ICMCTF2009 Session C3: Optical Characterization of Thin Films
Friday, May 1, 2009 8:00 AM in Room Royal Palm 4-6
C3-1 Spectroscopic Ellipsometry for Characterization of Thin Films and Surfaces: Harnessing Materials for Energy
M. Schubert (University of Nebraska-Lincoln)
Greenhouse gas emission, global climate concerns and modern lifestyles transform energy production, consumption and efficiency to be the most critical problem of our society. Harnessing materials for energy is seen momentarily as the only key to generating our future energy infrastructure in balance with rising global economic and ecologic imbalance. In this presentation we critically review our current perspective on spectroscopic ellipsometry – a nondestructive optical characterization technique – in energy materials research. Specifically, we address most recent progress towards boosting knowledge for thin-film solid state lightning, solar cell and battery storage applications: The optical Hall effect. In this newly developed approach, thin film heterostructures are studied under strong external magnetic fields, and unbound charge excitations are monitored which thereby reveal signatures of their density, effective mass and mobility, very much in similarity to the electrical Hall effect. However, unlike the electrical Hall effect, the optical Hall effect does not require electrical contacts, and can provide increased information if multiple layered structures are investigated. Examples include micro and nanostructured silicon solar cells – today’s photovoltaic conversion work horse, surface depletion and accumulation layer properties in InGaN – a novel candidate for multiple-junction solar cells, terahertz resonances in chiral metal nanowires –candidates for harvesting electromagnetic radiation energy, a study of the origin of the intercalated charge induced polaron formation in tungsten oxide – an energy storing material candidate, and magnetization properties in antiferromagnetically ordered ZnMnSe – a potential spintronic element.
C3-3 Effect of Laser Fluence on ZnCdS Thin Films
S. Singhal, A.K. Chawla, R. Chandra, H.O. Gupta (Indian Institute of Technology Roorkee, India)
II–VI Zn based ternary alloys are of great interest because of their applications in the production of devices such as short wavelength emitting laser diodes and light emitting diodes. Their electronic band structure and the wide range of band gap together with a low refractive index allow intense light transmission and also effective electron transport at high electric field. The interest in Zn based ternary alloys is also particularly concerned with the increasing demand for materials useful for the production of flat panel displays, high efficiency electroluminescent and field emission devices. Among the new and up to date techniques used for thin film deposition, pulsed laser ablation (PLA) is one of the most versatile methods to obtain layers of several materials that can be processed into a pellet target. One of the important features of this method is based on the possibility of maintaining the stoichiometry of the ablated target in the deposited layer. We have deposited the nanocrystalline thin films of Zn@sub 1-x@Cd@sub x@S (x = 0.4) by pulsed laser ablation on corning glass substrates. We have varied the laser fluences during deposition keeping all other parameters fixed. The deposited samples were characterized using X-ray diffraction for the phase purity and crystal structure. The stoichiometric composition of these alloys was estimated using lattice constant calculated from the XRD data. Surface morphology of the samples was examined using AFM and FE-SEM. Optical properties were studied at room temperature by transmittance, reflectance, and Photoluminescence measurements. It was observed from the PL data that with increase in laser fluence there is a decrease in the band gap. Transmission data shows a transmittance of more than 70% in the visible region recorded by UV-Vis-NIR spectrophotometer. TEM investigation of the samples reveals that the particles are spherical in shape with average diameter of 15-20 nm.
C3-4 Effect of Rapid Thermal Annealing on the Electrical and Optical Behaviors of Cu2O-Ag Nanocomposite Thin Films
C.-C. Tseng (National Chung Hsing University, Taiwan); J.H. Hsieh (Mingchi University of Technology, Taiwan); W. Wu (National Chung Hsing University, Taiwan)
Cu2O–Ag nanocomposite thin films were deposited by reactive co-sputtering on glass substrates. After deposition, some of these films were annealed using a rapid thermal annealing (RTA) system, with the variation of temperature and time. A UV-VIS-NIR photometer and a Hall measurement system were used to characterize the optical and electrical properties of these films with and without RTA. The results reveal that annealing by RTA can cause Ag nano-particles to emerge in the Cu2O matrix. Consequently, the optical properties of these films will change, as well as the electrical behavior. The effects of embedded Ag particles on photo-induced conductivity are discussed.
