p-DSPECs.
However, to further improve the performance of p-DSCs is very challenging. In p-ssDSCs, the limitation was confirmed from the poor electronically connection of the electron conductor (TiO2 or ZnO) inside the NiO-dye films. We further investigated the electronic property of surface states on mesoporous NiO film. We found that the surface sates, not the bulk, on NiO determined the conductivity of the mesoporous NiO films. The dye regeneration in liquid p-DSCs with I-/I3- as redox couples was significantly affected by surface states. A more complete mechanism is suggested to understand a particular hole transport behavior reported in p-DSCs, where hole transport time is independent on light intensity. The independence of charge transport is ascribed to the percolation effect in the hole hopping on the surface states.
 

2020
Atomic layer deposition towards novel device applications
Author
Giovanni Marin
University
Aalto University (Espoo, Finland)
Year
2020
Abstract

Atomic layer deposition (ALD) is a gas-phase thin film deposition technique that has gained increasing popularity in the last 20 years because of its unique properties. It is based on self-limiting chemical reactions that ensure the layer-by-layer growth of the film. This unique growth mode is fundamental to the fine control of both film thickness and structure. The film grows conformally on the substrate, following the morphology of the surface. ALD can grow films at low temperature, making possible the use of temperature-sensitive substrates. A slightly modified technique called molecular layer deposition (MLD) utilises organic precur-sors to deposit fully organic films. Hybrid inorganic-organic materials can be deposited with a combination of ALD and MLD. The aim of this research was to utilise the unique characteris-tics of ALD/MLD in two different applications, thermoelectrics and barrier coatings.

Thermoelectric devices were fabricated on flexible plastic, glass, and textile. Testing of the barrier properties of ALD-grown films were carried out on 3D printed plastic substrates. The conformality of the deposition is fundamental in both applications. The films needed to coat the single fibres within the textile substrate as well as the porous surface of the 3D printed plastic. The low deposition temperature made it possible to use cotton as well as various plas-tics as substrates. The fine control over the film thickness and structure, enabled the deposi-tion of inorganic-organic superlattice hybrid materials. Zinc oxide (ZnO) and hydroquinone (HQ) were chosen for the fabrication of the thermoelectric devices while aluminium oxide (AlOx) was the chosen barrier material. Hydroquinone was utilised as monomolecular layers within the ZnO matrix to lower thermal conductivity and enhance the thermoelectric perfor-mance.

The ALD-deposited AlOx coating was shown to successfully lower the vacuum degassing of the 3D printed plastics compared to commercial sealants. These superior performances open the way to inexpensive and personalised, 3D printed, laboratory tools coated with ALD which pro-vide degassing protection to the vacuum environment.

Thermoelectric devices were fabricated on several substrates (silicon, flexible plastic, flexible glass, and textile) using the n-type ZnO as thermoelectric. On textile, the device was made with both n-type (ZnO or ZnO-HQ) and p-type (poly(3,4-ethylenedioxythiophene) - PEDOT) components to improve performance. The ZnO-HQ superlattice outperformed the bare ZnO films, proving that the hybrid approach is worth pursuing to reduce thermal conductivity. The best device fabricated on textile, produced an open-circuit voltage around 150 mV at a ΔT of 20 °C with a power output in the order of pW. These numbers, although low, are paving the way for future application of the ALD/MLD in the fabrication of thermoelectric devices inte-grated into smart clothing.

2020
Time-resolved CVD of Group 13-Nitrides
Author
Polla Rouf
University
Linköping University (Linköping, Sweden)
Year
2021
Abstract

