ICMCTF2010 Session C2-1: Thin Films for Active Devices and Microstystems

Thursday, April 29, 2010 8:00 AM in Room Royal Palm 1-3

Thursday Morning

Start	Invited?	Item
8:00 AM	Invited	C2-1-1 Organic Semiconductor Lasers, Optical Amplifiers and Detectors Y. Yang, D. Amarasinghe, G. Tsiminis, J.W. Levell, M.E. Giardini, A. Ruseckas, G.A. Turnbull, Ifor Samuel (University of St. Andrews, United Kingdom) The attractive properties of organic semiconductors such as simple fabrication and scope for tuning properties make these materials appealing for thin film devices with a wide range of applications in electronics and optoelectronics. The great progress made in materials for organic light-emitting diodes has led to a newer field of organic semiconductor lasers. In addition to simple processing and tuneability, the strong absorption, high gain cross section, broad spectra and high solid state fluorescence quantum yields of organic semiconductors are attractive for applications as lasers. The high chromophore density in neat films enables high gain to be obtained, and distributed feedback lasers in which lateral corrugation of the film provides feedback are commonly used. The development of practical polymer lasers requires progress in reducing the threshold for lasing so that convenient pump sources can be made. Pump lasers have been steadily reduced in size from large frame lasers to a microchip laser and more recently a gallium nitride laser diode. The need for another pump laser has limited the development of polymer lasers. We have overcome this problem by matching an InGaN LED as pump source to a fluorene copolymer laser. The LED is a convenient low cost electrical pump source and the polymer gives tuneable emission at wavelengths not readily accessible by inorganic semiconductors. Hence this hybrid optoelectronic device provides the key advantages of electrical pumping and simple fabrication expected from an electrically pumped polymer laser. Another hybrid optoelectronic device will also be shown – a photodiode using a combination of silicon and an organic semiconductor thin film to give greatly enhanced ultraviolet response. The presence of gain in organic semiconductors can also be used to make optical amplifiers. These give broadband amplification of ultrafast optical pulses in the visible region of the spectrum. Finally, we will also show how organic thin films can be conveniently patterned by soft lithographic techniques.
8:40 AM		C2-1-3 Concepts for the Material Development for Solution Processable Phosphorescent Polymers and their Application in PLEDs Silvia Janietz, Hartmut Krueger, Manuel Thesen, Armin Wedel (FhG-IAP, Germany) Organic electronics is based on new conductive and semiconductive materials which, when put in solution, can be processed using manufacturing methods. By adapting established printing and other structuring processes, such as laser patterning, it is possible to mass produce very cost-effective electronics on large area, flexible substrates. A big advantage of organic electronics is that it allows the combining of a large number of different plastic electronic components and their production in one integrated manufacturing process. One example of organic electronics is the application of polymer based light emitting devices (PLEDs). PLEDs are very attractive for large area and fine-pixel displays, lighting and signage. The polymers are more amenable to solution processing by printing techniques which are favourable for low cost production in large areas. With phosphorescent emitters like Ir -complexes higher quantum efficiencies were obtained than with fluorescent systems, especially if multilayer stack systems with separated charge transport and emitting layers were applied in the case of small molecules. But polymers exhibit the ability to integrate all the active components like the hole-, electron-transport and phosphorescent molecules in only one or maximal 3 layers. Therefore this concept is very attractive for an alternative technology processing in comparison to the more complicated multilayer stack systems based on the evaporation of small molecules. Two different synthetic routes were developed . In one case, structure optimized and energy-level adapted hole-, electron- transport and phosphorescent molecules were selected and modified with polymerizable groups. Terpolymers were synthesized containing optimized ratio of the 3 selected components. The results of the OLED characterization will be discussed in detail in com parison with the chosen structure of the molecules. Another approach is to separate the transport molecules in two different layers. For this reason cross linkable polymer layers have to be applied to realize the multi layer structure stability during wet processing .
