Electron spin resonance signals in a-Si: Dangling versus floating bonds

In the literature on a-Si two types of defects are discussed as possible ca ndidates for the origin of the electron spin resonance signals: dangling bo nds (undercoordinated silicon atoms) or floating bonds (overcoordinated sil icon atoms). The problem of the true origin is so far unsettled. Structural models have been developed here which contain only one type of defect. Usi ng a tight-binding approach with an on-site Hubbard U term it is shown that for physically reasonable values of U (U = 0.1-1 eV) only dangling bonds g ive rise to local magnetic moments, whereas floating bonds give only tiny m oments and continuous random networks none. The local magnetic moments and local charges can be understood via bond angle distributions. The hyperfine data are also consistent with the dangling bond interpretation. The depend ence of the results on the potential used for the structural relaxation is investigated. [S0163-1829(99)02732-0].