ICMCTF2001 Session C3-1: Electro-optic, TCO and "Active" Films

Wednesday, May 2, 2001 8:30 AM in Room Sunset

Wednesday Morning

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8:30 AM C3-1-1 The Fluorescent Properties of ZnO Thin Film Prepared by RF Magnetron Sputtering
Y.M. Lu (Kung-Sen University of Tecnnology, Taiwan,R.O.C.); W.S. Huang, W.S. Tseng, W.Y. Liu (National Cheng-Kung University, Taiwan,R.O.C.)

Field emission displays(FED) are proposed to be the candidates to substitute traditional CRT display,and may have the chance to beyond the prime display products-liquid crystal displays(LCD).One of the essential technology needed to be solved is to develop high luminescence efficiency and low voltage excited flourescence materials. Zinc oxide is well known to be the most high efficiency luminescence material with green light and has the potential to be excited by low voltage . In this study , RF reactive magnetron sputtering system had been used to deposit transparent ZnO : Zn thin film on the glass and silicon substrates . Adjust the reaction parameters to get the optimum sputtering condition. Post heat treatment under 600? in air was employed to modify the crystal structure . The crystal characterization and optical properties of ZnO thin film had been studied .

From the results of this study , under the 200 walts RF radio frequency power , substrates temperatures set as 400° , sputtering pressure chosen as 3×10-2 torr , and oxygen partial pressure set as 15% , we can get the more perfect ZnO thin film . The as-deposited films have flat apparent surface . with (0002) preferred orientation . The photoluminescence(PL) emission extends between 540-560 nm region . After heat treatment under 600° and maintains 1 hour , the PL spectrum becomes narrower which imply more perfect crystallites favor the emission efficiency . The auger depth profile shows that the deposition composition is very homogeneous and the Zn element is always the dominant composition in the films.The oxygen vacancies may play the main role for light emission.

8:50 AM C3-1-2 Electrochromic Behaviour of Nickel Oxide Thin Films Deposited by Thermal Evaporation
I. Porqueras, E. Bertran (Universitat de Barcelona, Spain)
An electrochromic device can switch its optical properties by the application of an external electrical stimulus. The basic configuration of such a device consists on an electrochromic layer, an ionic conducting layer, and an ion storage layer. These three layers are placed between two electrodes to provide the external electrical signal, of which at least one has to be transparent. Being tungsten trioxide the most widely accepted material for the electrochromic layer, there are a few possible electrochromic complementary materials to act as ionic storage layer, and among them nickel oxide has ever attracted considerable interest. We analysed nickel oxide deposited by thermal evaporation at relatively low temperature (below 100°C) with and without ion bombardment, in order to test the suitability of such a layers for their use in an all-solid inorganic electrochromic device. We employed cyclic voltammetry, chronoamperometry and potentiometry with the simultaneous monitoring of the optical properties of the samples. The characterisation dealt with the activation potential ranges, the amount of inserted charge, colouration efficiency and optical and electrical compatibility with the tungsten trioxide layers. All these analyses provided criteria on the relation between the electrochromic properties of the thin films of nickel oxide and the deposition parameters employed, an opened a way to obtain cost-effective all-solid inorganic electrochromic devices.
9:10 AM C3-1-3 Comparative Study of Amorphous and Nanostructured Tungsten Tri-Oxide Produced by RF-Magnetron Sputtering Assisted by Oxygen Ion-Beam
A. Pinyol, E. Bertran (Universitat de Barcelona, Spain)
Thin films of amorphous and nanostructured tungsten trioxide (a-WO3 and ns-WO3) have been produced in RF-magnetron sputtering reactor coupled to an ion-beam source and using specific deposition conditions to act on the structure of the films. The deposition process has been performed using a W target in an Ar + O2 atmosphere at low temperatures (from room temperature to 100°C). Oxygen-ion bombardment has been carried out either alternating with the RF-magnetron sputtering during the WO3 film deposition (nanometric multi-layer structure) or as an oxygen surface treatment after deposition of WOx films. The samples have been electrically and optically characterised in order to determine the electrochromic properties. The structure and chemical composition of samples have been determined using surface analysis and spectroscopic techniques of characterization (TEM, SAED, XPS and FTIR). The results are discussed in terms of the influence of the deposition parameters on the properties, and their applications in electrochromism.
9:30 AM C3-1-4 High Quality Ta2O5 Layers for Electrochromic Applications
I. Porqueras, E. Bertran (Universitat de Barcelona, Spain)
A monolithic all-solid configuration for an electrochromic device working with lithium ions is claimed to provide the best combination of performance and long lifetime. Environmental stability and ageing considerations suggested the use of inorganic materials for such a device. Nowadays the layout of these devices is almost standard, still being the electrolyte layer one of its bigger weaknesses. The most promising material for such a function is Ta2O5, because it combines both the low optical absorption (to be optically neutral in the structure) and a reduced electrical conductivity. Yet, much work has to be done in order to clarify the ionic conduction of these layers. We are mainly interested in studying the conduction of lithium ions, but also the conduction of protons. The Isothermal Transient Ionic Current (ITIC) technique was applied to Ta2O5 samples made by physical vapour deposition at relatively low temperature (100°C) with ion bombardment. From the analysis of the data obtained for the as deposited samples the mobility, conductivity and number of protons in the material could we extracted. After these analyses Li ions were electrochemically inserted into the Ta2O5 samples, and it was found that the ITIC method could separate the contributions of protons and lithium ions to the conduction. Mobilities of both species were evaluated and compared to the electronic one to asses the applicability of these low temperature deposited Ta2O5 layers for electrochromic applications.
9:50 AM C3-1-5 Electroluminescence Study of Si Doped Glass Light Emitter
T. Ichinohe (Tokyo National College of Technology, Japan); S. Nozaki, H. Morisaki (The University of Electro-Communications, Japan); S. Masaki (Tokyo National College of Technology, Japan); K. Kawasaki (TDY Co.,Ltd., Japan)

