AVS2010 Session TF-WeA: Thin Films: Growth and Characterization

Wednesday, October 20, 2010 2:00 PM in Room Pecos

Wednesday Afternoon

Time Period WeA Sessions | Abstract Timeline | Topic TF Sessions | Time Periods | Topics | AVS2010 Schedule

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2:00 PM TF-WeA-1 Growth and Characterization of Thin Films for Organic Electronics Applications
Daniel J. Gaspar, Liang Wang, Zihua Zhu, Mark H. Engelhard, Barbara J. Tarasevich, James S. Swensen, Rick E. Williford, Mark E. Gross, Wendy D. Bennett, Dean W. Matson (Pacific Northwest National Laboratory)
There are two basic ways of generating organic thin films for electronics applications – vacuum-based processes and solution processes (each with many variations). Each has advantages and disadvantages in film purity, morphology, deposition rate, process control, molecular design and materials choices. This presentation will describe the deposition, characterization and performance of organic thin films deposited using variants of both methods for organic light emitting diodes (OLEDs), organic thin film transistors (OTFTs), and other applications utilizing electroactive organic thin films. Specific advantages in film purity and access to different classes of materials are discussed. Surface characterization using time of flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM and conductive AFM), ellipsometry and Fourier transform infrared spectroscopy (FTIR) are used to characterize thin films, highlighting the challenges in characterizing these often sensitive and usually amorphous films, as well as need to develop a solid experimental understanding of the composition and structure of thin films deposited for organic electronics applications to understand performance.
2:40 PM TF-WeA-3 Bio Modification of Titanium Surfaces with Grafted Sodium Styrene Sulfonate Thin Films
Gilad Zorn, David G. Castner (University of Washington)

Ti and its alloys are commonly used as biomaterials due to their unique mechanical properties and good corrosion resistance. Still, Ti implants can induce the formation of a fibrous layer that can compromise bonding at the interface with the living tissue. The lack of proper integration of Ti with tissue can lead to implant failure. Since the biological response to implanted biomaterials is initiated at their surfaces, the performance of Ti and Ti alloys can be improved by modifying their surfaces. A promising approach for surface bio-modification is grafting a bioactive polymer onto Ti implant surfaces. For optimal grafting, it is important to fully understand the nature of the bio-modified surfaces since it has a pivotal role in the biomaterial performance. The main thrust of this work is to graft bioactive sodium styrene sulfonate (NaSS) onto Ti surfaces to control and improve their response in biological environment.

Smooth Ti films evaporated onto silicon wafers were used as substrates. XPS showed that the surfaces of these films are covered with a layer of TiO2. The roughness of these surfaces, as measured by AFM, was 0.7nm. Methacryloxypropyltrimethoxysilane (MPS) was used as a cross linker between the Ti and the NaSS; the substrates were soaked in a solution of MPS in chloroform (5%v/v) for 1 hr at room temperature and then removed from the solution and heated at 140°C for 4 hrs. After attaching the MPS molecules, XPS surface composition and high resolution XPS data suggested that the Ti substrates were covered with a uniform thin film. Additional evidence for the MPS attachment to the Ti surfaces was the appearance of the CxHyOz fragments from the methacrylate group along with the decease of the Ti and TiOx fragments in the ToF-SIMS data. The NaSS grafting was then done at 90°C in an oxygen free environment for 15hrs using a 0.7M solution of NaSS monomer in dimethyl sulfoxide (DMSO). After NaSS grafting the XPS composition showed an increase of the C/Ti ratio and an appearance of sulfur and sodium. ToF-SIMS successfully detected the sulfonate group, C8H7SO3, and a decrease of the Ti containing fragments. Fourier transform infrared spectroscopy (FTIR) and near edge absorption fine structure (NEXAFS) indicated an ordered array of the grafted NaSS layer on the Ti surfaces and AFM showed a uniform coverage with a roughness of 1.11nm.

