Positron-Beam Observation of Dopant-Defect Complexes in Amorphized Silicon
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POSITRON-BEAM OBSERVATION OF DOPANT-DEFECT COMPLEXES IN AMORPHIZED SILICON R. D. Goldberg, T. C. Leung, I. V. Mitchell and Peter J. Schultz Department of Physics, The University of Western Ontario, London, Ontario, Canada, N6A 3K7. ABSTRACT The ability of positrons to probe local defect structures has been utilized to compare differences in residual damage structures in self-irradiated silicon for differing temperatures of the implanted substrate. Sb-doped < 100 > silicon was self irradiated with a 600 keV beam to fluences ranging from 3 x 1014 to 1 x 1016 ions/cm 2 at liquid nitrogen, room and slightly elevated (610 C) temperatures. A defect was identified near the end-of-range of the implanted ions whose concentration diminished at higher implant temperatures, but was only weakly dependent on total fluence. Comparison between Sb- and B-doped Si suggests that the defect is impurity based.
INTRODUCTION Calorimetry studies have shown that amorphous silicon produced by ion implantation is in an unrelaxed configuration [1]. Transition from the unrelaxed to relaxed state is marked by a homogeneous heat release from 100' C up to the region in which recrystallization occurs. Positron annihilation studies of both as-implanted [2, 3] and relaxed [3] amorphous (a-) Si have demonstrated the first evidence for a concentration of vacancytype defects. Prior to this there was little direct evidence for specific defect structures in a-Si, only the dangling bond having been identified by EPR [4]. In all the positronbeam studies of high-fluence self-ion irradiated silicon [3, 5], it has been observed that the depth-dependence of positron annihilation (usually characterized by the S-parameter [6] vs. incident energy) rapidly saturates at the defect-trapped value of S-,1.03 to 1.04xS 1 , where S! is the annihilation line shape parameter for positrons freely diffusing in crystalline (c-) Si. This saturated line shape parameter remains constant until returning to the bulk value, S, at high incident positron energy (depth in the substrate) and indicates that positrons are sensitive only to one predominant vacancy-type defect throughout the damaged region of the material. At lower fluences [5] or for relaxed a-Si [3] a smaller Svalue is observed through the damaged layer, but otherwise the shape of the S-parameter versus energy curve is similar. In one case of high fluence (1 x 1016 ions/cm2 ) 500 keV He ion implantation into Si, evidence for competing defect structures with dissimilar S-parameters was observed following relaxation at 300' C [5]. Recently, investigations of ion bombarded silicon have revealed differences in the accumulation of defects, prior to amorphization, for samples irradiated in different temperature regimes [7, 8]. At low temperatures, defect accumulation appears markedly different than at higher temperatures where dynamic defect annealing plays a significant role. In the present study, variable energy positrons are used to study damage structures in highfluence ion bombarded silicon. Analyses of the material imp
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