Browsing by Subject "THIN-FILMS"

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  • Zhang, Chao; Vehkamäki, Marko; Pietikäinen, Mika; Leskelä, Markku; Ritala, Mikko (2020)
    Novel area-selective molecular layer deposition (AS-MLD) of polyimide (PI) on Cu versus native SiO2 was studied. By use of 1,6-diaminohexane (DAH) and pyromellitic dianhydride (PMDA) as precursors, PI films can be selectively deposited on the Cu surface at 200-210 degrees C with a rate around 7.8 A/cycle while negligible growth takes place on SiO2. The selectivity was successfully demonstrated also on Cu/SiO2 patterns at 200 degrees C; after 180 MLD cycles, around 140 nm thick PI was deposited on Cu regions while
  • Seppälä, Sanni; Niinistö, Jaakko; Mattinen, Miika; Mizohata, Kenichiro; Räisänen, Jyrki; Noh, Wontae; Ritala, Mikko; Leskelä, Markku (2018)
    La2O3 thin films were deposited by atomic layer deposition from a liquid heteroleptic La precursor, La(iPrCp)2(iPr-amd), with either water, ozone, ethanol, or both water and ozone (separated by a purge) as the oxygen source. The effect of the oxygen source on the film growth rate and properties such as crystallinity and impurities was studied. Saturation of the growth rate was achieved at 225 °C with O3 as the oxygen source. With water, very long purge times were used due to the hygroscopicity of La2O3 but saturation of the growth rate was not achieved. Interestingly, when an O3 pulse was added after the water pulse with a purge in between, the growth rate decreased and the growth saturated at 200 °C. With ethanol lanthanum hydroxide was formed instead of La2O3 at 200–275 °C whereas hexagonal La2O3 films were obtained at 300 °C but the growth was not saturative. Using the separate pulses of water and ozone in the same deposition provided the best results from the four studied deposition processes. After annealing the films deposited with the La(iPrCp)2(iPrAMD)/H2O/O3 process showed pure hexagonal phase in all the films regardless of the deposition temperature, whereas mixtures of cubic and hexagonal La2O3 were seen with the other processes.
  • Pale, Ville; Nikkonen, Taru; Vapaavuori, Jaana; Kostiainen, Mauri; Kavakka, Jari; Selin, Jorma; Tittonen, Ilkka; Helaja, Juho (2013)
  • Ruoko, Tero-Petri; Hiltunen, Arto; Iivonen, Tomi; Ulkuniemi, Riina; Lahtonen, Kimmo; Ali-Löytty, Harri; Mizohata, Kenichiro; Valden, Mika; Leskelä, Markku; Tkachenko, Nikolai V. (2019)
    We employ atomic layer deposition to prepare 50 nm thick hematite photoanodes followed by passivating them with a 0.5 nm thick Ta2O5-overlayer and compare them with samples uniformly doped with the same amount of tantalum. We observe a three-fold improvement in photocurrent with the same onset voltage using Ta-overlayer hematite photoanodes, while electrochemical impedance spectroscopy under visible light irradiation shows a decreased amount of surface states under water splitting conditions. The Tadoped samples have an even higher increase in photocurrent along with a 0.15 V cathodic shift in the onset voltage and decreased resistivity. However, the surface state capacitance for the Ta-doped sample is twice that of the reference photoanode, which implies a larger amount of surface hole accumulation. We further utilize transient absorption spectroscopy in the sub-millisecond to second timescale under operating conditions to show that electron trapping in both Ta2O5-passivated and Ta-doped samples is markedly reduced. Ultrafast transient absorption spectroscopy in the sub-picosecond to nanosecond timescale shows faster charge carrier dynamics and reduced recombination in the Ta-doped hematite photoanode resulting in the increased photoelectrochemical performance when compared with the Ta2O5-overlayer sample. Our results show that passivation does not affect the poor charge carrier dynamics intrinsic to hematite based photoanodes. The Ta-doping strategy results in more efficient electron extraction, solving the electron trapping issue and leading to increased performance over the surface passivation strategy.
