Transparent Conducting Oxides
Wednesday, November 17, 2004 2:00 PM in Room 303C
TF-WeA-1 Structural, Electrical and Optical Properties of Transparent Conductive In@sub 2@O@sub 3@-SnO@sub 2@ Films
Y. Sato, R. Tokumaru (Aoyama Gakuin University, Japan); K. Utsumi, H. Iigusa (Tosoh Corporation, Japan); P.K. Song, Y. Shigesato (Aoyama Gakuin University, Japan)
Transparent conductive In@sub 2@O@sub 3@-SnO@sub 2@ films were deposited by dc magnetron sputtering using high density ceramic targets which contained various SnO@sub 2@ contents (0-100 wt.%) on unheated glass substrates. These films were post-annealed under various atmospheres (Air, Ar or Ar+3%H@sub 2@) at 200 @super o@C for 1 hr. The structural, electrical and optical properties of these films were investigated. XRD profiles of all the as-deposited films exhibited amorphous structure. After post-annealing under various atmospheres, XRD profiles of the films deposited using targets of up to 20% SnO@sub 2@ contents showed bixbyte crystal structures of In@sub 2@O@sub 3@, whereas all the films deposited using the targets of SnO@sub 2@ contents higher than 20 wt.% showed amorphous structure. Resistivity of the films deposited using the targets of up to 20% SnO@sub 2@ contents was reduced from about 400 to 200 @micro@@ohm@cm by post-annealing under each condition, because carrier density increased with increasing the number of the substitutional Sn@super 4+@ at In@super 3+@ site as the result of the crystallization. All the post-annealed films possessed high transmittance of over 80 % in the region of visible light. In the region of over 1000 nm, the transmittance decreased with increasing Sn concentration, where reflectance increased. Work functions of the all films were analyzed by electron spectroscopy in air (ESA, AC-2). The work function of the films showed clear negative relationship to the carrier density. This could be explained in terms of the shift of the Fermi level to the higher energy in conduction band with the increasing carrier density.
TF-WeA-2 Optimization of RF Magnetron Sputter Deposited Indium Tin Oxide (ITO) Transparent Conductors using a Taguchi Statistical Method
S.I. Jun, P.D. Rack (The University of Tennessee)
In order to optimize the electrical and optical properties of ITO (indium tin oxide) thin films a statistical analysis called the Taguchi design was employed. The sputtering parameters that were considered included RF magnetron sputtering power, oxygen partial pressure, total sputtering pressure, and substrate temperatures. The responses that were considered included sheet resistance, transmission, deposition rate, and etch rate (in 22%HCl+6%CH3COOH+72%H2O). From this design of experiments it was determined that the sheet resistance and transmittance are inversely proportional to each other as a function of the process parameters. The preferred orientation of crystalline ITO film is distinguishably changed with an increase of sputtering temperature and oxygen fraction (O2/O2+Ar) in the sputtering ambient. The change in crystallinity results from the content of incorporated oxygen, which significantly affects the electrical and optical properties of ITO films and causes a rearrangement of atoms to form a preferred closed-packed plane orientation. Finally the microstructure of the ITO films becomes denser with increasing oxygen fraction. As a result of this work, we have successfully achieved low sheet resistance (7.0 ohm/sq.) and high transmittance (~90%) for our targeted 300nm thick films. In this presentation we will briefly review our design of experiments and illustrate our experimental procedure. Subsequently, we will discuss how each process parameter affects each measured response. Finally, will correlate the observed electrical and optical properties of the ITO films to the measured crystal structure and microstructure.
