Papers

ICMCTF2007 Session C1: Recent Advances in Optical Thin Films

Thursday, April 26, 2007 1:30 PM in Room Tiki Pavilion

Thursday Afternoon

Start	Invited?	Item
1:30 PM		C1-1 Aerosol Assisted CVD Synthesis of Vanadium Oxides Thin Films C. Piccirillo, I.P. Parkin, R. Binions (University College London, United Kingdom) Vanadium oxides thin films have been widely studied in recent years due to their properties and their possible technological applications. V(IV) oxide VO₂ shows a metal to semiconductor transition, by conversion from the monoclinic form (M) to the rutile form (R). The transition takes place at 68°C and it is completely reversible. This behaviour makes the material suitable for application as intelligent window.¹. Other oxides such as V₂O₃ and V₂O₅ also show a phase change (Mott-Hubbard metal-insulator² and electrochromic transition respectively³). In this work we describe the deposition of different vanadium oxides phase using Aerosol Assisted Chemical Vapour Deposition (AACVD). In this method the precursor is dissolved into a suitable solvent (ethanol); an aerosol is generated ultrasonically and then a carrier of inert gas transports it to the deposition substrate. It was seen how the use of different experimental conditions (vanadium precursors, solvent and carrier gas flow rate) affects the vanadium oxidation state and hence the vanadium oxide formed. The nature of the phases in the deposited thin films was determined by XRD and Raman spectroscopies. Other features, such as the optical properties and the surface morphology, were also studied. ¹T. D. Manning, I. P. Parkin, C. Blackman, U. Qureshi, J. Mater. Chem. 2005, 42, 4560. ²K. Held, G. Keller, V. Eyert, D. Vollhardt, V. I. Anisimov, Phys. Rev. Lett, 2001, 86, 5345. ³ G. S. Nadkarni and V. S. Shirodkar, Thin Solid Films, 1983, 105, 115.
1:50 PM		C1-2 Effect of Hydrogen on Reactive Sputtering of Transparent Oxide Films V. Ondok, J. Musil (University of West Bohemia, Czech Republic) This article reports on the effect of hydrogen addition into Ar+O₂ discharge mixture on the process of dc reactive magnetron sputtering of oxides itself and moreover on the properties of the sputtered films: mainly on the electrical resistivity of transparent oxide films. Four oxide systems were investigated in detail: (1) In-Sn-O (ITO) films sputtered from alloyed InSn (90/10 at.%) target, (2) TiO₂ films sputtered from Ti (99.5) target, (3) Ti-Y-O films sputtered from alloyed TiY(95/5 at.%) target and (4) ZrO₂ films sputtered from Zr(99.99) target. Films were sputter deposited using an unbalanced dc magnetron with target 100 mm in diameter onto unheated glass (26x26x1 mm³) substrates. It was found that the addition of H₂ into Ar+O₂ sputtering gas mixture results in (i) decrease of the magnetron discharge voltage U_d, (ii) strong change of the dependence p_O2=f(Φ_O2) due to the incorporation of hydrogen into the surface of sputtered target, where p_O2 and Φ_O2 are the partial pressure of oxygen and flow rate of oxygen, respectively, (iii) dramatic decrease of the electrical resistivity of sputtered transparent oxide films due to the incorporation of hydrogen into the growing film, and (iv) stimulation of crystallization of ITO films due to the heat evolved in the exothermic reactions induced by hydrogen. These results are described in detail.
2:10 PM		C1-3 Production Scale Deposition of Multilayer Film Structures for Birefringent Optical Components M.K. Tilsch, K. Hendrix, K. Tan, D. Shemo, R. Bradley, R. Erz, J. Buth (JDSU) Many modern optical systems require polarization control of light in addition to spectral control. Light modulation applications include displays, optical data storage, digital imaging, metrology, instrumentation, optical communications and others. Within the information display sector, liquid crystal displays with polarization control components are prevalent. These components are often close to an image plane and thus have stringent defect requirements. Use of these components to control contrast and modulation drives the requirement of uniformity and part-to-part consistency. In a birefringent material the interaction of light with matter depends on the polarization state and orientation of the material. Bulk materials like quartz possess crystalline structures and hence exhibit molecular birefringence. Liquid crystal materials, stretched foils, obliquely deposited columnar films are examples for engineered materials that have a microstructure or molecular orientation that creates birefringence. But even a stack of thin isotropic layers has birefringent properties when illuminated at non-normal incidence angle. We discuss two deposition technologies which JDSU has scaled into volume production. The first is based on liquid crystal polymers and the second on vacuum multilayer thin-film deposition. Production scale deposition requires high throughput, high uniformity, batch-to-batch reproducibility, and low defect capability. Both processes are standardized on 200 mm wafers, which allows for modular manufacturing within the same work cell and clean room. The two technologies are used together to create specifically tailored birefringent optics. Independent control of in-plane and out-of-plane retardance is enabled. Excellent layer thickness control, knowledge of the optical properties, and suitable metrology enable JDSU to adjust processes to address various customer needs in a rapid manner.
2:50 PM		C1-5 Structural Analysis of Chemical Bath Deposited Cu(InAl)Se2 Thin Films B. Kavitha, M. Dhanam (Kongunadu Arts and Science College, India) Cu(InAl)Se2 (CIAS) thin films of different thicknesses were prepared by the chemical bath deposition technique(CBD) onto well-cleaned substrates from three different chemical baths at different temperatures. The thickness of the deposited films has been determined by gravimetry technique. The films are investigated with x-ray diffraction, scanning electron microscopy and Raman spectroscopy. The structural characterization was carried out by x-ray diffraction which confirms the polycrystalline nature of the films with tetragonal structure. The lattice constants, volume of the unit cell, crystallite size, dislocation density, number of crystallites per unit area and strain in the film have been evaluated. The composition of indium and aluminum has been analysed from XRD spectra. SEM analysis of the films enabled to conclude that the prepared films are uniform, smooth and polycrystalline. Raman scattering studies helped to find In/Cu ratio in the CIAS thin films. It has also been used to assign the fundamental lattice mode.
