SURFACE AND INTERFACE ANALYSIS, zyxwvutsrqpo VOL. zyxwvutsrq 21, zyxwvutsrq 95-100 (1994) Composition and Physical Properties of Thin a-C : N and a-C : N : H Films Deposited by Ion Beam Techniques Christian A. Zorman,* Jeansong Shiao, Susan Heidger and Richard W. Hoffinan Department of Physics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106 Thin a-C, a-C : N, a-C : H and a-C : N : H films were deposited onto silicon, glass and graphite substrates using ion beam techniques. Rutherford backscattering spectroscopy and elastic recoil detection analysis were performed in order to determine their atomic composition. The hardness of these films was measured using a Vickers hardness diamond indenter. The as-deposited a-C : H and a-C : N : H films were characterized using optical absorption spec- troscopy, while the a-C and a-C:N films were studied by Raman spectroscopy. A shift of the absorption edge toward the lower energy region and a lower gap of a-C : N : H films was observed. The Raman spectra of a-C : N films show an increase in the Z(D)/Z(G) ratio, as compared to those of non-nitrogenated diamond-like carbon films. A possible structure is suggested. INTRODUCTION The physical and chemical properties of diamond-like carbon (DLC) are similar in some ways to those of diamond, although its structure is amorphous and it sometimes contains other elements, e.g. hydrogen. The DLC films can be described as a covalent random network with both sp’ and sp3 carbon sites, often con- taining patches of graphitic crystallites. A study using electron energy-loss spectroscopy suggested that hydro- gen enhances the concentration of sp3 carbon sites.’ The optical gap also increases with an increase in hydrogen concentration.’ In recent years, researchers began to add third elements such as boron, nitrogen, oxygen or silicon to make alloys with DLC. In 1989, two researchers predicted a hypothetical carbon nitride material, b-C3N4, based on the structure of P-Si3N4, that may be harder than d i a r n ~ n d . ~ . ~ The predicted bulk modulus for B-C3N4 is 4.27 Mbar, near that of diamond. Since the prediction of this hypothetical carbon nitride, its synthesis has been attempted using various techniques. ’-’ The wear resistance, friction coefficient and hardness of amorphous carbon nitride films have been tested and shown to be equivalent to those of amorphous carbon (a-C) films.6-8 Many of the physical properties of nitrogen- containing diamond-like carbon (N-DLC) are not well understood, including details of the chemical bonding, local structure and optical properties. Optical methods in the UV-Vis-near-IR range can be used to probe the electronic transitions around the band gap. Raman spectroscopy is used to study the vibrational properties. Rutherford backscattering spectroscopy (RBS) and elastic recoil detection (ERD) spectroscopy can be used * Author to whom correspondence should be addressed. CCC 0 142-242 1 /94/02aoS 5 zyxwvutsrqponmlkji -06 Q 1994 by John Wiley & Sons, Ltd to study the composition of N-DLC films. Rutherford backscattering spectroscopy provides a non-destructive means of determining the nitrogen and carbon concen- trations throughout 1 he films, while ERD profiles the hydrogen content. Both of these techniques provide a straightforward means of depth profiling, without the pitfalls of sputtering. In this paper, a relationship between the composition, optical absorption and Raman parameters of N-DLC films is shown. A pos- sible structure is suggested. EXPERIMENTAL PROCEDURES Diamond-like carbon (DLC) and nitrogen-containing diamond-like carbon (N-DLC) films of both hydro- genated and non-hydrogenated varieties were prepared in a dual ion beam system using two Kaufman-type ion guns. The a-C:N films were deposited by single beam nitrogen sputtering of a graphite target, while the a-C films were deposited by the same process with argon as the sputtering gas. A direct single beam of methane was used to deposit the a-C:H films, while a nitrogen/ methane mixture was used to deposit the a-C:N:H films. The deposition system consisted of a turbomecha- nically pumped vacuum chamber maintained at a pre- deposition base pressure in the mid-lo-’ Torr range. The deposition gas was introduced and the system pres- sure was maintained at - 1 x Torr. Beam voltages were held constant and ranged between 100 and 1100 V. Deposition rates for a-C:N films ranged from 5 to 7 A min - ’, depending on the beam current. n-Type silicon ( 1 11) wafers, glass microscope slides and graphite squares were used for substrates. These substrates were presputtered in order to remove residual contaminants. The optical measiirements of the a-C:H and a- C : N : H films were carried out using a Varian Cary 219 zy Received 9 July 1993 Accepted 20 October 1993