  • Seppälä, Sanni; Vehkamäki, Marko; Mizohata, Kenichiro; Noh, Wontae; Räisänen, Jyrki; Ritala, Mikko; Leskelä, Markku (2019)
    Three heteroleptic Zr precursors were studied for atomic layer deposition (ALD) of ZrO2. Films were deposited from Zr(Cp)((t)BuDAD)((OPr)-Pr-i), Zr(MeCp)(TMEA), and Zr(Me5Cp)(TEA) with either water or ozone as the oxygen source {tBuDAD = N, N-bis(tertbutyl) ethene-1,2-diaminato, TMEA = tris[2-(methylamino) ethyl]aminate, TEA = triethoanolaminate}. Self-limiting film growth was confirmed for the Zr(Cp)((t)BuDAD)((OPr)-Pr-i)/O-3 process at 250 degrees C and for the Zr(M(e)5Cp)(TEA)/O-3 process at 375 degrees C, which is among the highest temperatures for advanced heteroleptic precursors. Excellent film purity with C, H, and N levels below the detection limit of the elastic recoil detection analysis was obtained with ozone as the oxygen source. All the studied processes showed the same trend that at low deposition temperatures films were tetragonal ZrO2 and at higher temperatures mixtures of tetragonal and monoclinic ZrO2. With water, the monoclinic phase appeared at higher temperatures than with ozone. In addition to the deposition temperature, the film thickness affected the phase; thinner films favored the tetragonal phase and monoclinic peaks were more clearly seen in thicker films. The high thermal stability and excellent film purity show that from the three studied Zr precursors, Zr(Me5Cp)(TEA) is a noteworthy precursor candidate for ALD of ZrO2. Published by the AVS.
  • Davodi, Fatemeh; Mühlhausen, Elisabeth; Settipani, Daniel; Rautama, Eeva-Leena; Honkanen, Ari-Pekka; Huotari, Simo; Marzun, Galina; Taskinen, Pekka; Kallio, Tanja (2019)
    Core-shell nanoparticles represent a class of materials that exhibit a variety of properties. By rationally tuning the cores and the shells in such nanoparticles (NPs), a range of materials with tailorable properties can be produced which are of interest for a wide variety of applications. Herein, experimental and theoretical approaches have been combined to show the structural transformation of NPs resulting to the formation of either NiFexCy encapsulated in ultra-thin graphene layer (NiFe@UTG) or Ni3C/FexCy@FeOx NPs with the universal one-step pulse laser ablation in liquid (PLAL) method. Analysis suggests that carbon in Ni3C is the source for the carbon shell formation, whereas the final carbon-shell thickness in the NPs originates from the difference between Ni3C and FexCy phases stability at room temperature. The ternary Ni-Fe-C phase diagram calculations reveal the competition between carbon solubility in the studied metals (Ni and Fe) and their tendency toward oxidation as the key properties to produce controlled core-shell NP materials. As an application example, the electrocatalytic hydrogen evolution current on the different NPs is measured. The electrochemical analysis of the NPs reveals that NiFe@UTG has the best performance amongst the NPs in this study in both alkaline and acidic media.
  • Mattinen, Miika; King, Peter J.; Bruener, Philipp; Leskelä, Markku; Ritala, Mikko (2020)
    Semiconducting 2D materials, such as SnS2, hold great promise in a variety of applications including electronics, optoelectronics, and catalysis. However, their use is hindered by the scarcity of deposition methods offering necessary levels of thickness control and large-area uniformity. Herein, a low-temperature atomic layer deposition (ALD) process is used to synthesize up to 5x5 cm(2)continuous, few-layer SnS(2)films on a variety of substrates, including SiO2/Si, Si-H, different ALD-grown films (Al2O3, TiO2, and Ir), sapphire, and muscovite mica. As a part of comprehensive film characterization, the use of low energy ion scattering (LEIS) is showcased to determine film continuity, coverage of monolayer and multilayer areas, and film thickness. It is found that on sapphire substrate, continuous films are achieved at lower thicknesses compared to the other substrates, down to two monolayers or even less. On muscovite mica, van der Waals epitaxial growth is realized after the post-deposition annealing, or even in the as-deposited films when the growth is performed at 175 to 200 degrees C. This work highlights the importance of the substrate choice for 2D materials and presents a practical low-temperature method for the deposition of high-quality SnS(2)films that may be further evaluated for a range of applications.
