Papers

ICMCTF1999 Session C3: Electro-optic and TCO Films

Wednesday, April 14, 1999 1:30 PM in Sunset Room

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		C3-1 Highly Transparent and Conductive Rare Earth-doped ZnO Thin Films Prepared by Magnetron Sputtering T. Minami, T. Yamamoto, T. Miyata (Kanazawa Institute of Technology, Japan) It has been reported that highly transparent and conductive ZnO thin films were prepared by doping with a group three element (B, Al, Ga, In) or a group four element (Si, Ge, Ti, Zr, Hf). This report presents the preparation of a transparent conducting thin-film of ZnO doped with a rare earth element such as Sc or Y. The films were prepared by rf or dc magnetron sputtering using a powder target. A mixture of ZnO and Sc₂O₃ or Y₂O₃ powders calcined at 900 °C under an Ar atmosphere and placed in an Al holder was used as the target. The sputtering deposition was carried out at a sputter gas pressure of 0.4-1.0 Pa under an Ar gas atmosphere and a substrate temperature of room temperature-350 °C. The resistivity of deposited ZnO:Sc thin films decreased as the Sc₂O₃ content was increased up to about 2 wt%; any further increase of the Sc₂O₃ content caused the resistivity to increase. A resistivity of 3.2X10^-4Ωcm was obtained in ZnO:Sc thin films prepared by dc magnetron sputtering on a glass substrate at a temperature of 200°C with a Sc₂O₃ content of 2 wt%. An average transmittance above 90% in the visible range was obtained for the ZnO:Sc thin films. The electrical and optical properties of ZnO:Sc thin films prepared by dc magnetron sputtering were comparable to those of magnetron sputtering prepared ZnO:Al and ZnO:Ga. In addition, a lower resistivity could always be obtained for ZnO:Sc thin films compared with ZnO:Y thin films. The obtained low resistivity may be attributed to the good crystallinity of ZnO:Sc thin films, resulting from the fact that the radius of Zn ion equals that of Sc ion.
1:50 PM		C3-2 Defect State Modification in Doped ZnO Films G.J. Exarhos, C.F. Windisch, Jr. (Pacific Northwest National Laboratory) The nature and density of resident defect states in ZnO films, formed during deposition or following post deposition modification, influence film conductivity and long wavelength reflectivity. Electron donor states have been introduced in transparent neat and doped films through gas phase and electrochemical reduction. Modified films obtained from, PVD or solution-based methods, have been characterized by means of Raman spectroscopy, XPS, TEM, spectroscopic ellipsometry, and electrochemical impedance spectroscopy (EIS). Results suggest the presence of multiple cation oxidation states within the lattice and a tendency of defects to migrate toward the microcrystallite grain boundary surface. The effects of attendant defects on optical properties are correlated with the measured optical response. A Mott-Schottky type analysis of the electrochemical data coupled with AFM measurements of the film morphology is used to correlate film thickness, morphology, and donor density. To first order, the average grain size can be extracted from the electrochemical data. Results suggest that electrochemical diagnostics can be used to selectively tailor films with targeted resistivity and optical response.
2:10 PM		C3-3 Emerging Transparent Conducting Oxides for Electro-Optical Applications C. Bright (Delta V. Technologies) The standard transparent conducting oxide (TCO) material used in Electro-Optical (EO) applications is tin-doped indium oxide (ITO). ITO has high conductivity, i.e., low resistivity, (~ 2 x 10-4 ohm-cm), high visible transmittance (~85%-90%), and low visible absorptance, k ≤(.01) which results in its popularity. However, many other TCO have been deposited and their performance reported. While the state of development of these materials is, in general, immature many new TCO are promising alternatives to ITO in specific applications. Often, attributes other than the transmittance and conductivity, are important in selecting the TCO material. The current performance of promising new TCO materials are reviewed and compared with the requirements of several EO applications. The benefits and drawbacks of these TCO versus ITO, in these applications, are discussed.
2:50 PM		C3-5 Investigations on the Function of the Blocker-Layer in Low-E Multilayer Systems on Glass O. Treichel, V. Kirchhoff (FEP, Dresden, Germany) Low-E multilayer systems are conventionally used to reduce the heat loss through window panes. Many different low-E systems use a silver layer as the functional layer. The silver layer is sandwiched between oxide layers. Often used oxides are e.g. SnO₂, TiO₂. To enhance the performance of the low-E system, a so-called blocker-layer is inserted between the silver and the top-layer [Glass/Oxide/Silver/Blocker/Oxide]. These systems are well known and applied in industrial production lines. Based on empirical results this blocker-layer functions as a protective layer for the silver layer, acts as a diffusion barrier and it influences the mechanical properties of the condensed multilayer system. However in most systems the precise physical and chemical behaviour of the blocker-layer is unknown. Measurements of the optical (optical spectroscopy) and microstructural (AFM) properties of different blocker materials are presented, as is a first model describing the function of the blocker.