Group 13 nitrides (AlN, GaN and InN) and their alloys are semiconductor materials with a wide bandgap span covering from UV down to IR range. Their excellent electronic properties make them extremely attractive materials for light emitting diodes (LEDs) and different kind of transistor structures, especially high electron mobility transistors (HEMTs). These materials are routinely deposited by chemical vapor deposition (CVD) at high temperatures. The most sought-after material among the group 13 nitrides is InN due to its high electron mobility making it extremely useful in transistor structures. InN needs to be deposited at low temperatures as it decomposes at high temperatures. This does not only limit the deposition temperature for InN growth but also for all the other materials that will be deposited on top of InN. In this thesis the deposition of group 13 nitrides is investigated by low temperature atomic layer deposition (ALD) via both a thermal and plasma route. This was conducted by both process development and by improving the deposition chemistry by developing new precursors.  Carbon impurities is one of the greater challenges when using the standard aluminum precursor trimethylaluminum (TMA) in ALD due to the strong Al–C bonds in the molecule. An in-situ removal of carbon impurities was investigated by introducing a cleaning pulse, after the TMA pulse. The cleaning pulse consisted of an H2, N2 or Ar gas pulse perpendicular to the surface. The introduction of the cleaning pulse reduced the carbon impurity in the AlN film from 3 at% down to under 1 at%. This made it possible to deposit AlN at higher temperature to obtain better crystalline quality and on the same time reduce the impurity levels. Kinetic simulations showed that the cleaning pulse cleans the surface from desorbed methyl groups resulting in a suppressed reabsorption pathway.  To further reduce carbon impurities, the strong M–C bonded precursors was replaced with a M–N bonded one. The precursor used were tris(dimethylamido)gallium together with ammonia (NH3) plasma to deposit GaN. The precursor showed ALD behavior and the resulting GaN film possessed significantly lower carbon impurities compared to M-C bonded precursor at low deposition temperatures. This precursor could also produce epitaxial GaN directly on 4H-SiC without a need of a seed layer. To further investigate the precursor impact on deposition chemistry and ultimately the film quality, three indium precursors were evaluated, indium(III)guanidinate, indium(III)amidinate and indium(III)formamidinate. All three precursors have more or less the same structure, only difference being the size of the substituent on the endocyclic carbon position (-NMe2, -Me and -H respectively). Experimental results showed that smaller groups on the endocyclic carbon position improved the InN film quality in terms of crystallinity, morphology, stoichiometry and optical properties. Density functional theory (DFT) calculations showed that smaller moieties on the endocyclic position will lead to less surface and steric repulsion with the exocyclic position. As the size is decreased the exocyclic groups can fold up closer towards the endocyclic position leading to elongated metal-ligand bonds which will result in easier removal of the ligand for the upcoming NH3 plasma pulse.  From these results a new ligand was developed to further improve the deposition chemistry where the endocyclic carbon atom in the ligand backbone of the foramidinate ligand was replaced by a N atom to form a triazenide ligand (iPr–N–N=N–iPr). The triazenide ligand possess no moiety on the endocyclic position compared to the ligands used previously and hence should result in improved material quality if extrapolated from our previous study. The ligand was placed on indium and gallium forming In(III)triazenide and Ga(III)triazenide respectively. Both precursors showed excellent thermal properties making them good ALD precursors. Their use for depositing InN and GaN was investigated with NH3plasma. The resulting films showed excellent quality where no carbon could be detected for either InN nor GaN using XPS and ERDA. Both InN and GaN showed epitaxial growth behavior on 4H-SiC at deposition temperature of 350 °C, a factor of three lower deposition temperature compared to CVD. Interestingly, several linear growth regimes (ALD windows) upon changing the temperature were observed, two and three for InN and GaN respectively. This indicated that the precursors decomposed upon increasing the temperature to form smaller fragments which increased the growth rate but on the same time the smaller precursor fragments saturated the surface. This was further confirmed by DFT calculations.    The In(III)triazenide and Ga(III)triazenide was further used to deposit the ternary InGaN phase. A new method was developed where both precursors were mixed in the bubbler and co-sublimed into the reactor via a single pulse. The composition of the films could be tuned via bubbler temperature, deposition temperature and premixed ratio of the precursors in the bubbler. Near In0.5Ga0.5N could be obtained at low deposition temperatures confirmed by both XPS, ERDA and bandgap measurement. Deposition at 350 °C on 4H-SiC resulted in epitaxial In1-xGaxN without a need of a seed layer. 

