Browsing by Subject "thin films"

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  • Kalam, Kristjan; Seemen, Helina; Ritslaid, Peeter; Rähn, Mihkel; Tamm, Aile; Kukli, Kaupo; Kasikov, Aarne; Link, Joosep; Stern, Raivo; Duenas, Salvador; Castan, Helena; Garcia, Hector (2018)
    Thin solid films consisting of ZrO2 and Fe2O3 were grown by atomic layer deposition (ALD) at 400 degrees C. Metastable phases of ZrO2 were stabilized by Fe2O3 doping. The number of alternating ZrO2 and Fe2O3 deposition cycles were varied in order to achieve films with different cation ratios. The influence of annealing on the composition and structure of the thin films was investigated. Additionally, the influence of composition and structure on electrical and magnetic properties was studied. Several samples exhibited a measurable saturation magnetization and most of the samples exhibited a charge polarization. Both phenomena were observed in the sample with a Zr/Fe atomic ratio of 2.0.
  • Mattinen, Miika; Hatanpaa, Timo; Sarnet, Tiina; Mizohata, Kenichiro; Meinander, Kristoffer; King, Peter J.; Khriachtchev, Leonid; Räisänen, Jyrki; Ritala, Mikko; Leskelä, Markku (2017)
    Molybdenum disulfide (MoS2) is a semiconducting 2D material, which has evoked wide interest due to its unique properties. However, the lack of controlled and scalable methods for the production of MoS2 films at low temperatures remains a major hindrance on its way to applications. In this work, atomic layer deposition (ALD) is used to deposit crystalline MoS2 thin films at a relatively low temperature of 300 degrees C. A new molybdenum precursor, Mo(thd)(3) (thd = 2,2,6,6-tetramethylheptane-3,5-dionato), is synthesized, characterized, and used for film deposition with H2S as the sulfur precursor. Self-limiting growth with a low growth rate of approximate to 0.025 angstrom cycle(-1), straightforward thickness control, and large-area uniformity are demonstrated. Film crystallinity is found to be relatively good considering the low deposition temperature, but the films have significant surface roughness. Additionally, chemical composition as well as optical and wetting properties are evaluated. MoS2 films are deposited on a variety of substrates, which reveal notable differences in growth rate, surface morphology, and crystallinity. The growth of crystalline MoS2 films at comparably low temperatures by ALD contributes toward the use of MoS2 for applications with a limited thermal budget.
  • Väyrynen, Katja; Hatanpää, Timo; Mattinen, Miika; Mizohata, Kenichiro; Meinander, Kristoffer; Räisänen, Jyrki; Link, Joosep; Stern, Raivo; Ritala, Mikko; Leskela, Markku (2019)
    Intermetallics form a versatile group of materials that possess unique properties ranging from superconductivity to giant magnetoresistance. The intermetallic Co-Sn and Ni-Sn compounds are promising materials for magnetic applications as well as for anodes in lithium- and sodium-ion batteries. Herein, a method is presented for the preparation of Co3Sn2 and Ni3Sn2 thin films using diamine adducts of cobalt(II) and nickel(II) chlorides, CoCl2(TMEDA) and NiCl2(TMPDA) (TMEDA = N,N,N ',N '-tetramethylethylenediamine, TMPDA = N,N,N ',N '-tetramethyl-1,3-propanediamine) combined with tributyltin hydride. The films are grown by atomic layer deposition (ALD), a technique that enables conformal film deposition with sub-nanometer thickness control. The Co3Sn2 process fulfills the typical ALD qualifications, such as self-limiting growth, excellent film uniformity, and conformal coverage of a trench structure. X-ray diffraction (XRD) shows reflections characteristic to the hexagonal Co3Sn2 phase, which confirms that the films are, indeed, intermetallic instead of being mere alloys of Co and Sn. The films are extremely pure with impurity levels each below 1.0 at.%. Ni3Sn2 films similarly exhibit the expected XRD reflections for the intermetallic phase and are of high purity. The Co3Sn2 film show magnetic hysteresis with high coercivity values exceeding 500 Oe, indicating great potential in terms of applicability of the films.