  • Montonen, Risto; Kassamakov, Ivan; Lehmann, Peter; Österberg, Kenneth; Haeggström, Edward (2018)
    The group refractive index is important in length calibration of Fourier domain interferometers by transparent transfer standards. We demonstrate accurate group refractive index quantification using a Fourier domain short coherence Sagnac interferometer. Because of a justified linear length calibration function, the calibration constants cancel out in the evaluation of the group refractive index, which is then obtained accurately from two uncalibrated lengths. Measurements of two standard thickness coverslips revealed group indices of 1.5426 +/- 0.0042 and 1.5434 +/- 0.0046, with accuracies quoted at the 95% confidence level. This agreed with the dispersion data of the coverslip manufacturer and therefore validates our method. Our method provides a sample specific and accurate group refractive index quantification using the same Fourier domain interferometer that is to be calibrated for the length. This reduces significantly the requirements of the calibration transfer standard. (C) 2018 Optical Society of America
  • Venäläinen, A.; Jalkanen, P.; Tuboltsev, V.; Savin, A.; Räisänen, J. (2018)
    Cluster deposited Pd films exhibit ferromagnetism in the temperature range from 1.8 to 400K. The magnetization properties are found to be dependent on the film thickness. The varying morphology of the resulting Pd film with respect to thickness suggests that cluster size, deposition energy, and substrate type are crucial for the resulting film magnetization. This is demonstrated by the characteristic ferromagnetic hysteresis with the temperature dependent saturation magnetization, remanence. and coercivity of palladium nanocluster aggregates. The temperature dependence of the saturation magnetization, remanence, and coercivity of Pd nanoclusters were measured using an ultra-high-sensitive magnetometer based on a superconducting quantum interference device, and the morphology of the samples was analyzed by tunneling electron microscopy. Published by AIP Publishing.
  • Trochowski, Mateusz; Kobielusz, Marcin; Mroz, Krystian; Surowka, Marcin Karol; Hämäläinen, Jani; Iivonen, Tomi; Leskelä, Markku; Macyk, Wojciech (2019)
    Synthetic procedures, including doping, sintering and surface coating, can noticeably affect the physicochemical properties of semiconductors. Introduced changes very often translate into photocatalytic and photoelectrochemical activity alterations. However, in this work we have focused on more subtle treatments, which result in lack of changes observed using XRD, UV-vis, porosimetry, TEM or SEM. We have subjected titanium dioxide (P25, UV100) to a treatment with reducing agents used in procedures of noble metal deposition (citrate, borohydride, and photoreduction), or surface decoration with small amounts of TiO2 by atomic layer deposition (ALD; 10 to 200 deposition cycles), which presumably should be neutral to its activity. Although the "classical" characterization methods did not show any differences between the original and treated samples, spectroelectrochemical (SE-DRS) determination of the density of states (DOS) and catechol adsorption tests revealed a significant influence of such treatments on the photocatalytic activity (photogeneration of HO radicals, water reduction, and herbicide degradation) and photoelectrochemical behaviour of the studied samples. We have shown that the applied slight surface modifications of titanium dioxide ("insignificant" at the first glance) may strongly affect the activity of this material. Such often overlooked effects must be taken into account during a comparative photoactivity analysis of various semiconductors, since an insignificant surface treatment may noticeably influence surface chemistry. We have also demonstrated that SE-DRS can be considered as a useful tool to study these effects, although it can be difficult to correlate a particular treatment with recorded changes in the density of states.
