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BANDGAP ENGINEERING IN HYDROGENATED SILICON FILMS MADE BY COMBINED HYDROGEN DILUTION AND ATOMIC HYDROGEN TREATMENTS K.C.HSU AND H.L.HWANG Department of Electrical Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan 30043, R.O.C. ABSTRACT Effective bandgap modifications of hydrogenated silicon are demonstrated in this work. These films were obtained by (combined) hydrogen dilution and atomic hydrogen treatments during the deposition. The Si-H bonding configurations were characterized by NMR measurements. The hydrogen dilution tends to create a very sharp line-shape in the NMR spectra as the dilution ratio is increased and the addition of atomic hydrogen treatment can produce the same NMR spectra even at a lower temperature. The optical bandgap of these Si:H samples showed the same tendency that. the bandgap narrowed as both the dilution ratio was increased and the addition of atomic hydrogen treatment. INTRODUCTION The bandgap engineering of hydrogenated silicon is very important in industrial applications of Si:H materials: such as solar cell and optoelectronic devices. It is well known that hydrogen incorl)oration in plasma-enhanced chemical vapor deposition (PECVD) plays an important role in determining the Si-H bonding structures of the hydrogenated silicon (Si:H) thin filns[1-7]. Recently, it was found that amorphous/microcrystalline (a-/Itc-) mixed-phase Si:H[1-3] as well as epitaxial Si films[4] could be obtained at a substrate temperature (Ts) lower than 3000C using high H2/(SitI 4 +I12) gas mixture and/or high rf power. We recently reported the evolution of Si-H microstructures in Si:H films prepared by diluted-hydrogen method and hydrogen-atom-treatment method[5-7]. For the hydrogenated silicon films prepared by the diluted-hydrogen method at the substrate temperature of 2500C, using a gas mixture of 90% hydrogen dilution (H2/(H 2+SiH 4 )=0.9) and a rf power density of 0.1 W/cm12 , a sharp line-shape of Lorentzian form was produced in the NMl spectra . As the substrate temperature was increased beyond 3000C, the sharp line-shape became dominant[5,6]. The Si:t films prepared by the hydrogen-atonm-treatment method, at a lower substrate

temperature of 2000C, exhibited the sirilar NMR spectra with a sharp line-shape dominating[7]. In this paper we report, the NMR results and the ol)tical bandgap of hydrogenated silicon films deposited at. different hydrogen dilution and hydrogen-atom-treatnment. EXPERIMENTAL The detailed sample preparation procedures are described elsewhere[5,6] and also summarized in Table I. The III NMR spectra of hydrogenated silicon films were measured with Larmor frequency 300.13 MHz. The optical bandgap were measured from a Tauc's plot of absorption spectra in the range of visible light. RESULTS AND DISCUSSION The IH NMR spectra of the hydrogenated silicon films were decomposed to determine the Si-H bonding configurations. The detailed NMR data and optical bandgap of the Si:H samples are summarized in Table I. The relative intensities of the sharp, narrow, and broad line-shapes