2013 Spring : Symposium A

Resume : Anodic aluminum oxide (AAO) templates which have well-arrayed nanopores are considered as one of the promising substrate to achieve high-performance large active area thin-film fuel cells (TF-SOFCs). However, the highly rough surface of AAO template can lower micro-structural integrity of a thin-film electrolyte and consequently electrochemical performance of TF-SOFCs significantly degrades. In this study, localized physical properties of AAO template-supported thin-film fuel cells are micro-structurally and electrically characterized with the aid of atomic force microscopy (AFM). The characterization results include the qualitative analysis of AAO pore regularity and the quantitative analysis in terms of electrical short between anode and cathode. A sub-100 nm thick yttria-stabilized zirconia (YSZ) layer is used as an electrolyte and fabricated with atomic layer deposition (ALD). The AAO template with 80 nm pores and 100 μm heights is used as a substrate, and an approximately 250 nm thick dense platinum layer is inserted between an ALD YSZ layer and an AAO template.

Resume : With an increasing need for cleaner production and conversion of hydrogen, proton conductors are becoming evermore relevant. La28-xW4+xO54+d, (LWO) is a material with useful properties for the hydrogen fuel economy, both as a proton-conducting electrolyte in an intermediate-temperature solid oxide fuel cell (IT-SOFC) and as a hydrogen gas separating membrane. One of the main areas for improvement with SOFCs lies in the thickness of the electrolyte. An increase in efficiency can be achieved by utilizing an electrolyte which is as thin as possible while still being pin-hole free, thus avoiding short-circuiting. Using ALD to deposit these kinds of materials it is possible to achieve very thin pin-hole free films even on porous materials. A process for depositing WOx was developed using the precursors (tBuN)2(Me2N)2W and H2O. ALD growth occurred from 275C – 325C, while CVD-growth was observed at 350C. It should be noted that the CVD films are still highly uniform, but have much higher growth rates and contain N. LWO thin films have been deposited by combining the processes for deposition of La2O3 (La(thd)3 + O3), and WOx ( (tBuN)2(Me2N)2W + H2O). The stoichiometry of the deposited material should have a La:W ratio in the 5.2 – 5.8 region. A 16:1 pulse relationship between La:W yielded the proton conducting phase of LWO, with an atomic stoichiometry of La:W = 5.6. All LWO samples have low roughness with Rq = 1-2 nm. XRD of heat treated samples confirms the presence of LWO.

Resume : CeO2 is a very attractive material widely known for its use in catalysis, energy storage systems, and gas sensing, caused by the presence of oxygen vacancies and facile conversion between Ce3+ and Ce4+. For most of these applications, slight changes in film thickness, composition, stoichiometry, ion motion or structural defects can dramatically affect its properties. The surface self-limiting Atomic Layer Deposition (ALD) characteristic growth offers great potential to ensure a nanoscale control of CeO2 properties over other deposition techniques such as chemical solution deposition, pulsed laser deposition or sputtering. For the first time, we prepared highly epitaxial as-deposited ALD-CeO2 thin films on several single crystal substrates (LAO, STO, YSZ) at temperatures below 300ºC obtaining a growth per cycle of ≈ 0.2Å/cycle. This extremely low growth rate has been identified as a key parameter to ensure epitaxial growth at these low temperatures. X-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), atomic force microscopy (AFM) and advanced scanning transmission electron microscopy (STEM) confirmed the formation of ultrasmooth, pure and epitaxial (00l) CeO2 films with a thickness range of 2 to 10 nm. Additionally, further tuning of ALD conditions for CeO2 deposition allowed us to obtain conformal coatings on 3D substrates such as vertically aligned carbon nanotubes and TiO2 nanotubes for photochemical and catalytic applications.

