Influence of dopant nature on the TCO properties of ZnO:M (M=Al, Ga, Sn, Si, Ge) thin films
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Influence of dopant nature on the TCO properties of ZnO:M (M=Al, Ga, Sn, Si, Ge) thin films J. Clatot1, G. Campet2, M. Jean3, M. Nistor4 and A. Rougier1 1 LRCS, 33 rue St Leu, 80039 Amiens, France 2 CNRS,Université de Bordeaux, ICMCB, 87 av. du Dr. Schweitzer, Pessac, F-33608 France 3 Institut des Matériaux, Université de Rouen, LASTSM, BP12, 76801 Saint Etienne du Rouvray Cedex, France 4 National Institute for Lasers, Plasmas and radiation Physics, L22, PO Box MG-36, 77125 Bucharest-Magurele, Romania ABSTRACT Aiming at clarifying the opto-electronic properties of ZnO based n-type Transparent Conducting Oxides, TCOs, properties of ZnO thin films are studied as a function of cationic doping. In addition to commonly reported, Al and Ga trivalent dopant, similar performances are reported for Si doping. In the visible region, ZnO:Si (3 %) thin film exhibit a transmittance higher than 80 % for a resistivity as low as 8x10-4 Ω.cm when grown at 100 °C under 1.0 Pa oxygen pressure. The influence of tetravalent cations as dopant is also investigated through Sn and Ge additions. It shows that not only the oxidation state plays a role but also the cation nature. Indeed, ZnO:Sn thin films are insulating whereas the ZnO:Ge thin films are conductive with resistivity values higher than the ones of ZnO:Si thin films. INTRODUCTION The development of flexible/plastic opto-electronic devices including solar cells flat panel displays and electrochromic devices, requires the optimization of transparent conducting electrodes deposited at low temperature (T < 150 °C). As a result of being abundant, environmental begnin, and having interesting physical properties, n-type ZnO based thin films have attracted significant attention [1-4]. Conductivity is commonly attributed to intrinsic or extrinsic doping [5-8]. Herein, focusing on this last approach, TCO properties of ZnO based thin films are discussed in relationship with the oxidation state of the cationic dopant, its nature and concentration. ZnO:M (M = Al, Ga, Sn, Si, Ge) thin films are deposited by Pulsed Laser Deposition, using a KrF excimer laser, at low substrate temperature (T < 150 °C). In a first step, comparison between the most commonly used extrinsic doping, namely Al and Ga addition, and Si doping is discussed. Finally, aiming at a better understanding of the influence of 4+ dopants, the TCO properties of ZnO:Sn and ZnO:Ge systems are investigated. EXPERIMENT Doped ZnO targets (1.5 cm diameter , 0.3 cm thick), with a density higher than 80 %, were prepared from a mixture of commercial ZnO, and Al2O3 or Ga2O3 or SiO2 or SnO2 or GeO2 powders and annealed at T = 1000 °C during 48 h in ambient atmosphere. A 3 % M/Zn target molar ratio was initially used. ZnO:M thin films were grown by Pulsed Laser deposition, PLD, using a KrF excimer laser beam (Lambda Physik Compex 102, λ = 248 nm) with a laser fluence of 1-2 J.cm-2. The substrates were 1 cm2 glass. The base pressure in the chamber was in the order of 1.0x10-4 Pa. The depositions were performed in oxygen atmosphere in the PO2 = 1
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