Papers

ICMCTF2007 Session C3-1: Optical Thin Films for Active Devices and Microsystems

Tuesday, April 24, 2007 8:00 AM in Room Tiki Pavilion

Tuesday Morning

Start	Invited?	Item
8:00 AM		C3-1-1 Transparent Conducting Oxide Films for Thin Film Silicon Photovoltaics W. Beyer (Forschungszentrum Jülich GmbH, Germany); J. Huepkes, H. Stiebig (Forschungszentrum Jülich IPV, Germany) Transparent conductive oxide (TCO) films are important functional layers in silicon (amorphous silicon, microcrystalline silicon) thin film solar cells. The active parts of these solar cells are packages of doped and undoped silicon layers usually deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane-hydrogen gas mixtures. The TCO films are applied as front contact and as part of the back contact to these active solar cell layers. The requirements for the TCO involve a high transparency in the range of the solar spectrum, a high conductivity (carrier mobility) as well as a high chemical stability of the bulk material as well as of the interfaces. Stability issues comprise, e.g., the influence of hydrogen plasma during silicon deposition, interface reactions as well as atmospheric influences. Furthermore, in order to fabricate the solar cell as an effective light trap for the purpose of saving material thickness of the silicon absorber layer, a texturing of the front contact layer is required. Rough interfaces between layers of different refractive index result in the scattering and subsequent trapping of light. Thus, high efficiency thin film silicon solar cells can be achieved. These issues are discussed with a focus on ZnO:Al transparent conducting oxide films.
8:40 AM		C3-1-3 Post-Deposition Annealing Effect on the Characteristics of Highly Transparent and Conducting ZnO-SnO₂ Thin Films Deposited by Filtered Vacuum Arc Deposition E. Çetinörgü (Tel Aviv University, Israel); S. Goldsmith, R.L. Boxman (Tel-Aviv University, Israel) ZnO and SnO₂ thin films were deposited on UV fused silica (UVFS) substrates using filtered vacuum arc deposition (FVAD). During deposition, the substrates were at room temperature (RT). As-deposited films were annealed at 400 and 600°C in Ar for 30 min. The structure, surface morphology and composition were determined as function of annealing temperature using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The XRD patterns of as-deposited ZnO films showed a strong c-axis orientation with a (002) peak, and the diffraction line intensity increased with annealing temperature, whereas, the XRD patterns of the SnO₂ films deposited on substrates at RT had amorphous structure, but polycrystalline SnO₂ thin films were observed after the annealing. The average transmission of as-deposited and annealed films in the visible range was 85% to 90%, and affected by interference. The films' optical constants in the 250-989 nm wavelength range were determined by variable angle spectroscopic ellipsometry (VASE). The complex refractive indexes of as-deposited, and 400 and 600°C annealed thin films decreased with the annealing temperature as function of wavelength. The optical band gap determined by the dependence of the absorption coefficient on the photon energy at short wavelengths was in the range 3.21 to 3.27 eV and 3.90 to 4.35 eV for ZnO and SnO₂ films, respectively. The lowest electrical resistivity was 1.1x10^-2 Ωcm for ZnO films and 7.8x10^-3 Ωcm for SnO₂ films deposited on RT substrates. The annealing greatly increased the resistivity.
9:00 AM		C3-1-4 Properties of Highly Oriented ZnO Nanowires Grown by Hydrothermal Method T.-Y. Tseng (National Chiao-Tung University, Taiwan); S.-N. Bai (Chienkuo Technology University, Taiwan); H.-H. Tsai (National Chiao-Tung University, Taiwan) A hydrothermal method was employed for the preparation of ZnO nanowires arrays and their properties were investigated. The two-step procedure was used for the growth of highly oriented ZnO nanowires. The experiment began with the deposition of an about 40 nm (002)-oriented ZnO seed layer on the Si substrate by a R.F. magnetron sputter. After the film deposition, the substrate was soaked in an aqueous solution containing Zn(OH)2 and complex agents. A vertical ZnO nanowire array is found to grow from the aligned seed layer in the aqueous solution at 95°C for 1-3 hours. From the X-ray diffraction pattern and SEM observation, the crystal orientation of ZnO nanowire arrays shows strong tendency to follow the orientation of the seed layer. Additionally, it is also demonstrated that the concentration of aqueous solution significantly affects the density of nanowires. The density of the ZnO nanowires increases with increasing concentration of aqueous solution. There are essentially two peaks appear in the cathodoluminescence (CL) spectra of the nanowires under room temperature. It is shown that a weak near-band-edge emission centered at 376 nm and a strong as well as board visible-band emission with a center at 575 nm. The CL emissions are believed corresponding to the exciton and defect transition.