2021
Multilayered ZnO-based thin films to control heat and electrical transport properties
Author
Fabian Krahl
University
Aalto University (Espoo, Finland)
Year
2021
Abstract

Interfaces between materials can have properties that differ greatly from the bulk state. In classical materials only a tiny fraction of atoms are at the interface while the vast majority is in the bulk of the material. The capability to engineer materials with an artificially high amount of interfaces opens up a pathway to amplify the interface effects and tailor the material properties by controlling the amount of interfaces. This approach to engineer materials step by step or layer by layer also allows for a controlled combination of very different materials into a hybrid material that would not form naturally and which can show fundamentally different and new properties.

In this thesis atomic layer deposition (ALD), molecular layer deposition (MLD) and pulsed laser deposition (PLD) are utilized to engineer ZnO-based thin films with high interface densities. The films are analysed with x-ray reflectivity (XRR), x-ray diffraction (XRD) and transmission electron microscopy (TEM) in regards to their internal structure. Time domain thermoreflectance (TDTR) is utilized to measure the thermal conductivity, the electrical properties are measured with a hall measurement setup. The latter is the focus in layered thin films of polycrystalline ZnO and amorphous InGaZnO4 in which a considerable increase in the charge carrier concentration following the interface density could be demonstrated.

The interfaces between a ZnO matrix, ZnO-benzene and AlOx layers are studied in detail in a hybrid ZnO/ZnO-benzene/AlOx system in which this work demonstrates, that these layers in ZnO can be as thin as a single atom/molecule, yet still form distinctive layers. However, these very thin layers of ZnO-benzene and AlOx are found to have little impact on the crystal growth of ZnO, but can act as effective barriers for ZnO crystal growth when 10 or more consecutive ALD/MLD cycles are utilized for each AlOx/benzene layer respectively. Finally the thermal conductivity in ZnO/benzene thin films is characterised, the database for the thermal conductivity in that system is significantly extended and thermal conductivities for irregularly layered structures are reported for the first time in ZnO/ZnO-benzene hybrid thin films. Analysis with multivariate data analysis of the database confirms that the interface density has the most pronounced effect on the thermal conductivity.

2021
Engineering Surfaces of Solid-State Nanopores for Biomolecule Sensing
Author
Shiyu Li
University
Uppsala University (Uppsala, Sweden)
Year
2021
Abstract

Nanopores have emerged as a special class of single-molecule analytical tool that offers immense potential for sensing and characterizing biomolecules such as nucleic acids and proteins. As an alternative to biological nanopores, solid-state nanopores present remarkable versatility due to their wide-range tunability in pore geometry and dimension as well as their excellent mechanical robustness and stability. However, being intrinsically incompatible with biomolecules, surfaces of inorganic solids need be modified to provide desired functionalities for real-life sensing purposes. In this thesis, we presented an exploration of various surface engineering strategies and an examination of several surface associated phenomena pertaining specifically to solid-state nanopores. Based on the parallel sensing concept using arrayed pores, optical readout is mainly employed throughout the whole study.
For the surface engineering aspect, a list of approaches was explored. A versatile surface patterning strategy for immobilization of biomolecules was developed based on selective poly(vinylphosphonic acid) passivation and electron beam induced deposition technique. This scheme was then implemented on nanopore arrays for nanoparticle localization. In addition, vesicle rupture-based lipid bilayer coating was adapted to truncated-pyramidal nanopores, which was shown to be effective for the minimizing DNA-pore interaction. Further, HfO2 coating by means of atomic layer deposition was employed to prevent the erosion of Si-based pores and to shrink the pore diameter, which enabled reliable investigations of DNA clogging and DNA polymerase docking.
For the surface associated phenomena, several findings were made. The lipid bilayer formation on truncated pyramidal nanopores via instantaneous rupture of individual vesicles was quantified based on combined ionic current monitoring and optical observation.  The probability of pore clogging appeared to linearly increase with the length of DNA strands and applied bias voltage, which could be attributed a higher probability of knotting and/or folding of longer DNA strands and more frequent translocation events at higher voltage. A free-energy based analytical model was proposed to evaluate the DNA-pore interaction and to interpret observed clogging behavior. Finally, docking of DNA polymerase on nanopore arrays was demonstrated using label-free optical method based on Ca2+ indicator dyes, which may open the avenue to sequencing-by-synthesis enabled by the docked polymerase.
 

