ICMCTF2004 Session C1: Recent Advances in Optical Thin Films

Monday, April 19, 2004 1:30 PM in Room Sunset

Monday Afternoon

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Start Invited? Item
1:30 PM C1-1 Optical Coatings Applied by Single Target Reactive Sputtering
Frank Breme (Satis Photonics AG, Switzerland); M. Peter, L. Jordi (Satis Vacuum, Switzerland)
Silicon-targets as a single source for reactive sputtering offer a lot of advantages. It is possible to build very compact sputtering systems, where the different materials are deposited by only using different gases like nitrogen, oxygen and argon. Possible materials in the visible range (400-700 nm) are Siliconoxides, Siliconnitrides and all kinds of different Siliconoxinitrides with a refractive index from 1.47-2.03. In the near infrared pure Silicon and Silicon-suboxides with refractive indices up to 3.2 can be deposited. High deposition rates up to 2 nm/sec. and short pumping times of about 3 min. lead to very fast cycle times. Different possible applications like ophthalmic antireflective-coatings, mirrors or NBP-filters in the NIR as well as the machine concept will be presented.
2:10 PM C1-3 Photoactive Coatings for a Photolytic Artificial Lung
P.M. Martin (Pacific Northwest National Laboratory); B.F. Monzyk, J.R. Busch (Battelle Memorial Institute); R.J. Gilbert (Massachusetts Institute of Technology); K.A. Dasse (Levitronics, Inc.); T.L. Stewart (Pacific Northwest National Laboratory)
Photoactive anatase TiO2 films have been developed for use in a photolytic artificial lung (PAL). The PAL is capable of facilitating gas exchange in the blood, thereby bypassing alveolar-capillary interfaces. The device will eventually be used in ex-vitro and in-vitro devices. The direct photolytic process, using UV laser radiation, converts water to liquid phase oxygen (dissolved oxygen), with commensurate clearance of carbon dioxide. The test cell consisted of an indium tin oxide coating, an anatase TiO2 coating (~ 2 microns thick), and a MnO2 overcoat deposited on fused silica by reactive magnetron sputtering. Blood flowed over the coated side and oxygen exchange occurred at the MnO2 interface. Far UV radiation was incident on the silica/ITO side. Electron-hole pairs were generated in the TiO2 layer by the laser radiation, which catalyzed a redox reaction with water in the blood. The MnO2 was also used as a catalyst to dissolve oxygen in the blood. Oxihemoglobin increased as much as 90 % with this process. The need for a highly porous microstructure in the TiO2 will be discussed, as well as recent efforts to increase the quantum efficiency of the device.
2:30 PM C1-4 Nitrogen-doped Titanium Oxide Films as Photocatalyst by Vapor Deposition
M.C. Yang, C.B. Shiu, T.S. Yang, M.S. Wong (National Dong Hwa University, Taiwan, R.O.C.)
TiO2 photocatalysts need illumination with ultraviolet (UV) light (λ < 387 nm) corresponding to the band gap of anatase TiO2 (3.2 eV), which only utilize small part of solar light. Hence, the quest of visible light photocatalysts that can yield efficient reaction under solar irradiation, and even poor illumination of interior lighting would be very valuable. Recently, Asahi et al. (Science 293 (2001) 269) reported that anionic N-doped titanium oxide films and powders were operative on the band-gap narrowing and photocatalytic activity under λ <436 nm. We used ion-assisted electron-beam evaporation system to prepare TiO2-xNx films. The TiO2-xNx films maintain the structure of anatase TiO2, characterized by their XRD patterns and Raman spectra. The films exhibit the visible-induced hydrophilicity and photocatalytic activities such as photodegradation of methylene blue and photoreduction of silver ion (Ag+) to metal (Ag).
2:50 PM C1-5 Uranium Oxide as a Highly Reflective Coating from 2.7 to 11.6 Nanometers
R.L. Sandberg, D.D. Allred, R.S Turley, S. Lunt, J.E. Johnson (Brigham Young University)
We present the measured reflectances (Beamline 6.3.2, ALS at LBNL) of naturally oxidized and reactively sputtered uranium and naturally oxidized nickel, gold, and iridium thin films from 2.7 to 11.6 nm at 5, 10, and 15 degrees grazing incidence. These show that uranium, as UO2, can fulfill its promise as the highest known single surface reflector for this portion of the soft x-ray region, being nearly twice as reflective as nickel in the 5-10 nm region. This is due to its large index of refraction coupled with low absorption. Nickel is commonly used in soft x-ray applications in astronomy and synchrotrons. (Its reflectance at 10 degrees exceeds that of Au and Ir for most of this range.) We prepared uranium and nickel thin films via DC-magnetron sputtering of a depleted U target and resistive heating evaporation respectively. Ambient oxidation quickly brought the U sample to UO2 (total thickness about 30 nm). For reactively sputtered uranium, oxygen was back filled into the evacuated chamber with argon to sputter uranium oxide. The nickel sample (50 nm) also acquired a thin native oxide coating (<2nm). Though the density of U in UO2 is only half of the metal, its reflectance is high and it is relatively stable against further changes. There are important discrepancies between the actual reflectance of UO2 and those predicted by the atomic scattering factor model indicative of the need to determine the actual constants of UO2.