  • Lan, Hangzhen; Salmi, Leo D.; Rönkkö, Tuukka; Parshintsev, Jevgeni; Jussila, Matti; Hartonen, Kari; Kemell, Marianna; Riekkola, Marja-Liisa (2018)
    New chemical vapor reaction (CVR) and atomic layer deposition (ALD)-conversion methods were utilized for preparation of metal organic frameworks (MOFs) coatings of solid phase microextraction (SPME) Arrow for the first time. With simple, easy and convenient one-step reaction or conversion, four MOF coatings were made by suspend ALD iron oxide (Fe2O3) film or aluminum oxide (Al2O3) film above terephthalic acid (H2BDC) or trimesic acid (H3BTC) vapor. UIO-66 coating was made by zirconium (Zr)-BDC film in acetic acid vapor. As the first documented instance of all-gas phase synthesis of SPME Arrow coatings, preparation parameters including CVR/conversion time and temperature, acetic acid volume, and metal oxide film/metal-ligand films thickness were investigated. The optimal coatings exhibited crystalline structures, excellent uniformity, satisfactory thickness (2-7.5 μm), and high robustness (>80 times usage). To study the practical usefulness of the coatings for the extraction, several analytes with different chemical properties were tested. The Fe-BDC coating was found to be the most selective and sensitive for the determination of benzene ring contained compounds due to its highly hydrophobic surface and unsaturated metal site. UIO-66 coating was best for small polar, aromatic, and long chain polar compounds owing to its high porosity. The usefulness of new coatings were evaluated for gas chromatography-mass spectrometer (GC-MS) determination of several analytes, present in wastewater samples at three levels of concentration, and satisfactory results were achieved.
  • Jogiaas, Taivo; Kull, Mikk; Seemen, Helina; Ritslaid, Peeter; Kukli, Kaupo; Tamm, Aile (2020)
    Nanolaminates of ZrO2 and HfO2 were grown by atomic layer deposition, using metal halides and water as precursors, on silicon and fused quartz substrates at 300 degrees C. The crystalline phase composition, optical refraction, and mechanical performance of the multilayers were influenced by the relative contents of the constituent metal oxides. The crystal growth in as-deposited HfO2 dominantly led to the monoclinic phase, whereas ZrO2 was partially crystallized as its metastable and hard tetragonal polymorph. The hardness and elasticity of the nanolaminate structures could be modified by varying the amounts of either oxide contributing to the crystallographic order formed in the solid films. The refractive indexes depended on the nanolaminate structure.
  • Zhang, Chao; Kalliomäki, Jesse; Leskelä, Markku; Ritala, Mikko (2018)
    This paper presents a new method for depositing patterned films by atomic layer deposition (ALD) using Parafilm as a mask to block the film growth on selected areas of the substrate surface. This offers an easy and efficient method for large area patterning from the millimeter to even meter scale as needed, for example, for protecting contact areas in integrated circuits and microelectromechanical system devices and in preventing film growth on the backside of substrates. It is shown here that Parafilm can protect the substrate against Al2O3, TiO2, and Ir film growth effectively. However, outgassing from the Parafilm affects the film growth on the unmasked areas differently for the three materials. For Al2O3, there are no significant effects on the growth rate and film quality on the nonmasked areas and the thickness profiles next to the Parafilm masked areas are narrow. For TiO2 and Ir, by contrast, the thickness profiles are wider and outgassing also slows down the Ir growth. Energy dispersive x-ray analysis confirms that Ir does not grow on Parafilm. Al2O3 and TiO2 films do grow on Parafilm, but after the ALD process, the Parafilm can be peeled off easily with tweezers and the areas under the Parafilm mask are without any deposition. (C) 2017 American Vacuum Society.
  • Miikkulainen, Ville; Vayrynen, Katja; Mizohata, Kenichiro; Räisänen, Jyrki; Vehkamäki, Marko; Ritala, Mikko (2019)
    Photoassisted atomic layer deposition (photo-ALD) is a variant of an ALD process where photons of ultraviolet or visible range are utilized to supply energy to, and to modify, the ALD surface reactions. In this paper, the authors report photo-ALD processes for titanium, zirconium, hafnium, niobium, and tantalum oxides by employing the corresponding liquid, volatile metal alkoxides as precursors in a single-source approach, i.e., without any additional reactant. The ALD reactor was equipped with a light source delivering photons over a continuous spectrum between 190 and 800 nm in wavelength. The deposition sequence consisted of a precursor pulse, a purge, a photon exposure, and another purge. The process characteristics and film properties were explored. NbT2O5 and Ta2O5 films were amorphous, whereas TiO2, ZrO2, and HfO2 showed an amorphous and polycrystalline structure, depending on the deposition conditions. With photo-ALD, area-selective deposition is realized by shadow masking. The character of the growth process, i.e., whether the chemistry is driven by photolytic or photothermal mechanism, is discussed based on deposition experiments with patterned substrates and optical filtering. Electrical characterization of photo-ALD HfO2 shows promising dielectric properties. Published by the AVS.