2021
Development of the Spatial Atomic Layer Deposition (SALD) technique for the fabrication of p-type thin films of highly conductive copper (I) oxide
Author
Cesar, Arturo Masse de La Huerta
University
Université Grenoble Alpes (Grenoble, France)
Year
2021
Abstract

Future trends in materials and devices are strongly based on novel fabrication methods that allow for mass production with low cost and high throughput. Such methods must be finely optimized to achieve nanometric control without incurring in high costs. This can be achieved by developing a process that reduces the number of steps required, as well as by reducing the amount of human involvement in the process, which would increase the quality and reproducibility of the output. But the improvement of fabrication technologies cannot be optimized without considering the materials desired, along with its most fundamental chemical and physical properties. Hence, to successfully design the instrumentation needed for novel fabrication technologies with nanometric precision, the design methodology must consider multiple different subjects related to the chemistry, physics, mechanics, electronics and automation, all working together to achieve the desired objective. In this doctoral work, such design methodology was implemented with a diverse number of tools and approaches to successfully optimize a nanofabrication method called Spatial Atomic Layer Deposition (SALD) to deposit thin films of a material that has potential applications as a component of non-silicon solar energy devices, photoelectrochemical water splitting devices, and thin film transparent electronics, among others: cuprous oxide (Cu2O ). Regarding the fabrication technology and the mechatronic design, SALD is a promising fabrication technique that allows fabrication of thin films with nanometric precision and with the ability to control their mechanical, electrical and crystallographic properties. Furthermore, the SALD approach used in this thesis and in the Laboratoire des Matériaux et du Génie Physique (LMGP) works in the open-air (no deposition chamber), and thus is potentially an industrial-compatible approach for large area, homogeneous thin film fabrication with a high throughput. Additionally, SALD can be used with conditions that make it compatible with flexible substrates and with rollto- roll (R2R) approaches. Finally, SALD provides flexibility on the deposition process so that it can be tuned to obtain different properties on the films fabricated with minimal change in the instrumentation. In this thesis, some of the potential benefits of the flexible parameters of the SALD system are explored and the impact of some of them on fabricated films is presented. Using Computational Fluid Dynamics (CFD) simulations, the fluid mechanics phenomena that occur during the deposition process in the SALD system were analyzed for different configurations of the reactor. The influence on the film properties were studied and validation with experimental depositions were performed. Afterwards, using the knowledge and guidelines obtained with the CFD simulations, and in order to lower the cost and complexity of modifying some of the mechanical components of the system, a workflow including Computer Aided Design (CAD) and additive manufacturing (also known as 3D printing) was established at the LMGP for the fabrication of one of the main components of the SALD system at LMGP: the deposition head. The use of additive manufacturing has followed a rapid increase on applications, and, in this work, it is the first time that such innovative fabrication technique is applied to thin-film nanofabrication processes, providing numerous potential applications in the field. In this thesis, such workflow is shown and explained, and the guidelines learned, and limitations discovered are presented as well. Finally, after making some modifications on the system and adding the necessary components such as new heating systems and containers for the needed precursor, Cu2O was successfully deposited with the SALD method. Cu2O is one of the few materials with promising electronic properties as a p-type transparent semiconductor. It is also a material that allows for mass production, if coupled with an industrial-compatible fabrication method (such as SALD), thanks to its non-toxicity, its chemical and environmental stability and its earth abundance. Here, the fabricated Cu2O films using the SALD system at LMGP are reported, and their p-type conductivity and crystallography are analyzed. In the work done during this doctoral project, a systematic approach was used to analyze, adapt and optimize the SALD system at LMGP for the deposition of Cu2O. Using CFD simulations, CAD tools, 3D printing and automation, the whole process was successfully installed in the system and highly conductive Cu2O films can be now deposited their further study or for their integration in numerous types of devices. Furthermore, the results of this work provide initial guidelines for the industrial design of an SALD-based high-throughput fabrication system, in which the design of its components is optimized for each material desired. Such design approach, combined with the flexibility and low cost of the SALD, the flexibility of the mechanical design and fabrication of some of its components, and the speed of the deposition procedure, make this work also useful to further increase the amount of materials compatible with SALD, as well as to further develop the SALD methodology into innovative fabrication processes of materials and devices. 