9:00 AM	Invited	C2-1-5 Light Emission from Organic Field-Effect Transistors Eva Feldmeier, Martin Schidleja, Chistopher Siol, Christian Melzer, Heinz von Seggern (Technical University of Darmstadt, Germany) The discovery of light emission from organic field-effect transistors (OFETs) in 2003 by Hepp et al. [1] and Ahles et al. [2] has opened an interesting field of research for the investigation of ambipolar charge transport in inorganic devices. The history and latest progress on organic light-emitting field-effect transistors will be presented. Based on the finding that ambipolar transport is possible for a variety of polymers and small molecules on electron-trap-free gate insulators, light emitting OFETs utilizing PMMA (poly(methyl methacrylate) and PS (polystyrene) as gate dielectrics and F8BT (poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) and different polyacences like teracene, ditetracene and pentacene as organic semiconductors [3] will be demonstrated. It will be investigated how electron and hole transport are initiated and finally result in recombination, which leads to a spatially controllable emission zone [3,4]. The influence of injection barriers from different source and drain metals is also addressed. It will be demonstrated that by selecting ohmic contacts strong light emission is already observable in pure electron and hole accumulation indicating strong thermal injection of charge carriers at the contacts. Some of the experimental results are compared to theoretical derived OFET characteristics from an OFET model by Schmechel et al. [5] generalized through the inclusion if charge carrier injection by a model proposed by Scott and Malliaras [6]. [1] A. Hepp, H. Heil, W. Weise, M. Ahles, R. Schmechel, H. von Seggern, “Light-emitting field-effect transistor based on a tetracene thin film”, Phys. Rev. Let. 91, 157406-1 (2003) [2] M. Ahles, A. Hepp, R. Schmechel, H. von Seggern, “Light emission from a polymer transistor“, Appl. Phys. Lett. 84, 428 (2004) [3] J. Zaumseil, R. H. Friend, and H. Sirringhaus, "Spatial control of the recombination zone in an ambipolar light-emitting organic transistor", Nat. Mater. 5, 69 (2006) [4] M. Schidleja, C. Melzer, H. von Seggern "The organic light-emitting field-effect transistor" submitted to Frequenz [5] R. Schmechel, M. Ahles, H. von Seggern, „A pentacene ambipolar transistor: Experiment and theory” J. Appl. Phys. 98, 084511 (2005) [6] J. C. Scott, G.G. Malliaras, Chem. Phys. Lett. 299, 115 (1999)
9:40 AM		C2-1-7 Polythiophene Thin Films Electrochemically Deposited on Sol-Gel Based TiO₂ for Photovoltaic Applications Rogerio Valaski (Inmetro); Natasha Yamamoto, Regina Mello, Marcela Oliveira, Lucimara Roman (UFPR, Brazil); Cristiano Legnani, Welber Quirino (Inmetro, Brazil); Marco Cremona (PUC-Rio, Brazil) Due to its higher stability in environmental conditions and its high absorption coefficient in the visible range of spectrum, polythiophene (PT) is one of the most promising organic semiconductors for photovoltaic applications. The electrochemical methods for organic films production present some interesting features as simultaneous organic synthesis and film deposition and the possibility of thickness control by the time deposition. Moreover, the overcoming of some intrinsic difficulties of the electrochemical method allows the development of efficient devices and has strategic importance due to the easiness and the lower costs of production. In this context the electrochemical deposition of PT on titanium dioxide (TiO₂) films has being a feasible alternative towards more efficient and more durable devices. TiO₂ is an n-type semiconductor with a large gap and it has been demonstrated that it is able to improve drastically the efficiency of organic photovoltaic cells. Among the different experimental techniques to produce TiO₂ layers the sol-gel method stands out due to be an easier way of production large area films without controlled atmosphere or considerably changes in the process. As most of organic semiconductors are p-type a TiO₂ intermediate layer between the electrode and the organic active layer produces a p-n junction. Furthermore, the difference between the material gaps produces energy steps which improve the dissociation of the excitons generated in organic layer by the radiation absorption. In this work, TiO₂ films produced by sol-gel technique were structurally, morphologically and electrically characterized by X-ray diffraction, AFM and Four Probe method . X-ray diffraction results lead to the conclusion that sol-gel TiO₂ films are predominantly composed by the anatase phase with electron mobility of 10¹ cm²/V.s, which is coherent with the values present in the literature. TiO₂ resistivity also varies in a range between 10⁶ and 10⁴ Ω.cm depending on temperature treatment.. Using a three electrode cell controlled by a potentiostat, PT films were electrochemically deposited under 3 V/Ag/AgCl and dedoped with a potential of 0.2V/Ag/AgCl. The electrolyte was a 0.02 M of (CH₃)₄NBF₄ in acetonitrile with 0.1 M of thiophene monomer. In order to study the influence of PT/TiO₂ bi-layer on the photovoltaic efficiency, IPCE (dynamic spectra under short-circuit condition) and J(V) curves were also measured in dark and under illumination for some organic photovoltaic devices (OPV).