Nanometer sized ultrafine structure material is expected for an application to a functional device. Especially, visible light emission at room temperature is extensively studied in a silicon (Si) nano crystallites film, and it is examined for an application to an optical function device.

We have studied the characteristics of visible luminescence at room temperature in the Si doped glass (Si-DG) film, which was deposited by co-sputtering of Si and SiO2.

In measurement of photoluminescence (PL) at room temperature in the Si-DG film, it was found that the post annealing was an indispensable process, in addition, the optimum temperature to obtain maximum PL intensity was found to be about 900 °C. According to an in-situ TEM observation, the composition in the film began to actively change in the heat treatment over 600 °C and then it was found that the crystallization of the Si ultrafine particles with 3-4 nm diameter was caused over 900 °C. This result agrees with the temperature dependence of the PL intensity, indicating that the nuclear growth of the Si nano crystallites in the Si rich part of the film contributes to the increase of the PL intensity.

A typical Si-DG film showed a broad PL spectrum with about 1 eV in a full width at half maximum. We succeeded to reduce down to 0.1 eV after fabricating an optical resonator, which consisted of a dielectric multilayer reflector with the reflectivity of 99.2 %.

In electroluminescence (EL), the light emission at about 5 V was recognized dimly in the dark with the naked eyes, and EL intensity increased with increasing the injection current. We found the extent of the EL spectrum with increasing the injection current; it seemed to be natural to consider the distribution of radiative centers in the film. The EL device with a p-Si substrate showed higher EL intensity rather than that with an n-Si substrate, indicating that the EL intensity of Si-DG film seems to be determined by hole injection.

