Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD Sunil Adhikary a, * , Xue M. Tian a , Sudip Adhikari b , Ashraf M.M. Omer b , Hideo Uchida a , Masayoshi Umeno a a Department of Electronics and Information Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan b Department of Electrical and Electronic Engineering, Chubu University, 1200 Matsumoto-cho, Kasugai 487-8501, Japan Available online 8 September 2005 Abstract The effects of CH 4 /C 2 H 4 flow ratio and annealing temperature on the defect states and optical properties of diamond-like carbon (DLC) films deposited by novel microwave surface-wave plasma chemical vapour deposition (MW SWP CVD) are studied through UV/ VIS/NIR measurements, atomic force microscopy, Raman spectroscopy and electron spin resonance analysis. The optical band gap of DLC has been tailored between a relatively narrow range, 2.65 – 2.5 eV by manipulating CH 4 /C 2 H 4 flow ratio and a wide range, 2.5 – 0.95 by thermal annealing. The ESR spin density varied between 10 19 to 10 17 spins/cm 3 depending on the CH 4 /C 2 H 4 flow ratio (1:3 to 3:1). The defect density increased with increasing annealing temperature. Also, there is a strong dependence of spin density on the optical band gap of the annealed-DLC films, and this dependency has been qualitatively understood from Raman spectra of the films as a result of structural changes due to sp 3 /sp 2 carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm). D 2005 Elsevier B.V. All rights reserved. Keywords: DLC films; Microwave surface-wave plasma CVD; Defect density; Optical band gap 1. Introduction Diamond-like carbon (DLC) consists of a network of tetrahedral (sp 3 ) and trigonal (sp 2 ) bonds, and is known to have a wide range of properties such as high hardness, chemical inertness, high thermal conductivity, high electrical resistivity and optical transparency from ultraviolet to infrared. These properties are attractive for optoelectronic device technology. However, the microstructure of DLC is compli- cated due to the presence of both j and k states, and large density of defect states found within the DLC band gap has been a limiting factor for its effective use in device application [1]. The control of the doping of p-type (e.g. Boron and Iodine) and n-type (e.g. Nitrogen and Phosphorous) dopants, bonding defects and optical band gap must be achieved to realize high performance electronic devices [2]. DLC thin films have been deposited by various methods such as pulsed laser deposition (PLD), ion beam sputtering (IBS), and r.f./microwave chemical vapour deposition (CVD), and the results suggested that properties of the films can be tailored by deposition parameters, such as precursor material, and method of deposition [2–4]. In order to improve DLC film quality and develop large area thin film deposition, recently a novel method, namely microwave (MW) surface-wave plasma (SWP) CVD has been employed [5,6]. In this paper, defect states and optical properties of DLC films deposited by MW SWP CVD are studied, together with thermal annealing dependence of the properties of the DLC films. 2. Experimental setup DLC thin films were deposited on n-type silicon and quartz substrates by MW SWP CVD at low temperature (< 100 -C). The details of the CVD system is described elsewhere [6,7]. In brief, unlike other plasma sources the 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.08.030 * Corresponding author. Tel.: +81 568 51 9244; fax: +81 568 51 1478. E-mail address: sunirus2003@yahoo.com (S. Adhikary). Diamond & Related Materials 14 (2005) 1832 – 1834 www.elsevier.com/locate/diamond