TF-WeA-3 Optimization of Process Parameters to Achieve High Quality as-Deposited Indium-Tin Oxide Films for Display Applications
S. Gupta (The University of Alabama)
High quality as-sputtered indium-tin oxide (ITO) films have been achieved by careful optimization of the process parameters@footnote 1@. Oxygen flow, deposition power, process pressure and deposition temperature were varied to achieve optimal resistivity and transmission characteristics of the ITO film. Resistivities below 300 µ@ohm@-cm and transmission of over 75% over most of the visible wavelength range of 400-800 nm, with a peak of 85% at about 550 nm, have been obtained for deposition temperatures below 180 @super o@C. For deposition temperatures of about 260 @super o@C, these numbers were improved to about 225 µ@ohm@-cm resistivity and over 90% transmission. The transmission numbers quoted are for the film plus the quartz substrate -- the film-only values exceed 95%. The thermal stability of the films was studied as a function of in-situ pre- and post-deposition annealing. It was seen that deposition of the ITO films at elevated temperatures improved thermal stability of the as-deposited films, and led to no further change in resistivity or transmission upon post-deposition annealing in atmosphere. ITO film properties vs. process parameters were also studied using two types of magnetron cathodes with maximum field strengths of a) 200 Oe, commonly used for non-magnetic materials, and b) 600 Oe, commonly used for magnetic materials. The purpose of changing cathode field strength was to determine whether lowering the plasma impedance by using a stronger field helped to lower the film resistivity, as reported by some researchers. We did not find any such correlation in our studies. @FootnoteText@ @footnote 1@ This study was performed at KDF, while the author was Director of Technology at KDF, 10 Volvo Drive, Rockleigh, NJ 07647.
TF-WeA-4 Analysis on Thermal Properties of Tin-doped Indium Oxide Films by Picosecond Thermoreflectance Measurement
T. Yagi, K. Tamano, Y. Sato (Aoyama Gakuin University, Japan); N. Taketoshi, T. Baba (National Metrology Institute of Japan); Y. Shigesato (Aoyama Gakuin University, Japan)
First time attempt of observation of thermal diffusion across tin doped indium oxide (ITO) thin films perpendicular to the film plane has been carried out using a picosecond thermoreflectance measurement@footnote 1@. ITO films sandwiched by molybdenum (Mo) films were prepared on fused silica substrate by rf magnetron sputtering using ITO and Mo multi-targets. Such Mo/ITO/Mo layered structure was fabricated without exposure to the atmosphere between each deposition. The Mo films with thickness of 70 nm are necessary because the wavelength of pulse laser used in this study is 780 nm at which wavelength ITO is transparent. The ITO films with different thickness of 30, 50 and 70 nm were prepared as the intermediate layer in order to estimate thermal resistance at the interface between Mo/ITO. The resistivity, carrier density and Hall mobility of the ITO film was 4.2e-4 ohm cm, 3.5e20 cm@super -3@ and 35 cm@super 2@/Vsec, respectively. The interface between the Mo films and the fused silica substrate was irradiated by picosecond laser pulse (2 ps). Heat generated by the pump laser pulse diffuses toward the top Mo surface across the three-layered films. Then the temperature changes at the Mo film surface, which was probed by reflectivity of another picosecond laser pulse. The thermal diffusion time increased with the increase in the thickness of the ITO layers. The thermal diffusion coefficient of the ITO films and thermal resistance at ITO/Mo were found to be 3e-6 m@super 2@/s and 1e-8 mK/W, respectively. This research was supported in part by the New Energy and Industrial Technology Development Organization (NEDO) in Japan. @FootnoteText@ 1: N. Taketoshi, T. Baba, E. Schaub, A. Ono: Rev. Sci. Instrum., 74, (2003), pp. 5226-5230.
TF-WeA-5 Direct Observation of Hydrogen Generated Free Carriers in ZnO Thin Films
C.A. Wolden (Colorado School of Mines); J.B. Baxter (University of California Santa Barbara); T.M. Barnes (Colorado School of Mines); E.S. Aydil (University of California Santa Barbara)
Zinc oxide is a versatile wide band-gap semiconductor that has generated tremendous interest due to its unique combination of optical, electronic and mechanical properties. Hydrogen is common to many ZnO processing environments, and recent first principle calculations have suggested that it behaves exclusively as an electron donor in ZnO. Although hydrogen in ZnO has been observed in bulk single crystals, its behavior has not been examined in commonly used polycrystalline thin films. We report on the use of in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to probe the changes in free carrier absorption when thin ZnO films are exposed to H or O atoms generated by H@sub 2@ and O@sub 2@ plasmas, respectively. Polycrystalline ZnO films were deposited on ATR crystals using metalorganic chemical vapor deposition or a plasma-assisted CVD technique. For both types of films, room temperature exposure to hydrogen plasma resulted in a sharp increase in free carrier infrared absorption and free carrier density. Carrier concentrations and mobilities were extracted from the infrared spectra using a model for the complex dielectric function. Hydrogen uptake was rapid and the carrier concentration reached its saturation level (~10@super 18@ /cm@super 3@) in less than a minute of H@sub 2@ plasma exposure. Room temperature oxygen plasma exposure of the hydrogenated films decreased free carrier absorption, but this decrease was only a fraction of the free carrier density increase after H exposure. Extended exposure to oxygen plasma at elevated temperatures (~200°C) was required to return the carrier concentration to levels that are comparable to those in the as-deposited films. The results are contrasted with observations from single crystal experiments, and the ramifications for device processing are discussed.
