Papers

ICMCTF2002 Session C5-1: p-type and n-type Semiconducting and TCO Films

Thursday, April 25, 2002 8:30 AM in Room Sunset

Thursday Morning

Start	Invited?	Item
8:30 AM		C5-1-1 Conducting Spinel Oxide Films with Infrared Transparency G.J. Exarhos, K.F. Ferris, C.F. Windisch, Jr. (Pacific Northwest National Laboratory); S.K. Sharma (University of Hawaii) Mixed transition metal spinel oxide films (AB₂O₄) deposited from solution or by means of magnetron sputtering have been found to exhibit resistivities as low as 0.3 milliohm-cm and optical transparency to wavelengths near 15 micrometers. These remarkable properties are achieved when metal cations selected from group VIII in the periodic table are resident in the spinel lattice. Both cation charge state and site occupancy were found to significantly influence properties in these relatively high refractive index and chemically robust films. Post deposition annealing of deposited films in air always led to decreased resistivity but little change in the optical transmission was seen. In order to verify the small polaron charge transport mechanism that drives the conductivity, Raman spectra, van der Pauw and Seebeck measurements were acquired as a function of film temperature and cation composition. Results indicate the importance of cation order-disorder transitions 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. Selected el ectronic structure modeling studies will be presented to demonstrate the effect of cation substitutions in the lattice on measured properties. Charge transport processes in these and similar oxides will be contrasted with those seen in oxides that conduct by means of a free carrier mechanism.ï£¿. .
9:10 AM		C5-1-3 High Rate Deposition of TCO Thin Films by Vacuum Arc Plasma Evaporation T. Minami, H. Toda, S. Ida, T. Miyata (Kanazawa Institute of Technology, Japan) Most TCO thin films in practical use have been deposited using d.c. magnetron sputtering. However, actual requirements call for achieving further target cost reductions and higher deposition rates. Although it is possible to lower target costs by substituting ZnO for ITO, there is a severe problem of spatial resistivity distribution on the substrate surface for transparent conducting ZnO films deposited by d.c. magnetron sputtering. In this paper, we describe a high rate TCO thin-film deposition method, vacuum arc plasma evaporation, without a spatial resistivity distribution problem. TCO thin films were deposited on glass substrates at temperatures up to 450°C by vacuum arc plasma evaporation equipment using fragments of sintered oxide as the source material. The use of sintered fragments can practically eliminate the processing costs of the targets used in magnetron sputtering. The depositions were carried out at a pressure of 0.08-0.3 Pa and Ar and O₂ gas flow rates of 12-30 and 0-15 ccm, respectively, with a cathode plasma power of 2-20 kW. ITO films were deposited at a pressure of 0.1 Pa and Ar and O₂ gas flow rates of 20 and 5 ccm, respectively, with a cathode plasma power of 4.5 kW. A deposition rate of 3.3 nm/s and a resistivity of 1.1X10^?4 Ωcm were obtained for ITO films deposited on large area substrates at 160°C. In addition, undoped and Al- or Ga-doped ZnO thin films were deposited on substrates at 150-350°C; a deposition rate of 2.5 nm/s was obtained at a pressure of 0.08 Pa and an Ar flow rate of 20 ccm with a cathode plasma power of 4.5 kW. It should be noted that a uniform distribution of resistivity on the substrate surface could be obtained in these ZnO films. It can be concluded that vacuum arc plasma evaporation is suitable for low resistivity TCO film depositions at high rate on large area substrates.
9:30 AM		C5-1-4 Electrical, Optical, and Mechanical Properties of Tin Oxide Coatings Deposited Using a Filtered Vacuum Arc R.L. Boxman, S. Goldsmith (Tel Aviv University, Israel); L. Rapoport (Holon Institute of Technology, Israel); E. Shuryan, T. David, Y. Kleinberger (Tel Aviv University, Israel) Tin Oxide films were deposited with a filtered vacuum arc deposition apparatus consisting of a 90 mm diam Sn cathode enclosed within a Cu cup, a 122 mm i.d. annular anode, a quarter-torus macroparticle filter having a 160 mm minor diameter and equipped with 5 field coils, a deposition chamber equipped with a pair of Helmholtz coils and beam deflection coils, and a water-cooled substrate holder. Typical arc current was 170 A. An oxygen background gas was maintained at a predetermined pressure in the 3.0-4.5 mTorr range. In some cases the substrate holder was connected as an auxiliary anode, with currents of 3 or 6 A. The films were deposited onto room temperature glass, polycarbonate, and PMMA substrates. The thickness, electrical conductivity, and optical transmission were routinely determined, and the nano-hardness and composition profile were determined for selected samples. It was found that there were different optimum oxygen pressures for maximizing the conductivity and the transmission. Defining the ratio of the optical extinction length L to the resistivity ρ as an electro-optical figure of merit, the best films were obtained with an auxiliary current of 3 A and a pressure of 3.8 mTorr, and had L=2.4 µm and ρ=0.0055 Ω-cm. The nano-hardness of the films was in the range of 4-8 GPa. Relatively thick films (e.g. >1 µm) deposited on the polymer substrates had a tendency to develop a fine crack pattern, sometimes hours after deposition. Excessive heating, due to the poor thermal conductivity of the substrate, is the suspected cause, and was alleviated by depositing for short periods interspersed with cooling periods.