2021
Atomic and Molecular Layer Processes for Industrial Applications in Semiconductors, Pharmaceuticals and Optics
Author
Tyler J. Myers
University
University of Colorado Boulder (Boulder, USA)
Year
2021
Abstract

Atomic layer deposition (ALD) is a thin film growth technique which deposits conformal, pin-hole free films with sub-nanometer precision. Molecular layer deposition (MLD) is an analogous process to ALD where molecular fragments are used to deposit all-organic or organic-inorganic hybrid films. Both ALD and MLD have been employed in numerous industries to advance technologies, notably in the semiconductor, energy storage, display and optics industries. In this thesis, I present three projects which utilize ALD and MLD processes for industrial applications in semiconductors, drug delivery and optical devices. The first project describes a study of the conversion of ZnO to Al2O3 using trimethylaluminum. Past instances of conversion are introduced, a number of analytical techniques are used to show evidence of the conversion mechanism and the generality of exchange reactions is discussed. Exchange reactions are becoming important to consider during ALD processes and as a processing tool in the semiconductor industry. The second project develops low-temperature MLD and ALD processes to coat nanoparticles. The construction of a new reactor built specifically for particle MLD is presented. Evidence of controlled polyamide MLD coatings is shown and we demonstrate MLD and ALD films may be used to modulate the release of pharmaceutical powders. The third project uses ALD to smooth surface roughness and improve the optical performance of Ag mirrors. Current smoothing techniques are abrasive and detrimental to mirror performance. The ALD process shows significant smoothing capabilities of both nano and microscale roughness and dramatically recovers reflectance performance lost due to optical scatter. These projects demonstrate the versatility of ALD and MLD processes and show precise thin film deposition techniques will continue to find use in numerous semiconductor and non-semiconductor industries.

2021
Transition metal dissolution from Li-ion battery cathodes
Author
Yonas Tesfamhret
University
Uppsala University (Uppsala, Sweden)
Year
2022
Abstract

Lithium-ion batteries (LIBs) have become reliable electrochemical energy storage systems due to their relative high energy and power density, in comparison to alternative battery chemistries. The energy density of current LIBs is limited by the average operating voltage and capacity of oxide-based cathode materials containing a variety of transition metals (TM). Furthermore, the low anodic stability of "conventional" carbonate-based electrolytes limits further extension of the LIBs voltage window. Here, ageing mechanisms of cathodes are investigated, with a main focus on TM dissolution and on strategies to tailor the cathode surface and the electrolyte composition to mitigate TM dissolution.
Atomic layer deposition (ALD) coatings of the cathode surface with electrically insulating Al2O3 and TiO2 coatings is employed and investigated as a method to stabilize the cathode/electrolyte interface and minimize TM dissolution. The thesis illustrates both the advantages and limitations of amorphous oxide coating materials during electrochemical cycling. The protective oxide layer restricts auto-catalytic salt degradation and the consequent propagation of acidic species in the electrolyte. However, a suboptimal coating contributes to a nonhomogeneous cathode surface ageing during electrochemical cycling. Furthermore, the widely accepted concept of charge disproportionation as the fundamental cause of TM dissolution is demonstrated to be a minor factor. Rather, a chemical dissolution mechanism based on acid-base/electrolyte-cathode interaction underlies substantial TM dissolution.
The thesis demonstrates LiPF6, and by implication HF, as the principal source of TM dissolution. In addition, the oxidative degradation of ethylene carbonate (EC) solvent contributes indirectly to generation of HF. Thus, an increase in electrolyte oxidative degradation products accelerates TM dissolution. Substituting EC and LiPF6 with a more anodically stable solvent (e.g., tetra-methylene sulfone) and a non-fluorinated salt (e.g., LiBOB or LiClO4) or addition of TM scavenging additives like lithium difluorophosphate (LiPO2F2) are here investigated as strategies to either i) mitigate TM dissolution, ii) supress TM migration and deposition on the anode surface, or iii) supress formation of acidic electrolyte degradation products and thereby TM dissolution. The thesis also highlights the necessity of taking precautions when attempting to replace the components, as reducing TM dissolution may come at the expense of electrochemical cycling performance.
 

