ICMCTF2006 Session C3-1: Optical Thin Films for Active Devices and Microsystems
Monday, May 1, 2006 10:30 AM in Royal Palm 4-6
C3-1-1 Oxide Thin Films for Transparent Electronics
H. Hosono (Tokyo Institute of Technology, Japan)
It is believed that high optical transparency is in general incompatible with high electronic conduction, since optical transparency requires bandgaps larger than 3.3 eV while it makes carrier doping much difficult. In this sense, Transparent Conductive Oxides (TCOs) are unique materials. The first TCO was developed as In@sub 2@O@sub 3@:Sn (ITO) in 1954, followed by findings of other TCOs, SnO@sub 2@, ZnO and so on. Now the TCOs are inevitable for solar cells and flat-panel displays. However, TCOs had been used only for passive applications such as window electrodes and transparent interconnections. It was because active devices such as pn junctions could not been fabricated due to absence of p-type TCO. The breakthrough was the finding of the first p-type TCO CuAlO@sub 2@ in 1997 by our group, which subsequently triggered the development of a series of p-type TCOs and transparent pn junction devices such as UV LEDs. These achievements changed our conception on TCOs to Transparent Oxide Semiconductors (TOSs), and therefore we consider that TOSs has potential to develop new functional optoelectronic devices that the present Si-based semiconductor technology cannot. In this study, we first discussed how to design new TOSs based on knowledge about electronic structures accumulated experimentally and theoretically. The validity of our material design concepts demonstrated by developing new TOSs including p-type TOSs. These new TOSs led to transparent electronic devices such as UV LEDs and transparent TFTs We concentrated on development of new functions and realization of new devices utilizing natural nanostructures embedded in crystal structures of TOSs. Although we do not achieve satisfactory results yet, we have found unique optoelectronic properties associated with low-dimensional electronic structures in layered oxychalcogenides and developed bright light-emitting devices using nano-porous semiconductor C12A7:e-.
C3-1-3 Stability in a High-Temperature Moist Environment of TCO Thin Films Deposited at Low Temperatures
T. Minami, T. Miyata, S. Tsukada (Kanazawa Institute of Technology, Japan)
In this paper, we describe the high-temperature moist environment stability of various transparent conducting oxide (TCO) thin films such as indium-tin-oxide (ITO), ZnO:Al (AZO) and ZnO:Ga (GZO) that were prepared on glass substrates at a low temperature below approximately 200@super o@C. TCO thin films prepared with a resistivity on the order of 10@super -3@ to 10@super -4@@ohm@cm and a film thickness in the range from approximately 20 to 500 nm by magnetron sputtering (MSP), pulsed laser deposition (PLD) and vacuum arc plasma evaporation (VAPE) were used in the stability test. The long term stability tests, up to 1000 h, were carried out under the following conditions; air at a relative humidity of 90% and a temperature of 60-80@super o@C: The electrical properties of TCO films were measured at intervals of 1 h. It was found that the resistivity of AZO and GZO thin films tested in air at 90% relative humidity and 60@super o@C increased with test time, whereas that of ITO remained relatively stable. The resistivity increases of AZO and GZO thin films were considerably affected by film deposition technique, deposition temperature and film thickness; preparation at a temperature below approximately 100@super o@C with a thickness below approximately 100 nm resulted in marked increases in resistivity, whereas the stability was relatively independent of deposition temperature in the range from approximately 100 to 200@super o@C and thickness at a value above approximately 200 nm. In most thin films that exhibited increased resistivity after being tested, the resistivity increase was ascribed to decreases of both carrier concentration and Hall mobility. In addition, the observed changes in electrical property could be explained by the grain boundary scattering resulting from the adsorption of oxygen on the grain boundary. In conclusion, the stability of TCO thin films that consist of a binary compound host material was found to be fundamentally dominated by the characteristic behavior of the host material in high-temperature moist environments.
