The Structure of Thermal Donors (NL8) in Silicon: a Model Derived From ¹⁷O and ²⁹SI Endor Experiments

Abstract

Results of new electron nuclear double resonance (ENDOR) experiments on the thermal donors (NL8) in silicon are presented. Hyperfine interactions (hi) with 170 were observed. Two types of oxygen were found, of each type there are at least two oxygen atoms. The size and tensor orientations of their hf interactions are consistent with the presence of four oxygen atoms with approx. <111>-symmetry in the core of the thermal donors.Heat treatment of Cz grown, oxygen-rich Si at about 450 0 C leads to the formation of oxygen-related, electrically active shallow donors, called 'thermal donors' (TD) [1]. Although there have been many attemps to determine the microscopic structure, it is still unknown. TD's are assumed to consist of small complexes containing oxygen, although this was contested recently [2]. From infrared measurements is known, that several different TD-species are formed with increasing annealing time [3]. Our previous ENDOR-investigations showed, that all singly ionised TD-species are superimposed in the one ESR line "NL8" [4]. In these ENDOR-measurements only 29 Si-interactions were observed, since the predominant 160 isotope has no nuclear moment. The magnetic 170 isotope has a natural abundance of only 3.7x10-2 %. In order to resolve 0 hyperfine (hf) interactions it was necessary to diffuse 170 enriched oxygen into oxygen free FZ-Si. B-doped Si was used with a B concentration of 4x10 5