  • Mattinen, Miika; King, Peter J.; Khriachtchev, Leonid; Meinander, Kristoffer; Gibbon, James T.; Dhanak, Vin R.; Räisänen, Jyrki; Ritala, Mikko; Leskelä, Markku (2018)
    Semiconducting 2D materials, such as SnS2, hold immense potential for many applications ranging from electronics to catalysis. However, deposition of few-layer SnS2 films has remained a great challenge. Herein, continuous wafer-scale 2D SnS2 films with accurately controlled thickness (2 to 10 monolayers) are realized by combining a new atomic layer deposition process with low-temperature (250 degrees C) postdeposition annealing. Uniform coating of large-area and 3D substrates is demonstrated owing to the unique self-limiting growth mechanism of atomic layer deposition. Detailed characterization confirms the 1T-type crystal structure and composition, smoothness, and continuity of the SnS2 films. A two-stage deposition process is also introduced to improve the texture of the films. Successful deposition of continuous, high-quality SnS2 films at low temperatures constitutes a crucial step toward various applications of 2D semiconductors.
  • Mäntymäki, Miia; Ritala, Mikko; Leskelä, Markku (2018)
    Lithium-ion batteries are the enabling technology for a variety of modern day devices, including cell phones, laptops and electric vehicles. To answer the energy and voltage demands of future applications, further materials engineering of the battery components is necessary. To that end, metal fluorides could provide interesting new conversion cathode and solid electrolyte materials for future batteries. To be applicable in thin film batteries, metal fluorides should be deposited with a method providing a high level of control over uniformity and conformality on various substrate materials and geometries. Atomic layer deposition (ALD), a method widely used in microelectronics, offers unrivalled film uniformity and conformality, in conjunction with strict control of film composition. In this review, the basics of lithium-ion batteries are shortly introduced, followed by a discussion of metal fluorides as potential lithium-ion battery materials. The basics of ALD are then covered, followed by a review of some conventional lithium-ion battery materials that have been deposited by ALD. Finally, metal fluoride ALD processes reported in the literature are comprehensively reviewed. It is clear that more research on the ALD of fluorides is needed, especially transition metal fluorides, to expand the number of potential battery materials available.
  • Majorin, Peter (Helsingin yliopisto, 2020)
    The ionized cluster beam (ICB) technique for thin film deposition can produce thin films of superior quality compared to conventional methods. This technique relies on depositing nanoclusters as building blocks for the thin film. To gain a deeper insight into the thin film formation process, molecular dynamics simulations can be used. In this thesis, Ge nanocluster deposition on a (001) Ge substrate at 300 K was studied with molecular dynamics. The Stillinger-Weber potential was used in all of the simulations, and the nanoclusters had a kinetic energy of 5 meV - 10 eV. The formed Ge thin films were analyzed for porosity, density, crystallinity, and coordination. The density analysis showed that porous films were formed with lower deposition energies of 5 meV - 0.5 eV, and dense amorphous films with higher deposition energies of 1.0 eV - 10 eV. The radial distribution function and a crystalline atom count showed that the degree of crystallinity of the thin film is gradually lost with increasing deposition energies. Almost no epitaxial growth of the deposited thin film was detected. In addition, small amounts of hexagonal Ge was detected on the surfaces of the deposited nanoclusters with deposition energies of 5 meV - 100 meV. By heating a free nanocluster we showed that hexagonal Ge is formed on nanocluster surfaces independently of the cluster-surface interactions. With higher deposition energies of 0.5 eV - 10 eV all nanocrystallinity was lost, and a density analysis showed up to 10% higher density than bulk crystalline density. The higher density was also accompanied by a higher mean coordination number of around 4.5. This unusually dense amorphous material has not been reported elsewhere, so it is likely that the parametrization of the Stillinger-Weber potential overestimates the coordination and density of the formed thin film. The fast heat dissipation used in the simulations may also be a part of this problem, and it is not known how well it matches experimental settings.
  • Hämäläinen, Jani; Mizohata, Kenichiro; Meinander, Kristoffer; Mattinen, Miika; Vehkamäki, Marko; Räisänen, Jyrki; Ritala, Mikko; Leskelä, Markku (2018)
    Abstract Rhenium is both a refractory metal and a noble metal that has attractive properties for various applications. Still, synthesis and applications of rhenium thin films have been limited. We introduce herein the growth of both rhenium metal and rhenium nitride thin films by the technologically important atomic layer deposition (ALD) method over a wide deposition temperature range using fast, simple, and robust surface reactions between rhenium pentachloride and ammonia. Films are grown and characterized for compositions, surface morphologies and roughnesses, crystallinities, and resistivities. Conductive rhenium subnitride films of tunable composition are obtained at deposition temperatures between 275 and 375 °C, whereas pure rhenium metal films grow at 400 °C and above. Even a just 3 nm thick rhenium film is continuous and has a low resistivity of about 90 µΩ cm showing potential for applications for which also other noble metals and refractory metals have been considered.