3:10 PM		C1-6 Effect of Substrate Temperatures on the Properties of Carbon-Coated Optical Fibers Prepared by Plasma Enhanced Chemical Vapor Deposition Method H.-C. Lin, Y.-M. Chou, S.-T. Shiue (National Chung Hsing University, Taiwan) The effect of substrate temperatures on the properties of carbon-coated optical fibers prepared by plasma enhanced chemical vapor deposition method is studied. The substrate temperatures were set at 20°C, 150°C, 300°C, and 450°C, respectively. The optical band gap, microstructure, surface roughness, water-repellency, and low-temperature surface morphology of carbon coatings were analyzed. Those results indicate that if the substrate temperature is changed from 20°C to 300°C, the optical band gap is almost the same. The hydrocarbon bonding transforms gradually from sp³ CH₂ bonds into sp³ CH₃ bonds. The increase of the sp³ CH₃ bonds results in decreasing the surface free energy of the carbon coating, so the water contact angle increases. On the other hand, when the substrate temperature is changed from 300°C to 450°C, the optical band gap decreases rapidly and it suggests that this result originates from the formation of graphite-like structure. The hydrocarbon bonding transforms from sp³ CH₃ bonds into sp² CH bonds. The increase of sp² CH bonds results in increasing the surface free energy of the carbon coating, so the water contact angle decreases. Additionally, when the substrate temperature is changed from 20°C to 450°C, the surface roughness and low-temperature surface morphology of carbon coatings is not obviously changed. Hence, it is found that the carbon coating deposited at substrate temperature of 300°C has a higher water resistance and is the best one for use as the hermetic optical fiber coating.
3:30 PM		C1-7 Photocatalytic Behaviour of Nanocrystalline TiO₂ Films Sputtered at Low Temperature J. Sicha, D. Herman, J. Musil (University of West Bohemia, Czech Republic) The article reports on low-temperature high-rate sputtering of hydrophilic TiO₂ thin films using dc dual magnetron (DM) sputtering in an Ar+O₂ mixture on unheated glass substrates. DM is operated in a bipolar asymmetric mode and is equipped with Ti(99.5) targets of 50 mm in diameter. The substrate surface temperature T_surf measured by a thermostrip is less than 180°C for all experiments. The effect of repetition frequency f_r is investigated in detail. It was found that an increase of f_r from 100 kHz to 350 kHz leads to the improvement of (a) an efficiency of the deposition process that results in a significant increase of the deposition rate a_D of sputtered TiO₂ films and (b) decrease of peak pulse voltage and sustaining of the magnetron discharge at lower sputtering gas pressures and higher target power densities. It was demonstrated that hydrophilic several hundreds nm thick TiO₂ films can be sputtered on unheated glass substrates at a_D=80 nm/min, T_surf=180°C when high value of f_r=350 kHz is used. Properties of thin hydrophilic TiO₂ film deposited on a polycarbonate is given.
3:50 PM		C1-8 Real Time Spectroscopic Ellipsometry for Thin Film Photovoltaics R.W. Collins, J. Li, N.J. Podraza, D. Sainju, J.A. Stoke (University of Toledo) Photovoltaics (PV) technologies based on thin films of hydrogenated amorphous silicon (a-Si:H) and polycrystalline cadmium telluride (pc-CdTe) have met with considerable success over the past few years. These technologies offer future promise of competitively-priced electric power due to automation of PV module production. The thin film PV devices are deposited by chemical and physical vapor deposition methods on low cost substrates such as stainless steel or glass. Optimized devices have become increasingly complex, and in the case of a-Si:H technology include as many as a dozen major layers and several minor layers, as well, designed to capture a broad range of the solar spectrum. In addition, complicated processing can be required, and in the case of pc-CdTe technology this includes a critical post-deposition anneal under CdCl₂ vapor. For a Si:H-based PV, real time optical probes based on spectroscopic ellipsometry (SE) have played a key role in research for establishing principles that guide the fabrication of optimized devices that are being used widely in the field. In this paper, we provide several examples of the use of real time SE in thin film PV research including: (i) development of deposition phase diagrams for optimizing devices made from hydrogenated silicon films, (ii) quantification of pc-CdTe nucleation, interface formation, and structural evolution, and (iii) analysis of losses due to plasmon absorption in Ag/ZnO back-reflector structures used to enhance efficiency in a second pass through the PV device. Finally, prospects for advances in real time optical instrumentation designed for PV research and production will be discussed.
4:30 PM		C1-10 Ellipsometer Analysis in the n-k Plane 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. Information about the reflecting (or transmitting) structure may be extracted assuming an appropriate mathematical model of the reflection (or transmission) process and appropriate mathematical means. Some of the advantages of the method are that measurements are sensitive to monolayer films, measurements may be made in air, the method is non-destructive, no sample preparation is required, and measurements are fast (a few seconds). A major challenge of the method has been the computation of reflecting surface physical parameters of interest from the raw measured data. This arises because the most common relationship is transcendental and cannot be inverted thus requiring numerical methods. The most popular methods are variations of least squares, for example Levenburg-Marquardt. These methods are plagued by local minima inherent in least squares. The work presented is a description of the geometry of the n-k plane for the case of a single measurement on a single layer film. This geometry lays the groundwork for a set of solution algorithms which are much more powerful and convenient than existing algorithms. An example algorithm is shown.