10:00 AM		C2-1-9 Microstructural and Optical Properties of Ga-Doped ZnO Semiconductor Thin Films Prepared by the Sol-Gel Process Chien-Yie Tsay, Chun-Wei Wu (Feng Chia University, Taiwan); Chien-Ming Lei (Chinese Culture University, Taiwan); Fan-Shiong Chen (Tatung University, Taiwan); Chung-Kwei Lin (Feng Chia University, Taian) ZnO-based semiconductor thin films attract considerable interest as an active channel layer in thin film transistors. In this study, transparent semiconductor thin films of Ga-doped ZnO (GZO), Ga dopant varying from 0 to 7 at.%, were deposited onto alkali-free glass substrates by the sol-gel process. The effect of Ga doping on crystallinity, microstructure, optical properties and electrical resistivity of ZnO thin films were systematically investigated. Moreover, chemical compositions and photoluminescence spectra of GZO thin films were examined. Each spin-coated film was dried at 300^oC for 10 min, and then annealed at 500^oC for 1 h in air ambiance. XRD results revealed that the undoped ZnO thin film exhibited preferential orientation along the (002) plane and ZnO thin films doped with Ga degraded the crystallinity. Both optical transmittances in the visible range and electrical resistivity of GZO thin films increased with increasing the dopant. In the present study, the 7 at.% Ga-doped ZnO thin film exhibited the best average transmittance of 90.5% and a resistivity of 2.0 × 10³Ω-cm.
10:20 AM		C2-1-10 Characteristics of Highly Transparent Conductive Ga-Doped ZnO on Polymer Substrates Prepared by Ion-Plating Deposition with DC Arc Discharge Tetsuya Yamamoto, Aki Miyake, Takahiro Yamada, Hisao Makino, Naoki Yamamoto (Kochi University of Technology, Japan) Highly transparent conductive polycrystalline Ga-doped ZnO (GZO) films with a thickness of about 100nm prepared on cyclo-olefin polymer (COP) or glass substrates at various substrate temperatures below 90°C by ion plating with DC-arc discharge were investigated. A systematic study has been made of the influence of substrate temperature T_s. An increase in T_s resulted in a beneficial effect on the crystallinity, electrical conductivity and absorption coefficient in the visible (VIS) range regardless of the type of substrate. The analysis of the XRD patterns of the films showed that all polycrystalline GZO films on the two different substrates have a preferred orientation along the c-axis regardless of T_s. The crystallity of GZO/COP is lower than that of GZO /glass at any given T_s. The minimum resisitivity for GZO on glass and COP substrates was as low as 5.0μΩm and 5.8μΩm, respectively. The total light transmittance of all the GZO films on the two different substrates was more than 86% regardless of T_s. The optical absorption loss in the visible range decreased according in order of increasing Hall mobility with increasing T_s. All the GZO films were found to be compressively stressed. Analysis of calculated residual stresses in GZO films based on the biaxial stress model shows that the residual stresses in GZO films on glass substrates are a decreasing function of T_s, whereas the residual stresses are an increasing function of T_s for GZO films on COP substrates. This finding indicates that for the GZO films on COP substrates deposited at higher temperature, thermal stress is dominant.