C3-1-4 Effect of Alumina Dopant on Structural, Electrical and Optical Properties of Sputtered ZnO Thin Films
S.-N. Bai (Chienkuo Technology University, Taiwan); T.-Y. Tseng (National Chiao-Tung University, Taiwan)
A systematic study of the influences of alumina dopant on the optical, electrical and structural characteristics of sputtered ZnO thin films is reported in this study. The ZnO thin films were prepared on Corning 1737F glass substrates by R.F. magnetron sputtering from a ZnO target mixed with Al@sub 2@O@sub 3@ of 0~4 wt.%. The X-ray diffraction (XRD) analysis demonstrate that the ZnO thin films with Al@sub 2@O@sub 3@ of 0~4 wt.% are highly (002) preferred orientation films, which have only one intense diffraction peak with a full width at half maximum (FWHM) lower than 0.5°. The electrical properties of the Al-doped ZnO thin films appear to be strongly dependent on the Al-doped concentration. The resistivity of the films decreases from 73.85W-cm to 2.2 10-3W-cm as Al-doped content increases from 0 to 4 wt.%. The optical transmittance of the Al-doped ZnO thin films is studied as a function of wavelength in the range 200~800 nm. It exhibits highly transparent in the visible-NIR wavelength region with some interference fringes and sharp ultraviolet absorption edges. The optical bandgap of the Al-doped ZnO thin films is shown a short-wavelength shift with the increase of Al content.
C3-1-6 Effect of Deposition Conditions on the Characteristics of Highly Transparent and Conducting ZnO-SnO@sub 2@ Thin Films Deposited by Filtered Vacuum Arc Deposition (FVAD)
E. Çetinörgü, S. Goldsmith, R.L. Boxman (Tel-Aviv University, Israel)
ZnO-SnO@sub 2@ transparent and conducting thin films were deposited on microscope glass substrates by filtered vacuum arc deposition (FVAD) system, operating at 200, 250, and 300 A for 60 and 120 s. The oxygen background pressure during deposition was in the range 4 to 8 mTorr, varied at 1 mTorr steps between samples. The cathodes were prepared with the following atomic ratios of Zn to Sn: 90%/10%, 70%/30%, and 50%/50%. The films were annealed in air at 500@super o@C for one hour. The micro and the macro properties of the films were investigated as a function of cathode composition, arc current, background oxygen deposition pressure, and deposition time, before and after annealing. X-ray diffraction analysis indicated that as-deposited and air-annealed thin ZnO-SnO@sub 2@ films were amorphous, independent of the cathode composition. The atomic ratio of Zn to Sn in the film obtained using the 50%/50% cathode as determined by EDX analysis, was 67%/33%. The highest deposition rate of the ZnO-SnO@sub 2@ films was 7 nm/s. The optical properties were determined from normal incidence transmission and spectroscopic ellipsometer measurements. Film transmission in the visible was 70 to 90%, indicating interference effects. The ellipsometric values of the refractive index n and the absorption coefficient k in the visible were 2.11 to 1.94 and 0.03 to 0.06, respectively. Assuming that the inter-band electron transition is direct the optical band gap is found to be in the range of 3.36±0.23eV. The lowest electrical conductivity was 2 x10@super -3@ @ohm@.cm for the 50% at. Sn films deposited with 200 A arc-current and 7 mTorr oxygen pressure. The electrical resistivity increased with decreasing Sn concentration and film thickness.
C3-1-5 High Quality Transparent Conductive Multilayer Films Deposited at Room Temperature
D.R. Sahu, J.-L. Huang (National Cheng Kung University, Taiwan)
Recently, there has been much attention paid to transparent conductive oxides (TCO) due to their potential applications in opto-electronics, flat panel displays, solar cells etc. This field aims to develop highly conductive and transparent film to enhance device performance at room temperature. We have fabricated multilayer transparent electrodes having much lower electrical resistance than the widely used transparent conductive oxide electrodes by simultaneous DC and RF magnetron sputtering. The multilayer structure consists of three layers (ZnO/Ag/ZnO). Ag films with different film thickness were used as metallic layers. Optimum thickness of Ag and ZnO film were determined for high optical transmittance and good electrical conductivity. Several analytical tools such as X-ray diffraction, spectrophotometer, Atomic force microscopy, Scanning electron microscopy and four point probe were used to explore the possible changes in electrical and optical properties. A high quality transparent electrode, having a resistance as low as 3 ohm/sq and a high optical transmittance of 90% was obtained at room temperature and can be reproduced by controlling the preparation process parameter. The electrical and optical properties of ZnO/Ag/ZnO multilayer were changed mainly by Ag film properties, which are effected by the deposition process of the upper layer. Details of the preparation process and possible causes of changes in properties will be discussed during presentation. The performance of the multilayer as transparent conducting materials will also be compared using a figure of merit.