Higher nitrides of hafnium, zirconium, and titanium synthesized by dual ion beam deposition
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I. INTRODUCTION Nitrides of the Group IVB metals Hf, Zr, and Ti are of great interest for their mechanical, chemical, and electrical properties. The mononitrides Hf N, ZrN, and TiN are shiny gold-colored materials with low resistivity ( 2 5 ^ 0 /u Q, cm), high hardness, low coefficient of friction (e.g., to steel), and excellent chemical and thermal stability.] In this paper we report the preparation and properties of higher nitrides with nitrogen/metal ratios exceeding unity. For Hf and Zr we present evidence for a metastable nitride phase with a composition of approximately Hf3 N 4 and Zr 3 N 4 . These two materials are insulating and transparent, straw colored. The existence of the higher Hf-N phase was previously reported by Johansson et al.1 in thin films prepared by reactive magnetron sputtering, and indications of the existence of higher nitrides of Hf and Zr have been reported in several earlier publications.3 To our knowledge this is the first report of the deposition conditions required to give the Zr 3 N 4 phase together with composition and structural information. Using the technique of dual ion beam sputter deposition,4 we have examined the conditions needed for the formation of both this higher nitride phase and the mononitride phase. In the case of Ti-N, we do not obtain a N/metal ratio as high as Ti 3 N 4 , under deposition conditions that produce a)
Present address: ASEA-STAL AB, Department PAP, S-61220 Sinspong, Sweden. b) Present address: Thin Film Group, Department of Physics and Measurement Technology, Linkoping University, S-58183 Linkoping, Sweden.
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J. Mater. Res. 1 (3), May/Jun 1986
Hf, N 4 and Zr 3 N 4 . Instead, a conducting dark film is obtained with a composition of about N/Ti = 1.25. The composition and structure of these thin films were analyzed by Rutherford backscattering (RBS) and transmission electron microscopy (TEM). In agreement with earlier x-ray analysis of Hf3 N 4 , 2 the structure of Hf3 N 4 and Zr3 N 4 is found to be very close to the Bl (NaCl) structure of the mononitrides, but exhibits a rhombohedral distortion. We propose that this phase has a significant number of vacancies in the metal sites. This model is supported by measurements of noble gas incorporation and by electron energy band calculations by Schwarz et al.,5 which predict an insulating phase for the composition Me3 N 4 (Me = Group IVB metal). II. EXPERIMENTAL Thin films of Group IVB nitrides were prepared over a wide range of deposition conditions using the dual ion beam technique, illustrated in Fig. 1, in which a metal target is sputtered by the target ion beam and the growing film is simultaneously bombarded by the substrate ion beam. Both ion sources are of the Kaufman multiaperture type. The technique and experimental equipment have been previously described,6 together with applications to aluminum-nitrogen cermets and compounds.4'7 In the preparation of aluminum nitride,7 the target beam was argon and the substrate beam was nitrogen. By controlling the relative intensity of the two ion beams, the
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