2022
Plasma-Enhanced Atomic Layer Deposition and Vapor Phase Infiltration of ZnO - From Fundamental Growth Characteristics to Piezoelectric Films
Author
Julian Pilz
University
Graz University of Technology (Graz, Austria)
Year
2022
Abstract

In this thesis, the growth of the semiconducting material ZnO by two methods - plasmaenhanced atomic layer deposition and vapor phase infiltration - is investigated. As ZnO is utilized in diverse applications such as UV-protection, gas sensors, or piezoelectrics, precise knowledge about the characteristics of the growth process is needed to obtain the desired properties for a specific application. Plasma-enhanced atomic layer deposition (PE-ALD) is a thin film technique which can deposit uniformal and conformal films with high thickness control at low temperatures. The presented studies show that PE-ALD is able to deposit ZnO with small amount of impurities as low as room temperature. Furthermore, by variation of the substrate temperature, ideal temperature regions for specific applications and the relationship between growth and resulting properties could be identified. In the beginning of the deposition, deviations from the ideal growth occur, which are identified as substrateenhanced island growth. The formation of crystallites is found to occur after this initial growth periode. The obtained knowledge about these growth characteristics is furthermore applied to piezoelectric devices. The piezoresponse of ZnO, sandwiched between electrodes, is hereby studied on both flexible and rigid substrates with a combination of macroscopic and scanning probe techniques. Vapor phase infiltration (VPI) is a technique for transforming polymers into hybrid organic/inorganic materials. It often uses the same precursors as ALD but instead of growing a thin film on a substrate, the polymer free volume is infiltrated with the precursors. In the thesis, the successfull infiltration of ZnO into polyisoprene is presented. Polyisoprene is an elastomeric polymer, a class of polymers which has not been widely studied as a substrate for VPI. The infiltration kinetics and the chemical mechanisms of this system are presented and it is shown that pre-heating of the polymer largely affects these due to changes in thickness and chemical structure. Concluding, the thesis gives fundamental insights into the growth characteristics for a future application of ZnO thin films or polymer/ZnO hybrids in diverse fields as well as a demonstration of ZnO in a piezoelectric device. 

2022
Group 11–14 Triazenides: Synthesis, characterization, and thermal evaluation for use in chemical vapor deposition
Author
Rouzbeh Samii
University
Linköping University (Linköping , Sweden)
Year
2022
Abstract

Abstract Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are corner-stone techniques for depositing thin films in semi-conductor manufacturing. To deposit semiconductor grade materials, these techniques rely on high-performance precursors. This thesis covers synthesis, characterization, and evaluation of 1,3-dialkyltriazenides of group 11–14 metals as precursors for CVD and ALD. Triazenides had previously not been used as precursors for ALD, nor any other CVD process. The gallium and indium triazenides were used for ALD of indium- and gallium nitride and yielded materials of superior quality over other precursors. The success of these precursors sparked subsequent investigation into triazenides of zinc, and the group 11- and 14 metals. These triazenides showed high volatility and thermal stability making them highly interesting as CVD and ALD precursors. 