Currently the mechanism of competitive protein adsorption on titanium surfaces before and after NaSS grafting is being studied. The XPS nitrogen signal indicates a higher amount of bovine serum albumin (BSA) or fibrinogen is adsorbed onto the titanium surfaces after modifying them with the NaSS layer.

3:00 PM TF-WeA-4 Probing Stability of the Molecule-Substrate Interface in Self-Assembled Monolayers by Ion-Beam-Induced Desorption
Piotr Cyganik (Jagiellonian University, Poland); Sabina Wyczawska, Frederik Vervaecke, Erno Vandeweert, Peter Lievens (Catholic University Leuven, Belgium)

Due to the ease of preparation and their relatively high stability, self-assembled monolayers (SAMs) are very promising candidates to be used in the development of micro- and nano-structured materials. With numerous detailed studies available nowadays for SAMs, the identification of SAMs adsorption geometry and stability of molecule-substrate interface still remains controversial and rather difficult to access experimentally. In this presentation we report experiments on ion-induced desorption and resonance enhanced ionization mass spectrometry of SAMs on Au(111) substrate.1 Althrough ion-induced desorption is commonly considered as a very invasive process when used for probing monomolecular films, our experiments demonstrate that this method can be successfully applied to monitor fine changes in the molecule-substrate interface stability of model SAMs systems based on thiols ( CH3‑C6H4‑C6H4-(CH2)n-S-Au(111), n = 2-6) and selenols (BPnSe, CH3‑C6H4‑C6H4-(CH2)n-Se-Au(111), n = 2-6) . Current desorption experiments will be discussed together with recent microscopic2 and spectroscopic3 analysis of the molecular structure and stability of these SAMs. We demonstrate that lower or higher ion-induced bond scission efficiency can be correlated with, respectively, higher or lower chemical stability of particular chemical bonds. Thus, a new method for probing the stability of the substrate-SAM interface can be proposed.


(1) S. Wyczawska, F. Vervaecke, et al. in preparation.

(2) P. Cyganik, K. Szelagowska-Kunstman, et al. J. Phys. Chem. C 2008, 112, 15466.

(3) K. Szelagowska-Kunstman, P. Cyganik, et al. Phys. Chem. Chem. Phys. 2010, 12, 4400.

4:00 PM TF-WeA-7 Benzene Adsorption on Self-Assembled Monolayers
Hanqiu Yuan, Daniel Killelea, Kevin Gibson (The University of Chicago)

Non-dissociative deposition of gas-phase species onto surfaces of alkanethiol self-assembled monolayers (SAMs) allows creation of new types of multi-component nanoscale materials. Systems such as these have garnered much attention due to their central role as model systems for studies of orientation-controlled adsorption and non-dissociative attachment of functional molecules on organic surfaces of technological importance, including molecular electronics.

Benzene (C66), perdeuterobenzene (C6D6) or a 50:50 mixture of these two isotopologues were deposited on SAM surfaces using a supersonic molecular beam. Supersonic molecular beam techniques permitted precise control over the dynamics of the deposition process by changes in the incident reagent’s translational energy (Etrans) and incident angle. The results presented here highlight the role these dynamical variables play in the adsorption, desorption and conformation of the resultant multilayer molecular film. A combination of in-situ infrared reflection absorption spectroscopy (FT-IRAS) and mass spectrometry was used to determine the surface coverage, molecular orientation and the sticking coefficient as a function of the surface coverage of the benzene molecules deposited on SAM surfaces. The interaction between adsorbates and the SAM substrate was also investigated by varying the SAM chain length and whether the SAM contains an odd or even number of carbon atoms. These results were compared to analogous results from adsorption on clean Au surfaces.

The results of these experiments uncovered the details of the adsorption process. The effects of Etrans and substrate temperature on sticking show the central role dynamics plays in the physisorption of molecules on surfaces. Most significantly, the sticking of gas-phase benzene was found to have a novel dependence on surface coverage, and non-Langmuirian uptake was observed.