3:10 PM C1-6 High Rate Deposition of Aluminum-doped Zinc Oxide Films by In-line Reactive Mid-frequency Magnetron Sputtering
R.J. Hong, B. Szyszka (Fraunhofer Institute for Surface Engineering and Thin Films (IST), Germany); X. Jiang (University of Siegen, Germany); V. Sittinger (Fraunhofer Institute for Surface Engineering and Thin Films (IST), Germany)
Aluminum-doped zinc oxide (ZnO:Al or AZO) have been fabricated by reactive mid-frequency (MF) magnetron sputtering from Zn/Al metallic targets (Al: 1-2 wt.%) with a dual magnetron (TwinMag TM) configuration. The highest deposition rate for transparent conductive AZO films deposited in a dynamic deposition mode is 280 nm/min. It is found that both deposition total pressure and oxygen partial pressure are very important for deposition high quality AZO films. Substrate temperature also plays a significant role on the structure and properties of the films. The films prepared at substrate temperature from 100 to 200°C exhibit columnar structures with high content of (002) texture component, resulting in high carrier concentrations, high Hall mobilities and thus good conductivities. It is shown that there exists only a narrow oxygen partial pressure window for depositing AZO films with low resistivity and high optical transmittance. At too high oxygen partial pressure the dopant was oxidized and at too low pressure the film became substoichiometric, which produced absorbing layers. The crystallinity of the film is improved and the columnar film growth becomes more dominant with the increase of total pressure. The film prepared at a total pressure of 970 mPa and a substrate temperature of 170°C shows a regular cone-shaped grain surface and densely packed columnar lateral structure. The improvement of crystallinity leads to an improvement of mobility. A minimum resistivity of 2.86 X 10-4 Ω cm with mobility of 32 cm2/V s is obtained. Based on the optimization of deposition process, high quality AZO films with fairly good homogeneity of ± 6% on an area of 1000 X 600 mm2 are obtained. The production films are investigated for the use as front electrode layers in thin film solar cells (a-Si:H cells) and shown excellent light scattering properties and high initial efficiencies.
3:30 PM C1-7 Dependence of ZnO Thin Films Properties on Filtered Vacuum Arc Deposition Parameters
T. David, S Goldsmith, R.L. Boxman (Tel-Aviv University, Israel)
The micro-properties (structure and composition), and macro-properties (electrical and optical properties) of ZnO thin films deposited on glass substrates using a filtered vacuum arc deposition system were investigated as a function of oxygen pressure (0.37-0.5 Pa) and arc current (100-300 A). The substrates were not heated or biased during deposition, and were not annealed. The films were polycrystalline, and the crystal plane orientation varied with the oxygen pressure and arc current, tending to be aligned parallel to the c-axis. The sizes of the crystallite grains of the films were 10-35 nm. The films were found to be compressively stressed, with stress in the range of 0-2.5 GPa. The stress decreased with increasing arc current, and was found to have a more elaborate yet well-defined dependence on the pressure. Film composition, determined by XPS, depended weakly on the deposition parameters. Oxygen to Zinc ratios were typically ~0.7. Film thickness, in the range of 80-780 nm, depended linearly on both deposition parameters. The electrical resistivity of the films was in the range of (1-4)x10-2 Ω cm, depending weakly on the deposition parameters. The electrical resistivity of large-grained films was higher than that of smaller-grained films. Stress free films also had higher resistivities. The optical transmittance depended strongly on both deposition parameters, improving in high-pressure and low-current regimes, and could be greater than 90% over the visual and near-IR range of the spectrum. However, large-grained and low-stressed films had lower optical transmission.
3:50 PM C1-8 Low Resistivity Indium-molybdenum Oxide Transparent Conductive Films I. Effects of Introducing H2 Gas during High Density Plasma Evaporation at Room Temperature
S.Y. Sun (National Cheng-Kung University, Taiwan, R.O.C.)
Indium molybdenum oxide (IMO) films were made from an oxidized target with In2O3 and MoO3 in a weight proportion of 95:5 by using high density plasma evaporation. Hydrogen was added to the gas mixture during the preparation of the IMO films. For IMO films fabricated at room temperature, it yielded the lowest resistivity of 5.0x10-4 © cm comparing with those without hydrogen. The decrease in the resistivity resulted mainly from significantly increasing mobility caused by MoO3 reduction effect and oxygen vacancies. Based on XPS results, reduction of MoO3 into Mo metal was evidenced. Uniform and ultra-flat IMO films with low Rms roughness of 0.6nm and high average transmittance of 83% over the visible wavelength were obtained.
4:10 PM C1-9 Studies on Electrical and Optical Properties of Reactive DC Magnetron Sputtered Silver Indium Oxide Thin Films: Role of Oxygen and Magnetron Power
U.K. Barik, A. Subrahmanyam (Indian Institute of Technology, India); S. Gunasekaran (GE india Technology Centre, India)