  • Bakos, Laszlo Peter; Justh, Nora; da Costa, Ulisses Carlo Moura da Silva Bezerra; Laszlo, Krisztina; Labar, Janos Laszlo; Igricz, Tamas; Varga-Josepovits, Katalin; Pasierb, Pawel; Farm, Elina; Ritala, Mikko; Leskelä, Markku; Szilagyi, Imre Miklos (2020)
    TiO2 and ZnO single and multilayers were deposited on hydroxyl functionalized multi-walled carbon nanotubes using atomic layer deposition. The bare carbon nanotubes and the resulting heterostructures were characterized by TG/DTA, Raman, XRD, SEM-EDX, XPS, TEM-EELS-SAED and low temperature nitrogen adsorption techniques, and their photocatalytic and gas sensing activities were also studied. The carbon nanotubes (CNTs) were uniformly covered with anatase TiO2 and wurtzite ZnO layers and with their combinations. In the photocatalytic degradation of methyl orange, the most beneficial structures are those where ZnO is the external layer, both in the case of single and double oxide layer covered CNTs (CNT-ZnO and CNT-TiO2-ZnO). The samples with multilayer oxides (CNT-ZnO-TiO2 and CNT-TiO2-ZnO) have lower catalytic activity due to their larger average densities, and consequently lower surface areas, compared to single oxide layer coated CNTs (CNT-ZnO and CNT-TiO2). In contrast, in gas sensing it is advantageous to have TiO2 as the outer layer. Since ZnO has higher conductivity, its gas sensing signals are lower when reacting with NH3 gas. The double oxide layer samples have higher resistivity, and hence a larger gas sensing response than their single oxide layer counterparts.
  • Heikkinen, Joonas J.; Kaarela, Tiina; Ludwig, Anastasia; Sukhanova, Tatiana; Khakipoor, Shokoufeh; Kim, Sung Il; Han, Jeon Geon; Huttunen, Henri J.; Rivera, Claudio; Lauri, Sari E.; Taira, Tomi; Jokinen, Ville; Franssila, Sami (2018)
    Carbon-based materials have attracted much attention in biological applications like interfacing electrodes with neurons and cell growth platforms due to their natural biocompatibility and tailorable material properties. Here we have fabricated sputtered carbon thin film electrodes for bioelectrical measurements. Reactive ion etching (RIE) recipes were optimized with Taguchi method to etch the close field unbalanced magnetron sputtered carbon thin film (nanocarbon, nC) consisting of nanoscale crystalline sp(2)-domains in amorphous sp(3)-bonded backbone. Plasma etching processes used gas mixtures of Ar/O-2/SF6/CHF3 for RIE and O-2/SF6 for ICP-RIE. The highest achieved etch rate for nanocarbon was >> 389 nm/min and best chromium etch mask selectivity was 135:1. Biocompatibility of the material was tested with rat neuronal cultures. Next, we fabricated multielectrode arrays (MEA) with carbon recording electrodes and metal wiring. Organotypic brain slices grown on the MEAs were viable and showed characteristic spontaneous electrical network activity. The results demonstrate that interactions with nanocarbon substrate support neuronal survival and maturation of functional neuronal networks. Thus the material can have wide applications in biomedical research.