4:20 PM TF-WeA-8 Ex and In Situ Analysis of the Growth of Ultrathin Organic Films from Ethanol Solutions
Tom Hauffman, Els Tourwé, Annick Hubin, Herman Terryn (Vrije Universiteit Brussel, Belgium)

In order to form stable self-assembling organic monolayers on numerous substrates, dipping deposition from organic solvents is a widespread technique. However, it is rarely investigated what is the influence of the solvent on the substrate and the molecules which are supposed to be deposited. In this study we present the self assembly of phosphonic acids from an ethanolic solution on aqueous based pretreated aluminium oxides. Following the deposition behavior with XPS and AFM throughout deposition time, it was concluded that the nature of the deposition is random and fluctuating. In order to understand better what is going on, the deposition was followed in situ with odd random phase multisine impedance spectroscopy. This technique gives the opportunity to follow the behavior of the organic molecules as well as the behavior of the buried substrate. It was observed that ethanol adheres on the surfaces, changing the water based chemistry which was obtained through the pretreatment. Furthermore, a competition between the adsorption of the phosphonic acids and ethanol was seen, explaining the non-stable behavior previously analysed with XPS and AFM.

This statement was proven by characterizing ethanol-stabilised aluminium oxide samples during different immersion times. Although here no monolayer was formed, the trend observed corresponded with continuous organic layer growth.

4:40 PM TF-WeA-9 Effect of Deposition Pressure on the Structural, Optical and Electrical Characteristics of Y2O3 Thin Films by Reactive Magnetron Sputtering
Vikas H. Mudavakkat, K. Kamala Bharathi, C.V. Ramana (University of Texas at El Paso)
Significant research efforts have been directed in recent years on the growth of Y2O3 films because of their interesting physical, electronic, and optical properties. The diverse range of potential applications of Y2O3 films includes storage capacitors, random access memory (RAM) and metal−insulator−semiconductor (MIS) devices, protective and antireflective coatings for IR detectors, and optical filters. In the present work,Y2O3 films have been produced by the magnetron sputter-deposition. The effect of pressure on the structure, optical and electrical properties ofY2O3 films has been investigated. The rate of deposition found to be significantly influenced by the overall pressure during deposition. Optical characterization carried out using transmittance analysis indicate that the samples at lower deposition rates showed weaker absorption in comparison to the samples with higher deposition rates. X-ray diffraction (XRD) showed that the as-is deposited films at room-temperature exhibit [111] oriented cubic structure. Electrical characterization indicate that films are insulating with a very high resistivity. The capacitance-voltage characteristics are also obtained forY2O3 films. The results will presented and discussed.
5:00 PM TF-WeA-10 Effect of Partial Pressure on Structural and Optical Properties of WO3 Thin Films
Rama S. Vemuri, Satya K. Gullapalli, Ramana V. Chintalapalle (University of Texas at El Paso)
Tungsten oxide (WO3) is a wide band gap semiconductor (~ 3.2 eV), which exhibits excellent properties suitable for the development of integrated chemical sensors and electrochromics. N-type conductivity coupled with selectivity and sensitivity to certain type of chemicals make WO3 thin films interesting for NOx and H2S sensors. The present work was performed to understand the effect of oxygen partial pressure on the microstructure, optical and electrical properties of WO3 thin films and optimize the conditions to produce materials suitable for sensor applications. WO3 thin films were produced by the reactive RF magnetron sputtering. The films were grown at various reactive gas pressures(2.3 – 5.6mTorr) by changing the oxygen flow rate while keeping the deposition temperature fixed at 400oC. Optical spectroscopy analysis of the grown films indicates that optical properties are sensitive to the oxygen partial pressure. The spectral transmission of the films increased with the increase in oxygen concentration. The band gap of these films was found to be increasing from 2.6 eV to 3.25 eV with increasing oxygen pressure. Electrical conductivity {~10-2 (Ω-cm)-1} measurements indicate that there is a correlation between the growth conditions, optical and electrical properties.
5:20 PM TF-WeA-11 Effects of Deposition Temperature on Al doped ZnO Thin Film for Solar Cells by dc Magnetron Sputtering
Wonkyun Yang, Junghoon Joo (Kunsan National University, Republic of Korea); Stephen M. Rossnagel (IBM Research)
Aluminum-doped zinc oxide films (AZO) were deposited on soda-lime glass substrates by dc magnetron sputtering for solar cell application. The resistivity and average transmittance were improved from 2.3 × 10-3 Ω·cm to 3.3 × 10-4 Ω·cm and from 77.3% to 86% at high deposition temperatures compared to films at room temperature. The mobility and carrier concentration increased, and the crystallinity and grain size also increased at high temperature during deposition. By post deposition annealing at 400℃ for very short time duration, the resistivity and transmittance of room temperature films were improved up to 4.8 × 10-4 Ω· cm and 90.5%. But we found the improved properties have no relation with the structural properties: crystallinity and grain size evaluated by XRD and SEM. The surface roughness of AZO films at high deposition temperature increased to 14 nm by larger grain size, but that by post deposition annealing needs an etching process due to no change of roughness.
5:40 PM TF-WeA-12 Influence of Growth Rate on the Epitaxial Orientation and Crystalline Quality of CeO2 Thin Films Grown on Al2O3(0001) by Oxygen Plasma-Assisted Molecular Beam Epitaxy
Manjula Nandasiri, Satyanarayana Kuchihbatla, Ponnusamy Nachimuthu, Tamas Varga, Vaithiyalingam Shutthanandan, Weilin Jiang, Suntharampillai Thevuthasan (Pacific Northwest National Laboratory); Sudipta Seal (University of Central Florida); Asghar Kayani (Western Michigan University)
Cerium oxide thin films were grown on Al2O3(0001) substrates with different growth rates (1-10 Å/min) by oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The growth rate induced epitaxial orientations and crystalline quality of CeO2 thin films were studied by in-situ reflection high-energy electron diffraction (RHEED), ex-situ atomic force microscopy (AFM), and high-resolution x-ray diffraction (HRXRD) techniques. CeO2 grows as three-dimensional (3-D) islands and two-dimensional (2-D) layers at growth-rates of 1-7 Å/min and ≥9 Å/min, respectively. The average surface roughness of 5-10 Å shows high-quality surfaces of CeO2 thin films. The formation of epitaxial CeO2(100) and CeO2(111) thin films occurs at growth rates of 1 Å/min and ≥ 9 Å/min, respectively. Glancing incidence XRD measurements have indicated that the films grown at intermediate growth rates (2-7 Å/min) consist of some polycrystalline CeO2 along with CeO2(100). The thin film grown at 1 Å/min showed six in-plane domains, characteristic of well-aligned CeO2(100) crystallites. All six of the repeating rectangle units of O atoms from the oxygen sub-lattice in Al2O3(0001) that bind to Ce atoms are nonequivalent which produces six in-plane domains. This also minimizes the lattice mismatch between the thin film and the substrate leading to well-aligned CeO2(100) crystallites. When increasing the growth rate from 1 Å/min to 2-7 Å/min, the lack of sufficient time to stabilize the Ce atoms on all the rectangle units of O atoms from oxygen sub-lattice in Al2O3(0001) results in poorly-aligned CeO2(100) crystallites that start to coexist along with well-aligned crystallites. Furthermore, the content of the poorly-aligned CeO2(100) crystallites increases with increasing growth rate up to 7 Å/min, and three out of six in-plane domains gradually decrease and eventually disappear. At growth rates ≥9 Å/min, CeO2(111) film with single in-plane domain was identified. In order to accommodate the CeO2(111) unit on top of the Al2O3(0001), the cerium sub-lattice undergoes compression in all three axes by ~24% resulting in poorly-aligned CeO2(111) crystallites. The formation of CeO2(100) 3D-islands at growth rates of 1-7 Å/min is a kinetically-driven process unlike at growth rates ≥9 Å/min which result in an energetically and thermodynamically more stable CeO2(111) surface.
Time Period WeA Sessions | Abstract Timeline | Topic TF Sessions | Time Periods | Topics | AVS2010 Schedule