Thin Solid Films 437 (2003) 266–271 0040-6090/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-6090(03)00683-7 Optical absorption threshold of low pressure chemically vapor deposited silicon oxynitride films from SiCl H –NH –N O mixtures 2 2 3 2 Dimitris Davazoglou* NCSR ‘Demokritos’, Institute of Microelectronics, P.O.B. 60228, Agia Paraskevi 153 10, Attiki, Greece Received 6 December 2002; received in revised form 16 April 2003; accepted 24 April 2003 Abstract Amorphous silicon oxynitride thin films were deposited on 4 inch (100) Si wafers in a conventional low pressure chemical vapor deposition reactor at 812 8C and 230 mTorr, using SiCl H –NH –N O gas mixtures. Depositions were carried out keeping 2 2 3 2 the flows of SiCl H and of NH constants at 20 and 60 sccm, respectively, while that of N O was varying between 0 and 180 2 2 3 2 sccm. The optical properties of deposits were studied with the aid of the Forouhi–Bloomer model (Phys. Rev. B34, 7018 (1986)) for amorphous semiconductors. From the analysis of the optical spectra the complex refractive index of the films as well as their thicknesses were derived. Moreover, an approximate picture of the energetic distribution of the density of electronic states near the absorption threshold of these films was obtained. This picture was found to be consistent with theoretical calculations found in the literature for crystalline silicon nitride and oxynitride. Thus, the increase of oxygen content in films does not cause a shift of valence and conduction band towards lower and higher energies, respectively, but it implies rather an increase of the band gap and a decrease of the energetic distance between their centers of gravity due to the appearance of a local maximum near the top of the valence band. This maximum is due to the additive effect of the N 2p and the O 2p non-bonding orbitals. The study of optical properties provides also useful information regarding the presence of electronic states within the forbidden gap due to film defects, coherent with electrical measurements.  2003 Elsevier Science B.V. All rights reserved. Keywords: Dielectrics; Optical properties; Chemical vapor deposition (CVD) 1. Introduction Silicon nitrides and oxynitride films are important materials in the Si based micro- and nano-technology w1x. Applications are numerous and among others include their use as diffusion barriers for various dopants w2x, and in the isolation of transistors w3,4x. High- temperature low pressure chemically vapor deposited (LPCVD) nitride and oxynitride films are being applied in the local oxidation processing sequence as oxidation barriers because they are not readily oxidized w5x. Silicon nitride and oxynitride are being used as sidewall spacers w6x in modern CMOS transistors. Moreover, they are used in Metal–Nitride–Oxide Semiconductor memory structures w7x or Oxide–Nitride–Oxide (ONO) struc- tures since their dielectric constant is higher than that *Corresponding author. Tel.: q30-210-650-3117; fax: q30-210- 651-1723. E-mail address: d.davazoglou@imel.demokritos.gr (D. Davazoglou). of SiO . Other applications are found in integrated 2 optoelectronics for the fabrication of optical waveguides w8,9x, in solar cells technology as anti-reflective coatings w10x and tunnel dielectrics w11x, etc. In all these appli- cations one hopes to combine the distinct beneficial properties of silicon oxide and silicon nitride in one film. Silicon oxide has a large band-gap (near 9 eV w1x) so it is an excellent insulator, it has a low refractive index value (1.46 w1x), it can be deposited in very pure form and its interface with silicon exhibits excellent electrical properties when thermally grown. On the other hand, silicon oxynitride has smaller band-gap than SiO , which can be tailored by controlling the oxygen 2 content, so its refractive index can be adjusted to the desired value. This property is very important in the design of integrated optical wave-guides and anti-reflec- tive stacks. In this work results are reported on our investigation of the optical properties of low pressure chemically vapor deposited (LPCVD) amorphous silicon oxynitride