9:50 AM		C5-1-5 Effect of r.f. Power, Deposition Time and Annealing on the Electrical, Micro-structural, and Mechanical Properties of ITO Thin Films on Glass Substrates N.R. Greenland (Michigan Technological University); K.H. Schulz (Mississipi State University); A.K. Kulkarni (Michigan Technological University) Indium-tin-oxide (ITO) is a transparent conductor deposited on glass for use in many opto-electronic devices. The processing parameters such as r.f. power, deposition time and annealing are shown to affect the microstructure and modify the sheet resistivity, hardness and Young's modulus of these films deposited on glass by r.f. sputter deposition technique. X-ray diffraction observations indicate a shift from a predominantly (222) and (440) oriented films to a predominantly (400) oriented films when r.f. power is changed from 50 W to 100 W. The higher r.f. power is shown to increase the deposition rate from 2nm/min to 4nm/min. This increase in r.f. power causes significant particle bombardment to preferentially etch away certain planes to result in (400) oriented films. The changes in the sheet resistivity are attributed to the corresponding changes in the coherent diffraction lengths estimated from the FWHM (full at width half maximum) determined from the X-ray diffraction peaks. The hardness and the Young's modulus determined by nanoindentation range from 3-5 GPa and 80-90 GPa respectively. Very slight variations are observed in the mechanical properties w.r.t. variations in the process parameters and annealing. However, the films deposited at 100W and annealed at 400C for an hour in air are softer compared to the films deposited at 50W and annealed in a similar fashion. A Laser-acoustic technique is also used to determine the impact of the process parameters on the Young's modulus. Significant variations in the slopes of phase velocity vs. frequency are observed w.r.t. thickness and r.f power.
10:10 AM		C5-1-6 Modelling of the Optical Behavior of Titanium and Titanium Oxide Multilayer Thin Films Deposited by DC Reactive Sputtering S.G. Springer, P.E. Schmid, R. Sanjinés, F. Lévy (Ecole Polytechnique Fédérale de Lausanne, Switzerland) In this work, we investigate some 300nm thick, multilayered films composed of titanium dioxide with thinner titanium interlayers equivalent to a few monolayers of Ti. The films were deposited by direct current reactive magnetron sputtering. They are semiconductors with a conductivity at room temperature up to 1000S/m and they reveal a transparency of more than 60% in the IR and visible range. We are discussing the modelling of optical properties of such multilayer thin films. The thickness of the layers, the interface roughness and the surface roughness have been taken into account as important modellig parameters. They were determined by profilometry and scanning probe microscopy. X-ray diffraction measurements provided information about nanocrystalline phase composition and preferential orientation. The texture information has been taken into account in an anisotropic effective medium approximation. The modeled data are compared with the optical properties measured by spectroscopic ellipsometry and UV-VIS-IR transmission spectroscopy.
10:30 AM		C5-1-7 Characterization on the Properties of n-ZnO/p-Si Photodiodes Using ZnO Thin Films Y.S. Choi, J.Y. Lee, S. Im (Yonsei University, South Korea) The photoelectric properties of n-ZnO/p-Si phodtodiodes obtained from RF sputtering are exhibited when n-ZnO has been prepared at an optimum deposition temperature of 480°C with various Ar:O₂ ratios of 2:1,4:1,6:1 and 8:1. Current-voltage characteristics of these diodes were observed with and without a continuous red laser(2mW,670nm) illuminating the surface of the n-ZnO/p-Si diodes. The best photo-electric performance was obtained using an Ar:O₂ ratio of 6:1. This photodiode exhibits high responsivity (0.25 A/W) and quantum efficiency (46.4%) under 5V reverse bias. The photo-response time of the photodiode is observed to be 35~40 ns by a digital oscilloscope as the diode is illuminated by a pulsed red laser (670nm) which basically has a rise-time limit of 35 ns. This work shows the potential of our simple n-ZnO/p-Si junction photodetector may be comparable to conventional p-i-n Si detectors.
10:50 AM		C5-1-8 Electrical and Optical Properties of Silver Oxide Thin Films Prepared by Reactive DC Magnetron Sputtering : The Role of Oxygen B. Ullash Kumar, S. Srinivasan (Indian Institute of Technology, India); C.L. Nagendra (Indian Space Research Organization, India); M.S. Ramachandra Rao, A. Subrahmanyam (Indian Institute of Technology, India) Silver Oxide (Ag₂O) thin films, in recent times, have attracted considerable attention mainly because of their p- type conduction and transmission in visible wavelengths and their potential in optical data storage. Oxygen seems to play a critical role in the electronic / physical properties in these films. The role of oxygen and the origin of the acceptor levels in these films have not been understood yet. In the present investigation, the role of oxygen on the electrical and optical properties of these thin films prepared by reactive DC Magnetron sputtering technique is reported. The magnetron target (7.4 cm diameter) is made up of pure silver (99.9%). The flow of Oxygen is varied in the limits 0 to 1.75 sccm. All growth parameters :sputter gas (Argon) flow rate (15.3 sccm), Magnetron power (25 watts), sputter time (300 sec) and target to substrate distance (5.0 cm) have been kept constant. The films are grown on cleaned soda lime glass substrates. The thickness of the films is measured by Digital thickness monitor (in situ) as well as Multiple beam interferometry technique. The flow rates of Oxygen corresponding to 0.54 sccm till 1.45 sccm have shown p-type conduction. With increasing oxygen in these films: (i) the resistivity(measured at 25^oC) of the films increases slowly ( and these films are insulating for 1.75 sccm of oxygen), (ii) the Hall mobilities of the p-type films decrease from 126.7 to 35.0 cm²v^-1s^-1 and (iii) the optical transmission exhibits complicated behavior. The optical band gap of these films is in the range : 3.5 to 3.78 eV. X ray diffraction experiments show that the films are polycrystalline in nature; a systematic shift is observed in the silver peak with increasing oxygen content in the film. These data are being analyzed for an understanding of the conduction mechanism and optical transmission behavior.