C3-5 Laser Damage Thresholds of Optical Coatings
D. Ristau (Laser Zentrum Hannover e.V., Germany)
Since the very beginning of laser technology, Laser Induced Damage Thresholds (LIDT) of optical components were always an obstacle for the application of laser systems operating at high power levels. Also, further progresses in the development of new high power laser concepts are often directly limited by the availability of advanced optical components with high quality and LIDT-values. Nowadays, in the course of the development of optical materials with excellent quality and power handling capability, the problem of laser induced damage has shifted from the bulk to the surface of the optical component. The optical surface is objected to various production steps and environmental influences, which modify its structure and composition. Especially, the thin film coating, which is deposited on the optical surface to adapt its reflectance and transmittance to the application, contributes predominantly to the reduction of the LIDT-values. As a consequence, the measurement and optimization of the power handling capability of thin films is considered as one of the primary research areas in modern optics technology and is supported by an extensive scientific community. @paragraph@ In the present paper, a brief review will be given on selected fundamental damage mechanisms in thin films considering different operation conditions of modern laser systems. Also, the current standards for the measurement of LIDT will be described, and examples illustrating some practical aspects of high power optical coatings will be presented. Finally, a summary of the present state of the art will be given, and recent trends in laser technology will be discussed in respect to research in laser induced damage.
C3-7 Numerical Ellipsometry: Analysis of Thin Metal Layers Using n-k Plane Methods with Multiple Incidence Angles
F.K. Urban, D. Barton (Florida International University); T. Tiwald (J.A. Woollam Co., Inc.)
Ellipsometry is an optical analytical method based on measuring the change in polarization state of reflected or transmitted polarized light. A major challenge for those utilizing this 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. A recent study examined existing methods for modeling thin metal layers on various substrates. Previously we have applied Complex Analysis in the n-k plane to improve ellipsometry modeling for growing films on substrates. The work presented here applies this advanced methodology to multiple angle measurements over a wavelength range spanning visible for thin absorbing metal films deposited on various substrates. Results show that the new methods work well across light incidence angle and across wavelength. Relative advantages for this kind of film-substrate combination will be presented.
C3-9 Investigation of Scattering Mechanisms in Transparent Conductive Ga-doped ZnO Films with Thicknesses of Less than 100 nm
T. Yamada, A. Miyake, H. Makino, N. Yamamoto, T. Yamamoto (Kochi University of Technology, Japan)
Low resistivity and highly transparent Ga-doped ZnO (GZO) films with different thicknesses of less than 100 nm were deposited on alkali-free glass substrate by an ion-plating method using a direct-current arc discharge, where the deposition conditions are optimized in terms of a reduction in resistivity@footnote 1@. Complex dielectric functions of the GZO films in a photon energy range from 0.73 to 3.8 eV were characterized by an ellipsometric model analysis constructed from the Drude and Tauc-Lorentz functions, assuming that the GZO films are a homogeneous. From the Drude model analysis for free-carrier response, optical carrier concentration, N@sub opt@, and mobility, μ@sub opt@, of the GZO films were obtained, and they were compared with carrier concentration, N@sub Hall@, and Hall mobility, μ@sub Hall@, determined by Hall effect measurements. While the N@sub opt@ was in near agreement with the N@sub Hall@, the μ@sub opt@ exhibited the values higher than μ@sub Ha ll@ in all the films. The difference in the mobility is attributed to grain boundary scattering of carriers. From relationship between the electrical and optical properties of the GZO films with increasing film thickness of less than 100 nm, scattering mechanisms of carrier were discussed. @paragraph@@footnote 1@T. Yamada, A. Miyake, S. Kishimoto, H. Makino, N. Yamamoto and T. Yamamoto, Appl. Phys. Lett., 91, 051915 (2007).