Resume : Transparent conductive oxides (TCOs) are nowadays widely investigated mainly due to a possible replacement of indium tin oxide (ITO) as a transparent electrode material. Among various TCOs, zinc oxide, especially doped with group III elements (Al, Ga) is very promising for such applications. Recently, it was found that titanium dioxide films in a crystalline structure of anatase and doped with niobium (TiO2:Nb) reveal very low resistivities, ~10-4 Ohmcm, comparable to those for ITO films. Transparent conductive TiO2:Nb films that show good optical and electrical properties grown by magnetron sputtering or pulsed laser deposition were already reported. Till now, however, there are very few papers reporting TiO2:Nb growth by atomic layer deposition (ALD). In this work, firstly, we concentrate on the ALD growth conditions that lead to anatase TiO2 growth. Next, we investigate the Nb doping mechanism during the TiO2:Nb deposition. Based on secondary ion mass spectroscopy, x-ray diffraction and scanning electron microscopy, we check the uniformity of Nb distribution as well as the possible formation of Nb foreign phases in TiO2:Nb films depending on the Nb content. Finally, we show the optical and electrical properties of ALD-grown TiO2:Nb films and indicate the optimal growth conditions leading to the TiO2:Nb films with the lowest resistivities. This work was partially supported by the European Union within the European Regional Development Fund, through the Innovative Economy grant (POIG.01.01.02-00-108/09).

Resume : The demand for cheap, reliable transparent conductive oxides (TCOs) has increased due to highly competitive solar cell and electronic markets striving to replace the current industrial standard ITO (Indium Tin Oxide). The interest is both on performance and cost - the price of Indium is rising, and better-performing devices would yield a precious advantage. Doped ZnO is widely predicted to be the successor to ITO as, when doped, it has properties that are comparable to that of ITO. Many electronic devices have a low thermal budget and a demand for reproducible high conformality across the sample. Atomic Layer Deposition (ALD) is therefore ideal. The inherent layer-like structure of ALD depositions can however be problematic. As these are deposited at low temperature there is often little diffusion and cyclic inhomogenieties may occur in the doping distribution. We have therefore investigated some of the known ways to counter this issue without annealing, such as changing the precursor and using surface inhibition. A significant variation in resistivity was evident depending on the film thickness in titanium doped zinc oxide. Increasing the thickness from 100 to 200 nm decreases the specific resistivity by one order of magnitude. The lower resistivity stems from an increase in mobility – with almost one order of magnitude difference for the samples with low Ti content, while the carrier concentrations change minimally. The lowest resistivities are in samples with 1-3at% Ti.

Resume : The commercial application of new generation solar cells depends on the use of low-cost, atmospheric-based, fast, low-temperature and roll-to-roll compatible deposition methods. One such method is Atmospheric (or Spatial) Atomic Layer Deposition (AALD). This implements Atomic Layer Deposition in atmospheric conditions by separating the precursors in space rather than in time. One area we have worked on to show the benefits of using AALD for solar cells is the Mg-doping of ZnO for hybrid solar cells with poly(3-hexylthiophene-2,5-diyl) (P3HT) as the p-type light absorbing material. The uniform 50 nm AALD ZnMgO films were deposited in under 7.5 minutes. By varying the carrier nitrogen gas flow rate through the precursors, fine control was achieved over the film composition, with the Mg content varied from 0% to 53%. This resulted in the band gap being increased over a wide range, from 3.3 eV to 5 eV. Through the significant increase in the conduction band level, the open-circuit voltage of ZnMgO-P3HT devices increased from 300 mV to 800 mV. This resulted in the maximum power conversion efficiency obtained being 270% larger than in undoped ZnO-P3HT devices. The significant improvement in device performance and easy fabrication of the Mg-doped ZnO make the AALD process extremely attractive for the commercial application of new generation solar cells.

