The crystalline-to-amorphous transition in ion-bombarded silicon

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Abstract
Hydrogen-free, but defect-rich a-Si can be obtained by ion bombardment of c-Si. We have studied in detail the formation of such material using carrier-removal measurements in the characterization of the bombardment damage. In order to develop an overall view of the disordering process these data are discussed extensively together with results obtained on similar films by Rutherford back-scattering, electron spin resonance, electron microscopy and optical measurements. We conclude that amorphous material generally evolves from an intermediate crystalline phase supersaturated with point defects. The transition occurs locally at the sites of energetic ion impacts into critically predamaged crystalline material. As a consequence, an amorphous layer is built up from small clusters with dimensions typically of the order of 50 Å. From the net expansion of the bombarded layers we conclude that regions of lower atomic density are locally present, very likely a consequence of a structural mismatch between individual amorphous clusters. In this way a heterogeneous defect structure may build up in these films which determines their electronic properties.