C3-10 Optical and Photoluminescence Studies of Gold Nanoparticles - Embedded ZnO Thin Films
A. Patra, V. Damodara Das, S. Kasiviswanathan (Indian Institute of Technology Madras, India); S. Kumar (Guru Nanak Dev University, India)
Metal oxides find application in variety of fields like photo-catalysis, gas sensing and corrosion protection. Some of the metal oxides like ZnO exhibit two diametrically opposite characteristics viz., high optical transparency in the visible region and high conductivity. On the other hand, exotic properties exhibited by physical systems of small sizes and dimension have attracted much attention. Au nanoparticles (AuNP) for instance exhibit surface plasmon resonance enhanced light absorption, which leads to local heating. Thus AuNP can act as potential nanoheaters in photothermal therapy and it is of interest tune the resonant frequency at around to that of commercial He-Ne lasers. We report herein, the synthesis of AuNP embedded ZnO films by sandwiching a thermally evaporated Au film between sputtered two ZnO layers The films have been characterized using high resolution transmission electron microscopy (HRTEM), glancing angle X-ray diffraction (GXRD), optical absorpti on and photoluminescence (PL). The peaks in GXRD patterns were identified to be due to the reflections from various planes of ZnO and elemental Au. Formation of AuNP within ZnO matrix has been confirmed by HRTEM images. Pure ZnO film have exhibited a prominent PL peak in the UV region, whereas AuNP embedded ZnO film showed a peak of reduced intensity. This has been attributed to the absorption of the emitted radiation by the AuNP. Further, the size dependence of the plasmon absorption has been studied by forming nanoparticles of various sizes. The resonant frequency reported so far in sandwiched films is low. The UV-VIS-NIR spectrum of the samples has exhibited strong peaks at 608, 638, and 676 nm for AuNP of average sizes 27, 40 and 67 nm respectively. Thus, tuning the resonance peak to He-Ne laser wavelength is possible by varying the particle size. Moreover, antibodies and other biological molecules can be readily attached to the surface of AuNP. Also, absorption cross-sec tion and light scattering by AuNP is several orders higher than that of conventional absorbing dyes and strongly fluorescing dyes respectively. These properties of AuNP have been exploited in photothermal therapeutic applications. We have calculated theoretically the rise in temperature at the AuNP site due to heat generation as a function of incident light intensity. The rise in temperature has been estimated to be 15 K for a particle size of 80 nm.
C3-11 Anisotropic Optical Properties of Sculptured Thin Films Grown by Glancing Angle Deposition
D. Schmidt (University of Nebraska-Lincoln); B. Booso (University of Dayton); T. Hofmann (University of Nebraska-Lincoln); A. Sarangan (University of Dayton); E. Schubert, M. Schubert (University of Nebraska-Lincoln)
Three-dimensional (3D) structure design on the nanoscale is in the focus of modern material science and engineering because intriguing applications are foreseen for such nanostructured films in various fields ranging from optics, electromechanics or electromagnetics. We utilize glancing angle physical vapor deposition, which allows for “bottom-up” fabrication of three-dimensional nanostructures arranged in sculptured thin films (STFs). Geometric shadowing and varying particle incidence azimuth is exploited to grow nanostructures with different 3D morphologies such as (slanted) columns, screws or spirals. We will present the anisotropic (structure-related) optical properties of STFs from various metals grown on silicon substrates by electron-beam evaporation at an oblique angle of incidence. Generalized spectroscopic ellipsometry is employed to determine the anisotropic optical constants of the thin films in the spectral range from 400 nm to 1000 nm. All nanostructured thin films show strong form birefringence and large dichroism and differ drastically from their bulk material. Columnar thin films from metal are found to be monoclinic@super 1,2,3@. @paragraph@@super 1@D. Schmidt, B. Booso, T. Hofmann, A. Sarangan, E. Schubert, and M. Schubert, Appl. Phys. Lett. 94, 011914 (2009). @paragraph@@super 2@D. Schmidt, B. Booso, T. Hofmann, A. Sarangan, E. Schubert, and M. Schubert, Opt. Lett. (submitted, 2009). @paragraph@@super 3@D. Schmidt, T. Hofmann, A. C. Kjerstad, M. Schubert, and E. Schubert, Mat. Res. Soc. Symp. Proc. (Fall 2008).