Paramagnetic Centers in Microcrystalline Silicon

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 τ = 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.