9:20 AM		C3-1-5 Optical Properties of Zinc Oxide Films Grown by Atmospheric-Pressure Chemical Vapor Deposition Using Zn and H₂O as Source Materials T. Terasako (Ehime University, Japan); M. Yagi (Takuma National College of Technology, Japan); M. Ishizaki, Y. Senda, H. Matsuura, S. Shirakata (Ehime University, Japan) Zinc oxide (ZnO) is one of the promising materials for ultraviolet-light-emitting devices because of its wide direct bandgap of 3.37 eV at room temperature. For this kind of application, the first prerequisite is to establish film growth technique. Among various film growth methods, CVD methods are effective in obtaining high-quality films at low cost. In this paper, ZnO films have been grown on c-plane sapphire substrate by atmospheric-pressure chemical vapor deposition (AP-CVD) using Zn and H₂O as source materials for the first time. Photoluminescence (PL) and photoacoustic (PA) properties have been studied in terms of substrate temperature and source feeding ratio of H₂O to Zn (VI/II). Substrate temperature (T_S) was changed in the range from 550 to 800^oC. Source temperature for Zn and that for H₂O were maintained at 700 and 54^oC, respectively. The VI/II ratio was changed from to 5.6 to 29.8 by adjusting flow rate ratio of nitrogen carrier gaseous for both sources. Growth time was two hours. The film grown under VI/II=5.6 at T_S=700^oC exhibited a near band-edge (NBE) emission around 3.2eV and a green band emission around 2.5eV. The latter emission is reported to be associated with oxygen vacancies. Both the PL spectra of the films grown under VI/II=11.2 and 14.9 were dominated by the NBE emission. The film grown under VI/II=29.8 exhibited an orange band related to interstitial oxygen atoms. This remarkable VI/II dependence of PL indicates that stoichiometory control, which seems to be difficult for other growth methods, can be achieved by the AP-CVD using Zn and H₂O as source materials.
9:40 AM		C3-1-6 Electroluminescence of Zinc Oxide Films Prepared via Polymeric Precursor and via Sol-gel Method. S.A.M. Lima (Instituto de Quimica - UNESP, Brazil); M. Cremona (PUC-Rio, Brazil); C. Legnani, W.G. Quirino (Inmetro, Brazil); M.R. Davolos (Instituto de Quimica - UNESP, Brazil) Zinc oxide (ZnO) is an old material facing new novel applications due to its outstanding optical, electrical, and structural properties. In this work, we compare the electroluminescence (EL) of ZnO thin films obtained via two different wet routes: (1) polymeric precursor method (modified Pechini method), and (2) sol-gel method. We also compare the use of two different insulator layers - SiO₂ and lithium fluoride (LiF) - in the thin film electroluminescence device (TFELD). The final architecture of the devices was indium tin oxide (ITO)/insulator/ZnO/insulator/aluminum. To deposit both Pechini and sol-gel solutions spin-coating technique was used. After the deposition, the samples were submitted at different thermal treatments in order to burn the organic material and to obtain a uniform ZnO thin film with an average thickness of about 500nm and 400nm for the Pechini and the sol-gel precursor respectively. Silica layers were grown by rf-sputtering achieving a final thickness of 90 nm each. LiF layers were deposited by using an electron beam equipment and the measured thickness was 50 nm each. Finally, a 200 nm thin film of aluminum was thermally deposited as counter electrode. The ZnO films were characterized by XRD, FTIR and UV-Vis spectroscopy. The EL of the devices was measured in a PTI fluorimeter while different DC/AC voltages were applied at the device. The devices prepared via Pechini's solution exhibit two emission bands attributed to ZnO transitions, one with maximum around 420 nm, and the other with maximum around 700 nm. On the other hand, the devices prepared via sol-gel solution exhibit one band, centered at 700 nm, when LiF is used as insulator layers, and three bands, centered at 350 nm, 434 nm and 700 nm, when SiO₂ is used instead. The current-voltage curves measured for all devices indicate that they have characteristics of diode.
