Dielectric response and structure of amorphous hydrogenated carbon films with nitrogen admixture Lenka Zajíčková a, ⁎, Daniel Franta a , David Nečas a , Vilma Buršíková a , Mihai Muresan a , Vratislav Peřina b , Christoph Cobet c a Department of Physical Electronics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic b Institute of Nuclear Physics, Academy of Science of the Czech Republic, Řež, Czech Republic c Institute for Analytical Sciences, Albert-Einstein-Str. 9, 12489 Berlin, Germany abstract article info Article history: Received 31 August 2010 Received in revised form 26 January 2011 Accepted 3 February 2011 Available online 18 February 2011 Keywords: Diamond-like carbon Amorphous hydrogenated carbon Optical properties Band structure The optical properties and structure of a-C:H films were modified by addition of nitrogen into the CH 4 /H 2 deposition mixture. Three films prepared in capacitively coupled rf discharge were compared: (a) hydrogenated diamond like carbon film with hydrogen content of 34% and indentation hardness of 21.7 GPa, (b) hard a-C:H:N film with nitrogen content of 13% and indentation hardness of 18.5 GPa and (c) soft a-C:H:N film with nitrogen content of 10% and indentation hardness of 6.7 GPa. It is shown how the parametrized density of states model describing dielectric response of electronic interband transitions can be applied to modified a-C:H:N and how it can be combined with correct treatment of transmittance measured in infrared range using additional Gaussian peaks in joint density of phonon states. This analysis resulted in determination of film dielectric function in wide spectral range (0.045–30 eV) and provided also information about the density of states of valence and conduction bands and lattice vibrations. © 2011 Elsevier B.V. All rights reserved. 1. Introduction There are many forms of amorphous carbon with great variety of physical properties depending on the specific mixture of sp 3 and sp 2 sites. If they contain hydrogen its content additionally influences the material structure and properties. An a-C or a-C:H of higher sp 3 content belongs to the class of materials often called diamond-like carbon (DLC). Casiraghi suggested more precise classification of a-C:H films into four types [1,2]: (a) polymer-like a-C:H (PLCH) with the highest H content (40–60 at.%), up to 70% sp 3 but soft and of low density, (b) harder diamond-like a-C:H (DLCH) with intermediate H content (20-40 at.%), (c) hydrogenated tetrahedral amorphous carbon films (ta-C:H) in which the C–C sp 3 content is increased (as compared to DLCH) whilst keeping a fixed H content 25–30 at.% H and (d) graphite-like a-C:H (GLCH) with less than 20 at.% of H, a high sp 2 content and sp 2 clustering. PLCH films are soft, have low density and the optical gap ranges from 2 to 4 eV. Even if DLCH films have lower overall sp 3 content, they have more C–C sp 3 bonds than PLCH. Thus, they have better mechanical properties. Their optical gap is between 1 and 2 eV. Films defined as ta-C:H differ from DLCHs by their Raman spectra, higher density (up to 2.4 g/cm 3 ) and Young's modulus (up to 300 GPa) [3–5]. Their optical gap can reach 2.4 eV [6]. The gap of GLCH is under 1 eV. The optical gap in the above material classification was deter- mined from the decrease of absorption coefficient to 10 4 cm − 1 , so called E 04 , or from Tauc plot. This approach provides information about the film transparency but does not describe the nature of sp 3 /sp 2 carbon materials because two different types of states in valence and conduction bands form the density of states (DOS), the states related to σ and π electrons. Therefore, two different band gaps, corresponding to σ →σ ⁎ and π →π ⁎ interband transitions, should be distinguished. In this aspect a-C(:H) material differ from amorphous semiconductors such as a-Si [7] and classification using the optical gap can be confusing. The a-C(:H) coatings have wide range of applications but it is necessary to optimize their adhesion to substrate material and decrease intrinsic stress in the films. Approaches solving this problem include the fabrication of an intermediate layer between the coating and substrate, and the reduction of the internal stress of the coating. Intermediate metal or compound layers such as Ti, Zr, W, Nb, or WC have shown potential to improve the adhesive strength [8]. The great disadvantage of this technique is that two or more steps, sometimes using different deposition techniques, are necessary. A prospective trend is to use a single step process by mixing some dopants into the structure of DLC. The residual stress of the DLC coating has been reduced by including additional elements. Positive results were achieved by Rabbani [9] for nitrogen admixture. Thin Solid Films 519 (2011) 4299–4308 ⁎ Corresponding author. Tel.: +420 54949 8217; fax: +420 541211214. E-mail address: lenkaz@physics.muni.cz (L. Zajíčková). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.02.021 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf