Electron Collisions in the Plasma as a Major Factor in the Growth of Amorphous Hydrogenated Carbon
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ELECTRON COLLISIONS IN THE PLASMA AS A MAJOR FACTOR IN THE GROWTH OF AMORPHOUS HYDROGENATED CARBON S. LIN AND BERNARD J. FELDMAN Department of Physics and Astronomy, Center for Molecular Electronics, University of Missouri - St. Louis, St. Louis, MO 63121 ABSTRACT We have measured the dependence of the optical bandgap of amorphous hydrogenated carbon as a function of self-bias voltage and external bias voltage. It is observed that there is a much stronger variation in the optical bandgap with self-bias voltage than external bias voltage. We explain this observation in terms of collisions in the plasma between electrons and precursor molecules and ions that break carbon-hydrogen bonds. Increased self-bias voltage causes increased collisions, more broken CH bonds, precursor ions and molecules more deficient in hydrogen, leading to grown films with lower hydrogen concentrations and thus smaller optical bandgaps. This interpretation is further supported by plasma emission spectroscopy where the intensity ratio of CH to H lines decreases with increasing self-bias voltage, demonstrating that increased self-bias voltage does lead to increased electron collisions in the plasma and more broken CH bonds.
INTRODUCTION Bubenzer, Dischler, Brandt, and Koidl first systematically studied the parameters associated with the growth of amorphous hydrogenated carbon (a-C:H) and realized that the negative selfbias voltage between the cathode and the anode, VS, was a significant parameter[l]. In particular, they observed that the optical bandgap decreases with increasing VS. This result has been recently confirmed by Tamor, Haire, Wu, and Hass[2], and Varhue and Pastel(3). The self-bias voltage has also been strongly correlated with many other properties of a-C:H films; hydrogen concentration[1, 3,4,5], index of refraction[l], photoluminescence spectra[4], density[l,3,5], Raman spectra[2,5], and electron spin resonance(4]. Couderc and Catherine also reported very similar results and explained this relationship in terms of increasing VS causing increased sputtering of the growing film--primarily by ions accelerated through the sheath region breaking carbon-hydrogen bonds in the growing film, thus decreasing its hydrogen concentration, which in turn decreases its optical bandgap[5]. More recently, Koidl, Wild, Dischler, Wagner, and Ramsteiner established by plasma emission measurements that both sputtering and ion fragmentation occur at the growing surface--namely, ions accelerated through the sheath potential both break bonds on the growing surface and also disintegrate when they collide with the growing surface[6]. We have followed up on these studies by adding an external dc voltage between the cathode and anode, VE, which should cause virtually identical effects to that of an increased VS. It is the observation that this is not the case that has motived us (1) to reexamine the properties of the plasma by emission spectroscopy, and (2) to look for other causes of the relationship between VS and the optical bandgap. Mat. Res. Soc. Symp
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