ENVIRONMENTS OF ION-IMPLANTED DOPANTS IN AMORPHOUS-SILICON AT VARIOUSSTAGES OF ANNEALING

Glancing angle X-ray absorption fine structure (XAFS) spectroscopy has been used to record the varying environments of Ga and As impurities ion-implanted in amorphous silicon (a-Si) - both glow discharge (gd) d eposited and amorphised by ion beam damage (ii). Apart from small diff erences in coordination number and Debye-Waller factor due to the pres ence of hydrogen in gd material the environments for the same impurity in the two forms of a-Si are almost indistinguishable. The electronic activity of gd a-Si is therefore directly attributable to the lower d ensity of intrinsic defects compared to ii a-Si. The coordination numb ers of impurities in as-implanted material are found to differ from th ose of a-Si at various stages of anneal. Ga, for instance, falls from 3.8 (room temperature) to 3 (400-degrees-C) demonstrating a substantia l change in the balance between tetrahedral and naturally bonded sites . Coordination numbers of both Ga and As increase for annealing at 700 -degrees-C and the onset of crystallisation is identified in the appea rance of outer shells of atoms in the impurity environments. Crystalli sation, however, is only partial (approximately 25%) at this temperatu re even though thermal epitaxial regrowth of the majority of the Si ma trix is complete. Clearly recrystallisation of a-Si is inhibited in th e vicinity of naturally bonded dopant sites.