Role of nitrogen in optical and electrical band gaps of hydrogenated/hydrogen free carbon nitride ﬁlm Abhijit Majumdar a, ⁎, Robert Bogdanowicz a, b , Subrata Mukherjee c , Rainer Hippler a a Institute of Physics, University of Greifswald, Felix-Hausdorff-Str. 6, 17489 Greifswald, Germany b Department of Metrology and Optoelectronics, Gdansk University of Technology, Narutowicza Str.11/12 80-952 Gdansk, Poland c FCIPT Division, Institute for Plasma Research A10-B, GIDC, Gandhinagar-382044, India abstract article info Article history: Received 18 May 2012 Received in revised form 10 November 2012 Accepted 13 November 2012 Available online 28 November 2012 Keywords: Nitrogen doping Carbon nitride Optical band gap Electrical band gap We report the optical and electrical band gap energy of amorphous hydrogenated carbon nitride (a-HCN x ) and carbon nitride (a-CN x ) as a function of nitrogen concentration (N/C). The optical band gap of a-HCN x and a-CN x ﬁlms has been determined by means of Ellipsometry and UV–VIS. Both optical and electrical band gaps increase with elevated nitrogen concentration. Experimentally obtained electrical band gap is compared with the same one calculated from single particle band gap or carbon nanotube model to observe the depen- dence like behavior. Moreover, resistivity of the a-HCN x ﬁlm shows a higher value in comparison to that of the a-CN x ﬁlm as the nitrogen concentration increases from 0.07 to .54 at room temperature. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Band gap is one of the most useful aspects of the band structure, as it strongly inﬂuences the electrical and optical property of the material. An electron can transfer from one band to the other by means of carrier generation and recombination processes. Band gap and defect states created in the band gap by doping can be used to create semiconductor devices such as solar cells, diodes, transistors, laser diodes and others. Carbon nitride ﬁlms have received considerable attention recently due to their interesting properties (high hardness, high transparency, and chemical inertness) predicted by Liu and Cohen [1]. The amorphous hydrogenated/hydrogen free carbon nitride (a-HCN x and a-CN x ) ﬁlms are expected to be applied widely as a low friction coefﬁcient, low band gap protective material on hard disks and read heads [2,3]. Simi- larly there are interesting applications such as biosensor [4,5], IR detec- tors [6], and anti-biomaterial [7,8]. Many reports have been devoted to the study of nitrogen bonding in a-CN x ﬁlms. In particular, X-ray photoelectron spectroscopy (XPS) [2,9–15] and vibrational spectroscopy (Raman and Fourier transform infrared spectroscopy) [15–19] measurements have been the most common techniques used to investigate this subject. Different bond- ing conﬁgurations are mostly close in energy to each other and for that the proper interpretation of these data is widely varied. Optical band gap of hydrogenated carbon nitride has not been studied in detail except few reports on optical band gap of hydrogen free carbon nitride and amorphous carbon ﬁlms [20,21]. The optical gaps of carbon nitride ﬁlms ranges between 1.2 eV and 3 eV obtained from the literature. In the case of hydrogenated carbon nitride ﬁlms (a-HCN x ) a small decrease of the optical gap is observed as the nitrogen content increases [22–25]. In the case of a-CN x ﬁlms it is clear that the nitrogen incorporation deter- mines both the nitrogen content and the prevalent C\N bonding conﬁg- uration in the deposited ﬁlm, and therefore the optoelectronic properties, that can be ranged from conductive to highly resistive. The large varia- tions of the electronic structure contradict its microstructure in which, consists of a mixture of sp 2 , sp 3 and sp 1 conﬁgurations, consequently the resistivity values may vary between those of graphite, diamond and organic polymers. As the nitrogen content is further increased most of the network becomes terminated by NH or CNsp 1 bonds limiting the clus- ter size increase and in consequence the decrease in the optical gap [25]. For the hydrogen free carbon nitride ﬁlms (a-CN x ) the gaps have no direct correlation with the N%; in some cases the optical energy gap decreases as the nitrogen content increases [26–31] and the semimetallic [32] behav- ior has been observed, while in other reports the trend is exactly the op- posite [33–37]. Here we would like to mention that the electronic structure of carbon nano-tube (CNT) cannot be comparable with that of the thin ﬁlm. We just showed a dependence like behavior of a-HCN x and a-CN x ﬁlms with respect to single particle band gap model with- out changing any parameters of the original equation (effective mass model) [38]. In this model both electrical band gap (E eg =E g +E ee ) and optical band gap (E og =E g +E ee - E eh , where E ee and E eh are the electron–electron and electron–hole interaction energies, re- spectively) depend on the single particle band gap, E g . They can be modiﬁed depending on the dielectric function of ε of the material Thin Solid Films 527 (2013) 151–157 ⁎ Corresponding author. Tel.: +49 3834864784; fax: +49 3834864701. E-mail addresses: abhijit_majumdar2005@yahoo.com, majumdar@uni-greifswald.de (A. Majumdar). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.020 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf