Papers

ICMCTF2008 Session C4: Optical Characterization of Thin Films

Thursday, May 1, 2008 8:00 AM in Room Royal Palm 1-3

Thursday Morning

Start	Invited?	Item
8:00 AM		C4-1 Synthesis, Structural, and Optical Properties of ZnO Nanowires S.-N. Bai (Chienkuo Technology University, Taiwan); T.-Y. Tseng (National Chiao Tung University, Taiwan) ZnO nanowire arrays have been grown on ZnO film-coated silicon (100) substrates by a hydrothermal growth method, and the deposited nanowires are found to have a uniform size distribution with sharp hexagonal-shaped tips. The structural and optical properties of the nanowires were investigated using atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and cathodoluminescence (CL) techniques. The SEM and XRD results demonstrate that the well-aligned ZnO nanowires are single crystalline formed along the c axis preferentially oriented. EDX analysis shows that the Al diffusion in ZnO nanowires at oxygen and nitrogen atmospheres after post-deposition annealing is quite different. After post-deposition annealing, the Al diffusion ratio in ZnO nanowires prepared at nitrogen atmosphere is higher than that prepared at oxygen atmosphere. Moreover, CL measurement was used to analyze the optical characteristics of the ZnO nanowires. The optical properties of the nanowires were measured after annealing in oxygen and nitrogen atmospheres at 550°C under various times. The room temperature CL spectrum shows two dominant emission bands in this study; one is a strong UV emission band that is well-known results from the radiative free exciton recombination, and the other is a weak deep-level emission band that maybe attributed to the oxygen defects. It is found that the un-doped and alumina-doped ZnO nanowires show different behaviors. The dependence of the optical properties on the annealing condition is discussed.
8:20 AM		C4-2 Effect of Sputtering Gas on Structural and Optical Properties of Nanocrystalline Tungsten Oxide Thin Films A.K. Chawla, R. Chandra (Indian Institute of Technology, Roorkee, India) Transition metal oxides (TMO) are of great interest because of their electrochromic, gaschromic & photochromic effects. Recently the electrochromic devices based on TMO have been extensively studied to regulate the radiated energy through glass with modifying their optical properties for the application in ‘smart windows’. Among the semiconducting TMO, tungsten oxide holds an important place in electrochromic display devices because of their marvel performance in electrochromic behaviour. Varieties of preparation techniques are available for the preparation of thin films of WO₃ including electrodeposition, spray pyrolysis & vacuum thermal evaporation. However there are few reports available on the effect of oxygen concentration on nanocrystalline WO₃ thin films by RF magnetron sputtering. The nanocrystalline thin films of Tungsten oxide were deposited by RF magnetron sputtering on glass substrates using two different inert gases (Argon and Helium) with different percentages (10-40%) of oxygen gas. X-ray diffraction spectra collected from the films was used to evaluate the particle size and the quality of the films in terms of phase purity and crystalline orientation. It was observed that the films deposited in Helium-Oxygen mixture have smaller particle sizes as compared to the samples deposited in Argon-Oxygen mixture. The effect of the nature of the inert gas and oxygen gas were studied in detail. The surface microstructure of the nanocrystalline films was studied by Atomic Force Microscope. Optical transmission was measured in the 200-3000 nm wavelength range using a UV-Vis-NIR spectrophotometer and is above 80% in the visible region.
8:40 AM		C4-3 Study on Characteristics of Indium Zinc Oxide Thin Films With Magnetron Sputtering C.-C. Lin (Minghsin Universityof Science and Technology, Taiwan) The transparent conductive indium-zinc-oxide (IZO) films on Corning glass substrates with size of 750mmx620mmx0.6mm for display application have been fabricated using magnetron sputtering technique. The crystal structure of as-sputtered films is observed amorphous and then those films are in-situ processed in sputtering system at different deposition temperatures of 100°C, 200°C, 300°C, 400°C and 500°C. Thickness of the sputtered films is measured to be 3000A. The influence of oxygen atmosphere and post annealing processing on physical characteristics of indium-zinc-oxide films has also been investigated and demonstrated. The physical characterization of as-sputtered and as-annealed films were observed using XRD, four-point resistivity, UV-VIS spectrometer, AFM and SEM. With the increase of annealing temperatures, surface resistivity of films slowly increase, this is due to the decrease of carriers and the substitution of Zn atom by indium atom result in the increase of oxygen vacancy in concentration. The increase of concentration of oxygen vacancy leads to the increase of concentration of carriers, while the existence of a great deal of oxygen vacancy occurs lattice disorder and lower carrier mobility in thin films. Resistivity of as-sputtered films is 4.42x10-4-cm, while resistivity of annealed films at 100-500°C is about 4.48-4.98 x10-4-cm. Transmittance of as-sputtered and as-annealed films is about >80.0% in the range of 400nm-700nm, while that of as-annealed films in high-concentration oxygen atmosphere is >86.0%. The observed characterization of transparent conductive indium-zinc-oxide films in this experiment is suitable to fabrication OLED devices.
9:00 AM		C4-4 Characterization of The Semi-Conducting Polymeric Thin Films C.L. Chung, C.H. Po (I-Shou University, Taiwan) The Conjugative polymer is one of the potential materials due to its electric conductivity located within semi-conductive range. For the purpose of the photo-electronic applications, the conjugated polymer poly(9,9-dioctylfluorene) (PFO) and poly(9,9'-dioctyl-fluorinecobithiophene) (PFT2) are highly attractive and shows high electroluminescent (EL) quantum yields along with blue light emission and superior chemical stability, making it an excellent candidate for a wide variety of electroptic devices. Polyfluorene belongs to the family of "stiff-chain" polymers, and in combination with its flexible alkyl substituents, the stiff backbone affords well-characterized nematic liquid crystalline behaviors. In this paper, the effects of the annealing and rubbing conditions on the topography and microstructure and optical prosperities are explored in detail by using AFM, wide angle xay diffraction (WAXD) with hot stage, differential scanning calorimeter (DSC), and UV-PL. The different treatment conditions induce different topography and the crystalline structures seem to change the chrematistics of optical absorption and photoexcited emission. In addition, all these films consist of same size of nanograins. The films are therefore inherently inhomogeneous in this length scale.
9:20 AM		C4-5 Preparation and Characterization of Potential Bilayer Reflectance Standards for the VUV and EUV Z. Strother (Georga Tech); D.D. Allred (Physics and Astronomy); N. Brimhall, L. Strein (Brigham Young University) Determining the optical properties of materials in the VUV/EUV (Vacuum/Extreme Ultraviolet) range (-8 to 80 nm) would be benefited from stable, reliably, easy-to-prepare and characterize mirrors being available to researchers. Simple, single-surface mirrors with high reflectance over broad spectral ranges, analogous to the role that materials like Al and Ag fill in the IR and visible, do not exist. Multilayers are often used in lower wavelengths ranges to calibrate optical systems but these are expensive, are not broadband and often unstable. In place of these we are developing bilayers which exhibit strong interference fringes as a function of reflectance angle for large portions of the spectra. They show large interference fringes near the Brewster’s angle and acceptably large reflectance at near-normal incidence. One class of standards is thermally oxidized silicon wafers, that is 20-40 nm thick SiO₂ on Si. These shows large fringes in the ~8 to 30 nm range. Another class is sputtered Ru-Si bilayers which can exhibit strong interference fringes at 30.4 and acceptable performance at 58.4 nm. These wavelengths are of particular interest to us. They are important for space-based, magnetosphere-observing EUV observatories. We will present our preparation and characterization of the standards. We discuss their advantages and limitations, showing to what extend they meet the criteria of being readily prepared even in small laboratories, easily characterized by a variety of techniques, and being sufficiently robust, stable and inert for long-term use.
9:40 AM		C4-6 Atomic Force Microscopy and Spectroscopic Ellipsometry Characterization of Polymer-Fullerene Blend Films A.M.C. Ng, K.Y. Cheung, A.B. Djurisic, W.K. Chan (University of Hong Kong) Different polymer-fullerene blends have been widely used as the active layer in bulk heterojunction organic solar cells due to its high power conversion efficiencies. The film morphology as well as the micro/nanoscale phase separation of the two components has a significant effect on the solar cell performance since the exciton diffusion length is typically very short in organic materials. The effectiveness of this bulk heterojunction is then highly dependent on the phase separation and the respective exciton diffusion lengths of the two materials involved. However, the phase separation also affects the morphology and surface roughness of the film, which can affect the optical properties determination using spectroscopic ellipsometry. In this work, we have used spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) to characterize the properties of blend films commonly used in organic solar cells, namely poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C₆₁ butyric acid methlyl ester (PCBM) blends. The blend films were prepared using different solvents, and for different ratios of P3HT:PCBM in order to change the surface roughness and phase separation in the blends. The obtained SE results were analyzed with and without the surface roughness correction to examine how the morphology affects the optical properties. The effect of different substrate (silicon and glass), different solvent and different composition on the obtained results are discussed.
