Composition and Phase Control for Molybdenum Nitride thin Films
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ABSTRACT Molybdenum nitrides are active and selective hydrodenitrogenation (HDN) catalysts. The catalytic properties of molybdenum nitrides were found to be dependent on the structural properties. The purpose of research described in this paper was to synthesize molybdenum nitride thin films with well defined structures and stoichiometries using ion beam assisted deposition. The films were deposited by evaporating Mo metal, and simultaneously bombarding the growing film with low energy nitrogen ions. The phase constituents of the films were determined using x-ray diffraction and the film composition was obtained by Rutherford backscattering spectrometry. The film composition and phase constituents were strong functions of the ion-to-atom arrival rate ratio, ion energy and ion angle of incidence. Differences in the film composition for different arrival rate ratios and ion angles of incidence were interpreted based on reflection and sputtering effects. Our results suggest that phase formation was governed by the effective energy density per deposited atom. Evaluation of the effective energy density per deposited atom and its physical significance in ion beam assisted deposition is discussed.
INTRODUCTION Molybdenum nitride has shown promise for use as a selective hydrotreatment catalyst. 1, 2 Choi et al. 3 have illustrated that the composition and structure at the surface is very different from that in the bulk. Understanding the relationship between surface structure, composition, and catalytic function is vital for improved performance of the catalyst. We have used ion beam assisted deposition to synthesize molybdenum nitride thin are used to develop films with well-defined structures and stoichiometries. These 4films 5 detailed structure-function relationships for molybdenum nitride. ,
EXPERIMENTAL Our effort involved synthesis of a series of clean, well-defined molybdenum nitride films with varying stoichiometries and structures using ion beam assisted deposition. The experimental system consisted of a stainless steel ultra-high vacuum chamber with a base pressure < 9x10- 9 torr. The set-up and charge cleaning procedures are described in detail elsewhere. 6 A nitrogen backfill pressure of 2.2x10-5 torr was maintained throughout the depositions. Silicon (100) and graphite were used as substrates. High purity molybdenum metal was deposited at a rate of 0.3 nm/sec, and the growing film was simultaneously bombarded with nitrogen ions ( 250-1000 eV) to reactively synthesize molybdenum nitride films. The nitrogen ion flux and the ion angle of incidence were varied to achieve different nitrogen to molybdenum arrival rate ratios. The composition of the nitrogen ion beam was 69 Mat. Res. Soc. Symp. Proc. Vol. 354 0 1995 Materials Research Society
taken as 11% N+ and 89% N2+ based on values reported in the literature. 7 The films were grown to -400 nm thickness, and then passivated in 0.1% 02 in He for 30 minutes to prevent bulk oxidation. The film composition was analyzed using Rutherford backscattering spectrometry,
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