3:30 PM		C3-7 Dependence of Surface Roughness of Indium-Tin-Oxide (ITO) Thin Films on the Thickness of the Films and the Impact of the Substrate Surface Roughness on the Electrical and Optical Properties of ITO Thin Films A.K. Kulkarni, K.H. Schulz, R.A.G. de Almeida (Michigan Technological University) ITO thin films(100-200nm thick) are deposited on glass and plastic( PET and Polycarbonate) substrates by rf sputtering technique. The sheet resistance, optical transmittance and the microstructure of these films are reported in recently published papers ^1,2. Here we report our recent results on the dependence of the surface roughness of ITO thin films deposited for different times(10,15,30,60,and 120 min.).The surface roughness was determined by an atomic force microscope. We observe a general trend of increasing surface roughness with increasing deposition times. However, the surface roughnesses of ITO films deposited on polycarbonate substrates remain independent of the deposition times. This observation is attributed to the higher initial surface roughness of the polycarbonate substrate(15-18 nm) compared to the initial surface roughness of glass and PET substrates(3-6 nm). The impact of the substrate and the film surface roughness on the electrical and optical properties of ITO films is elucidated. ¹ A.K.Kulkarni, K.H.Schulz, T.S.Lim, M.Khan, Thin Solid Films,308-309(1997)1-7 ²A.K.Kulkarni, T.Lim, M.Khan and Kirk H.Schulz, J.Vac.Sci.Technol.A16(3)(1998)1636-1640
3:50 PM		C3-8 Infrared Emittance Modulation Devices Using Electrochromic Crystalline WO₃, Polymer Conductor, and Nickel Oxide.ⁿ C.L. Trimble, M.J. DeVries (University of Nebraska-Lincoln); J.S. Hale (J. A. Woollam Co., Inc.); D.W. Thompson, T.E. Tiwald, J.A. Woollam (University of Nebraska-Lincoln) Prototypical small area electrochromic devices were fabricated, and their reflectivity measured from 1 to 30 µm. Devices consistently show a 10-15% modulation over this infrared spectral range The emittance performance was calculated, based on the reflectivity modulation. One difference between these devices and the more frequently explored visible light transmission devices is the utilization of crystalline WO₃ instead of heavily disordered amorphous WO₃. The crystalline WO₃ and NiO charge storage films are characterized by Cyclic Voltometry, coulometric experiments, XRD, and spectroscopic ellipsometry. Cyclic voltometry curves show little degradation of the WO₃ over 10³ cycles. Cycle times are approximately 15 minutes for a full color bleach cycle. A DLC antireflection coating was applied to the backside of a low doped Si substrate to greatly increase the emmitance of the devices. This shows promise in improving performance of future devices. By varying the thickness of the DLC the device can be designed for maximum emittance at a given wavelength. For application as thermal control surfaces the DLC coated devices in this report have been designed to perform optimally at the peak of the blackbody curve for a 25 C object. ⁿThis research is supported by the Ballistic Missile Defense Organization, Contract number DASG60-98-C-0054.
4:10 PM		C3-9 Stability and Sensing Mechanism of High Sensitivity Chlorine Gas Sensors Using Transparent Conducting Oxide Thin Films T. Miyata, T. Minami (Kanazawa Institute of Technology, Japan) Recently, we reported that high sensitivity chlorine (Cl₂) gas sensors could be prepared using (Zn₂In₂O₅)_1-x-(MgIn₂O₄)_x multicomponent transparent conducting oxide thin films. In this paper, we describe the sensing mechanism and stability of these high sensitivity gas sensors in regard to Cl₂ gas. The (Zn₂In₂O₅)_1-x-(MgIn₂O₄)_x thin films were prepared by rf magnetron sputtering using (Zn₂In₂O₅)_1-x-(MgIn₂O₄)_x powder targets. The transient response of resistivity, carrier concentration and Hall mobility of the thin films was measured by the van der Pauw method at high temperatures of 200 to 400°C in various ambient gases. The resistivity of (Zn₂In₂O₅)_1-x-(MgIn₂O₄)_x thin films increased when exposed to Cl₂ gas, whereas it decreased when exposed to various inflammable gases. The increase and decrease of resistivity of (Zn₂In₂O₅)_1-x-(MgIn₂O₄)_x thin films are attributable to the decrease and increase, respectively, in both carrier concentration and Hall mobility. Therefore, the mechanism causing the increase of resistance in these thin films is attributable to the trapping of free electrons by Cl₂ being adsorbed on grain boundaries and/or the thin film surface. The transient response of resistivity of the (Zn₂In₂O₅)_1-x-(MgIn₂O₄)_x thin film was stable for continuous operation in excess of 2500 hours in a mixture of air and Cl₂ gas with a concentration of about 500 ppm.
4:30 PM		C3-10 Case Studies in Scale-up of Industrial Thin Film Coating Processes E. Kurman (Flex Products, Inc.)