Silver-indium oxide (AIO) thin films are one of the potential candidates for p-type transparent conducting thin films. The suited crystal structure for p-type conduction is Delafossite. In the present investigation, the silver-indium oxide (AIO) thin films have been prepared at room temperature [30C] on soda lime glass as well as on p-Si substrates by reactive dc magnetron sputtering technique. The target is pure metal alloy of Silver: Indium=50: 50 atomic %. The oxygen flow rate and DC magnetron power are varied in the range 0.00-2.86 sccm and 40-80 Watts respectively. The X-ray diffraction data show that the films are polycrystalline and amorphous (nano) in nature for 0.00 - 2.36 sccm and 2.86 sccm oxygen flow rates respectively. The electrical resistivity measurement shows that these films are semiconducting. The electrical (resitivity, carrier concentration, Hall coefficient, Hall mobility, work function) and optical (optical band gap, refractive index) properties of these films have been reported in this paper. The refractive index (at 632.8 nm) of these films varies in the range 1.13-1.22. The work function has been measured on these films by contact potential method using Kelvin Probe1. The nano-nature of the films is confirmed by SEM measurement.

1C. Suresh Kumar, A. Subrahmanyam, J. Majhi, Rev. Sci. Instrum. 67, 805 (1996).

4:30 PM C1-10 Microstructures of Ga-Doped ZnO Films Prepared by a DC-arc Ion Plating Method
T. Yamamoto (Kochi University of Technology, Japan); T. Sakemi (Sumitomo Heavy Industries, Ltd., Japan); K. Awai (Sumiju Technical Center Co., Ltd, Japan)

Transparent and conductive Ga-doped ZnO (GZO) films (200-nm thick) have been deposited at a glass substrate temperature of 200°C using a DC-arc ion plating system. Effects of the oxygen flow rate on film properties were examined. The resistivity as low as 2.7*10-4Ω cm with a high carrier concentration of 8*1020 cm-3, Hall mobility of 27 cm2 /Vs and high average transmittance above 90 % in the visible range were obtained from the film deposited under reducing conditions using ZnO:Ga2O3 tablets with a Ga2O3 content of 3 wt%. Under reducing conditions, for the oxygen flow rate of 0-15 sccm, carrier concentration decreases and Hall mobility increases very slowly as there increases the oxygen flow rate.

The surface morphology of the GZO films prepared at different oxygen flow rates, 0, 15 and 20 sccm, were observed by a scanning electron microscope (SEM). Little deference of grain size was found among the three films. The grain size was estimated to be 20-40 nm. In order to clarify the grain structure, GZO films were analyzed by a cross-sectional transmission electron microscopy (TEM). The films were confirmed to consist of columnar grains with a diameter of 20-40 nm that pass through the film thickness were observed. Moreover, no segregation of Ga metals in intragrains was found on the basis of the analysis of data obtained by energy dispersive X-ray spectroscopy (EDS). Thus, the mean free path of carrier electrons based on a highly degenerated electron gas model was theoretically calculated. Under the reducing conditions, the mean free path of the GZO films was almost 5 nm, being significantly smaller than the grain diameter of 20-40 nm. From both the experimental data and theoretical results, we concluded that the grain boundary scattering mechanism in GZO films deposited under reducing conditions appears not to play an important role on electrical properties.

4:50 PM C1-11 Thin Film Transistor using H-doped ZnO Channel Layers
S. Im, H.S. Bae (Yonsei University, South Korea)
ZnO-based thin film transistors (TFTs) have been fabricated. ZnO films of ~100 nm thickness were deposited as channel layers on SiO2/p-Si substrate by r.f. sputtering at room temperature (RT). For source/drain contacts, Al was evaporated through a shadow mask. Rapid thermal annealing (RTA) was performed at 300~350 C for 1~2 min to reduce the contact resistance of Al/ZnO interface and also to increase the conductivity of ZnO films as well. In particular, the higher field effect mobility of 0.3 cm2/Vs was obtained from a TFT that was treated using RTA in forming gas ambient (10% H2 + 90% N2) than from another TFT (mobility ~ 0.1 cm2/Vs) treated in N2 ambient. The role of H atoms as an N-type dopant in ZnO channel layer is discussed in detail.
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