  • Woll, K.; Bergamaschi, A.; Avchachov, K.; Djurabekova, F.; Gier, S.; Pauly, C.; Leibenguth, P.; Wagner, C.; Nordlund, K.; Muecklich, F. (2016)
    Established and already commercialized energetic materials, such as those based on Ni/Al for joining, lack the adequate combination of high energy density and ductile reaction products. To join components, this combination is required for mechanically reliable bonds. In addition to the improvement of existing technologies, expansion into new fields of application can also be anticipated which triggers the search for improved materials. Here, we present a comprehensive characterization of the key parameters that enables us to classify the Ru/Al system as new reactive material among other energetic systems. We finally found that Ru/Al exhibits the unusual integration of high energy density and ductility. For example, we measured reaction front velocities up to 10.9 (+/- 0.33) ms(-1) and peak reaction temperatures of about 2000 degrees C indicating the elevated energy density. To our knowledge, such high temperatures have never been reported in experiments for metallic multilayers. In situ experiments show the synthesis of a single-phase B2-RuAl microstructure ensuring improved ductility. Molecular dynamics simulations corroborate the transformation behavior to RuAl. This study fundamentally characterizes a Ru/Al system and demonstrates its enhanced properties fulfilling the identification requirements of a novel nanoscaled energetic material.
  • Majtyka, Anna; Nowak, Anna; Marchand, Benoit; Chrobak, Dariusz; Ritala, Mikko; Räisänen, Jyrki; Nowak, Roman (2016)
    The present paper pertains to mechanical properties and structure of nanocrystalline multiferroic BeFiO(3) (BFO) thin films, grown by atomic layer deposition (ALD) on the Si/SiO2/Pt substrate. The usage of sharp-tip-nanoindentation and multiple techniques of structure examination, namely, grazing incidence X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectrometry, enabled us to detect changes in elastic properties (95 GPa
  • Grosjean, Sylvain; Hodapp, Patrick; Hassan, Zahid; Woell, Christof; Nieger, Martin; Bräse, Stefan (2019)
    Modular synthesis of structurally diverse functionalized azobiphenyls and azoterphenyls for the realization of optically switchable materials has been described. The corresponding synthesis of azobiphenyls and azoterphenyls by stepwise Mills/Suzuki-Miyaura cross-coupling reaction, proceeds with high yields and provides facile access to a library of functionalized building blocks. The synthetic methods described herein allow combining several distinct functional groups within a single unit, each intended for a specific task, such as 1) the -N=N- azobenzene core as a photoswitchable moiety, 2) aryls and heteroaryls, functionalized with carboxylic acids or pyridine at its peripheries, as coordinating moieties and 3) varying substitution, size and length of the backbone for adaptability to specific applications. These specifically designed azobiphenyls and azoterphenyls provide modular bricks, potentially useful for the assembly of a variety of polymers, molecular containers and coordination networks, offering a high degree of molecular functionality. Once integrated into materials, the azobenzene system, as a side group on the organic linker backbone, can be exploited for remotely controlling the structural, mechanical or physical properties, thus being applicable for a broad variety of 'smart' applications.
  • Ganti, Srinivas; King, Peter J.; Arac, Erhan; Dawson, Karl; Heikkilä, Mikko J.; Quilter, John H.; Murdoch, Billy; Cumpson, Peter; O'Neill, Anthony (2017)
    We introduce a new approach to creating low-resistance metalsemiconductor ohmic contacts, illustrated using high conductivity Au island metal films (IMFs) on Ge, with hot carrier injection initiated at low applied voltage. The same metallization process simultaneously allows ohmic contact to n-Ge and p-Ge, because hot carriers circumvent the Schottky barrier formed at metal/n-Ge interfaces. A 2.5x improvement in contact resistivity is reported over previous techniques to achieve ohmic contact to both n- and p- semiconductor. Ohmic contacts at 4.2 K confirm nonequilibrium current transport. Self-assembled Au IMFs are strongly orientated to Ge by annealing near the Au/Ge eutectic temperature. Au IMF nanostructures form, provided the Au layer is below a critical thickness. We anticipate that optimized IMF contacts may have applicability to many material systems. Optimizing this new paradigm for metalsemiconductor contacts offers the prospect of improved nanoelectronic systems and the study of voltage controlled hot holes and electrons.