APCVD - In-Situ Growing and Investigation of Electrochromic WO 3 Films
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ABSTRACT The paper presents the results on developing a technological process for in-situ growing in CVD reactor of W0 3 electrochromic thin films. Films were deposited at 290 °C on In 2 0 3:Sn and Sn02 covered pyrex substrates by deposition in APCVD reactor using W(CO) 6 as precursor. The basical process is pyrolitical decomposition of W(CO) 6 and in presence of oxygen bearing gases W0 3 films were grown. The process proceeds at atmospheric pressure. Optimization of the process parameters such as oxygen flow rate, argon passing the precursor chamber and the diluent argon flow permitted achievement of high growt rates in the order of 30 A/s.Reflection High Energy Electron Diffraction (RHEED), Scanning Electron Microscopy (SEM), Infrared Spectroscopy(IRS) were applied for structural studies. Results show that films grown are of amorphous phase , with spherically shaped, randomly distributed structural unites. Additional heating at 450 0C for 15 minutes in oxygen bearing gas atmosphere leads to W0 3 films of better transmittance and they posses monoclinic polycrystalline structure with a certain degree of texturing. The observed peak at 700 cm' in the IR spectrum is additional evidence for W0 3 phase structure. INTRODUCTION
The term "electrochromism" can be applied to a broad range of phenomena in which a color change is induced in a material by an externally applied electric field or current. The concept is very similar to the well known electroluminescence process, except that a metastable absorption center is created in the material instead of a emission process .A great deal of interest in this type of material arose in recent years, primarily as a result of an observation that large densities of coloration can be generated in a controlled and reversible manner in thin films of certain transition metal oxides, such as W0 3, MoO 3 etc, at ambient temperature [1-3]. Electrochromic materials are very promising to realize smart windows for energy control utilizations usually in building or automobile industry, display devices that utilize some of the unique properties of this phenomenon such as high contrast, no angular dependence, continuously variable density of coloration, and memory [4]. Large- area transmissive electrochromic device are the switchable, solar- gain-control building windows, the so called "smart windows" have been under an intensive studying and application during the last several years [5-7]. With transferring to a large scale applications the cost effectiveness starts to play an important role. Then the technology for fabrication of the material is of exclusive importance. Different technics are utilize for W0 3 fabrication, such as magnetron sputtering, vacuum evaporation, sol-gel technics, and others. Chemical vapor deposition in its PECVD variation has been successfully used for smart windows [8].This technics has the potential for significant cost reduction , due to the well known advantages of the CVD process. The purpose of this 155 Mat. Res. Soc. Symp. Proc. Vol. 415 01996 Materials Rese
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