10:10 AM C3-1-6 Infrared Transparent Spinel Films With p-Type Conductivity
G.J. Exarhos, C.F. Windisch, Jr., K.F. Ferris (Pacific Northwest National Laboratory)
Spinel oxide films containing at least two transition metal cations were found to exhibit p-type conductivity with high optical transparency from the visible to wavelengths near 12 micrometers. Resistivities as low as 0.001 ohm-cm were measured on 100 nm thick rf sputter deposited films that contained nickel and cobalt. Deposited films also were magnetic. Optical spectra, Raman scattering results and XPS measurements indicated the presence of trivalent nickel localized on octahedral sites within the spinel lattice which is thought to promote these properties. Electronic band structure calculations corroborated the experimental results. A resistivity minimum was found at the composition NiCo2O4 in films deposited from aqueous or alcoholic solutions followed by subsequent annealing at 400 degrees C in air. Solution deposited films richer in nickel than this stoichiometry always were found to phase separate into nickel oxide and the spinel phase with concomitant loss in conductivity. However, the phase stability region could be extended to higher nickel contents when rf-sputter deposition techniques were employed. To achieve these compositions, a pure oxygen carrier gas was used and substrate temperatures were held below 200 degrees C. Sputter deposited spinel films having a nickel to cobalt ratio of 2 were found to exhibit the highest conductivity. Results suggest that the phase stability region for these materials can be extended through appropriate choice of deposition conditions. A possible mechanism that promotes high conductivity in this system is thought to be charge transfer between the resident di- and trivalent cations which may be assisted by the magnetic nature of the oxide film. Continuing studies are focused on improving charge transport and optical transparency, and understanding the conductivity mechanism in these new conducting oxide systems.
10:30 AM C3-1-7 Synthesis and Characterization of Chemical Solution Derived Aluminum Doped Zinc Oxide Thin Films for Transparent Conducting Electrodes
S.B. Majumder, M. Jain (University of Puerto Rico); S.B. Krupanidhi (Indian Institute of Science, India); R.S. Katiyar (University of Puerto Rico)
Transparent conducting oxides are now a days being investigated for their possible uses in flat television screen, larger and high resolution screens for portable computers, low emissivity and electro-chromic windows, and also for thin film photovoltaics. Doped tin oxide thin films are attractive candidates for these applications, however, aluminum (Al) doped zinc oxide (ZnO) thin films offer less expensive and non-toxic alternative material. In the present study we have investigated the d.c conductivity and optical properties of Al doped ZnO ( Al content varies from 0 to 5 at %) thin films prepared by chemical solution deposition technique. The measured transmittance of these films in the visible range is above 90% and have a sharp UV absorption edge about 380 nm. Assuming a direct band gap transition, the absorption edge analysis of the undoped ZnO thin films yielded an optical bandgap energy in the range of 3.26 to 3.28 eV depending upon the adopted annealing conditions. The correlation between the optical properties and electrical conductivities has been investigated as a function of Al doping , post deposition annealing ambients and temperatures. .
10:50 AM C3-1-8 Variation of the Mechanical Properties of Indium Tin Oxide (ITO) Transparent Thin Films as a Function of Processing Conditions
N.R. Greenland, K.H. Schulz, A.K. Kulkarni (Michigan Technological University)
Indium-Tin-Oxide (ITO) is a transparent conducting material, which is currently deposited as a thin film on glass for use in opto-electronic devices. ITO thin films are used in a variety of different applications where a transparent conducting thin film is required with strong adhesion to the substrate and which has a sufficient impact resistance for most applications. Previous studies have shown that the microstucture of the film is closely related to the mechanical properties. It is clear that processing conditions under which the films are grown will have a significant impact on the mechanical properties and microstructure of the ITO films. However, there have been relatively few studies that have statistically examined the effects of various processing conditions (r.f. power, deposition time, oxygen partial pressure, substrate preparation and type) on the mechanical properties of the film. In this study, the effect of deposition conditions on the mechanical properties of ITO films was investigated. Films were created by r.f. sputtering on glass substrates. The oxygen partial pressure, r.f. power, deposition time, and substrate preparation techniques were varied and the effect of these changes on film hardness and Young's modulus was determined. In order to perform a statistical analysis of the data, a fractional factorial design was used to select the deposition conditions. For these experiments, oxygen partial pressure was varied from 5 to 15%, the r.f. power was varied from 50 to 100 watts, deposition time was varied from 30 to 180 minutes, and different substrate preparation conditions were used. Film hardness was measured using an MTS Nano Indenter XP. This paper will discuss these results and will show the major factors involved in film processing which have the biggest effect on the mechanical properties.
