Influence of Oxygen Annealing on Electrical Properties of ZnO:Cl Thin Films
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Influence of Oxygen Annealing on Electrical Properties of ZnO:Cl Thin Films Tamar Tchelidze1, Ekaterine Chikoidze1,2, Francois Jomard2, Ouri Gorochov2, and Pierre Galtier2 1 Faculty of Physics, Tbilisi State University, Tbilisi, Georgia 2 GEMAC, CNRS, 1.A.Briand, Meudon, France ABSTRACT The influence of oxygen treatment on transport properties of pure ZnO and ZnO:Cl thin films grown by MOCVD were studied. The experimentally obtained values of carrier concentrations after oxygen treatment at different temperatures, were compared with the results obtained from thermodynamical analysis of the system: ZnO:Cl-Oxygen vapor pressure, using method of quasi-chemical reactions (QCR).
INTRODUCTION Nowadays significant issues remain in p-type doping of ZnO and related alloys. But ZnO, which has been recognized as an intrinsic, n-type semiconductors more then 70 years ago [1,2] has broad range of applications in optoelectronic, acoustic devices and lasers light-emitting diodes. As a semiconductor oxide material with low resistivity, high transmittance up to UV spectral range, and with a good chemical stability under strong reducing environments, ZnO is thus a promising Transparent Conductive Oxide (TCO) and a possible alternative to tin oxide and indium oxide to be used as transparent electrode for photovoltaic solar cells, electrodes on flat panel displays [3]. Up to now, metal elements like Al [4,5], Ga [6,7], Ba [8], Sc [8], Cu [9], Fe [9], Sn [9], In [9,10] substituting to Zn, have been widely used for this purpose. While apart from for a few works [11-13] where Fluorine doping of ZnO is studied little is known about the doping of ZnO with anion impurity in substitution to oxygen. Lately, the use of non-metal dopants in substitution to oxygen was suggested as a better way to achieve high carrier concentration and mobility while keeping good transparency, thanks to the weaker perturbation of the ZnO conduction band expected in this configuration [14]. Following above mentioned idea and the fact that to our knowledge, with the exception of one report [15], there are no data in literature about chlorine doping of ZnO, we decided to study this issue. The details about the growth and electro-optical characterization of ZnO:Cl thin layers will be published elsewhere, while in presented manuscript we present the influence of oxygen post annealing on the defects and free carriers concentrations in ZnO:Cl.
DEFECTS AND FREE CARRIERS CONCENTRATIONS IN ZnO:Cl AFTER OXYGEN TREATMENT In order to carry out thermodynamic analysis of the system ZnO:Cl ñ oxygen vapor Krˆgerís method of quasi-chemical reactions (QCR) is used [16]. The aim is to obtain the concentrations of defects and charge carriers as a function of post heat treatment temperatures,
surrounding gas pressure, and chlorine concentration in the system. The principal processes we take into account with the corresponding mass action laws are as follows: The creation of Schottky and Frenkel pairs: (1) O → V Zn + VO , K S = [VZn ] [VO ] , K F = [VZn ] [Zni ] ,
O →
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