2022
Engineering inorganic nanostructured composites for boosting H2 and O2 evolution reactions
Author
Getachew Solomon
University
Luleå University of Technology (Luleå, Sweden)
Year
2022
Abstract

Hydrogen is considered a promising energy source with zero emission of CO2; it can provide higher energy density compared to other sources of energy. The amount at which H2 is produced, and the method of production need further improvement for the advancement of hydrogen energy technologies. Water electrolysis using renewable energy sources such as electrical, solar, and wind energy is one of the alternative technologies that can produce pure H2. However, water electrolysis itself is not an easy process, it requires a highly active catalyst capable of converting water into hydrogen, and oxygen.
This Ph.D. dissertation mainly focuses on developing efficient, robust, and low-cost catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and Oxygen reduction reaction (ORR). The work describes different strategies for improving the performance of the catalyst, such as creating nanocomposite, Nobel metal decoration, core-shell structures, hierarchical nanostructure, and cocatalyst and protective layers, which are vital for improving the efficiency of the catalyst. Consequently :
Nanocomposites composed of Ag2S nanoparticle, MoS2, and reduced graphene oxide (RGO) flake, with a 0D/2D/2D interface were synthesized. Ag2S nanoparticles were homogeneously distributed and embedded in a layer of semi-crystalline MoS2 nanosheets. The ternary catalyst results in a superior performance due to the intimate contact created by the 2D-2D interface (MoS2/RGO) and due to the uniformly grown Ag2S nanoparticles, which provides the ease of hydrogen adsorption by modulating the electronic properties, and exposure of highly rich active sites
Nobel metal decorated (Ag-decorated vertically aligned MoS2 nanoflakes) were developed and investigated for OER and ORR. Results of this work revealed that, due to the presence of silver, the catalyst shows more than 1.5 times an increase in the roughness-normalized rate of OER. Based on the rate constant values obtained during the ORR test, Ag-MoS2 proceeds through a mixed 4 electron and a 2 + 2 serial route reduction mechanism, suggesting that the presence of silver decreases the electron transfer number and increases the peroxide yield. 
A core-shell structure of hydrous NiMoO4 micro rods conformally covered by Co3O4 nanoparticles was developed and employed as an OER catalyst, showing a remarkable catalytic activity towards OER with a record low overpotential of 120 mV at 10 mA/cm2. Here, the strong interactions between core (hydrated NiMoO4) and shell (Co3O4) help to tune the electronic properties by modifying the active sites densities of the surface.
A hierarchical nanostructure composed of NiMoO4 nanorods and MoS2 nanosheets was synthesized on interconnected nickel foam substrates. The as-prepared hierarchical structure exhibits excellent OER performance due to its numerous exposed active sites for adsorbing oxygen intermediates which are beneficial for promoting the enhancement of the OER catalytic performance
Cu2O photocathode protected by a very thin layer of TiO2 and an amorphous Vox were synthesized and used for HER, with aim of improving the photostability of Cu2O. Photooxidation of Cu2O nanowires are minimized by growing TiO2 protective layer and an amorphous VOx cocatalyst. After optimization of the overlayer and the cocatalyst, the photoelectrode exhibits a stable photocurrent density for an extended illumination time. 
Besides, advanced characterization tools were used for tracking ORR reaction intermediates and OER active sites. RRDE, Operando Raman, and synchrotron-based photoemission spectroscopy analysis were utilized together with Post OER characterization tools to reveal the reason behind the higher catalytic activity of the catalyst. 
In summary, the presented outcomes can significantly contribute to the fundamental insight towards improving the efficiency of HER, OER, and ORR catalyst, by offering a clear and in-depth understanding of the preparation and characterization of cheap and efficient catalysts.
 

2022
Electronic and Self-healing Properties of Polymer-Inorganic Hybrids Enabled by Vapor Phase Infiltration
Author
Oksana Yurkevich
University
CIC nanoGUNE Nanomaterials group (San Sebastián, Spain)
Year
2022
Abstract

Enabling self-healing of materials is crucially important for saving resources and energy in numerous emerging applications. A plethora of recently published research works is dedicated to the development of strategies which allow for self-healing of materials, especially of those with certain technological importance. Given that most of the approaches are based on chemical processes, the vast majority of these works focus on the self-healing of organic materials, specifically polymers. At the same time, there is a growing demand for adapting such functionality to inorganic materials due to their importance in most developed electronics, including flexible electronics. The few existent examples of self-healing of inorganic materials rely on the incorporation of liquid healing agents, such as liquid metals or liquid precursors, into the devices. However, the development is in its infancy and further progress remains very challenging, mainly because of a lack of feasible healing agents and suitable ways to supply them to the damaged site.