10:00 AM		C3-1-7 Luminescence Enhancement by an In₂O₃ Buffer Layer Inserted for the ZnGa₂O₄ Phosphor Screen S.-H. Yang, C.-Y. Lu (National Kaohsiung University of Applied Sciences, Taiwan); S.-J. Chang (National Cheng Kung University, Taiwan) White emitting ZnGa₂O₄ phosphor screen for field emission display was prepared by inserting an In₂O₃ buffer layer between the phosphor and indium-tin oxide (ITO) coated glass substrate. Radio-frequency (RF) magnetron sputtering was used for thin films deposition. Argon and oxygen were used as sputtering gases. (222) was the preferred crystallization plane of In₂O₃. Smooth surface and high crystalline of In₂O₃ with transmittance over 85% was deposited under oxygen flow ratio of 20%, sputtering pressure of 19 mtorr and sputtering power of 160 W. The resistivity of In₂O₃ film was about 1.55 mΩ-cm. The preferred crystallization plane of ZnGa₂O₄ phosphor was (311), which was deposited under optimal parameters, 5 mtorr, 100 W in Ar atmosphere. The crystallization of ZnGa₂O₄ deposited on In₂O₃ was evidently better than deposited on ITO; a smooth ZnGa₂O₄ surface with particle size of about 8.47 Å was obtained. The resistivity of phosphor screen of ZnGa₂O₄/In₂O₃/ITO glass was inactive with the variation of sputtering pressure and power but was sensitive to the change of oxygen flow ratio for In₂O₃ deposition. The ZnGa₂O₄ grown on In₂O₃ had a lower surface resistivity due to the surface recombination rate increased. When the grain size of In₂O₃ increased, the photoluminescence intensity of ZnGa₂O₄/In₂O₃/ITO glass phosphor screen was subsequently increased. A significant enhancement on luminescence of the ZnGa₂O₄ phosphor screen was observed when a high transparent, low resistivity and lattice matched In₂O₃ buffer layer was inserted, and a broad-band white luminescence with CIE coordinates at x = 0.31, y = 0.38 was achieved.
10:20 AM		C3-1-8 Optical Characterizations of Complete TFT-LCD Display Devices by Phase Modulated Spectroscopic Ellipsometry M. Gaillet (HORIBA Jobin Yvon, France); L. Yan, E. Teboul (HORIBA Jobin Yvon (USA)) Performances such as brightness and contrast ratio of a Thin Film Transistor-Liquid Crystal Display (TFT-LCD) are directly related to the optical properties of the materials constituting the device. Therefore a complete and accurate characterization of each individual layers of the TFT-LCD system is an important step in its fabrication process. The sandwich-like structure of the TFT-LCD device, as well as the optical properties of the very thin layers involved in its fabrication, make the complete characterization of the device not trivial. Spectroscopic ellipsometry (SE) is recognized as one of the most sensitive and accurate optical techniques to measure multiple thin film thickness and their optical constants. In this work, we present ellipsometric results obtained by a commercially available phase modulated spectroscopic ellipsometer (PMSE) on a complete TFT-LCD structure characterized from UV to NIR. Due to its unique optical set-up, PMSE presents the capability to measure very accurately ultra thin films on transparent substrate as often found in displays applications. Results show that the indium tin oxide (ITO) is inhomogeneous in depth. This property is very well known and depends on the deposition process. Also, strong uniaxial anisotropy was determined for the liquid crystal device over the entire measured spectral range. Finally, doping effects on the optical properties of the a-Si layer of the TFT device were also measured.
10:40 AM		C3-1-9 Studies on Electrochromic Smart Windows Based on Titanium Doped WO₃ Thin Films A. Karuppasamy, A. Subrahmanyam (Indian Institute Of Technology Madras, India) The tungsten oxide (WO₃) thin films are being used as electrochromic windows for energy efficiency; several attempts are being made to enhance the electrochromic efficiency. In the present paper, we report the results of two electrochromic (EC) devices consisting of five thin film layers; Glass/ITO/Ti:WO₃/Ta₂O₅/NiO/ITO and Glass/ITO/Ti:WO₃/ LiAlF₄/NiO/ITO. The Ionic conductor (Ta₂O₅) and ion storage layer (NiO) were deposited by electron beam evaporation and the active electrochromic (EC) layer (Ti:WO₃) was deposited by reactive pulsed dc magnetron sputtering. Pure Titanium and Tungsten metal targets were co-sputtered with controlled argon and oxygen keeping the sputtering power constant. The smart window performance of the EC film has been studied in three steps. First, the material properties of the EC film were investigated by XRD, AFM, UV-Vis spectrophotometer and Kelvin probe (for surface work function measurements); the thickness and the optical constants were estimated from the reflectance measurements. Secondly, the electrochromic behavior of the EC film was characterized by cyclic voltammetry (CV). The CV measurements were performed using a potentiostat with a standard three-electrode configuration consisting of the sample as the working electrode.1.0 M LiClO₄ in PC and 1.0 MHCl were used as electrolytes. Finally, the EC device consisting of the five layers have been fabricated and tested. The optical modulation (ΔOD), coloration effiiency (CE) and switching time (t_s) for the proton based device was found to be better with typical values; optical modulation = 60 %, CE = 80 cm²/C(at λ = 550nm) and t_s~1s (for 1cm x 1cm device).