The Changing Environments of Dopants in Amorphous Silicon at Various Stages of Annealing

Abstract

ABSTRACTX-ray Absorption Fine Structure (XAFS) spectroscopy measured at glancing angles of incidence has been used to record the varying environments of Ga and As ion-implanted in amorphous silicon at dose levels of 4 1014 atoms cm-2. Both glow discharge deposited films and silicon wafers previously amorphised by ion beam damage were used and apart from small differences relating to the presence of hydrogen in the former the environments of the same impurity in the two forms of amorphous Si were almost indistinguishable for the same beat treatment. The fact that only glow discharge deposited material is electronically active points to differences in the intrinsic defects in these two forms of a-Si. The local atomic structure of impurities in as-implanted specimens, however, was found to alter significantly with annealing. By 400°C, for instance the coordination numbers of both Ga and As approach 3 Si's indicating very few tetrahedral sites are present in the amorphous state. By 700°C, however, the coordination numbers of either impurity Ga and As have increased and the crystallisation local to the impurities can be detected in the XAFS by the appearance of outer shells of atoms. The fraction of crystalline substitutional sites, however, is only around 33% even though the thermal epitaxial regrowth of the remaining Si matrix is practically complete. Recrystallisation of Si is clearly inhibited in the vicinity of dopants by the persistence of 3-fold sites.