Resume : Vertically-aligned carbon nanotube (VACNT) forests grown on metallic substrates are of great interest as high surface area electrodes for applications such as supercapacitors [1]. Direct growth on metal substrates by thermal chemical vapor deposition (TCVD) on a prior ex-situ catalyst (i.e. iron oxide nanoparticles) layer has proven to be challenging due to phenomena like catalyst coarsening and alloying to the metal substrate. At high temperatures (i.e. 750ºC), the small catalyst nanoparticles tend to agglomerate into larger ones through atomic interdiffusion, which is known as “Ostwald ripening”. Efforts have been done in solving this problem by improving the thermal stability of the catalyst nanoparticles through different approaches, although an efficient and direct solution to catalyst agglomeration is still open. Atomic layer deposition (ALD) is a technique to deposit conformal thin films on surfaces regardless of whether the materials are flat or possess high-aspect ratio features, with atomic thickness control through self-limiting surface reactions [2]. ALD of metal oxides such as alumina (Al2O3) has been reported as a technique for stabilization of the catalyst nanoparticles used in catalytic reactions [3]. In the present work an ultrathin Al2O3 layer was deposited on monodisperse catalytic iron oxide nanoparticles by ALD in order to prevent their agglomeration and coalescence at high temperatures, and also to act as a diffusion barrier in the growth of VACNTs. Inconel 600, a nickel-based super-alloy that keeps its microstructural integrity even at high temperatures, was chosen as substrate material, ensuring the formation of VACNTs forest-metal contacts [4]. References: [1] C. Du, J. Yeh, N. Pang, Nanotechnology, 2005, 16, 350-353 [2] M. Knez, K. Nielsch, L. Niinistö, Adv. Mater. 2007, 19, 3425-3438 [3] J. Lu, B. Fu, M.C. Kung, G. Xiao, J. W. Elam, H.H. Kung, P. C. Stair, Science, 2012, 335, 1205-1208 [4] S. Talapatra, S. Kar, S. K. Pal, R. Vajtai, L. Ci, P. Victor, M. M. Shaijumon, S. Kaur, O. Nalamasu, P. M. Ajayan, Nature Nanotechnology, 2006, 1, 112-116

Resume : Aluminum oxide (Al2O3) deposited by atomic layer deposition (ALD) has been demonstrated as surface passivation layer in solar energy conversion devices [1, 2]. The applied thickness ranges from only one layer to several tens of nanometer [2]. Al2O3 can be deposited by thermal ALD and plasma enhanced ALD (PEALD) using trimethylaluminium (TMA, Al(CH3)3) as MO-precursor. For low temperature applications PEALD is preferred over thermal ALD [2]. We report about our results of producing Al2O3 using thermal ALD and PEALD in the SENTECH SI ALD LL system. The layers were deposited in the temperature range between room temperature and 200°C. We discuss ellipsometric data (thickness, refractive index, growth rate) as well as X-Ray Photoelectron Spectroscopy (XPS) results. The distribution of refractive index and thickness over 4” wafers will be shown. The XPS data are analyzed in terms of O/Al ratios and carbon contaminations. Comparing the 200°C process we observe similar refractive indices and O/Al ratios but increased growth rates and reduced carbon contaminations in the PEALD samples. This work is supported by BMBF program “Zukunft des Technologietransfers in strukturschwachen Regionen” (grant numbers 03IN2V4A, 03IN2V4B). [1] F. Le Formal, N. Tétreault, M. Cornuz, T. Moehl, M. Grätzel, K. Sivula: Chem. Sci. 2 (2011) 737. [2] G. Dingemans, W. M. M. Kessels: J. Vac. Sic. Technol. A 30 (2012) 040802.

Resume : Elemental silicon has the potential to be used as negative electrode material in lithium ion batteries with a fivefold larger capacity than the current standard material, graphite. However, the volume changes taking place upon charge and discharge typically cause mechanical tensions and eventually fractures in the solid, which precludes practical application. We propose silicon nanotube arrays as a platform in which the hollow central cavity serves to alleviate this problem. We demonstrate the thermal reduction of silicon oxide films, deposited using ALD, by lithium vapors under argon. The reaction proceeds quantitatively and homogeneously to pure silicon. It is also applicable to arrays of silica nanotubes created in porous anodic alumina. The tubular morphology is maintained, so that the tubes can be electrically contacted and tested electrochemically in a lithium-containing electrolyte. Their cyclic voltammograms display the features associated with the incorporation and removal of lithium ions.