11:10 AM C3-1-9 Transparent Conducting Impurity-Co-Doped ZnO:Al Thin Films Prepared by Magnetron Sputtering
T Minami, S Suzuki, T Miyata (Kanazawa Institute of Technology, Japan)
In this paper, we describe the effects of impurity-co-doping on transparent conducting Al-doped ZnO (ZnO:Al) films prepared by magnetron sputtering. ZnO:Al thin films co-doped with Co or Cr were prepared on glass substrates by conventional dc magnetron sputtering. A mixture of ZnO powder and dopant powders, Al2O3 and CoCl2 or Cr2O3, was used as the target. The sputter deposition was carried out at pressures of 0.2 Pa in a pure Ar gas atmosphere. The resistivity of Cr-co-doped ZnO:Al films was found to be relatively independent of Cr content up to 3.3 atomic%; it increased gradually with further increases of Cr content. However, the etching rate of ZnO:Al,Cr films in a HCl solution decreased markedly as the Cr content was increased above about 1 at.%. In addition, the etching rate in a KOH solution was also found to decrease as the Cr content was increased above about 1 at.%. A practical etching rate as well as a low resistivity of 3X10?4Ωcm were obtained in transparent conducting ZnO:Al,Cr films prepared with an Al content of 3.2 at.% and a Cr content of 1 at.% under optimized target preparation and sputter deposition conditions. In addition to Cr impurity co-doping, Co-co-doped ZnO:Al thin films were prepared in the same manner as above. A practical etching rate as well as a low resistivity of 6.4X10?4Ωcm were obtained in transparent conducting ZnO:Al,Co films prepared with an Al content of 3.2 at.% and a Co content of 2 at.% under optimized target preparation and sputter deposition conditions. It can be concluded that a chemical property, etching rate, of transprent conducting ZnO:Al films could be improved by co-doping Cr or Co without significantly altering the original electrical and optical properties.
11:30 AM C3-1-10 Characterization of Sputtered WO3 Thin Films for Electrochromic Devices
V. Teixeira, A. Monteiro, B. Almeida, H.N. Cui (Universidade do Minho, Portugal); E. Bertran (Universitat de Barcelona, Spain)
An electrochromic layered coating is characterised by its ability to show reversible changes of the optical properties under the action of electrical fields. The interest in such devices has increased recently because of their potential applications in a wide variety of optical modulation devices including energy efficient smart windows for solar control in architectural and automative applications. In this contribution we study the microstructure, the crystalline structure and the optical properties of tungsten oxide thin films deposited on glass and indium-tin oxide (ITO) coated glass. Films were produced by reactive DC magnetron sputtering of tungsten in plasma of argon and oxygen at different sputtering pressures and substrate temperatures for a constant target current, substrate bias and substrate-target distance. The film thickness and microstrucuture were studied by scanning electron microscopy. The surface microtopography was analysed by atomic force microscopy (AFM). X-ray diffraction (XRD) analysis was carried out to determine the degree of crystallinity and crystalline structure, grain size and lattice parameters of the produced thin films. Spectral transmittance in the visible and near infra-red of the deposited oxide films was measured using a double-beam spectrophotometer. Refractive index and absorption coefficients were calculated. From Bruggeman effective medium approximation we have calculated the packing coating density. The surface electrical resistivity of the tungsten oxide sputtered coatings was also determined by the four-point probe method. The device performance were studied by cyclic voltammetry using electrochemical Li intercalation/deintercalation, and ensuing optical data were recorded by spectrophotometry. The absorption mechanism in the electrochromic layered coating coloring under charge insertion is attributed to an excess charge localized on a transition metal site being transferred to a neighboring site upon photon absorption.
11:50 AM C3-1-11 Effects of Doped Tin Concentration on the Properties of Indium Tin Oxide Thin Films Deposited at Low Temperature
J.W. Bae, J.S. Kim, G.Y. Yeom (Sungkyunkwan University, Korea)
Tin-doped indium oxide( ITO ; 2 ~ 10 at.% tin ) thin films were deposited on polycarbonate and polyimide substrates at low temperature(room temperature ~ 120°C) by a dual ion beam assisted evaporator system and the effect of the tin oxide composition and dual ion beam flux/energy ratio on the electrical, physical, and optical properties were investigated. The tin composition of the source was varied by mixing indium oxide and tin oxide using a ball-mill for 24 hours in ethanol, drying at 50°C for several hours in dry oven, and by sintering at 1500°C for 10 hours under atmospheric condition. Two rf ion sources mounted in the chamber were used for ion bombardment using Ar or Kr and for oxygen ion generator, respectively. Atomic percentages of tin in the deposited ITO thin films were investigated by X-ray photoelectron spectroscopy. Carrier density and Hall mobility changed with the doped tin concentration were measured by Hall effect measurement. Crystallinity, sheet resistance, and transmittance were observed by a X-ray diffractometer, a four point probe, and an UV-spectrometer, respectively. Using this method, low temperature ITO with the characteristics of electrical resistivity lower than 4x10 Ohms-cm and the optical transmittance at 550nm higher than 85 % for 200nm thick ITO could be obtained.
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