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Paramagnetic Centers in Microcrystalline Silicon M. M. de Lima, Jr.,1,2 S. Morrison,3 A. LeGeune,3 F. C. Marques2, and P. C. Taylor.1 1 Department of Physics, University of Utah, Salt Lake City, UT 84112, USA, 2 Departamento de Física Aplicada, Instituto de Física “Gleb Wathaghin”, Universidade Estadual de Campinas – Unicamp, Campinas – SP, Brazil. 3 M V Systems, Golden, CO 80401, USA. ABSTRACT We have studied hydrogenated microcrystalline silicon, µc-Si:H, using dark and lightinduced electron spin resonance (ESR). In dark ESR measurements only one center is observed. The g values obtained empirically, assuming axial symmetry, from powder pattern lineshape simulations are g|| = 2.0096 and g⊥ = 2.0031. We suggest that this center is related to defects in the crystalline phase. Another signal, which occurs only after illumination, at low temperatures, is best described by two powder patterns indicating the presence of two centers. One center is slightly asymmetric (g|| = 1.999, g⊥ = 1.996) and the other has a large unresolved broadening such that unique g-values cannot be obtained. The average g value for this center is 1.998. We interpret the light-induced electron spin resonance (LESR) signal as coming from electrons and holes trapped in the band tails at the crystalline grain boundaries. INTRODUCTION The paramagnetic centers in µc-Si:H have been systematically studied since 1994.1 However, a unique interpretation concerning their microscopic origin has not been established so far. The most relevant signal that is usually observed in the dark ESR of µc-Si:H has a zero crossing g value, g0, of 2.005, and it is often associated with silicon dangling bonds (DB). Nevertheless, this center has a clear asymmetric line-shape, which is very different from that of Si DBs in a-Si:H. Some authors have attributed this asymmetry to the presence of two different centers.2,3 One center is supposed to be a Si DB with a g value of 2.0052, while the other (g0 = 2.0043) was initially attributed to electrons trapped in the conduction band tails of the amorphous phase.2 Another interpretation for the second center is the presence of Si DBs in an oxygen-rich environment.3 Recently, this signal was explained as a consequence of Pb centers with g|| = 2.0022 and g⊥ = 2.0078.4 Another very important signal that occurs in µc-Si:H has g0 = 1.998 and it appears after illumination, for high quality samples. Originally, this line was ascribed to free electrons in the conduction band, since its g0 is almost the same as that for conduction electrons in crystalline silicon.1 However, others have attributed the LESR signal to electrons trapped in the localized, conduction band tail states.5-7 µc-Si:H is a material that shows different morphological structures: 1) (usually) randomly oriented crystalline grains; 2) an amorphous phase; 3) boundaries between the amorphous and crystalline phases; and 4) boundaries between different crystalline grains. In principle, the paramagnetic defects in this material could occur in any of these regions making their