Paramagnetic Defects and Photoluminescence in Carbon Rich a-SiC:H Films: Role of Hydrogen and Excess of Carbon
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Paramagnetic Defects and Photoluminescence in Carbon Rich a-SiC:H Films: Role of Hydrogen and Excess of Carbon A.V. Vasin1, A.A. Konchits1, S.P. Kolesnik1, A.V. Rusavsky1, V.S. Lysenko1, A.N. Nazarov1, Y. Ishikawa2, and S. Ashok3 1 NASU, Laskharyov Institute of Semiconductor Physics, Kiev 03028, Ukraine 2 Japan Fine Ceramic Center, Atsuta-ku, Nagoya 456-858, Japan 3 Penn State University, 212 EES Building, University Park, PA, 16802 ABSTRACT We have studied the effect of excess of carbon in a-Si1-xCx:H on local structure reconstruction, evolution of paramagnetic defects and photoluminescence (PL) after vacuum annealing over the temperature range 300-850oC. Two series of samples with stoichiometric (Si0.5C0.5) and carbon-rich (Si0.3C0.7) compositions were studied by Electron Paramagnetic Resonance (EPR), Photoluminescence (PL) and Raman scattering. It is found that there exist two effects responsible for the PL efficiency of a-Si1-xCx:H films: “killing” effect of carbon-related paramagnetic defects and “enhancing” effect of carbon-hydrogen bonds in Si:C–Hn configuration. A microstructure model is proposed for explaining the non-monotonic behavior of integrated PL intensity and concentration of paramagnetic centers and Si:C–Hn bonds as a function of annealing temperature. This model evolves from the following principal processes during thermal treatment of a-Si1-xCx:H: thermally activated release of weakly bonded hydrogen, migration of hydrogen within material and interaction of hydrogen with carbon-related defects. INTRODUCTION Thin-film hydrogenated amorphous silicon-carbon alloy (a-Si1-xCx:H) has attracted interest as a wide band gap material with variable electrical, optical and light emitting properties. One of the main disadvantages of such amorphous hydrogenated semiconductor is a large concentration of metastable defects and a large density of mid-gap electron states associated with these defects. It is believed that metastable defects are responsible for instability of electronic properties of a-Si1-xCx:H films [1]. The origin of these defects and the physical mechanism of electronic instabilities related to these defects are still under discussion in the literature. In our previous paper [2], it was demonstrated that low temperature vacuum annealing reduces the concentration of paramagnetic defects and strongly increases the intensity of photoluminescence of near-stoichiometric a-Si1-xCx:H. This phenomenon was explained in terms of the release of weakly bonded hydrogen and its interaction with unpaired spins. It was also suggested that paramagnetic defects are related to sp2-coordinated carbon. To get more detailed information about the properties of paramagnetic defects, we focus in the present report on the study of the effect of excess of carbon on the evolution of paramagnetic defects and light-emitting properties of a-Si1-xCx:H after vacuum annealing. For this purpose we have performed a comparative study of the effect of vacuum annealing on stoichiometric and carbon-rich a-Si1-xCx:H films.
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