10:40 AM		C2-1-11 Optical and Electrical Properties of Transparent Conductive Ga-Doped ZnO Thin Films Hisao Makino, Takahiro Yamada, Naoki Yamamoto, Tetsuya Yamamoto (Kochi University of Technology, Japan) Ga-doped ZnO (GZO) is one of promising candidate as transparent electrodes in optoelectronic applications, such as flat panel displays and solar cells. It has been well known that electrical properties of GZO films strongly depend on its deposition parameters and conditions of post-deposition thermal annealing. In this paper, we studied optical absorption (OA) and photoluminescence (PL) of GZO films in order to explore mechanisms of change in electrical properties of GZO films. 150 nm thick GZO films were deposited on alkali-free glass substrates by an ion-plating with DC-arc discharge at a substrate temperature of 110 ^oC. During the deposition, high purity oxygen gas was introduced into the deposition chamber, and the oxygen flow rate was varied from 8 to 15 sccm. The minimum resistivity of 2.8 μΩm was obtained at the oxygen flow rate of 11 sccm. Carrier concentration monotonously decreased with increasing the oxygen flow rate. The OA spectra showed a fundamental optical gap in ultra violet region and a free-carrier absorption in the mid-infrared region. In addition, an absorption band in visible wavelength region was clearly observed for the GZO films deposited under the oxygen flow rate of 8 sccm . The absorption band decreased with increasing the oxygen flow rate. The absorption band is most probably due to intrinsic defects, like as interstitial Zn, oxygen vacancy or others in the GZO film caused by lacking of oxygen during the deposition. Post-deposition thermal annealing also decreased the absorption band. The intensity of the absorption band decreased with increasing thermal annealing temperature. We found that the evolution of absorption band showed some correlation with the decrease of carrier concentration. In PL spectra, we observed a broad emission band, which can be assigned as near band edge (NBE) emission. By the thermal annealing, the PL intensity decreased with increasing the annealing temperature. In addition, a deep level emission band was appeared in lower energy side of the NBE emission band after the annealing in air. It was suggested that several mechanisms to kill carrier in GZO films simultaneously contribute to the change of carrier concentration. It is crucial to control intrinsic defects in transparent conductive GZO films.
11:00 AM		C2-1-12 Effect of Organic-Buffer-Layer on Electrical Properties and Environmental Reliability of GZO Films Prepared by RF Plasma Assisted DC Magnetron Sputtering on Plastic Substrates Toshio Hinoki, Chika Kyuhara, Hideaki Agura, Kenji Yazawa (Oike & Co., Ltd., Japan); Kentaro Kinoshita, Koutoku Ohmi, Satoru Kishida (Tottori University, Japan) ZnO thin films are expected as one of candidate transparent conductive oxide films alternative to indium tin oxide films. Considering an application for large size transparent electrode such as electronic display panels and solar panels, ZnO coated plastic sheets have an advantage of easily producing large area transparent electrodes by using a roll-to-roll mass production system. To prepare ZnO transparent conductive thin films on plastic sheets, a low temperature deposition at less than 100°C is required. Recently, we have reported on Ga-doped ZnO (GZO) thin films prepared by RF plasma assisted DC magnetron sputtering without intentionally heating substrate. In this study, we have attempted to deposit GZO thin films on polyethylene telephthalate (PET) sheets. Effects of organic-buffer-layer (OBL) formed between GZO films and PET substrates on electrical properties and environmental reliability of GZO films have been investigated. GZO films were deposited by RF plasma assisted DC magnetron sputtering on OBL coated PET substrates, which ware not intentionally heated during the deposition. The reductive gas of Ar + H₂ (10%) was locally supplied just below the substrate. To generate RF plasma near a substrate, a 2.5-turns-coil was located between a target and a substrate. Several kinds of organic silicone were used as an OBL. The sheet resistance was measured by 4-prove method. To confirm an environmental reliability of GZO films, an accelerated aging test was performed under the storage condition at 60°C and relative humidity of 95% for 250 hours. The resistivity of the GZO film prepared on the OBL-coated PET substrate is decreased to one half compared to that on the OBL-uncoated PET substrate. A moderate OBL thickness is 0.02 - 0.2 μm. The resistivity of the GZO film is decreased by providing the RF plasma, especially at the position facing the erosion area in the source target. A similar result has been obtained for GZO films on glass substrates. The total transmittance of the GZO film and the PET substrate is improved from 79% to 83% by installing the OBL. After the accelerated aging test, the sheet resistance maintains almost the same as the initial value for the GZO film on the OBL-coated PET substrate. Contrary, for the film prepared on the OBL-uncoated PET substrate, the sheet resistance increases by 10 times. The OBL is thought to play a roll of relaxing the stress between the GZO film and the PET substrate under high temperature and high humidity environment, and of suppressing the formation of crack in GZO films. One can be also thought that the OBL increases an adhesion strength between GZO films and PET substrates.