TF-WeA-8 A Note on the Measurement of Carrier Density by Hall Effect in p- Type Transparent Conducting Oxide (TCO) Thin Films
A. Subrahmanyam (Indian Institute of Technology Madras, India)
P- type transparent conducting oxide (TCO) thin films are the emerging new materials with a vast potential in transparent electronics. Many research groups are working on the preparation and characterization of these films but with a limited success. The Silver Indium oxide (AIO) and Copper Aluminum oxide (CAlO) belonging to the Delafossite crystal structure have shown p- type conductivity, however, the preparation conditions have a very narrow window for the growth parameters to realize p- type conduction. These materials being metal oxides, any small oxygen deficiency leads to metal rich films with sufficient free electrons. Thus, there exists a bipolar conduction in these p-type TCO films. The conventional Hall effect measurement with bipolar conduction (with comparable free carrier concentrations) and the electrons and holes having different mobilities leads to an incorrect sign of the Hall coefficient and erroneous estimation of the majority carrier concentration. In the present paper, Hall effect data from several groups (including the author's group) on several thin films (prepared with different growth conditions) of silver indium oxide (AIO) and copper aluminum oxide (CAlO) have been analyzed. The analyses clearly reveal that a lot of caution is to be exercised while measuring and interpreting the Hall effect data. The concept of Quantitative Mobility Spectrum Analysis (QMSA) for evaluating the mobility and carrier concentration in bipolar semiconductors is discussed in light of the TCO thin film samples. The contact potential difference (CPD) measurements by Kelvin probe technique to estimate the work function (and thus the Fermi level) in these p- type TCO thin films seems to be one of the potential techniques to estimate the p- type nature of conduction.
TF-WeA-9 Electrical and Optical Properties of Reactive DC Magnetron Sputtered Silver Doped Indium Oxide Thin Films
S. Srinivasan, A. Subrahmanyam, B. Ullash Kumar (Indian Institute of Technology, India); C.L. Nagendra (Indian Space Research Organization (ISRO), India)
In the present study, the role of oxygen in the formation of semiconducting Silver doped Indium oxide thin films is reported. This material has potential application in opto-electronic devices. The Silver doped Indium oxide thin films have been prepared on quartz and soda lime glass substrates at room temperature (300 K) by reactive DC Magnetron sputtering technique. The target is a pure Silver: Indium metal of composition 5: 95 At %. The oxygen flow rates in the growth chambers have been varied in the range of 12.5-14.0 sccm. The sputtering power is kept constant at 50 watts. The X-ray diffraction pattern shows the amorphous nature of these thin films. The electrical resistivity measured at room temperature increases (2.158Ã-10-2 â?"1.545Ã-10-1 â"¦-cm) with increasing oxygen flow rates. The films show an n-type behavior as shown by the Hall effect measurements. The Hall mobility is observed to be decreasing (12.071- 1.984 cm2v-1s-1) with increasing the oxygen flow rates. The thickness and refractive index of these films have been measured by using a spectro-photometric technique and the associated computation algorithm. The thickness of the film as determined by the technique is in the range 165-220 nm. The refractive index and absorption index of these films varies in the range 1.9- 2.1 and 0.01-0.02 over the spectral range from 2500-400nm. The work function has been measured on these films by contact potential method using Kelvin Probe. These data are being analyzed for an understanding of transport mechanism of these films.