Papers

AVS2002 Session TF-MoA: Transparent Conductive Coatings

Monday, November 4, 2002 2:00 PM in Room C-101

Monday Afternoon

Start	Invited?	Item
2:00 PM		TF-MoA-1 Polaron Conductors with Infrared Transparency G.J. Exarhos, C.F. Windisch, Jr., K.F. Ferris (Pacific Northwest National Laboratory); S.K. Sharma (University of Hawaii) Mixed transition metal spinel oxide films (AB₂O₄) deposited from solution or by means of reactive magnetron sputtering are found to exhibit resistivities on the order of milliohm-cm and optical transparency to wavelengths approaching 16 micrometers. These extraordinary properties are achieved when metal cations selected from group VIII in the periodic table are resident within the spinel lattice. Results from temperature-dependent Raman spectroscopy, Hall and Seebeck measurements, XPS, and x-ray diffracti on indicate the importance of cation disorder on the conductivity and suggest processing avenues to further tailor film properties. These include partial substitution of lithium for cations resident on the tetrahedral lattice sites and gross replacement of first row transition metal cations with those deeper in the periodic table. Electronic structure modeling approaches provide a rational path to optimizing properties in these materials. Charge transport processes in p-type spinel oxides will be contrasted with those of free-carrier driven TCO films such as cation-doped ZnO. Prospective applications of the spinel films to applications requiring high transmissivity at long wavelengths will be discussed.
2:40 PM		TF-MoA-3 p-Type Semiconducting Cu₂O-CoO Thin Films Prepared by Magnetron Sputtering S. Suzuki, T. Miyata, T. Minami (Kanazawa Institute of Technology, Japan) In this report, we describe the preparation by magnetron sputtering of p-type semiconducting thin films consisting of a new multicomponent oxide, Cu₂O-CoO. The Cu₂O-CoO films (thickness, 200-450 nm) were deposited on glass substrates at a temperature of 200 to 400°C by r.f. magnetron sputtering using a powder target. The sputtering deposition was carried out at a pressure of 0.2 to 4.0 Pa in an Ar+O₂ gas atmosphere with an r.f. power up to about 80 W. A mixture of Cu₂O and CoO powders calcined at 1000°C in air for 1 h was used as the target: CoO contents of 0 to 100 mol.%. The obtained electrical and optical properties were strongly dependent on the deposition conditions as well as the CoO content of the target. The resistivity of Cu₂O-CoO thin films deposited at 200°C in a pure O₂ gas atmosphere at a pressure of 2.0 Pa with an r.f. power of 80 W decreased as the CoO content was increased, reached a minimum at about 80 mol.%, and then increased markedly with a further increase in CoO content. A minimum resistivity of 3.9X10^?3Ωcm was obtained in a Cu₂O-CoO thin film prepared with a CoO content of 80 mol.% and identified as the polycrystalline delafossite CuCoO₂ by x-ray diffraction analyses. However, the resistivity exhibited a spatial distribution on the substrate surface that depended on the deposition conditions. The multicomponent oxide Cu₂O-CoO thin films prepared in the CoO content range from 0 to 100 mol.% found to be p-type, or positive hole conduction, as evidenced from Seebeck effect. From transmission spectra measurements, the band-gap energy of Cu₂O-CoO films are roughly estimated to be about 1.8 eV. This is the first report of the preparation of new p-type semiconducting Cu₂O-CoO thin films.
3:00 PM		TF-MoA-4 CVD Formed p-type ZnO Thin Films X. Li, Y. Yan, T.A. Gessert, C. Perkins, H.R. Moutinho, T.J. Coutts (National Renewable Energy Laboratory) We have fabricated zinc oxide (ZnO) films that demonstrate a p-type behavior by using a metalorganics chemical vapor deposition (MOCVD). In our low pressure MOCVD, without any plasma enhance, the Zn precursor: diethylzine (DEZ) is reacted with the oxygen and nitric oxide (NO) gas at the temperature range between 200° and 500°C. The p-type behave is only observed on those films formed by DEZ and NO gas. In this reaction, the NO gas is used to supply both O and N to form an N-doped ZnO (ZnO:N) film. The highest N concentration obtained in our ZnO:N films is ~3 at.%, which is highest N-incorporation level reported for ZnO:N films. Hole concentrations of the films are in the range of 1.0x10¹⁵ cm^-3 - 1.0x10¹⁸ cm^-3, with mobility varied from the values of 260 cm² V^-1s^-1 to 0.7 cm² V^-1s^-1. The minimum film resistivity achieved is ~20 Ω-cm.