In this thesis we have developed a concept for the self-healing of metal oxides, which is the most challenging type of material in this research area. This concept consists of growing metal oxide nanoparticles inside the bulk of halogenated polymers via vapor phase infiltration and their subsequent entropy-driven migration to externally induced defect sites, which eventually leads to the recovery of the defect. The hybrid material, i.e., the polymer matrix with dispersed NPs, can serve as a reservoir with healing agents for the repair of a cracked MeO film. The self-healing of inorganic materials and structures was realized also without liquid agents by making use of the mobility of inorganic NPs within polymers, as the spatial distribution of NPs can be tuned by means of harnessing both enthalpy and entropy.


Herein we present an expansion of the pool of self-healing materials to semiconductors such as indium, zinc, indium tin and zinc indium oxides, thereby allowing to increase the reliability and sustainability of future functional materials. We revealed that not only the morphology, but also the electrical properties of ITO can be largely restored upon healing. Such properties are of immediate interest for the further development of transparent flexible electrodes.
 

2022
Atomic Layer Deposition of Two-Dimensional Metal Dichalcogenides
Author
Miika Mattinen
University
University of Helsinki (Helsinki, Finland)
Year
2022
Abstract

Two-dimensional (2D) materials rank among the most scientifically exciting materials of the early 21st century. Transition metal dichalcogenides (TMDCs) have emerged into the spotlight due to the semiconducting nature of many TMDCs, which is in contrast to the most actively studied 2D material, semimetallic graphene. Research on the basic properties of TMDCs has been very active and fruitful, resulting in unveiling of many new phenomena and properties. Furthermore, there is a strong drive to realize the technological potential of TMDCs. For use in practical applications, TMDCs need to be synthesized as uniform films of controlled thickness on large and complex substrates. In order to realize cost-effective industrial production, the synthesis needs to be done at low temperatures using methods that are highly controllable, scalable, and repeatable. Atomic layer deposition (ALD) is an advanced gas-phase thin film deposition technique capable of fulfilling the requirements of many demanding applications. ALD has already proven its industrial applicability in fields ranging from electroluminescent displays to microelectronics, photovoltaics, and corrosion protection. To realize the potential of ALD in the deposition of TMDCs, suitable ALD precursors possessing adequate reactivity, volatility, and thermal stability have to be identified and evaluated. In this thesis, 29 precursor candidates were tested for seven metals. Successful ALD processes were developed for five 2D sulfides: MoS2, SnS2, WS2, HfS2, and ZrS2. In addition, ALD processes were developed for oxides of molybdenum and tungsten. The oxides may be converted into the respective 2D sulfides. Furthermore, α-MoO3 is a 2D material by itself. The sulfide processes varied in terms of their growth behavior and morphology of the films. All of the films crystallized in 2D structures. In the case of SnS2 and WS2, crystallization required mild post-deposition annealing, which preserves the smooth morphology of the as-deposited amorphous films and gives an additional degree of freedom in processing. Particular attention was paid to the role of the substrate in the growth of TMDCs in tuning the film growth, morphology, and crystallinity. HfS2, MoS2, SnS2, and ZrS2 films were observed to grow in a van der Waals epitaxial manner on mica, which is a promising approach to achieve high film quality under mild conditions. Once the deposition processes are developed, the produced films should be evaluated for the target applications. A major challenge is to improve the performance of large-area TMDC films grown under application-relevant conditions up to the level of TMDC flakes that have been manually exfoliated from bulk crystals. The possible applications of ALD TMDCs are comprehensively reviewed in the literature part of the thesis. In the experimental part, results on photodetector (HfS2, SnS2, and ZrS2), field-effect transistor (SnS2), and hydrogen gas sensor (MoOx) devices are shown. It is anticipated that the processes developed in this thesis can be used also for other applications. For example, the rough MoS2 and disordered WS2 films should be promising for energy storage and conversion applications. 

2022
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