10:00 AM		C4-7 Material-, Topography-, and Feature-Recognition: Challenges on Optical Property Testing of Surfaces and Thin Films U. Beck, A. Hertwig, J. Krueger (BAM, Germany) The optical properties of surfaces and thin films are of interest mainly for two reasons. Firstly, optical features are essential to the rapidly growing number of photonic applications such as small area parts (for laser technology and optics) and large area components (for solar cells and low-energy glazing). Secondly, optical characterization is fast and non-destructive with in-situ, in-line, and on-line capabilities for quality control and screening. For optical material properties (refractive index, extinction coefficient, reflectance and transmittance, laser ablation thresholds) and topographic features (size of features and defects, roughness, thickness), a variety of examples is discussed using spectroscopic ellipsometry (SE), femto-second pulse laser technology (fs-laser), white light interference microscopy (WLIM), atomic force microscopy (AFM), and X-ray reflectometry (XRR). Testing methods are applied to material properties, i.e. laser ablation thresholds (fs-laser) vs. optical constants (SE), to topographic features (WLIM vs. AFM), and to layer thickness (SE vs. XRR). It is shown that multi-probe and supplementary techniques are useful or even required for the adequate characterization of the quantity or feature of interest. Nowadays, multi-layers, graded layers, buried layers, modified and structured surfaces, layers consisting of nano- and micro-sized particle composites are state of the art. In addition to point investigations, for larger areas, imaging and mapping are essential to material properties in a broad spectral range, to geometric quantities (thickness, roughness), and to other features (pigments, particles, inclusions). Imaging ellipsometry is compared with topographic investigations by WLIM and AFM. Focusing on the probe dependence, an evaluation is provided for micro- and nano-scaled properties and features.
10:40 AM		C4-9 Optical Characterization of Colloidal Crystals Based on Dissymmetric Metal Coated Oxide Nanospheres S. Portal, M.A. Vallve, O. Arteaga, M. Rubio-Roy, E. Pascual, A. Canillas, J. Ignes-Mullol, E. Bertran (University of Barcelona, Spain) The optical properties of 2D photonic crystal monolayers constituted of silica nanospheres partly covered with a metallic coating were studied in this work. The silica nanoparticles were synthesized by Stöber method and had a diameter of about 300 nm with a low size dispersion. They were arranged in a hexagonal compact structure and deposited on silica and silicon substrates by Langmuir-Blodgett process. The resulting films were constituted of a single monolayer of nanoparticles. The final step of the sample elaboration consisted of evaporating a metal layer on top of the nanoparticles crystal. The thickness of the metal coating was controlled over the 0.1 nm range. An anisotropy in the metallic coverage was introduced by tilting the deposition angle: elongated metallic coating shapes were obtained in a preferential direction depending on the angle. The morphology of the nanoparticles and the film structure were characterized by SEM. Ellipsometric measurements in reflection and transmission modes were performed in order to determine the sample anisotropy and to relate it to the deposition angle. The Surface Plasmon Resonance of the samples was also investigated by the same analysis mean. These dissymmetric 2D photonic crystals show interesting unidirectional optical and surface properties. Models were proposed for the sample structure and simulations of the optical properties were also realised.
11:00 AM		C4-10 Optics of Quasiparticle Phase Transitions in Nanostructured Ag Thin Films M. Gnanavel (University of Hyderabad, India); D.B. Mohanb (University of Coimbra, Portugal); C.S. Sunandana (University of Hyderabad, India) Thin films (2nm- 100 nm thick) of Ag obtained from vacuum thermal evaporation exhibit thickness-dependent optical transmission characteristic of a variety of nano scale structures. While ultrathin films(<10 nm) give evidence of Mie particles^1,2 thicker films exhibit red-shifted plasmon resonances that transform from surface-to-volume resonance. Partially iodized Ag films exhibit an interesting plasmon-exciton "transition"³. These results could be understood from the perspective of surface thermodynamics of the metallic nanostructures. ¹D. Bharathi Mohan and C.S. Sunandana, J. Appl. Phys. 100, 064314 (2006). ² D. Bharathi Mohan and C.S. Sunandana, Physics Chemistry and Applications of Nanostructures(Nanomeeting 2007) Eds. V E Borisenko, S V Gopananko and V S Gurin, World Scientific, New Jersey. ³D. Bharathi Mohan, K. Sreejith and C. S. Sunandana, Appl. Phys.(B) (at press).
11:20 AM		C4-11 Ellipsometer Analysis in the n-k Plane for Select Combinations of Metals, Semiconductors, and Insulators F.K. Urban, D. Barton (Florida International University) Ellipsometry is an optical analytical method based on measuring the change in polarization state of reflected or transmitted polarized light. A major challenge of the method has been the computation of reflecting surface physical parameters of interest from the raw measured data. These methods in common use are plagued by local minima and algorithm performance. The mathematics of the light reflection has long taken advantage of some aspects of Complex Analysis. The authors have used much more of Complex Analysis to treat the case of a metal film on an insulator substrate (nickel on BK7 glass). The work presented here explores this advanced treatment considering other combinations of metal, semiconductor, and insulator materials. While there are differences between these material combinations, all could be treated successfully with the new method. This result lays the groundwork for a set of solution algorithms which are much more powerful and convenient than existing algorithms. It also eliminates the multiple solution problems inherent in least squares methods.