3:20 PM		TF-MoA-5 Raman Spectra of Cobalt Nickel ITCO Films C.F. Windisch Jr., K.F. Ferris, G.J. Exarhos (Pacific Northwest National Laboratory); S.K. Sharma (University of Hawaii) Cobalt-nickel spinel oxide films have recently shown promise as infrared transparent conducting oxide (ITCO) materials, with resistivity as low as 10^-3 ohm cm and transmittance (for a 100 nm-thick film) approaching 80% at 5 µm. While our current research is focused on optical behavior, the materials have been the subject of previous study, mainly due to their capacity to catalyze the water electrolysis reaction. Attempts to characterize the underlying structural and mechanistic causes for the remarkable electrical and magnetic properties, however, were only partly successful. In particular, there is still uncertainty regarding not only the electrical conduction mechanism, but also the identities of the various charge states of the Ni and Co ions and their distributions among the octahedral and tetrahedral sites in the spinel lattice. In all likelihood, the characteristics of the ions and the specifics of the conduction mechanism are intimately related. In order to understand how the unique electrical and optical properties of the cobalt-nickel spinel oxide films result from chemical structure, Raman spectra were obtained as a function of temperature and composition. Shifts in peak frequencies and changes in bandwidth as a function of temperature and laser power, particularly under cryogenic conditions, could not be explained by simple heating effects alone. Details of this behavior, as well as the spectral changes observed as a function of the Co/Ni ratio in the spinel, point to the presence of a localization of charge states and the important role of small polaron hopping in the electrical conduction mechanism. This work was supported by the Materials Sciences Division of Basic Energy Sciences through the DOE Office of Science and the ARO through DARPA contract AO J209/00. Pacific Northwest National Laboratory (PNNL) is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RLO 1830.
3:40 PM		TF-MoA-6 Plasma Chemistry Aspects versus Material Properties Related to Textured Zinc Oxide Deposition and Etching using an Expanding Thermal Plasma R. Groenen, E.R. Kieft, M. Creatore (Eindhoven University of Technology, The Netherlands); J.L. Linden (TNO, The Netherlands); M.C.M. Van de Sanden (Eindhoven University of Technology, The Netherlands) Zinc oxide (ZnO) is a transparent conducting oxide (TCO) of considerable technological interest. As recently shown, a new approach for low temperature ZnO deposition is developed, using an expanding thermal argon plasma created with a cascaded arc.¹ (Co)precursors are oxygen and diethylzinc. Films are deposited on glass substrates at a temperature from 300 down to 150°C at a rate up to 1 nm/s. A rough surface texture, which is essential for application as front electrode in thin film solar cells, is obtained during deposition. Quadrupole mass spectrometry is used to correlate film properties and gas phase composition. Gas specific calibrations have been performed to make quantitative measurements of the stable reaction product concentrations. First insights into the chemical processes taking place in the plasma have been obtained. There appears to be a close relation between detected species and the observed texture development, which allows for further improvement of the material light trapping properties. Especially the role of hydrogen is under investigation. In this respect, a new method for dry ZnO etching using an expanding thermal argon - hydrogen plasma created with a cascaded arc has been demonstrated, obtaining etch rates up to over 10 nm/s. In-situ ellipsometry, as well as ex-situ FTIR, AFM, SEM, and Hall measurements have been applied to characterize the effect of etching on film properties. ¹R. Groenen, et al., Appl. Surf. Sci. 173 (2001) 40.
