Vibrational spectroscopy of mixed hexagonal-cubic boron nitride thin films M.G. Beghi a , C.E. Bottani a , A. Miotello b , P.M. Ossi a, * a INFM—Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Milano, Italy b INFM—Dipartimento di Fisica, Universita ` di Trento, Povo (TN), Italy Abstract The attainment of cubic boron nitride by physical vapour deposition methods is influenced by several parameters. We present new results obtained at about 343 K on the system boron-nitrogen with an RF sputtering machine, adopting both DC and RF bias and in argon or argon plus nitrogen atmospheres. We also prepared samples by reactive ion assisted deposition (IBAD). The films were deposited on silicon substrates, and characterized by Auger electron spectroscopy (AES) infrared (IR) and Raman spectroscopies and nanoindentation. Sig- nificant differences are found between DC and RF biased samples, in particular concerning film structure. Evidence of sp 3 bonded phase formation, as given by IR spectroscopy, is found only in samples produced with a RF bias. Raman spectra of all films show a broad band typical of a completely disordered structure. Strong Raman signals from the substrate prove that the films, except the IBAD ones, are highly transparent in the visible.  1997 Elsevier Science S.A. Keywords: Cubic boron nitride; Auger electron spectroscopy; Infrared spectroscopy; Raman spectroscopy; Reactive ion assisted deposition 1. Introduction Boron nitride (BN) is one of the most interesting III-V compounds. Similar to carbon, BN is known to exist in various allotropic forms [1]. These are the hexagonal BN, which is the equilibrium phase equivalent to graphite, and rhombohedral BN, which consist of networks of sp 2 bonded B and N atoms. hBN is attractive because of its softness and it can find application as a solid lubricant. Two other phases with sp 3 coordination are known, namely cubic cBN, with zinc-blend structure, analogous to diamond, and hexagonal wBN, with wurtzite structure. Both cBN and wBN show a tetrahedral arrangement of B and N atoms, but they differ in the tetrahedra stacking. Also a disordered hexagonal, sp 2 bonded phase was observed, turbostratic tBN, with random stacking of hexagonal planes, which appear strongly dis- torted. Interest has focused on cBN, due to its unique properties, such as low density, high thermal conductivity and mechan- ical strength, stability at high temperatures, elevated wear resistance, possibility of easy doping with silicon (n-type) and beryllium (p-type); thus the material appears a good alternative to diamond for several applications. Low pressure deposition of cBN thin films, both by che- mical and by physical vapour deposition techniques was attempted, but it has usually proved difficult to obtain cBN in a large fraction: low pressure processes, in the absence of simultaneous ion bombardment of the growing film, invariably result in hBN, or tBN, no cubic phase being produced. Among ion assisted techniques IBAD [2,3] and ion plat- ing [4] have been traditionally adopted. Predominantly cBN films are produced by RF sputtering [5], applying RF bias with process atmospheres composed of pure Ar, as well as by RF reactive sputtering [6] starting from a B 4 C target. The process temperature is always elevated, mostly ranging between 800 and 1100 K [7,8]. Usually, mixtures of nitro- gen and inert gas ions (mostly Ar) are employed; it is observed that ion to atom arrival rate ratios of the order of the unity often correspond to cBN attainment. The relevant parameter appears to be the momentum transferred to the film by the bombarding ions per depositing atom, which scales with the square root of ion energy [9,10]; however, this means that elevated residual stresses of compressive nature develop in the films, whose mechanical stability and adhesion to the substrate are often unsatisfactory [11]. Surprisingly enough, in view of the involved ion kinetic energies, also the substrate temperature influences cubic phase nucleation; at intermediate and low temperatures it is very difficult to obtain cBN; the lower the substrate tem- Thin Solid Films 308–309 (1997) 107–112 0040-6090/97/$17.00  1997 Elsevier Science S.A. All rights reserved PII S0040-6090(97)00380-5 * Corresponding author.