Ellipsometric Investigation of Metal-Organic Chemical Vapor Deposition of Niobium Oxide Films
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TOBIAS GERFIN AND MICHAEL GRAETZEL Institut de Chimie Physique, Ecole Polytechnique F6d6rale de Lausanne, CH-1015 Lausanne, Switzerland
ABSTRACT Niobium oxide films have been prepared by metal-organic chemical vapor deposition (MOCVD) in two different systems via thermal decomposition of Nb(OEt) 5 . A detailed study by exsitu variable-angle spectroscopic ellipsometry indicates a wavelength dependence of the refractive index that could be described by a Cauchy type dispersion formula and typical values of 1.9 to 2.1 at 632.8nm. The band gap of these films were at about 3.5 and 4eV for the indirect and direct transition. Preliminary results of in-situ ellipsometry at four different wavelengths in the visible and UV will be discusssed. The films were amorphous, extremely smooth and pinhole free and therefore they are useful candidates for interdiffusion barrier layers.
INTRODUCTION Niobium oxide is a material with many different stoichiometries each having distinct properties, e.g. Nb 2 0 5 is a semiconductor with a band gap of about 3.5eV, whereas NbO is a metal. The first compound can be used as dielectric or antireflection coating, because of its high dielectric constant and refractive index, i.e. 11-100 and 2.2-2.6, respectively. Nb 2 0 5 films have been prepared by several techniques, such as sputtering, electron beam evaporation or chemical vapor deposition (CVD). However, although CVD or MOCVD are very attractive for the deposition of metal oxide films on industrial scale, the growth of Nb 2 0 5 has not been studied in detail. Duffy and coworkers [I ] grew niobium oxide films with Nb(OEt) 5 at a substrate temperature of 450'C. The same precursor was used very recently by Hara et al. [2] for the investigation by in-situ ellipsometry at a wavelength of 546.1 nm. Suhr and coworkers [3] used a different class of precursors for plasma enhanced MOCVD of niobium and niobium oxide, i.e. Nb(C 5 H5 )(CO) 4 and Nb(CH 3 C5 H5 )(CO) 4 . Ellipsometry is a well established, non-destructive technique for measuring thicknesses and optical constants [4]. In the last several years, spectroscopic and in-situ ellipsometry have made 45 Mat. Res. Soc. Symp. Proc. Vol. 363 01995 Materials Research Society
enormous progress and the measurements during growth in MOCVD have given much insight into the deposition and nucleation process [5, 6]. It is realistic to suppose, that, in the near future, it will be used not only in CVD but also in PVD as real-time control of film deposition [7]. The present work was started to grow interdiffusion barrier layers that hinder direct contact of liquid electrolytes to transparent conducting oxides being used as electrodes in electrochemical and photovoltaic applications. Therefore, the growth kinetics of niobium oxide films using Nb(OEt) 5 as precursor have been studied to optimize the growth conditions.
EXPERIMENTAL PART Figure I shows the setup of both MOCVD systems. Nb(OEt) 5 (Haeraeus, puriss.) was evaporated at 130 0 -140'C and transported in a flow of nitrogen to the reactor. The transport
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