Papers

AVS1996 Session TF-FrM: Transparent Conducting Films

Friday, October 18, 1996 8:20 AM in Room 107B

Friday Morning

Start	Invited?	Item
8:20 AM		TF-FrM-1 Properties of Transparent Conducting Oxides formed from CdO and ZnO, Alloyed with SnO\sub 2\ and In\sub 2\O\sub 3\ X. Wu, T. Coutts, W. Mulligan (National Renewable Energy Laboratory) We report on the structural, electrical, and optical properties of several ternary alloy thin-films. The alloys are zinc oxide and cadmium oxide, each of which was reacted with both indium oxide and tin oxide to form sputtering targets. The films were deposited by radio-frequency sputtering. X-ray diffraction spectroscopy showed that cadmium stannate, cadmium indate, and zinc stannate films were all single-phase spinel, but only when deposited at room temperature in pure oxygen and then annealed in argon/cadmium sulfide. The fourth alloy (zinc indate) exhibited the hexagonal phase, when prepared under identical conditions. We show that single-phase material is essential for the highest electrical conductivity and lowest optical absorbance. Cadmium stannate has one of the lowest resistivities of any TCO, has low absorbance in the visible spectrum and is an excellent compromise between electrical and optical requirements. Zinc stannate has a resistivity more than one order of magnitude higher than cadmium stannate but has even lower absorbance because of a larger bandgap. We conclude that there is no "best" TCO, and the decision which to use depends on the specific application and the weighting given to electrical versus optical properties. The non-toxicity of the zinc-bearing alloys may also be an attractive feature for many applications.
9:00 AM		TF-FrM-3 Visible and Infrared Spectroscopic Ellipsometry on ITO, and Electrochromic WO\sub 3\ and Ni(OH)\sub 2\ J. Hale, D. Thompson, J. Woollam (University of Nebraska, Lincoln) Electrochromic devices are being investigated as a means for modulating the infrared emmisivity for thermal control of space station structures. Thus optical "switches" operating in the mid-infrared are desired. Indium-tin-oxide (ITO) is used for the conducting transparent electrode, and its properties must be strictly controlled to effect proper usefulness. In-situ spectroscopic ellipsometry is used to control the sputter deposition thickness, and the in-situ annealing to achieve the desired electrical resistivity. A modified form of the Drude optical model is the basis for a simple, quick, accurate and non-destructive optical determination of electrical resistivity, as verified by direct electrical measurements. Ex-situ variable angle spectroscopic ellipsometry covering the spectral range from ultraviolet to 14 micron infrared is used to determine the complex dielectric function (index of refraction and extinction coefficient) for various amounts of proton intercalation. These optical constants permit precise engineering of optimal devices for space applications. \super *\ Research supported by NASA Lewis Research Center, Grant NAG-3-1802.
9:20 AM		TF-FrM-4 Large Area ZnO Films for Flat Panel Display Applications Y. Li, G. Tompa (Structured Materials Industries, Inc.); S. Liang, C. Gorla, Y. Lu (The Rutgers University) ZnO thin films, as transparent conductive coatings have broad applications in displays, optical sensors and solar cells. We have developed a CVD process technology for 3" x 4" sheets and highly transparent conductive Ga-doped ZnO films for application to Flat Panel Displays. The films exhibit low resistivity, ~ 2.6 x 10\super -4\ Ohm-cm, high optical transparency (>85%) in the visible range, good adhesion, and highly stability. Highly conductive and transparent ZnO thin films were also deposited on quartz, sapphire and silicon substrates using the same techniques. The film properties were investigated as a function of process conditions, such as flow, temperature, pressure, doping level. The microstructural properties of the films, such as surface and interface morphology, crystallinity, uniformity, and composition were studied using scanning electron microscopy, high resolution transmission electron microscopy, X-ray diffraction, and secondary ion mass spectroscopy. The electrical and optical properties of the films were investigated by four-point probe measurements, photoluminescence spectroscopy, and optical absorption spectroscopy. The effects of post annealing processes on the film quality are also presented.
9:40 AM		TF-FrM-5 Highly Transparent and Conductive ZnO-In\sub 2\O\sub 3\ Thin Films Prepared by Atmospheric Pressure CVD T. Minami, H. Kumagai, T. Kakumu, S. Takata, M. Ishii (Kanazawa Institute of Technology, Japan) Highly transparent and conductive ZnO-In\sub 2\O\sub 3\ thin films prepared by magnetron sputtering have been recently reported. In this paper, we describe the preparation of transparent conducting ZnO-In\sub 2\O\sub 3\ films by an atmospheric pressure chemical vapor deposition (CVD) technique. Zn(C\sub 5\H\sub 7\O\sub 2\)\sub 2\, In(C\sub 5\H\sub 7\O\sub 2\)\sub 3\ and H\sub 2\O, which are low in cost and easy to handle, were used as Zn, In and O source materials, respectively. These sources, contained in separate stainless steel tanks, were introduced into the reactor with a nitrogen carrier gas. The supply rate of each source was controlled by changing