4:00 PM		TF-MoA-7 Super-Smooth Indium-Tin Oxide Thin Films by Negative Sputter Ion Beam Technology M.H. Sohn, D. Kim, N.W. Paik, S.J. Kim (Plasmion Corporation); S. Gupta (KDF) A new ionized PVD, Negative Sputter Ion Beam technology, is described for the deposition of super-smooth indium-tin oxide (ITO) thin films with highly transparent and conductive properties at near-room temperature deposition. A limited amount of cesium vapor injected onto a conventional sputtering target surface lowers the work function of the target and produces a negatively charged sputter ion beam. The negative sputter ion beam carries the kinetic energy defined by the potential difference between the cathode and substrate. A negatively-charged sputter ion beam was produced by retrofitting an ITO magnetron sputtering cathode with a cesium vapor injector capable of releasing controlled amounts of cesium vapor into the plasma during deposition. Experiments were performed in a down-sputtering scanning batch tool (KDF 902GT). Using this highly energetic deposition process, ITO thin films have been obtained at near-room temperature (less than 50 °C) with super smooth surface (< 1 nm RMS), resistivity of 4 x 10^-4Ωcm, and transmittance higher than 90% (at wavelength 550 nm). Baseline ITO depositions were also carried out under the same sputtering conditions with no cesium injected, as a comparison. In this paper, film properties such as resistivity, transmittance over the visible spectrum, and surface roughness will be detailed as a function of cesium partial pressure during deposition. The significance of a high quality, low temperature ITO coating process applied to polymer substrates will be discussed.
4:20 PM		TF-MoA-8 ZnO Thin Film Synthesis and Device Application R.S. McLean, M.H. Reilly, P.F. Carcia (DuPont Central Research and Development) ZnO is a wide bandgap, n-type semiconductor that can be doped degeneratively with Group III elements, making it useful as a transparent conducting electrode. Recently, there have been reports of successful p-type doping of ZnO, which would enable device applications ranging from uv lasers to transparent thin film transistors. ZnO is also attractive because polycrystalline films can be grown at low temperatures, compatible with temperature-sensitive plastic substrates, thus presenting the opportunity for fabricating good quality electronic devices on flexible substrates. In this paper we discuss synthesis and electronic properties of ZnO films on polyester substrates and device application. ZnO films were grown by rf magnetron, rf diode, and ion beam sputtering on substrates ostensibly at room temperature. Films grown at low oxygen partial pressure, p(O₂), had the lowest resistivity, ~0.01 ohm-cm, with a dependence of resistivity on p(O₂) that was exponential-like. Hall effect mobility measured in films with resistivity between 0.01 and 10 ohm-cm, was ~ 6-12 cm²/V-s, independent of deposition technique. All film were polycrystalline with prominent c-axis orientation and small, uniform grain size, ~ 25 nm. Film stress increased with p(O₂) and more energetic deposition conditions, suggesting that bombardment by negative oxygen ions likely caused high stress. Finally we fabricated a ZnO, transparent, thin-film transistor, and its properties will also be discussed.
4:40 PM		TF-MoA-9 ITO Films with Low Resistivity and Low Internal Stress S. Takayama (Hosei University, Japan) Indium tin oxide (ITO) is widely used to make transparent conducting films for various display devices. Recently, ITO films with low resistivities, low internal stresses, and high optical transmittance are required in relatively low-temperature processes for use in future display applications. However, to my knowledge, there are no reports of ITO films with both low resistivity and low internal stress having been successfully obtained in relatively low-temperature processes at less than 200°C. For this purpose, in this report, Indium tin oxide (ITO) films (260 to 280 nm in thickness, prepared by d.c.magnetron sputtering on a glass substrate at room temperature) were annealed in air, vacuum, and oxygen gas atmosphere. The structure of all the present as-deposited ITO films was not amorphous, but poly-crystalline. The electronic properties were measured by Hall effect measurements using the Van der Pauw method at room temperature. The internal stress was measured by using a thin-film X-ray diffractometer (XRD, Rigaku RINT 2500). It was found that, among the above post-annealing treatments, oxygen gas annealing significantly reduced both the resistivity and the internal stress in ITO films at fairly low temperatures of 100°C -150°C. Resistivities and internal stresses as low as 7 x 10^-4Ωcm and 38 MPa, respectively, were obtained by annealing in oxygen gas atmosphere at 100°. It was also revealed that the (111) crystal orientation becomes dominant and that whole grains grow dramatically as a result of post-oxygen-annealing, even at 100°C. It was tentatively concluded that the decrease of both resistivity and internal stress of post-annealed ITO films in oxygen gas atmosphere resulted from a large grain growth at relatively low temperatures. Finally, the optical transmittance of all of the post-annealed films in oxygen gas atmosphere was measured and found to be nearly 90% in the visible region of the solar spectrum.