11:20 AM		C2-1-4 Synthesis and Characterization of 1-Phenyl-3-(4-phenyl-[1,2,3]triazol-1-yl)-4-Styryl-Azetidin-2-One Films for Light Emitting Applications Aman Mahajan, Vipan Kumar, Ratish Kumar Bedi (Guru Nanak Dev University, India) Triazole derivatives have attracted considerable interest in OLEDs because of their electron transporting as well as hole blocking and/or exciton confining properties. To improve the quantum efficiency and lower the operating voltage of OLEDs, a novel derivative of triazole namely 1-phenyl-3-(4-phenyl-[1,2,3]triazol-1-yl)-4-styryl-azetidin-2-one (TAZ) has been designed as an electron transporting material. This organic material has been synthesized by [2+2] ketene-imine cycloaddition reaction popularly known as Staudinger reaction of 1-azadiene with azidoketene to yield the desired compound, which is then subjected to Cu-catalyzed click chemistry with phenylacetylene in dichloromethane to yield the TAZ compound. Thin films of TAZ have been prepared by vacuum evaporation technique onto glass substrate kept at different temperature under different experimental conditions. These films have been systematically studied for their structural, optical and electrical properties. X-ray diffraction and FESEM studies of these films show increase in crystallinity with increase in substrate temperature. The electrical resistance of these films are found to lie in 106-107 ohm at room temperature. Analysis of optical absorption measurements on these films indicate that the interband transition energies lies in 3.4-3.6 eV. These wide band gaps show that TAZ layers can work as hole blocking (HB) material in OLED.
11:40 AM		C2-1-8 Effect of Cationic Surfactant on the Sensing Behavior of Nanocrystallite CuO Films Deposited by Spray Pyrolysis Ratish Kumar Bedi (Guru Nanak Dev University, India); Iqbal Singh (Khalsa College, Amritsar, India) P-type nanocrystalline CuO thin films have been deposited by chemical spray pyrolysis from cupric nitrate trihydrate (Cu(NO₃)₂.3H₂O) solution in water with cetyltrimetylammonium bromide (CTAB) as cationic surfactant, at different substrate temperature of 300 and 400˚C. The films were studied by X-ray diffraction (XRD) to obtain the information about structural properties in detail, the grain size (D), strain and lattice parameter were calculated. In the X-ray diffraction profile of thin films, the more intense contribution is systematically the (002) peak in all the samples. The surface properties and elemental analysis were characterised using scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The SEM micrographs showed different distribution of surface particles dependent on the substrate temperature and CTAB addition influences the shape, size with homogeneous distribution on the substrate. The sensitivity of the thin films was measured using two probe technique for ammonia vapors at room temperature. The presence of cationic surfactant resulting in an increase in the sensitivity of CuO thin film sensor for ammonia vapours at room temperature.