Dangling bonds on silicon

The electron-paramagnetic-resonance signal that appears when Si is crushed, cleaved, or abraded is shown to be proportional to the areas of microcracks induced in the specimen. These are shown to be more prevalent than previously realized. Detailed consideration shows that a wide variety of previously inexplicable data can now be understood. These include some effects of oxygen and hydrogen, variability of signal width, effects of abrasive particle size, and kind of cleavage. The origin of the unpaired electrons is considered and it is concluded that they may be in localized states on the surfaces of the microcracks, such states being apparently a case of Anderson localization. The atoms on the crack surfaces are subject to spatially varying overlap forces and stress fields whose energy range exceeds the normal bandwidth, thus inducing localization. The temperature dependence of the paramagnetism of such states is discussed, including correlation corrections, and shown to yield approximately T^-1 as observed experimentally. A similar explanation applies to Ge. Cleancleaved Si surfaces display negligible surface paramagnetism due to pairing of surface electrons on alternate atom sites. The results suggest that for amorphous Si and Ge, localized states on the surfaces of small atom aggregates should be considered as a possible source of the observed paramagnetism.