the tank temperature under a constant carrier gas flow rate. The resistivity of In\sub 2\O\sub 3\ or ZnO films deposited on glass substrates at a temperature of 350\super o\C using In(C\sub 5\H\sub 7\O\sub 2\)\sub 3\ or Zn(C\sub 5\H\sub 7\O\sub 2\)\sub 2\ and H\sub 2\O, respectively, was on the order of 10\super -1\ \Omega\ cm. The resistivity of ZnO-In\sub 2\O\sub 3\ films deposited at a constant In source temperature of 170\super o\C decreased as the Zn source temperature was increased; the minimum resistivity was obtained in films deposited at a Zn source temperature of about 80\super o\C. The Zn content (Zn/(Zn+In) atomic ratio) in the minimum resistivity ZnO-In\sub 2\O\sub 3\ film was about 0.25. A resistivity of 4x10\super -4\ \Omega\ cm and an average transmittance above 85% in the visible range were obtained in an amorphous ZnO-In\sub 2\O\sub 3\ film deposited at a growth temperature of 350\super o\C.
10:00 AM		TF-FrM-6 Film Properties of ZnO:Al Prepared by Co-sputtering of ZnO:Al and Either Zn or Al Targets K. Tominaga, H. Manabe, N. Umezu, I. Mori, K. Ushiro, I. Nakabayashi (The University of Tokushima, Japan) ZnO:Al films are expected as an substitute of indium-tin oxide films. To obtain those ZnO:Al transparent films with lower resistivity, many efforts have been made. We investigated in this paper the influence of additional supply of Zn or Al during film growth on film properties. To do this, we prepared ZnO:Al films on both glass and sapphire substrates by a planar magnetron sputtering system with facing targets. The films were prepared under a condition that ZnO:Al target was sputtered concurrent with either of a Zn or Al target. At an adequate supply of Zn, film resistivity shows a minimum, both carrier concentration and Hall mobility show maxima, and absorption edge is shifted to higher energy side and the optical transmittance improved near the absorption edge wavelength. Then cathodoluminescence increased and film growth was also improved. When Al target was cosputtered, carrier concentration alone was increased. The cathodoluminescence intensity increased when the film resistivity had a minimum. These results indicate that additional Zn or Al atoms during film deposition can improve the film crystallinity, in addition to the increased donor concentration in the film. The improvement of film crystallinity is thought to be due to the compensation of Zn defects by depositing additional Zn and Al atoms.
10:20 AM		TF-FrM-7 Thermally Stable Amorphous Indium-Tin-Oxide Thin Film on Polymeric Substrates S. Fukuda, F. Yamazaki, T. Okamura, T. Miyashita (Mitsui Toatsu Chemicals, Inc., Japan) Polymeric webs coated with indium-tin-oxide (ITO) have many applications in a wide industrial area, such as touch panels, electro-magnetic shields, flexible electrodes for electro-luminescence devices etc. ITO prepared on a glass substrate above 500K is known as crystalline. On the other hand, ITO deposited on polymer is amorphous, since the deposition is usually carried out at lower temperatures. The amorphous ITO easily crystallizes after a heat treatment, e.g. at 430K for 24hrs, and has a poor mechanical property. This limits the applications of amorphous ITO to the field where a high thermal stability and a high mechanical strength are not required. We found, however, that amorphous ITO prepared on polymer under specific conditions shows a high thermal stability and unique properties. In this report, we will describe the relationship of the deposition conditions with the thermal stability and mechanical and electronic properties. ITO films were prepared with DC magnetron sputtering on polyethylene terephtalate and polyimide at room temperature. Sintered ITO targets with tin oxides of 0 to 20 wt% were used. Argon was introduced at a flow rate of 10 sccm. Then oxygen flow rate was varied. Crystalline structure was determined by X-ray diffraction. Electronic properties were also measured. Carrier mobility and thermal stability increased with oxygen flow rate. The increase of tin oxide content was also effective to enhance the thermal stability. Carrier density increased by orders of magnitude with a heat treatment. The application of the stabilized amorphous ITO to electro-luminescence devices and touch panels will be also presented.
10:40 AM		TF-FrM-8 Hetero-Epitaxy of RuO\sub 2\ on Sapphire Substrate with CeO\sub 2\ Buffer Layer by Pulsed Laser Ablation C. Chen (University of Houston); Q. Jia, Y. Lu, J. Smith (Los Alamos National Laboratory) Metallic conductive ruthenium oxide thin film has been epitaxially grown on a sapphire substrates with a buffer layer of the cedium oxide by pulsed laser deposition. TEM, HREM, RBS and X-ray diffraction have been used to characterize the growth behavior of the as-grown films and their relationship between the films and substrate. Both electron diffration and X-ray diffraction studies indicate that the epitaxial relationship is (h00)RuO\sub 2\ // (h00)CeO\sub 2\ // (01 2)sapphire. The cross-sectional high resolution transmission electron microscopic studies reveal that interfaces between the RuO\sub 2\ and CeO\sub\ 2\ films, and the CeO\sub\ 2\ film and the sapphire substrate are atomically sharp. No precititates and/or any additional phases have been found among films and/or their interfaces under the growth conditions.